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The development of muscle strength is an essential component of any rehabilitation program. Strength is the ability of a muscle to generate force against some resistance. Muscle weakness or imbalance can result in abnormal movement or impaired performance. Physiological improvements occur only when an individual physically demands more of their tissues than is normally required—the ‘overload principle’. The muscles and tendons must be stressed above the normal load in order effectively to increase their performance and strength. Progressive resistive exercise (PRE) is the most common strengthening technique used for reconditioning the muscle after injury. It is important to allow the overloaded tissue to recover and to allow adequate time for recovery.

Muscle training after injury

Muscle strength is proportional to the cross-sectional area of the muscle (i.e. to the diameter and number of muscle fibers). The larger the cross-sectional area, the greater the force that the muscle can generate. The degree of force generated varies inversely to the speed at which the muscle contracts. Maximum force is generated by isometric contractions in which a large number of motor units are used. The faster a muscle contracts, the less force it can generate, as fewer motor units are used. Strength training increases the strength not only of the muscles but also of their attachments. The strength of tendons, ligaments, and the skeleton does not increase as quickly as that of the muscles since their metabolism is slower, and this fact should be borne in mind when training growing individuals. During rehabilitation after injury, strength training should be carried out to the pain threshold. In order to shorten recovery time, training of muscles in the injured area can begin along the following lines. Muscle contraction is the basis for all movement and exercise. There are three kinds of muscle work:

isometric (or static) work involves contraction without a change in the length of the muscle (for example,holding a weight stationary in an outstretched hand); concentric work implies the muscles contract and shorten in length simultaneously so that their attachments are drawn closer together (for example, the contraction of quadriceps muscles when climbing stairs); eccentric work implies that the muscles contract and lengthen simultaneously so their attachments are drawn apart (for example, the action of the quadriceps muscles when walking downstairs).

During activity that involves change from eccentric to concentric muscular work, or vice versa, there is a risk of tearing a muscle or tendon. Concentric work mainly accelerates a moving object, eccentric work decelerates it. Injuries often occur during deceleration.

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