Chapter 12. Improving Muscle Performance

At the completion of this chapter, the reader will be able to:

1. Outline the various roles of human skeletal muscle.

2. List the various roles of muscle in the human body.

3. Differentiate among muscle strength, endurance, and power.

4. Describe strategies to increase muscle strength.

5. List the different types of resistance that can be used to strengthen muscles.

6. List the different types of muscle contractions and the advantages and disadvantages of each.

7. Outline the various types of exercise progression and the components of each.

8. Describe strategies to increase muscle endurance.

9. Describe strategies to increase muscle power.

10. Explain the basic principles behind plyometrics.

11. Define delayed onset muscle soreness (DOMS) and explain why it occurs.

12. Define senescence sarcopenia.

13. List the changes that can occur with muscles during aging.

14. Describe the concept of specificity of training.

Movement of the body or any of its parts involves considerable activity from those muscles directly responsible. Muscle is the only biological tissue capable of actively generating tension. This characteristic enables human skeletal muscle to perform the important functions of maintaining upright body posture, moving body parts, and absorbing shock. For body motions to take place, the muscles producing movement must have a stable base from which to work from. Muscles serve a variety of roles depending on the required movement:

• Prime agonist. A muscle that is directly responsible for producing movement.
• Synergist (supporter). Performs a cooperative muscle function in relation to the agonist. Synergists can function as stabilizers or neutralizers.

• Stabilizers (fixators). Muscles that contract statically to steady or support some part of the body against the pull of the contracting muscles, against the pull of gravity, or against the effect of momentum and recoil in certain vigorous movements.
• Neutralizers. Muscles that act to prevent an undesired action of one of the movers.

• Antagonist. A muscle that has an effect opposite to that of the agonist.

Types of Muscle Contraction

The basic function of a muscle is to contract. The word contraction, used to describe the generation of tension within muscle fibers, conjures up an image of shortening of muscle fibers. However, a contraction can produce shortening or lengthening of the muscle, or no change in the muscle length. Thus, three types of contraction are commonly recognized (see Chap. 1): isometric, concentric, and eccentric.

• Isometric contraction. Isometric exercises provide a static contraction with a variable and accommodating resistance without producing any appreciable change in muscle length.1
• Concentric contraction. A concentric contraction (Fig. 12-1) produces a shortening of the muscle. This occurs when the tension generated by the agonist muscle is sufficient to overcome an external resistance and to move the body segment of one attachment toward the segment of its other attachment.1
• Eccentric contraction. An eccentric contraction (Fig. 12-2) occurs when a muscle slowly lengthens as it gives in to an external force that is greater than the contractile force it is exerting.1 In reality, the muscle does not actually lengthen, it merely returns from its shortened position to its normal resting length. Eccentric muscle contractions, which are capable of generating greater forces than either isometric or concentric contractions,2–4 are involved in activities that require a deceleration to occur. Such activities include slowing to a stop when running, lowering an object, or sitting down. Because the load exceeds the bond between the actin and myosin filaments during an eccentric contraction, some of the myosin filaments probably are torn from the binding sites on the actin filament while the remainder are completing the contraction cycle.5 The resulting force is substantially larger for a torn cross-bridge than for one being created during a normal cycle of muscle contraction. Consequently, the combined increase in force per cross-bridge and the number of active cross-bridges results in a maximum lengthening muscle tension that is greater than the tension that could be created during a shortening muscle action.5,6

Four other contractions are worth mentioning:

• Isokinetic contraction. An isokinetic contraction occurs when a muscle is maximally contracting at the same speed throughout the whole range of its related lever.1 Isokinetic contractions require the use of special equipment that produces an accommodating resistance. Both high-speed/low-resistance and low-speed/high-resistance regimens result in excellent strength gains.7–10 The major disadvantage of this type of exercise is its expense. In addition, there is the potential for impact loading and incorrect joint axis alignment.11 Isokinetic exercises may also have questionable functional carryover.12
• Econcentric contraction. This type of contraction combines both a controlled concentric and a simultaneous eccentric contraction of the same muscle over two separate joints.13 Examples of an econcentric contraction include the standing hamstring curl, in which the hamstrings work concentrically to flex the knee while the hip tends to flex eccentrically, lengthening the hamstrings. When rising from a squat, the hamstrings work concentrically as the hip extends and work eccentrically as the knee extends. Conversely, the rectus femoris work eccentrically as the hip extends and work concentrically as the knee extends.
• Isolytic contraction. An isolytic contraction is an osteopathic term used to describe a type of eccentric contraction that makes use of a greater force than the patient can overcome. The difference between an eccentric contraction and an isolytic contraction is that, in the former the contraction is voluntary, whereas in the latter it is involuntary. The isolytic contraction can be used in certain manual techniques to stretch fibrotic tissue (see Chap. 10).

Muscle Performance

The ability of a muscle to carry out its various roles is a measure of muscle performance. Muscle performance can be measured using a number of parameters. These include strength, endurance, and power.

• Strength. Strength may be defined as the amount of force that may be exerted by an individual in a single maximum muscular contraction against a specific resistance, or the ability to produce torque at a joint. Dynamometry is the process of measuring forces that are doing work. Muscle strength can be measured as follows (see Chap. 4):
  • Manual muscle testing (MMT): MMT is an acceptable standardized process utilized to find gross strength deficits and to isolate muscle groups and actions.
  • Using a dynamometer: A dynamometer is a device that measures strength by using a load cell or spring-loaded gauge. These measurements are more objective than MMT. Examples of a dynamometer include the following:
    • Handheld: This is used to assess the grip strength of a patient, or to measure muscle group strength by having the patient exert maximal force against the dynamometer.
  • Isometric: This measures the static strength of the muscle group by stabilizing the extremity using stabilization straps or verbal instruction.
  • Isokinetic: This measures the strength of a muscle group during a movement with constant, predetermined speed.
• Endurance. Muscular endurance is the ability of a muscle, or group of muscles, to continue to perform without fatigue. The nature of muscular endurance encourages the body to work aerobically. This phenomenon, called steady state, occurs after some 5–6 minutes of exercise at a constant intensity level.14 During steady state, the rate of mitochondrial adenosine triphosphate (ATP) production is closely matched to the rate of ATP hydrolysis and demonstrates the existence of efficient cellular mechanisms to control mitochondrial ATP synthesis in a wide dynamic range.15 Endurance exercise training produces an increase in mitochondrial volume density in all three muscle fiber types16 and thus muscle aerobic power. With a higher mitochondrial density in trained muscle, the rate of substrate flux per individual mitochondrion will be less at any given rate of ATP hydrolysis.15 Therefore, the required activation of mitochondrial respiration by adenosine diphosphate (ADP) to achieve a given rate of ATP formation will be less, resulting in increased ADP sensitivity of muscle oxidative phosphorylation.15
• Power. Mechanical power is the product of force and velocity. Muscular power, the maximum amount of work an individual can perform in a given unit of time, is the product of muscular force and velocity of muscle shortening. Muscular power is an important contributor to activities requiring both strength and speed. Maximum power occurs at approximately one-third of maximum velocity.17 Muscles with a predominance of fast-twitch fibers generate more power at a given load than those with a high composition of slow-twitch fibers.18 The ratio for mean peak power production by type IIb, type IIa, and type I fibers in skeletal tissue (see Chap 1) is 10:5:1.19

These three components of muscle performance are important in functional activities as they can allow the patient to interact with their environment in a more efficient and pain-free way through increased movement control and capacity.

