Chapter 6

#### Objectives

After completion of this chapter, the physical therapist should be able to do the following:

• Define muscular strength, endurance, and power, and discuss their importance in a program of rehabilitation following injury.

• Discuss the anatomy and physiology of skeletal muscle.

• Discuss the physiology of strength development and factors that determine strength.

• Describe specific methods for improving muscular strength.

• Differentiate between muscle strength and muscle endurance.

• Discuss differences between males and females in terms of strength development.

Following all musculoskeletal injuries, there will be some degree of impairment in muscular strength and endurance. For the therapist supervising a rehabilitation program, regaining, and in many instances improving, levels of strength and endurance are critical for discharging and returning the patient to a functional level following injury.

By definition, muscular strength is the ability of a muscle to generate force against some resistance. Maintenance of at least a normal level of strength in a given muscle or muscle group is important for normal healthy living. Muscle weakness or imbalance can result in abnormal movement or gait and can impair normal functional movement. Resistance training plays a critical role in injury rehabilitation.

Muscular strength is closely associated with muscular endurance. Muscular endurance is the ability to perform repetitive muscular contractions against some resistance for an extended period of time. As we will see later, as muscular strength increases, there tends to be a corresponding increase in endurance. For the average person in the population, developing muscular endurance is likely more important than developing muscular strength because muscular endurance is probably more critical in carrying out the everyday activities of living. This statement becomes increasingly true with age.

#### Types of Skeletal Muscle Contraction

Skeletal muscle is capable of 3 different types of contraction: isometric contraction, concentric contraction, and eccentric contraction. An isometric contraction occurs when the muscle contracts to produce tension, but there is no change in muscle length. Considerable force can be generated against some immovable resistance even though no movement occurs. In a concentric contraction, the muscle shortens in length while tension increases to overcome or move some resistance. In an eccentric contraction, the resistance is greater than the muscular force being produced, and the muscle lengthens while producing tension. Concentric and eccentric contractions are considered dynamic movements.56

Recently, econcentric contraction, which combines both a controlled concentric and a concurrent eccentric contraction of the same muscle over 2 separate joints, has been introduced.19,30 An econcentric contraction is possible only in muscles that cross at least 2 joints. An example of an econcentric contraction is a prone, open-kinetic-chain hamstring curl. The hamstrings contract concentrically to flex the knee, while the hip tends to flex eccentrically, lengthening the hamstring. Rehabilitation exercises have traditionally concentrated on strengthening isolated single-joint motions, despite the fact that the same muscle is functioning at a second joint simultaneously. Consequently, ...

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