The one who selects exercises for muscular strength and muscular
endurance must be aware of the meaning of each of these elements
of physical conditioning and understand the relationship that exists
between them. Muscle strength is the
force a muscle or muscle group can exert against a resistance in
one maximum effort. Muscle endurance is
the ability to perform repeated contractions of the muscle(s) or
to sustain a contraction against a submaximal resistance for an
extended period of time. These elements are related so that training
with an emphasis on strength will have an effect on endurance. However,
different adaptations occur within the muscle, with different training
emphases. For this reason conditioning programs should be selected
to be specific to the needs of a particular activity and should
be patterned after the demands placed on the muscle in the activity.
The most important factor in maximum strength development is the
amount of resistance employed to overload the muscle. In endurance
development the emphasis is placed on the number of repetitions
of the movement.
As generally practiced, this form of exercise involves lifting
weights—for example, dumbbells, disk weights, or stack
weights, through a specified range of motion. The resistance to
the contracting muscles is not only the actual magnitude of the
weight lifted but is the product of the weight and length of the
resistance arm of the anatomical lever involved. Hence the maximum
resistance occurs only when the resistence force is acting at right
angles to the lever.
The return movement of concentric-type exercises, when done in
a slow, controlled manner, uses eccentric contraction of the antagonist
muscles. Because a muscle can sustain more tension in eccentric
contraction than it can develop in concentric contraction, exercise
physiologists had thought that eccentric contraction exercises,
or negative lifts, should be more effective in strength development
than concentric exercises. The results of research to date do not
support this theory. Eccentric exercises have been shown to be as
effective as concentric exercises, but no more effective. Eccentric
exercise has been found to contribute significantly to delayed onset
muscle soreness. With high repetitions or high loads, eccentric
exercise may also lead to muscle damage. Strengthening the muscle
concentrically has been found to reduce the effects of eccentric
exercise. It is often the eccentric phase of lifting that produces
the pain that leads to the saying “no pain, no gain.” In
fact, microdamage to the muscle from lifting eccentrically does
not contribute to muscle strength and may impair the ability of
the muscle to develop adequate force in the short term. Intentionally
damaging the muscle in the name of muscle strength or hypertrophy
should be discouraged (Folland et al. 2002; Proske and Morgan 2001).
to Muscle Strength and Endurance
This has long been recognized as the essential physiological
requirement for strength development. A muscle must be exercised
at or near maximal strength and endurance capacity for a specified
period of time if strength and endurance are to develop. The strength
of a muscle exercised against normally encountered resistances will
Strength or endurance training activities must be specific to
the demands of the particular activity for which strength or endurance
is being developed. The full range of joint action, the speed, and the
resistance demands of the movement pattern should be duplicated
in the training activity.
The resistance against which a muscle group is exercised must
be increased in a planned manner, called periodization. A strength
training program may be periodized over a few weeks or a single day.
The key is the variation in resistance. Because the overload program
increases strength, the original overload eventually becomes inadequate
and must be supplemented with progressive increases in resistance.
The American College of Sports Medicine recommends that resistance
be increased from 2 to 10 percent when the current load can be lifted
one to two repetitions beyond the set number for that period. Strength
training programs should be designed based on individual abilities,
needs, and goals (Kraemer et al. 2002).
A regular program of exercises should be established and followed
at least three to five times per week. For muscular endurance the
maximum number of repetitions used depends on the specific demands
of the activity. The number of repetitions that an individual can
do will depend on the actual work being performed. Because work
performed will be based on repetitions, displacement, weight of
the resistance, and weight of the segment, it will differ greatly
from individual to individual.
The American College of Sports Medicine recommends that beginning
and intermediate lifters train two to three times per week; advanced
lifters may train as much as four to five times a week (Kraemer
et al. 2002). Other investigators have suggested that beginners
lift 60 percent of their maximum capability, doing from one to four
sets of each lift. Advanced lifters should work at 80 percent of
their maximum (Bird et al. 2005; Peterson et al. 2005).
Use of momentum to initiate motion in the resistance should be
minimized. Where possible, the motion should be done in a slow,
controlled manner using muscle force and not momentum. One must
remember, however, that speed of motion is also related to specificity.
Studies have shown that high-speed resistance training with light
loads can lead to improvement in performance in ballistic movements
such as throwing and running (McEvoy and Newton 1998; Morrissey
et al. 1995).
The longer the resistance arm, the more strenuous the motion.
Segments should be kept close to the body until one is certain that
individuals are capable of more strenuous effort.
When the weight is supported by the feet, in an impact or nonimpact
situation, the knees must always remain over the feet (the knee
should never be twisted). When there is an alternative, low-impact
work (e.g., one foot remaining on the floor) is more appropriate
than high-impact work.
Closed Kinetic Chain
Most strengthening exercises are done in one of two conditions.