Exercise Prescriptions

Before initiating an exercise program, it is necessary to conduct a needs analysis to evaluate the physical requirements and physical attributes of the patient. Determining the selection of exercise is dependent upon the goals and objectives and the needs analysis. As with prescriptions for medications, a successful exercise prescription requires the correct balance between the dose (exercise variables) and the response (specific health or fitness adaptations).20 The dosage of an exercise refers to each particular patient's exercise capability, and is determined by a number of variables (Table 12-1).21 For these variables to be effective, the patient must be compliant and be able to train without exacerbating the condition.22

Depending on the specific program design, resistance training is known to enhance muscular strength, power, or endurance and can provide a potent stimulus to the neuromuscular system. Other variables such as speed, balance, coordination, jumping ability, flexibility, and other measures of motor performance have also been positively enhanced by resistance training.23

It is worth remembering that when the individual trains for two different types of adaptations (e.g., aerobic fitness versus strength), the training stimuli can interfere with one another and result in less improvement in one or both of the effects. For example when strength loads are combined with aerobic training, the aerobic adaptation is not detrimentally affected, but there is a negative impact on strength development.25,26

Each exercise session should include a 5–15 minute warm-up and a 5–15 minute cool-down period.

• Warm-up
  • includes low-intensity cardiorespiratory activities.
  • prevents the heart and circulatory system from being suddenly overloaded.
• Cool-down
  • includes low-intensity cardiorespiratory activities and flexibility exercises.
  • helps prevent abrupt physiological alterations that can occur with sudden cessation of strenuous exercise.

The length of the warm-up and cool-down sessions may need to be longer for deconditioned or older individuals. The type of exercise prescribed determines the type of warm-up.27 The possible benefits of a warm-up prior to physical activity are listed in Table 12-2. The most effective warm-up consists of both general (walking, biking, jogging, and gentle resistive exercises) and specific (movements that are appropriate for the particular activity to be undertaken) exercises.27

Once the warm-up is completed, it is recommended that a flexibility program be incorporated to increase joint movement and muscle extensibility (see Chap. 13).

The initial exercise is prescribed at a level that the patient can perform, before progressing in difficulty. The early goals of exercise are concerned with increasing circulation, preventing atrophy, increasing protein synthesis, and reducing the level of metabolites.22

The following factors should be considered with exercise prescriptions.

Frequency. Training frequency refers to the number of times strength-training sessions are completed in a given period (e.g., the number of work-outs per week). Optimal training frequency depends on several factors such as experience, training volume and status, intensity (see later), exercise selection, level of conditioning, recovery ability, and the number of muscle groups trained per workout session. Strength is most effectively enhanced by a program featuring high resistance and few reps. More training sessions can be accomplished and provide adequate rest if a split routine is used. A split routine divides the training session into groupings, split between the upper body and lower body exercises. Another alternative is to perform a push and pull exercise program, in which the strength training is divided into exercises in which the individual pushes a weight (e.g., triceps extension, bench press), then pulls a weight (e.g., biceps curl, latissimus dorsi pull down).

Based on the studies of isokinetic and eccentric/concentric exercise,28,29 muscle strength recovery follows a steady, nonlinear, and predictable increase over time.21 In the rehabilitation population, strengthening exercises are typically performed on a daily basis initially, with the weight and frequency governed by the individual's response to the exercise. As healing progresses, evidenced by a decrease in pain and swelling and an increase in range of motion, the exercises should be performed every other day. If the training loads are near the maximum capacity, more time for recovery will be required to minimize soreness and provide adequate rest.

Once sufficient strength is attained, even if the patient only performs the strength training at a minimum of once per week, the strength can be fairly well maintained over a 3-month period.30

Repetitions. Initial selection of a starting weight may require some trial and error to find the correct number of reps. For any given exercise, the amount of weight selected should be sufficient to allow 8–12 reps per exercise for three sets with a recovery period between sets of 60–90 seconds to elicit improvements in muscular strength and endurance as well as muscle hypertrophy.31

Duration. Duration refers to the length of the exercise session. Physical conditioning occurs over a period of 15–60 minutes depending on the level of intensity. Average conditioning time is 20–30 minutes for moderate intensity exercise. However, individuals who are severely compromised are more likely to benefit from a series of short exercise sessions (3–10 minutes) spaced throughout the day.

Rest periods. In most functional exercises, fatigue of the muscle being exercised is the goal. However, fatigue may also occur due to lack of coordination, insufficient balance, poor motivation, or the addition of compensatory movements. In addition, fatigue may also be associated with specific clinical diseases, e.g., multiple sclerosis, cardiac disease, peripheral vascular dysfunction, and pulmonary diseases. Because fatigue is detrimental to performance, rest is an important component of any exercise progression. The rest period must be sufficient to allow for muscular recuperation and development while alleviating the potential for overtraining.32 In general, the heavier loads lifted, the longer the rest period between sets. The rest period between sets can be determined by the time the breathing rate, or pulse, of the patient returns to the steady state, or by using the following guidelines:

• muscular endurance: less than 30 seconds rest
• muscular hypertrophy: 60–90 seconds
• muscular strength: 2–5 minutes
• muscle power: 2–5 minutes

A 48-hour rest period between concurrent training sessions is generally recommended.23

One of the biggest challenges for a clinician is to find a balance between achieving a maximum training effect while alleviating fatigue. Fortunately, fatigue has a much faster decay time than the training effect. Thus, to remove the fatigue, the volume of training should be reduced while maintaining the training effect by holding the intensity of the training load constant.35

Intensity. Intensity refers to the power output (rate of performing work) or how much effort is required to perform the exercise. In clinical terms, intensity refers to the weight or resistance lifted by the patient. It is now recognized that an individual's perception of effort (rated perceived exertion or RPE) is closely related to the level of physiological effort.36,37 The Borg Scale is commonly used to help determine a patient's RPE, as an individual's perception of effort is closely related to the level of physiological effort—a high correlation exists between a person's RPE multiplied by 10, and their actual heart rate.36,37 For example, if a person's RPE is 15, then 15 × 10 = 150; so the heart rate should be approximately 150 beats per minute. A cardiorespiratory training effect can be achieved at a rating of “somewhat hard” or “hard” (13–16). Note that this calculation is only an approximation of heart rate, and the actual heart rate can vary quite a bit depending on age and physical condition. The original scale introduced by Borg37 rated exertion on a scale of 6–20, but a more recent one designed by Borg included a category (C) ratio (R) scale, the Borg CR10 Scale® (Table 12-3).

Patient responses that can modify the intensity include increases in pain level, muscle fatigue, time taken to recover from fatigue, cardiovascular response, compensatory movements, level of motivation, and degree of comprehension.

