The most common system of resistance training used with dynamic exercise against constant or variable resistance is progressive resistance exercise (PRE). A later section of the chapter, which covers systems of training using mechanical resistance, addresses PRE.

Dynamic constant external resistance (DCER) exercise is a form of resistance training where a limb moves through a ROM against a constant external load, provided by free weights such as a handheld or cuff weight, torque arm units weight machines, or pulley systems. This terminology (DCER exercise) is used in lieu of the term “isotonic exercise” because although the imposed load (weight) does not change the torque imposed by the weight and the tension generated by the muscle do change throughout the range of movement. If the load is less than the torque generated by the muscle, the muscle contracts concentrically and accelerates the load; if the load exceeds the muscle’s torque production, the muscle contracts eccentrically to decelerate the load.

DCER exercise has an inherent limitation. When lifting or lowering a constant load, the contracting muscle is challenged maximally at only one point in the ROM where the maximum torque of the resistance matches the maximum torque output of the muscle. A therapist needs to be aware of the changing torque of the exercise and the changing length-tension relationship of the muscle and modify body position and resistance accordingly to match where in the range the maximum load needs to be applied. Despite this limitation, constant external resistance has been and continues to be an effective form of muscle loading for training-induced improvements in muscle performance; and it is a mainstay of resistance exercise programs.

Variable-resistance exercise, a form of dynamic exercise, addresses the primary limitation of dynamic exercise against a constant external load (DCER exercises). Specially designed resistance equipment imposes varying levels of resistance to the contracting muscles to load the muscles more effectively at multiple points in the ROM. The resistance is altered throughout the range by means of a weight-cable system that moves over an asymmetrically shaped cam, by a lever arm system, or by hydraulic or pneumatic mechanisms. How effectively these machines vary the resistance to match muscle torque curves is questionable.

Dynamic exercise with elastic resistance products (bands and tubing) can also be thought of in the broadest sense as a variable-resistance exercise because of the inherent properties of the material and its response to stretch. Furthermore, when dynamic exercise is performed against manual resistance, a skilled therapist can vary the load applied to the contracting muscle throughout the ROM. The therapist adjusts the resistance based on the patient’s response so the muscle is appropriately loaded at all portions of the range.

Special Considerations for DCER and Variable-Resistance Exercise

During either DCER or the variable-resistance exercise, the velocity and excursion of limb movement is controlled exclusively by the patient (with the exception of manual resistance exercise and performing exercises on units that have range-limiting devices). Exercises must be performed at a relatively slow velocity to avoid momentum and uncontrolled movements, which could jeopardize the safety of the patient.

Hydraulic and pneumatic variable-resistance equipment and elastic resistance products do allow safe, high-velocity resistance training. Because most daily living and occupational activities occur at moderate to fast velocities, the training-induced improvements in muscle strength that occur only at slow velocities may not prepare the patient for activities that require rapid bursts of strength or quick changes of direction.

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