A guiding principle of exercise prescription that has been one of the foundations on which the use of resistance exercise to improve muscle performance is based is the overload principle. Simply stated, if muscle performance is to improve, a load that exceeds the metabolic capacity of the muscle must be applied; that is, the muscle must be challenged to perform at a level greater than that to which it is accustomed. If the demands remain constant after the muscle has adapted, the level of muscle performance can be maintained but not increased.

Application of the Overload Principle.The overload principle focuses on the progressive loading of muscle by manipulating, for example, the intensity or volume of exercise. Intensity of resistance exercise refers to how much weight (resistance) is imposed on the muscle, whereas volume encompasses variables such as repetitions, sets, or frequency of exercise, any one or more of which can be gradually adjusted to increase the demands on the muscle.

• In a strength training program, the amount of resistance applied to the muscle is incrementally and progressively increased.
• For endurance training, more emphasis is placed on increasing the time a muscle contraction is sustained or the number of repetitions performed than on increasing resistance.

To ensure safety, the extent and progression of overload must always be applied in the context of the underlying pathology, age of the patient, stage of tissue healing, fatigue, and the overall abilities and goals of the patient. The muscle and related body systems must be given time to adapt to the demands of an increased load or repetitions before the load or number of repetitions is again increased.

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