Speed of Exercise. The speed of the exercise should depend on the imposed demands of an individual. In some cases, increasing the speed of an exercise following the initial phase of learning a particular exercise and developing proficiency in the performance of the exercise is beneficial.38 In addition, higher velocity training appears to improve peak power measures.23

Variation. Variation in training is a fundamental principle that supports the need for alterations in one or more program variables over time to allow for the training stimulus to remain optimal (Table 12-4).23 The concept of variation has been rooted in program design universally for many years. The most commonly examined resistance training theory is periodization. Periodization is the systematic process of planned variations in a resistance-training program over a specified training cycle to prevent overtraining and to perform at peak or optimum levels at the right time.39 Two models of periodization are classic (linear) and undulating (nonlinear)23:

• Classic. This model is characterized by high initial training volume and low intensity. As training progresses, volume decreases and intensity increases in order to maximize strength, power, or both.
• Undulating. The nonlinear program enables variation in intensity and volume within each 7- to 10-day cycle by rotating different protocols over the course of the training program. Nonlinear methods attempt to train the various components of the neuromuscular system within the same 7- to 10-day cycle. During a single workout, only one characteristic is trained in a given day (e.g., strength, power, or muscular endurance).

It has been shown that systematically varying volume and intensity is most effective for long-term progression.23,39 Periodization consists of several phases including the macrocycle, mesocycle, and microcycle. The macrocycle for an athlete can, for example, last for 1 year ending with the end of a competitive season. The macrocycle is divided into several mesocycles. A mesocycle defines distinct variations in the resistance and exercise program. For example, a mesocycle for an athlete can be divided into three distinct phases27:

• Conditioning. This phase emphasizes developing aerobic and anaerobic fitness, strength, and power.
• Precompetition. This phase emphasizes correct technique.
• Competition. During this phase, the emphasis is on competitive performance while maintaining basic conditioning.

The final breakdown of periodization is the microcycle, which involves changes in training parameters such as intensity, work–rest ratios, sets, reps, exercise order, and specific exercises, and generally lasts for 1–2 weeks. A common format is a 4-week mesocycle that consists of three microcycles of 1 week each, in which the loads are progressively increased, followed by a 1-week microcycle of reduced volume and intensity.

When prescribing a progressive resistive exercise (PRE) regimen, the clinician should consider the individual's current health and fitness status, goals, access to appropriate equipment, and time available for training.40 Training programs prescribed for competitive athletes, which often include exercises designed specifically to improve the development of explosive power, are generally inappropriate for children, untrained adults, elderly persons, or patients with chronic disease(s). Exercise progression in the orthopaedic population, including the post surgical population, is determined by the stage of healing and the degree of irritability of the structure, which are factors of patient response, as healing relates to signs and symptoms.

Specificity of Training

It appears that the muscle response to resistance training for people who have a broad range of conditions and who might consult a physical therapist is similar to muscle responses reported in young people without impairment.41 Resistance training can have a beneficial effect in populations where pain is a particular problem, such as people with low back pain and people with osteoarthritis. In addition, resistance training can have a beneficial effect on conditions such as high blood pressure, fracture rehabilitation, and cardiovascular disease. The effect of resistance training on other impairment parameters, such as bone mineral density, fat mass, and aerobic capacity, remains inconclusive.41 There is also evidence to suggest that improvements in the ability to generate muscle force can carry over into an improved ability to do everyday tasks. However, the effects are generally quite modest, and there are a number of examples in the literature where significant improvements in activity were not demonstrated after resistance training.41 Part of the problem with drawing conclusions from the literature is the lack of details provided as to the specifics of the exercises prescribed. Specificity of training is an accepted concept in physical therapy rehabilitation. This concept involves the principle of the specific adaptation to imposed demand (SAID). Thus, the focus of the exercise prescription should be to improve the strength and coordination of functional or sports-specific movements with exercises that approximate the demands of the desired activity (speed, agility, strength, power, endurance, etc.).

Three important concepts are related to the SAID principle:26

• Overload. When cells, tissues, and organs are loaded beyond what they normally are required to do, they will adapt to permit them to deal with these new loads more effectively. These training effects are specific to the energy supply systems that have been utilized, the locale of the stimulus, the specific muscle groups, the joint action(s), the type of contraction, and the speed of contraction. When loads are applied they should be specific to the desired effect (SAID), and be appropriate to the individual in terms of frequency, intensity, and duration. It is important to remember that once an adaptation has taken place to a specific load, this load is no longer an “overload” and the load must be progressively increased to get further improvements.
• Underload. When cells, tissues, and organs are loaded at a level below what is normally performed, the body will adapt by decreasing its ability in the underloaded component (detraining). Detraining occurs quickly after the cessation of training—a reduction in the VO2 max can be detected within four weeks of detraining.

• Overtraining. Overtraining occurs because of an inappropriate amount of rest, recovery, and unloading. It is manifested as an inability to recover from exercise, lowered resistance to injury, and chronic fatigue or exhaustion, and is caused by a loss in the body's adaptive capability after chronic high-volume loading or in response to a too large increase in either duration or intensity of training. The signs and symptoms of overtraining include loss of appetite, inability to sleep, lethargy, muscle soreness, chronic fatigue, declining performance, and altered metabolism.42–44

The SAID principle can be applied by exercising the muscles along each extremity and within the trunk in functional patterns.45 The exercise component of the intervention should be as specific as the manual technique used in the clinic. Muscles can be classified into tonic and phasic groups according to how muscles develop from the myotomes46 (Table 12-5).

Speed-strength training applies the principles of specificity of training and is typically used with highly conditioned athletes who want to take their performance to the next level. Speed-strength training involves taking some of the basic movements of a task and increasing the resistance. For sports, such as baseball and golf, athletes can use devices, such as an oversized ball or weighted golf club, to train the arms and trunk to work against a greater resistance. Sprinters have long benefited from the use of a small parachute to increase wind resistance, or by dragging a tire fastened by a rope. The theory behind speed-strength training is that once the higher resistance is removed the athlete's speed is improved when they perform the activity under normal resistance. Wherever possible, strength testing by the clinician should assess the function of a muscle. If a power muscle is assessed, its ability to produce power should be assessed. In contrast, an endurance muscle should be tested for its ability to sustain a contraction for a prolonged period, such as occurs with sustained postures.

In addition to speed-strength training, agility drills, rapid reflex, and specific skill training should form the core of many sport-specific exercise programs.

Delayed Onset Muscle Soreness

Familiar to most individuals at some time, muscular soreness is one of the drawbacks to participating in an exercise program involving activity beyond what one usually experiences. The purpose of the recovery phase is to restore the muscles in the body to preexercise levels through promotion of venous turn and removal of metabolites from muscle, reestablishment of fluid balance, replacement of depleted fuel energy reserves, and relaxation in muscles that were active.26 The recovery period is also necessary to replenish muscle glycogen following prolonged exercise of the continuous or integral nature. If the recovery period is insufficient, acute or DOMS can result. Acute soreness is apparent during the later stages of an exercise bout and during the immediate recovery period.38 This results from an accumulation of end products that occurs with exercise, H+ ions, and lactate, but generally disappears between 2 minutes and 1 hour after cessation of exercise.38 DOMS, on the other hand, appears 24–56 hours after the exercise bout.49 DOMS can exhibit as anything, from minor muscle soreness to debilitating pain and swelling, but is most commonly described as causing a reduction in joint range of motion, shock attenuation, and peak torque.50 A recent review of DOMS confirms that the mechanisms, treatment strategies, and impact on athletic performance remain uncertain.51 Recognized mechanisms include lactic acid and potassium accumulation, muscle spasms, mechanical damage to the connective tissues, inflammation, enzyme efflux secondary to muscle cell damage, and edema.51 There is certainly a marked increase in the proportion of disrupted muscle fibers after eccentric as compared with concentric exercise, which has been roughly correlated to the degree of DOMS.52 Eccentric exercise is also linked to morphologic and metabolic signs of muscle alteration: myofibrillar damage along the Z-band,53,54 mitochondrial swelling,53,54 increased intramuscular pressure,53,54 and impaired glycogen resynthesis.