If the distal endpoint of the limb, or kinetic chain, is fixed against
something, the limb operates in a closed
kinetic chain. As mentioned in Chapter 1, in this situation
the segments moved by joint action will be the segments proximal
from the joint, or closer to the body. For instance, if the feet
are planted against the plate of a squat sled, when the knee and
hip are extended, the whole body will move while the plate remains
stationary. If, on the other hand, the participant is sitting on
a bench with a weight strapped to the ankle, knee extension will
produce motion only in the lower leg. This would be an open kinetic chain. In another example,
a push-up would be a closed kinetic chain exercise, and a bench
press would be an open kinetic chain. Both types of strength training
exercises have been found to be effective. When selecting the appropriate
kinetic chain exercise, it is important to consider joint stability
or existing injury and investigate the different patterns of muscle
activation for each kinetic chain type.
The order in which strength training exercises are done should
be carefully planned to achieve balanced conditioning while maximizing
both workout effectiveness and safety. To effectively train muscle,
both concentric and eccentric exercises should be included. When
considering exercise order, multijoint exercises such as squats
should be done before single-joint exercises, such as knee extensions.
Large muscle groups, such as those in the lower extremity, should
be exercised before the smaller muscle groups, such as those in
the upper extremity. When exercise of varying intensities is to
be performed, the higher intensity exercise should be done first,
followed by the lower intensity work. Rest periods of from 1 to
3 minutes should be taken between sets, depending on the intensity
and velocity of the exercise (Kraemer et al. 2002).
All muscle strengthening and endurance workouts should be preceded
by warm-up and followed by cooldown exercises. The warm-up prepares
the muscles and joint tissues by increasing their temperature and
permits the neuromuscular system to adjust the threshold levels,
thereby making the muscles and joints less susceptible to strains
and tissue tears. The cooldown helps speed recovery, removing accumulated
Once muscular strength and endurance are developed, they may
be maintained with less frequent workout sessions. The loss of strength
and endurance progresses at a slower rate than its gain. Therefore,
much will be retained for an extended time. Once the difficult development
phase has been accomplished, retention of strength and endurance
are possible with exercise sessions once every one or two weeks,
provided maximum contractions are used.
An appropriate balance between joint flexibility and muscle strength
must be maintained for each muscle and joint as well as a balance
between agonist and antagonist muscle pairs. Imbalance can lead
to injury or permanent deformity.
Individual differences in anthropometry must be considered in
exercise. Different limb lengths produce different angular kinematics.
Consequently, all individuals should not be expected to perform
to the same cadence. In the same manner, all equipment is not equally
well fitted to all individuals. Care must be taken in working with
weight machines to see that machine lever lengths are adjusted to
match each individual. Many exercise machines are not designed to
fit either very large or very small bodies.
As resistance training becomes more popular for young people,
care must be taken to prevent stress that may cause developmental
problems or injury. As was discussed in Chapter 2, the skeletal
structure is not fully formed until the late teen years. Those who
work with children are cautioned that undue, high-resistance stress
on soft epiphyseal growth plates can have long-term effects such
as changes in normal growth patterns. Light weights and low repetitions
On the other end of the age spectrum, resistance training has
been shown to be beneficial in elderly populations. A variety of
studies have shown that balance, strength, bone density, and mobility
can all be improved through resistance training. When designing
a resistance training program for the elderly, the movement special-ist
is encouraged to follow the guidelines set forth by the American
College of Sports Medicine (Kraemer et al. 2002). Risk factors,
discussed in the following text, that can cause problems in healthy
adults may be magnified in the elderly, and caution should be used.
Exercise, when properly used, is a powerful tool for positive
health and well-being. It is the role of the kinesiologist to understand
and to properly evaluate exercise motions and to choose those that
provide a safe, enjoyable, and appropriate exercise. Because exercise
is most effective when the systems of the body are required to respond
to stress, potential risk is involved in exercise programs for strength
and endurance. To minimize risk, some motions should be avoided.
In general, in choosing exercises for strength and endurance the
following guidelines should be followed:
- 1. Avoid motion to the limit
of flexion and hyperextension in the cervical or lumbar regions
to protect the vulnerable areas of the vertebral column. Therefore,
when exercising, never hyperextend the trunk without stabilizing
the pelvis and keeping the head in line with the neck.
- 2. Avoid combinations of motions
in the vertebral column in more than one plane. Combination motions
in more than one plane executed simultaneously or in sequence increase
the compressional force applied to the discs.
- 3. Avoid forcing the knee joint
into extreme flexion or hyperextension. Either action may weaken
ligaments and other structures in the joint.
- 4. Never hold the breath while
exercising. Breathe normally through the exercise to avoid increased
undue stress on the cardiovascular system through increased intrathoracic
Endurance Exercise Programs
Most exercise programs use some form of resistance applied either
in an isotonic, isokinetic, or isometric manner. Isotonic and isokinetic
exercises can be done either concentrically or eccentrically; isometric
exercises use a static contraction. Purely isotonic exercises are
very difficult to produce and are not common in most exercise programs.
Most common activities have elements of both concentric and eccentric
exercise without being either purely isotonic or isokinetic.
