The SAID principle (specific adaptation to imposed demands) suggests that a framework of specificity is a necessary foundation on which exercise programs should be built. This principle applies to all body systems and is an extension of Wolff’s law (body systems adapt over time to the stresses placed on them). The SAID principle helps therapists determine the exercise prescription and which parameters of exercise should be selected to create specific training effects that best meet specific functional needs and goals.

Specificity of training, also referred to as specificity of exercise, is a widely accepted concept suggesting that the adaptive effects of training, such as improvement of strength, power, and endurance, are highly specific to the training method employed. Whenever possible, exercises incorporated in a program should mimic the anticipated function. For example, if the desired functional activity requires greater muscular endurance than strength, the intensity and duration of exercises should be geared to improve muscular endurance.

Specificity of training should also be considered relative to mode (type) and velocity of exercise as well as patient or limb position (joint angle) and the movement pattern during exercise. For example, if the desired functional outcome is the ability to ascend and descend stairs, exercise should be performed eccentrically and concentrically in a weight-bearing pattern and progressed to the desired speed. Regardless of the simplicity or complexity of the motor task to be learned, task-specific practice must always be emphasized. It has been suggested that the basis of specificity of training is related to morphological and metabolic changes in muscle as well as neural adaptations to the training stimulus associated with motor learning.

In contrast to the SAID principle, carryover of training effects from one variation of exercise or task to another has also been reported. This phenomenon is called transfer of training, overflow, or cross-training. Transfer of training has been reported to occur on a very limited basis with respect to the velocity of training and the type or mode of exercise.It has also been suggested that a cross-training effect can occur from an exercised limb to a nonexercised, contralateral limb in a resistance training program.

A program of exercises designed to develop muscle strength has also been shown to improve muscular endurance at least moderately. In contrast, endurance training has little to no cross-training effect on strength. Strength training at one speed of exercise has been shown to provide some improvement in strength at higher or lower speeds of exercise. However, the overflow effects are substantially less than the training effects resulting from specificity of training.

Despite the evidence that a small degree of transfer of training does occur in resistance exercise programs, most studies support the importance of designing an exercise program that most closely replicates the desired functional activities. As many variables as possible in the exercise program should match the requirements and demands placed on a patient during specific functional activities.

Reversibility Principle. Adaptive changes in the body’s systems, such as increased strength or endurance, in response to a resistance exercise program are transient unless training-induced improvements are regularly used for functional activities or unless an individual participates in a maintenance program of resistance exercises.

Detraining, reflected by a reduction in muscle performance, begins within a week or two after the cessation of resistance exercises and continues until training effects are lost.For this reason, it is imperative that gains in strength and endurance are incorporated into daily activities as early as possible in a rehabilitation program. It is also advisable for patients to participate in a maintenance program of resistance exercises as an integral component of a lifelong fitness program.

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