Given the role of eccentric exercise in DOMS, prevention of DOMS involves the careful design of any eccentric program, which should include preparatory techniques, accurate training variables, and appropriate aftercare, including a cool-down period of low-intensity exercise to facilitate the return of oxygen to the muscle. It is a widely held belief among athletes and coaches that massage is an effective therapeutic modality that can enhance muscle recovery and reduce soreness following intense physical activity.55 However, the actual scientific literature does not tend to support the positive efficacy of manual massage as a postexercise therapeutic modality in the athletic setting.55–57

To help prevent DOMS, there is some controversy as to whether a cool-down or warm-up should be used. A number of studies advocate a short (5–10 minutes) low-intensity warm-up (30–65% of VO2 max) to assist in maintaining venous return and removal of metabolites above levels in passive rest.58,59 However, one randomized controlled trial found that whereas a 10 minute warm-up reduced perceived muscle soreness (measured on a 100-mm visual analogue scale) 48 hours after exercise (mean effect of 13 mm, 95% CI 2 to 24 mm), a 10 minute cool-down had no apparent effect (mean effect of 0 mm, 95% CI 11 to 11 mm).60

If the prevention of DOMS is unsuccessful, the intervention should include, as appropriate, rest, local measures to reduce edema (e.g., cryotherapy, elevation of the involved limb(s)), drug therapy (typically nonsteroidal anti-inflammatory agents), or further exercise (aerobic submaximal exercise with no eccentric component, e.g., swimming, biking, or stepper machine, pain-free flexibility exercises, and high-speed [300 degrees per second] concentric-only isokinetic training).21,61

Types of Resistance Used in Exercise Programs

Resistance-training programs have the potential for many positive benefits provided the training is appropriately progressed to meet the needs of a person as his or her strength increases. Resistance can be applied to a muscle by any external force or mass, including any of the following.

Gravity

Gravity alone can supply sufficient resistance for a weakened muscle. With respect to gravity, muscle actions may occur

• in the same direction as gravity (downward);
• in the opposite direction to gravity (upward);
• in a direction perpendicular to gravity (horizontal);
• in the same or opposite direction to gravity, but at an angle.

The direction in which the muscle is working determines the role that gravity plays and the role that the muscle must play in order to counteract the forces of gravity. For example, if an arm muscle is working to lower a book to a table, the biceps muscle works eccentrically against the force of gravity to control the speed of the lowering. If the muscle works to lift the book from the table, the biceps muscle must work concentrically against the force of gravity.

Body Weight

A wide variety of exercises have been developed that do not require any equipment but rely solely on the patient's body weight for the resistance (e.g., push-up).

Small Weights

Cuff weights, dumbbells, and surgical tubing (elastic resistance) are economical ways of applying resistance. Small weights are typically used to strengthen the smaller muscles or to increase the endurance of larger muscles by increasing the number of reps. Free weights also provide more versatility than exercise machines, especially for three-dimensional exercises, as the movements do not occur in a straight line or plane. The disadvantage of free weights is that they offer no variable resistance throughout the range of motion and so the weakest point along the length–tension curve of each muscle limits the amount of weight lifted. According to a recent study, 96% of rehabilitation professionals use elastic resistance with their patients, and 85% of home exercise programs prescribed by rehabilitation professionals require elastic-resistance bands or tubing.9,32,62–67 Elastic resistance offers a unique type of resistance that cannot be classified within the traditional subcategories of strengthening. The amount of variable resistance offered by elastic bands or tubing is a factor of the internal tension produced by the material. This internal tension is a factor of the elastic material's coefficient of elasticity, the surface area of the elastic material, and how much the elastic material is stretched.32 It is commonly believed that the resistance provided by these bands or tubing increases exponentially at the end range of motion. However, the forces produced by elastic resistance are linear until approximately 500% elongation, at which point the forces increase exponentially.32 As the elastic resistance is not stretched more than 300% in prescribed exercises, the exponential increase should not be attained. In addition, the torque production of elastic-resistance exercises is similar to that produced by a concentric/eccentric dumbbell exercise: a bell-shaped curve.68

Medicine Balls

Medicine balls provide the opportunities to improve strength, balance, and coordination through dynamic movements by training the body as a functional unit and strengthening the core and trunk musculature. Both upper and lower body medicine ball exercises can be performed using a variety of speeds, and medicine ball weights and sizes.

Exercise Machines

In situations where the larger muscle groups require strengthening, and where exercises need to be performed in straight and supported lines, a multitude of specific exercise machines can be used. These machines are most often used in the more advanced stages of a rehabilitation program when more resistance can be tolerated, but can also be used in the earlier stages depending on the size of the muscle undergoing rehabilitation. Examples of these machines include the multi-hip, the lat pull-down, the leg extension, and the leg curl machine. Exercise machines are often fitted with an oval-shaped cam or wheel that mimics the length of tension curve of the muscle (Nautilus, Cybex). Although these machines are a more expensive alternative to dumbbell or elastic resistance, they do offer some advantages:

• They provide more adequate resistance for the large muscle groups that cannot be achieved with free weights/cuff weights, or manual resistance.
• They are typically safer than free weights as control/support throughout the range is provided.
• They provide the clinician with the ability to quantify and measure the amount of resistance that the patient can tolerate over time (as compared with elastic resistance).

The disadvantages of exercise machines are as follows:

• the inability to modify the exercise to be more functional or three-dimensional and
• the inability to modify the amount of resistance at particular points of the range.

Manual Resistance

Manual resistance is a type of active exercise in which another person provides resistance manually. An example of manual resistance is proprioceptive neuromuscular facilitation (PNF) (see Chap. 10).

The advantages of manual resistance, when applied by a skilled clinician, are as follows69:

• Control of the extremity position and force applied. This is especially useful in the early stages of an exercise program when the muscle is weak.
• More effective reeducation of the muscle or extremity, using diagonal or functional patterns of movement.
• Critical sensory input to the patient through tactile stimulation and appropriate facilitation techniques (e.g., quick stretch).
• Accurate accommodation and alterations in the resistance applied throughout the range. For example, an exercise can be modified to avoid a painful arc in the range.
• Ability to limit the range. This is particularly important when the amount of range of motion needs to be carefully controlled (postsurgical restrictions).

The disadvantages of manual resistance include the following:

• The amount of resistance applied cannot be measured quantitatively.
• The amount of resistance is limited by the strength of the clinician/caregiver or family member.
• Difficulty with consistency of the applied force throughout the range, and with each rep.