During the late 1940s DeLorme and Watkins (1951) increased our
awareness of the overload principle by their emphasis on progressive resistance training, and
by the concept of the repetition maximum (RM).
A repetition maximum is the maximum resistance a muscle group can
lift a given number of times before fatiguing. Their principles
for using sets of repetition maximum to develop concentric muscle
strength gains are still followed in most such programs today.
Because an isometric muscle contraction
is defined as involving no change in muscle length, it follows that
an isometric exercise is one that involves no motion either. Isometric
exercise occurs when muscles contract in a static contraction in
opposition to a fixed or immovable load. Isometric exercise, although
providing some strength gain, does not increase strength throughout
the range of motion. Further, not all of a muscle’s fibers
are activated during isometric contraction. This makes isometric
exercise somewhat limited in application. A primary use for isometrics
currently is in the rehabilitation of muscles surrounding injured
There are many drawbacks and even potential hazards when using
isometric exercises. Isometric exercises increase muscle strength
only and are not effective for development of other fitness components
such as flexibility and cardiovascular endurance. Moreover, the
strength development achieved is greatest at the specific angle
of the static contraction. If the objective is the development of
strength throughout the range of motion, then the isometric exercise
must be done at many joint angles spread over the entire range.
Finally, a static contraction activates the slow-twitch muscle fibers
predominantly and therefore does not act to enhance speed of movement
in any way.
Inasmuch as the extreme effort exerted in isometric exercises
causes considerable internal pressure, especially if the breath
is held, it is not wise for persons out of condition, such as the
middle-aged or the elderly, to engage in them. The strain might
be injurious to anyone with a cardiovascular impairment or with
a weakness in the abdominal wall.
The term isotonic is a combination
of the terms for “constant” and “tension. ” Hence,
isotonic exercise is that in which muscular tension remains the
same throughout. In fact, this state is very difficult to produce and
very difficult to measure. In most exercise forms, the tension produced
in muscles will vary with muscle length and with the variation in
external torque. In other words, as the segment in motion moves
through the range of motion, torque will vary with joint angle.
A quick review of Chapter 13 will refresh the student’s
memory of why this is true. In the 1970s fitness equipment companies
started developing weight-training machines to produce isotonic
exercise. A number of systems employ an elliptical cam that provides
a constantly changing resistance moment arm (Figure 16.3). The change
in the resistance arm is calculated to coincide with both the changing muscle
angle of pull and changing muscle length and is intended to provide
for a constant resistance torque throughout the motion. This comes
close to meeting the constant tension requirements of an isotonic
exercise. Equipment of this type, designed to provide this very
specific type of resistance, requires a separate machine for each
muscle group and joint action.
A resistance machine with an isotonic cam. The length
of the resistance arm changes to vary the resistance as the muscle
angle of pull changes.
Isokinetic exercise means exercise
done with a constant rate of motion. An isokinetic exercise theoretically is
one in which there is no acceleration and in which the velocity
remains constant. To accomplish this, machines such as the Cybex
(Figure 16.4) have been developed. With such a machine, the angular
velocity of segment motion may be preset to any selected rate within
the limits of the machine. Because the machine accommodates to the
force applied, it permits the individual to apply maximum force
throughout the range of motion without increasing the velocity of
the motion. Maximal resistance may be applied throughout the exercise
without sudden acceleration or deceleration forces. Given that optimal
training requires practice of a skill at the appropriate speed (often
several thousand degrees per second), use of an isokinetic device
permits maximum strengthening throughout the range of motion at
required speeds without endangering joints. Isokinetic devices have
proved valuable in the diagnosis and rehabilitation of athletic
and other joint injuries. Such devices are also widely used in research.
Isokinetic exercise for knee extensors using a Cybex
Training programs for running, jumping, and throwing events have
combined eccentric and concentric exercises into plyometric programs.
These programs utilize eccentric contractions immediately followed
by explosive concentric contraction. One example of a plyometric
program is drop jumping. In this exercise, the individual steps
or drops off a bench, allowing flexion at the knees in a normal
landing. Immediately following knee flexion, one or both legs extend
in a maximum explosive jump. This training works on explosiveness
by taking advantage of the stretch reflex, series elastic components
of muscle, and training for the synchronous firing of motor units. This
combination results in a more forceful jump.
Any number of movements can be used as plyometric exercise. Beside
the drop jump, countermovement jumps are a common technique. A countermovement
jump is simply a vertical (or horizontal) jump using a rapid squat–extend
motion. Almost any motion that involves joint motion in one direction
followed immediately by joint motion in the other direction can
become a plyometric exercise. The key is the rapid agonist stretch
followed immediately by the concentric agonist contraction. A higher
velocityeccentric contraction will lead to a greater plyometric
Plyometric training has been found to be a valuable tool for
increasing both strength and power. In addition to increasing the
ability of the muscle to generate force, plyometrics act on the
neuromuscular system to ready the body for dynamic, changing situations
or perturbations. This has been shown to help stabilize joints against
injury (Myer et al. 2006; Toumi et al. 2004).