Resistance training machines are designed to provide either constant or variable resistance, with the latter intended to generate a machine resistive torque (MRT) that mirrors the natural fluctuations in human torque capability (HTC) across joint angles. Yet, achieving a precise match between MRT and HTC remains a persistent challenge. This study aimed to validate a novel variable-cam resistance system, the Variable Moment Arm Cam® (VMAC®), by examining torque output and muscle activation during leg extension across the full range of motion (100–0°), using repeated testing and direct comparison with an isokinetic dynamometer. Twenty-two young men completed four randomized sessions, two on the variable-cam system and two on the dynamometer, each separated by 72–96 h. Torque and muscle activity were recorded during six isometric contractions at 100°, 80°, 60°, 40°, 20°, and 0°. The variable-cam system produced torque and activation patterns broadly comparable to the dynamometer, with acceptable agreement across angles. Validity was highest at 60°, aligning with the region of peak torque, whereas greater variability emerged at the extremes of flexion and extension. Muscle activation profiles were similar between devices, though more variable than torque, underscoring the inherent complexity of neuromuscular assessment.
Costa-Machado et al. (Tue,) studied this question.
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