Improving Muscle Performance

Progression is defined as “the act of moving forward or advancing toward a specific goal over time until the target goal has been achieved.”23 A progressive program is recommended to condition the patient for a return to activity and to prevent overload injuries. Such a program should address the following areas:70–74

• Flexibility. Attempts should be made to improve general body flexibility, with an emphasis on the specific activity or exercise. The general flexibility exercises should address the entire kinematic chain and not just the joint in question (e.g., shoulder rotation and elbow motion in the arm, low back, hip rotation, and hamstrings in the legs).
• Strengthening. The exercises to improve strength should be applied in appropriate amounts and locations to address sport- or functional-specific activities.
• Power. Power is incorporated through the use of rapid movements in appropriate planes with weights and ballistic activities.
• Endurance. Endurance can be built up with aerobic exercises.

Optimal resistance training progressions should always be based on sound rationale and should always be individualized to meet specific training goals. Each progression is made more challenging by altering one of the parameters of exercise including the intensity, duration, and frequency which are modified according to patient response.

Increasing Strength

Strengthening of a muscle occurs when the muscle is forced to work at a higher level than that to which it is accustomed. The extent of the functional and health benefits from exercise training depends on several factors including initial performance and health status, along with the exercise prescription variables previously discussed such as frequency, duration, intensity, variation, and rest intervals.75

To increase muscle strength most effectively, a muscle must work with increasing effort against progressively increasing resistance.3,76 In a strength training program, the term load refers to the amount of weight used in a specific exercise. Most strength and conditioning programs describe the load as a percentage of a RM. The RM can be expressed as the greatest amount of load lifted one time (1 RM). As the load becomes heavier, the number of times an individual can lift (perform a rep) the load decreases, whereas if the load becomes lighter, more reps can be accomplished. If resistance is applied to a muscle as it contracts so that the metabolic capabilities of the muscle are progressively overloaded, adaptive changes occur within the muscle, which make it stronger over time.4,77 These adaptive changes include the following2,3,78–82:

• An increase in the size of the muscle (hypertrophy). In normal individuals, an increase in strength after a resisted exercise program is thought to initially occur as a result of neural adaptation, followed by hypertrophy of muscle fibers if the exercise program is continued for a longer period. Mitochondria are the main subcellular structures that determine the oxygen demand of muscle. There is consensus that there is a dilution of mitochondrial volume density through an increase in myofibrillar (i.e., contractile protein) volume density as a consequence of strength-type exercise training.14,83 This increase in myofibrillar volume density, or hypertrophy that occurs with strength training is regarded as the main cause for the overall increase in the anatomical cross-sectional area (CSA) of an entire muscle group. The fiber hypertrophy is typically greater for fast-than for slow-twitch muscle fibers.84
• An increase in the force per unit area. Strength training has also been shown to lead to an increase in the force per unit CSA of the muscle. This effect has been attributed either to an increase in neural drive85 or to an actual increase in muscle specific tension due to a denser packing of muscle filaments.86 A denser packing of contractile tissue along the tendon could theoretically increase the angle of pennation of muscle fibers.87
• A reduction in the time to peak force.88 This can be defined as the time from the onset of muscle activation until peak force is attained.
• An increase in the efficiency of the neuromuscular system. This increased efficiency results in
  • an increase in the number of motor units recruited,
  • an increase in the firing rate of each motor unit,
  • an increase in the synchronization of motor unit firing, and
  • an improvement in the endurance of the muscle.
• Stimulation of slow-twitch fibers (when performing workloads of low intensity) and stimulation of fast-twitch IIa fibers (when performing workloads of high intensity and short duration).
• Rhythmic activities increase blood flow to the exercising muscles via contraction and relaxation.
• The power of the muscle improves.
• Improved bone mass (Wolff's Law).
• Increase in metabolism/calorie burning/weight control.
• Increased intramuscular pressure results from a muscle contraction of about 60% of its force generating capacity.
• Cardiovascular benefits when using large muscle groups. Strength training of specific muscles has a brief activation period and uses a relatively small muscle mass, producing less cardiovascular metabolic demands than vigorous walking, swimming, etc.
• An increase in the rate of force development.88
• Conversely, a muscle can become weak or atrophied through:
  • Disease.
  • Neurologic compromise.
  • Immobilization. Continuous immobilization of skeletal muscle tissues can cause some undesirable consequences. These include weakness or atrophy of muscles. Muscle atrophy is an imbalance between protein synthesis and degradation. After modest trauma, there is a decrease in whole body protein synthesis rather than increased breakdown. With more severe trauma, major surgery, or multiple organ failure, both synthesis and degradation increase, the latter being more enhanced.
  • Disuse.

Strengthening Exercises Based on Contraction Type

As outlined earlier in the chapter, a muscle can contract in a variety of ways. Each type of contraction has its advantages and disadvantages.

Isometric Exercises. Studies have demonstrated that a 6-second hold of 75% of maximal resistance is sufficient to increase strength when performed repetitively.89,90 Isometric exercises have an obvious role when joint movement is restricted, either by pain or by bracing and casting. The primary role in this regard is to prevent atrophy and a subsequent decrease of ligament, bone, and muscle strength. Isometric exercises have the following disadvantages:

• Strength gains are not increased throughout the range (unless performed at multiple angles).
• They do not activate all of the muscle fibers (primary activation is of slow-twitch fibers).
• There are no flexibility or cardiovascular fitness benefits.
• Peak effort can be injurious to the tissues because of vasoconstriction and joint compression forces.
• There is limited functional carryover.12
• Considerable internal pressure can be generated, especially if the breath is held during contraction. This can prove injurious to patients with weakness in the abdominal wall (hernia) or cardiovascular impairment (increase in blood pressure through the Valsalva maneuver)91 even if performed correctly.

Concentric Exercises. These contractions are commonly used in the rehabilitation process and in activities of daily living. The biceps curl and the lifting of a cup to the mouth are examples, respectively. Concentric exercises are dynamic and allow the clinician to vary the load from constant, using free weights, to variable, using an exercise machine. The speed of contraction can also be manipulated depending on the goal of the intervention. A number of programs have been designed for the progression of concentric exercise programs. Some of these PRE programs are summarized in Tables 12-6 and 12-7. Determining the RM is dependent upon the goals of the individual and his or her functional demands. A 10 RM testing load is often recommended to estimate the 1 RM. The rehabilitation program designed by DeLorme is based on a rep maximum of 10 (10 RM).79 The Oxford technique92 reverses the percentage of maximum in the three sets (Table 12-6), whereas the MacQueen technique93,94 differentiates between beginning, intermediate, and advanced levels. In contrast, the program designed by Sanders is based on a formula that uses a percentage of body weight to determine starting weights.95 The daily adjusted progressive resistive exercise (DAPRE) program, designed by Knight, allows for individual differences in the rates at which patients progress in their rehabilitation programs.96 (Table 12-7) Finally, the Berger technique97 selects an amount of weight that is sufficient to allow 6–8 RM in each of the three sets (the initial selection of a starting weight typically requires some trial and error), with a recovery period of 60 to 90 seconds between sets. If at least three sets of 6 RM cannot be completed, the weight is considered too heavy and is reduced. Conversely, if it is possible to do more than three sets of 8 RM, the weight is considered too light and is increased.

A typical exercise prescription for high school, collegiate, and professional athletes includes three or more sets of a 6–12 RM per exercise performed 3 days/week.40 For others, a 10–15 RM is generally recommended.23 Initially, the strengthening exercises are performed on a daily basis, with the variables dependent on the patient's response to the exercise. This is progressed to every other day, and then to at least three times per week, but no more than four times per week.

