This study examined the moment-producing capabilities of the gastrocnemius during isokinetic knee flexion tasks. Nine healthy men were tested using a Biodex isokinetic dynamometer. Each one completed 3 maximum repetitions at 3 angular velocities, 30, 75, and 150º/s, with his ankle braced in either full dorsiflexion or full plantar flexion. A computer model was used to simulate the experimental tasks. Experimentally, the moment produced at the knee joint with the ankle dorsiflexed was significantly higher than the moment with the ankle plantar-flexed at all 3 angular velocities, p < 0.05. This suggests that lengthening the gastrocnemius allowed for greater contribution of the gastrocnemius to the total moment produced at the knee during isokinetic knee flexions. The simulations supported the experimental data and suggested that, with the ankle dorsiflexed, the gastrocnemius acts on a more favorable part of the muscle’s force-length curve compared with the plantar-flexed condition. The results of the experimental work, along with the simulations, demonstrated that lengthening the gastrocnemius significantly increased the moment produced at the knee joint during isokinetic knee flexion tasks. These results have implications for instructions given to persons who perform leg curls for muscle strengthening, and for the design of knee flexion exercise machines.
The authors are with the Biomechanics Laboratory, 39, Recreation Bldg., Dept. of Kinesiology, The Pennsylvania State University, University Park, PA 16802-3408.