Variable Resistance Training (Machines, Cams)

Variable resistance equipment operates through a lever arm, cam, or pulley system. Its purpose is to alter the resistance to match the increases and decreases in strength (strength curves) throughout the exercise's range of motion, so that the muscle is forced to contract near-maximally throughout the range of motion, resulting in maximal gains in strength.

Many exercise machines have been produced that attempt to overcome shortcomings of the traditional isotonic mode. Special cams and linkages were designed to provide resistance according to the capacity of the involved muscles to exert force. For single-joint exercises, the resistance was said to vary in accord with the strength curve for the exercise concerned. (The source illustrates the variable-radius cam: a larger horizontal moment arm x produces a larger force, a smaller x a smaller force, for a hanging weight stack W.)

There are 2 shortcomings of these designs (Komi, 1992):

1. Because it was apparently not feasible to design a variable resistance mechanism adaptable to subjects of different sexes and anthropometric dimensions, the variable resistance provided by these machines was based on average values.
2. The basis for designing these mechanisms for multiple-joint exercises is less obvious because strength curves for multiple-joint exercises have yet to be reported. (Multiple-joint exercises such as squatting and deadlift with free weights are superior if performed correctly.)

The arm-curl strength curve illustrates the underlying rationale: strength is optimized at an elbow angle of about 100 degrees (≈100%), with relative strength of ~60%–67% at 60°, ~98% at 120°, ~95% at 140°, and ~71% at 180° (Wilmore & Costill, 1994).

• Operates via lever arm, cam, or pulley to vary resistance across ROM.
• Goal: muscle contracts near-maximally throughout ROM for maximal strength gains.
• Cam principle: large moment arm (x) → large force; small x → small force.
• Shortcoming 1: designed on average values, not individual anthropometry (Komi, 1992).
• Shortcoming 2: multiple-joint strength curves not established; free-weight squats/deadlifts superior if done correctly.
• Arm-curl strength curve: optimal at ~100° elbow angle.

• dcer / isotonic-training — variable resistance attempts to fix the varying-torque limitation of these.
• length-tension-relationship — strength curves reflect length/angle-dependent force.
• accommodating-resistance — a related machine approach controlling speed rather than load.
• specificity — average-value cams may not match the individual.