Eccentric Exercises. The clinical indications for the use of eccentric exercise are numerous21 (Table 12-8).

Isokinetic Exercises. An isokinetic exercise is one in which the length of the muscle is changing while the contraction is performed at a constant velocity.100 Isokinetic exercise requires special equipment that produces an accommodating and variable resistance. The main principle behind isokinetic exercise is that peak torque (the maximum force generated through the range of motion) is inversely related to angular velocity, the speed that a body segment moves through its range of motion. Put more simply, the resistance provided by an isokinetic machine moves at some pre-set speed, regardless of the torque applied to it by the individual, thus, the key to isokinetic exercise is not the resistance but the speed at which the resistance can be moved.101

Advantages for this type of exercise:

• Both high-speed/low-resistance and low-speed/high-resistance regimens result in excellent strength gains.7–10
• Both concentric and eccentric resistance exercises can be performed on the machines.
• Provides objective and quantifiable measurements of muscular strength.
• The machines provide maximum resistance at all points in the range of motion as a muscle contracts.
• The gravity-produced torque created by the machine adds to the force generated by the muscle when it contracts resulting in a higher torque output than it actually created by the muscle.

Disadvantages of this type of exercise:

• Expense. Isokinetic machines, which rely on hydraulic, pneumatic, and mechanical pressure systems, are very expensive, with purchase prices ranging between $50,000 and $80,000.
• Requires maximal effort to work effectively.
• The increased potential for impact loading and incorrect joint axis alignment.11
• Questionable functional carryover.12

Increasing Muscle Endurance

It is well established that endurance training results in enhanced performance and delayed onset of fatigue during endurance exercise. Endurance exercise training also leads to a shift of skeletal muscle mitochondria toward an increased use of lipids as a substrate source both at the same absolute and at the same relative exercise intensity.14,102

Muscular endurance training is typically prescribed during the general preparation phase of training to prepare the body for the increased work demands that will be required, and to program the body's neuromuscular coordination systems. It is worth remembering that working at a level to which the muscle is accustomed improves the endurance of that muscle but does not increase its strength.

To increase muscle endurance, exercises are performed against light resistance for many reps (no fewer than 20 per set) so that the amount of energy expended is equal to the amount of energy supplied. The amount of weight used during muscular endurance training can be determined by using a RPE scale, in which 1 is very light exertion and 10 is intense exertion. Using the example of a bench press, if an athlete plans to work at around level 3 and his maximum weight for the bench press is 220 pounds, he should reduce his workout weight by 70% (154 pounds) and increase the number of reps.

The number of reps used is a factor of the speed of one rep, and how many reps the athlete can complete in 60 seconds. For example, if under normal training conditions the athlete takes 3 seconds to raise the weight during a biceps curl and 3 seconds to lower it, the muscle is under tension for 6 seconds and the athlete is working at a speed of 10 reps per minute. By increasing the speed of the rep to 4 seconds (the time the muscle is under tension), the athlete must achieve at least 15 reps in order to build muscular endurance.

The major drawback of muscular endurance training is the increased potential for overuse injuries. This can be offset by manipulating one or more of the training variables, such as sets, loads, tempo, rest periods between sets, number of exercises, hand position, and grip width.

Increasing Muscle Power

Power is increased by having a muscle work dynamically against resistance within a specified period. In the context of rehabilitation, plyometric training can be viewed as the bridge between pure strength and sports-related exercises.103

Plyometrics

It has been demonstrated that when a concentric contraction is preceded by a phase of active or passive stretching, elastic energy is stored in the muscle. This stored energy is then used in the subsequent contractile phase. For example, during some functional activities, such as jumping, the movement involves an eccentric loading of a muscle or muscle group, followed immediately by a concentric muscle action, as part of a stretch-shortening cycle.81 The eccentric/concentric actions utilize the stretch reflex or stretch-shortening cycle in which the muscle is preloaded with energy during the eccentric phase (much like stretching a rubber band apart) and the release of that stored energy for subsequent muscular actions (releasing the rubber band).104 The stored elastic energy within the muscle is used to produce more force than can be provided by a concentric contraction alone (see later).105–107

The goal of plyometric training is to decrease the amount of time required between the yielding eccentric muscle contraction and the initiation of the overcoming concentric contraction.108

The training system of plyometrics is credited to Yuri Verhoshanski,109 the renowned Soviet jump coach of the late 1960s, although the actual term plyometrics was first introduced in the mid-1970s by an American track coach Fred Wilt.110 The term plyometrics, when broken down to the roots of the words, is a little confusing. Plyo comes from the Greek word pleythein, which means to increase, and metric, which means to measure. Plyometrics is associated with an enhancement of the ability of the muscle–tendon unit to produce maximal force in the shortest amount of time through activation of the myotatic reflex.106,111–113 The tendon portion of the muscle–tendon unit has been found to be the main contributor to muscle–tendon unit length changes and the storage of elastic potential energy.114

Movement patterns in both athletics and activities of daily living involve repeated stretch-shortening cycles, in which an eccentric movement must be stopped and converted into a concentric movement in the opposite direction. When a muscle contracts in a concentric fashion, most of the force produced comes from the muscle fiber filaments sliding past one another. Force is registered externally by being transferred through the series elastic component of the muscle, and as the muscle lengthens like a spring during the eccentric contraction, the series elastic component is also stretched and contributes to the overall force production.108

Acceleration and deceleration are the most important components of all task-specific activities.22 These activities use variable speed and resistance throughout the range of contraction, stimulating neurologic receptors and increasing their excitability. The nerve receptors involved in plyometrics are the muscle spindle, the Golgi tendon organ, and the joint capsule/ligamentous receptors (see Chap. 3). These neurologic receptors play an important role in fiber recruitment and physiologic coordination. Plyometric activities serve to improve the reactivity of these receptors. Two other reflex mechanisms, which result from neural signals generated by muscle receptors that project back to the muscle of origin as well as other muscles, can be initiated during plyometric exercise and may assist with motor coordination and joint stability:115

• Length feedback. These signals, generated by muscle stretch, occur around the same time frame as the stretch reflex, and serve to link muscles that are synergists through excitatory feedback and those with opposite actions by reciprocal inhibition.116 Length feedback also links monoarticular muscles with excitatory feedback and contributes to joint stiffness.116
• Force feedback. These are signals generated by muscle force, which are provided by stimulation of the Golgi tendon organ, and which connect muscles that cross different joints and exert torque in different directions through inhibitory feedback. Force feedback regulates coupling between joints.116

Together, length and force feedback induced during the loading phase of a plyometric activity have the potential to improve neuromuscular control.115

The physiology of plyometrics can be broken down into a number of phases:

1. A loading phase (eccentric, deceleration, setting, yielding, or cocking phase), in which the muscle–tendon units of the prime movers and synergists are stretched as a result of kinetic energy or loading applied to the joint and begin to perform negative work.115 Stretching of the muscle–tendon unit during this phase elicits the stretch-shortening cycle, which results in enhanced force production and performance when compared to the absence of stretch.46 Stretch of active muscle during the loading phase elicits two mechanisms associated with the stretch-shortening cycle: muscle “potentiation” and activation of the muscle spindle115:

   • Muscle potentiation. An alteration of the muscle contractile properties that leads to higher force production through an increase in the proportion of cross-bridges attached to actin and a decrease in the cross-bridge detachment rate.
   • Muscle spindle activation. Sensory information from the muscle spindle is passed through a monosynaptic reflex loop to provide excitatory feedback to the same muscle (see Chap. 3). This results in short-latency reflex muscle activity (myotatic or stretch reflex). However, the stretch reflex may not be elicited in all muscles that are stretched during a plyometric activity. Monoarticular muscles are consistently activated, but biarticular muscles are not. Differences in reflex muscle activity between monoarticular and biarticular muscles may be explained by differences in muscle length changes during loading. In certain activities, some of the fascicles of biarticular muscles undergo lengthening (eccentric action), while the other muscle fascicles act nearly isometrically. This suggests that monoarticular muscles may benefit more than biarticular muscles from stretch reflex force augmentation for enhanced work output.

