Isotonic Training Method

Isotonic training methods have traditionally used resistance in the form of dumbbells, barbells, stacked or plate-loaded machines, and pulley weights. Research during the past half century has attempted to determine the optimum combination of weights and repetitions to maximize strength gains.

Early contributions by DeLorme and Watkins (1948) have been slightly modified in recent years to suggest that isotonic training be performed at five to seven repetitions maximum (RM), with three sets completed per exercise at one session. The frequency of training should be approximately three or four sessions per week. The RM concept of DeLorme and Watkins represented the maximum number of repetitions that could be continuously performed at a given resistance; thus 6-RM referred to the resistance for which only six repetitions could be achieved. This system provided a suitable method of incorporating the overload principle, as persons could exercise at their individual loads yet still conform to the RM concept.

Several limitations exist for this type of training. When using a barbell, the weight of the bar does not change throughout the movement, but the effective resistance — determined statically by the torque about the axis of rotation located at the elbow joint — varies considerably. This torque is minimal at the starting position so that the momentum generated through the remainder of the curl causes a reduction in the experienced load, compared with the static load at subsequent positions. This is especially true during fast repetitions (power training), but not so critical when slow movements are performed. (Because the effective resistance is not constant, free-weight and machine exercise, though usually called isotonic, is more accurately described as DCER — see dcer.)

This is an action-type definition cross-linked to physiology: in concentric (positive) action the muscle shortens; in eccentric (negative) action it is forcibly lengthened (see muscle-action-terminology).

• Equipment: dumbbells, barbells, stacked/plate-loaded machines, pulley weights.
• DeLorme & Watkins (1948), modified: 5–7 RM, 3 sets per exercise per session, ~3–4 sessions/week.
• RM = max reps continuously performable at a given resistance (e.g., 6-RM = load allowing only 6 reps).
• Provides a method for applying the overload principle at individual loads.
• Limitation: effective resistance (torque) varies through ROM despite constant bar weight.

• dcer — the more accurate term for this "isotonic" free-weight/machine training.
• variable-resistance — machines designed to compensate for the varying torque.
• muscle-action-terminology — concentric/eccentric action-type definitions.
• repetition-maximum — the RM concept originates with DeLorme & Watkins.
• progressive-overload — RM loading is a vehicle for overload.