2. A coupling (amortization, transmission, pay-off, or reversal) phase: This phase marks the transition between the loading phase and the unloading phase.46 The coupling phase, the definitive phase of plyometric exercise, is generally a period of quasi-isometric muscle action.115 If this transition phase is not continuous, the activity will no longer be considered plyometric because the benefits of the stretch-shortening cycle will be lost.115

3. An unloading phase: The unloading (rebound, shortening, push-off, or propulsion) phase of a plyometric exercise occurs immediately after the coupling phase and involves shortening of the muscle–tendon unit. In the biphasic analysis of plyometric jumps, the unloading phase begins at the start of upward movement of the center of mass and ends when ground contact ceases.117 Most plyometric activities terminate in a momentum phase, during which body segments continue to move as a result of the forces generated in the unloading phase.115

By reproducing these stretch-shortening cycles at positions of physiologic function, plyometric activities stimulate proprioceptive feedback to fine-tune muscle activity patterns. Stretch-shortening exercise trains the neuromuscular system by exposing it to increased strength loads and improving the stretch reflex46 (see Chap. 3). The degree of performance enhancement during the momentum phase is dependent on the magnitude of the forces and quickness of movement during the plyometric activity.115 In particular, higher forces are associated with a shorter coupling phase118 and greater energy storage in the series elastic component.119 Performance is also a consequence of the total contact duration (loading through unloading phases), because as the contact duration becomes shorter, higher forces and joint moments are generated120 and the tendon contribution to work is increased.115,121

The mechanisms by which performance is enhanced during plyometric exercise depend on a number of factors:

• The activity must impart higher forces and faster speeds of movement.
• Prolonged contact times should be avoided. The goal of plyometric training is to decrease the amount of time required between the loading phase and the initiation of the overcoming concentric contraction. Prolonged contact times may result when the intensity is too high during the loading phase or when the transition between the loading and unloading phases is not continuous.115
• Plyometric exercises must be initiated at a lower intensity and progressed to more difficult, higher-intensity levels according to tolerance. Before initiating plyometric exercises, the clinician must ensure that the patient has an adequate strength and physical condition base.46 Initially, the patient is instructed to perform fewer sets and reps. Later, the patient is permitted to do more sets, but not more reps.

Plyometric exercises were traditionally designed to enable lower extremity muscles—primarily the thighs, quadriceps, hamstrings, and calves—to attain maximal power using high-intensity workouts in short spurts of hops, leaps, or bounds. Therefore, care should be taken when applying the physiological principles derived from lower extremity investigations to upper-body and trunk applications, as it is unknown whether the upper extremity and trunk will respond in similar manner.115 Plyometrics are typically introduced into the rehabilitation program in the later stages as many plyometric exercises, even at low intensities, expose joints to substantial forces and movement speeds.115 Plyometric exercise is indicated for those patients who desire to return to activities that include explosive movements. Contraindications for initiating plyometric exercise are acute inflammation or pain, immediate postoperative status, and joint instability.46 Joint pathologies such as arthritis, bone contusion, or chondral injury are relative contraindications, depending on the ability of the tissue to tolerate the high forces and joint loading required in many plyometric activities.115 Musculotendinous injury is also a relative contraindication until the tissue is able to handle the rapid and high forces of a plyometric exercise.115

Guidelines for initiating plyometric exercise in rehabilitation are poorly developed. Most of the criteria have been established for high-intensity exercise in uninjured athletes and are grounded in opinion rather than research.115 For example, it has been suggested that plyometric exercise should be initiated only after achieving the ability to perform one rep of a parallel squat with a load of 1.5–2.5 times body mass on the back and/or squat 60% of body mass 5 times within 5 seconds (lower extremity), and a bench press with one-third of body weight and/or perform five hand clap pushups (upper extremity).103 In addition, success in the static stability tests91 (Table 12-9) and dynamic stability tests (vertical jump for the lower extremities and medicine ball throw for the upper extremities) have also been used as a measure of preparation.21

Many different activities and devices can be used in plyometric exercises. Plyometric exercises may include diagonal and multiplanar motions with tubing or isokinetic machines. These exercises may be used to mimic any of the needed motions and can be performed in the standing, sitting, or supine positions. Generally, 48–72 hours of rest is recommended for recovery between plyometric training sessions.122

Lower Extremity Plyometric Exercises. Lower extremity plyometric exercises involve the manipulation of the role of gravity to vary the intensity of the exercise (Table 12-10). Thus, plyometric exercises can be performed horizontally or vertically.

• Horizontal plyometrics are performed perpendicular to the line of gravity. These exercises are preferable for most initial clinical rehabilitation plans because the concentric force is reduced and the eccentric phase is not facilitated.21 Examples of these types of exercises include pushing a sled against resistance, and a modified leg press that allows the subject to push off and land on the foot plate.
• Vertical plyometric exercises (against or with gravitational forces) are more advanced. These exercises require a greater level of control.21 The drop jump is an example—the subject steps off a box, lands, and immediately executes a vertical jump.

The footwear and landing surfaces used in plyometric drills must have shock-absorbing qualities, and the protocol should allow sufficient recovery time between sets to prevent fatigue of the muscle groups being trained.123

Upper Extremity Plyometric Exercises. Plyometric exercises for the upper extremity involve relatively rapid movements in planes that approximate normal joint function. For example, at the shoulder this would include flexion or abduction at the shoulder, trunk rotation and diagonal arm motions, and rapid external and internal rotation exercises.

Plyometrics should be performed for all body segments involved in the activity. Hip rotation, knee flexion and extension, and trunk rotation are power activities that require plyometric activation. Plyometric exercises for the upper extremity include push-offs (Fig. 12-3), corner push-ups, and weighted ball throws (Fig. 12-4). Medicine and other weighted balls are very effective plyometric devices. The weight of the ball creates a prestretch and an eccentric load when it is caught. This combination creates resistance and demands a powerful agonist contraction to propel it forward again. The exercises can be performed using one arm or both arms at the same time. The former emphasizes trunk rotation and the latter emphasizes trunk extension and flexion, as well as shoulder motion.

A variety of positive changes in athletic performance and neuromuscular function have been attributed to plyometric training, predominantly in the lower extremity:115

• Increased maximal vertical jump height.124
• Increased leg strength, especially when combined with weight training.124
• A faster rate of force development during jumping.125
• Delayed onset of muscle fatigue during jumping.126
• Correction of neuromuscular imbalances.127

Due to the scarcity of research on upper extremity performance plyometric training, the improvements in upper extremity performance remain largely anecdotal. It is also unknown whether patients recovering from injury will respond to plyometric exercise in a manner similar to uninjured subjects.115

Contraindications to Strengthening, Endurance, and Power Exercises

Absolute contraindications to exercise include unstable angina, uncontrolled hypertension, uncontrolled dysrhythmias, hypertrophic cardiomyopathy, and certain stages of retinopathy. Patients with congestive heart failure, myocardial ischemia, poor left ventricular function, or autonomic neuropathies must be carefully evaluated before initiating an exercise program.

A number of precautions need to be taken when exercising patients who have a compromised cardiovascular or pulmonary system (see Chap. 15):

• An appropriate level of intensity must be chosen.
• Too high a level can overload the cardiorespiratory and muscular systems and potentially cause injuries.
• Exercising at the level it is too high causes the cardiorespiratory system to work anaerobically, not aerobically.
• A sufficient period of time should be allowed for warm-up and cool-down to permit adequate cardiorespiratory and muscular adaptation.

Increasing Joint Stabilization

According to Voight,128,129 the standard progression for stabilization exercises involves:

• Static stabilization exercises with closed chain loading and unloading (weight shifting). This phase initially employs isometric exercises around the involved joint on solid and even surfaces, before progressing to unstable surfaces. The early training involves balance training and joint repositioning exercises, and is usually initiated (in the lower extremities according to weight-bearing restrictions) by having the patient placing the involved extremity on a 6–8 inch stool, so that the amount of weight bearing can be controlled more easily. The proprioceptive awareness of a joint can also be enhanced by using an elastic bandage, orthotic, or through taping.130–135 As full weight bearing through the extremity is restored, a number of devices such as a mini-trampoline, balance board, stability ball, and wobble board can be introduced. Exercises on these devices are progressed from double limb support, to single leg support, to support while performing sports-specific skills.

• Transitional stabilization exercises. The exercises during this phase involve conscious control of motion without impact, and replace isometric activity with controlled concentric and eccentric exercises throughout a progressively larger range of functional motion. The physiological rationale behind the exercises in this phase is to stimulate dynamic postural responses, and to increase muscle stiffness. Muscle stiffness (see Chap. 2) has a significant role in improving dynamic stabilization around the joint, by resisting and absorbing joint loads.136
• Dynamic stabilization exercises. These exercises involve the unconscious control and loading of the joint, and introduce both ballistic and impact exercises to the patient.

A delicate balance between stability and mobility is achieved by coordination between muscle strength, endurance, flexibility, and neuromuscular control.137

The neuromuscular mechanism that contributes to joint stability is mediated by the articular mechanoreceptors (see Chap. 3). These receptors provide information about joint position sense and kinesthesia.134,135,138,139 Initially, closed-chain exercises are performed within the pain-free ranges or positions. Open chain exercises may be built upon the base of the closed chain stabilization to allow normal control of joint mobility.

The emphasis during these exercises is to concentrate on functional positioning during exercise rather than isolating open and closed chain activities.137 The activities should involve sudden alterations in joint positioning that necessitate reflex muscular stabilization coupled with an axial load.135,137 Such activities include rhythmic stabilization (an isometric contraction of the agonist followed by an isometric contraction of the antagonist) performed in both a closed and an open chain position,140 and in the functional position of the joint.137 The use of a stable, and then unstable, base during closed chain exercises encourages co-contraction of the agonists and antagonists.140

Weight shifting exercises are ideal for this. For example, the following weight shifting exercises may be used for the upper extremity:

• Standing and leaning against a treatment table or object.
• In the quadruped position, rocking forward and backward with the hands on the floor or on an unstable object.
• Kneeling forward in the three-point position (with one hand on the floor). A Body Blade® can be added to this exercise to increase the difficulty (Fig. 12-5).
• Kneeling in the two-point position (high-kneeling) (Fig. 12-6).
• Weight shifting on a Fitter® while in a kneeling position (Fig. 12-7).
• Weight shifting on a Swiss ball with the feet elevated on a support, and both hands on the Swiss ball in the push-up position (Fig. 12-8). The exercise can be made more difficult by raising the height of elevation of the feet, such as placing the feet on a chair.
• Slide board exercises in the quadruped position moving hands forwards and backwards, in opposite diagonals and in opposite directions.

Integration of the Entire Kinematic Chain

Although force-dependent motor firing patterns should be reestablished, special care must be taken to completely integrate all of the components of the kinetic chain to generate and funnel the proper forces to the appropriate joint. It should be clear that for a kinematic chain to operate efficiently, there must be an optimal sequential activation of the limb segments involved. This then allows for an efficient generation and transfer of force along the kinematic chain.141 When choosing a mode of kinematic exercise, the variables of each type of exercise must be considered. The clinician should understand the principles of exercise application and the differences between open kinematic chain (OKC) and closed kinematic chain (CKC) movements, in order to accomplish a specific intervention goal (Table 12-11). A number of studies have illustrated the importance of the sequential activation of these links.142,143 The benefit of closed kinematic chain exercises (CKCE) over open kinematic chain exercises (OKCE) is based on the premise that CKCEs, particularly in the lower extremities, appear to replicate functional tasks better than OCKEs. This is because the CKCEs appear to allow the entire linkage system of the kinematic chain to be exercised together.45,74,144–150 In addition, CKCEs have been shown to enhance joint congruency, decrease the shearing forces, and stimulate the articular mechanoreceptors using axial loading and increased compressive forces.147,151–155 Thus, CKC activities are purported to help reinforce the synchronization of the necessary muscle-firing patterns for both antagonist and agonist muscle groups used during stabilization and ambulation.45 However, there also appears to be much in the literature to suggest that OKCEs have a beneficial effect on function,8,156,157 especially when combined with specific closed chain exercises or when used to strengthen individual muscles.144

A comprehensive rehabilitation program thus should integrate the entire kinematic chain, using a blend of OKCEs and CKCEs. This integration must occur during functional exercises, with the emphasis determined by the activity to be restored.

In addition, under the concept of specificity, rather than isolating OKCEs or CKCEs, it may be wise to emphasize functional positioning during the exercise training, while striking a balance between mobility and stability.144,159

Several objectives must be met if the rehabilitation of the functional kinematic chain is to be comprehensive.144:

1. The first objective in the rehabilitation program, the healing phase, is the restoration of functional stability, which is the ability to control the translation of the joint during dynamic functional activities, through the integration of both the primary and the secondary stabilizers.160

2. The second objective, the functional phase, is to restore sports- or functional-specific movement patterns. The functional phase begins once the patient has near-full, pain-free ROM.

3. The final objective is assessing the readiness of the patient to return to his or her prior level of function or level of athletic performance.

• 1. What are the four biomechanical properties of skeletal muscle?

• 2. Give three examples of a muscle that cross two or more joints.

• 3. What are the three main types of muscle contraction?

• 4. Define the characteristics of an isotonic contraction.

• 5. True/false: Rapid lengthening contractions generate more force than do slower lengthening contractions.

Answers