STRETCH-INDUCED ENHANCEMENT OF MECHANICAL WORK PRODUCTION IN FROG SINGLE FIBERS AND HUMAN MUSCLE

The relations between the velocity of prestretch and the mechanical en ergy liberated during the subsequent isovelocity release were studied in contractions of frog single fibers and human muscles. During isomet ric contractions of frog single fibers, a ramp stretch of varied veloc ity (amplitude, 0.02 fiber length; velocity, 0.08-1.0 fiber length/s) followed by a release (amplitude, 0.02 fiber length; velocity, 1.0 fib er length/s) was given, and the amount of work liberated during the re lease was measured. For human muscles, elbow flexions were performed w ith a prestretch of varied velocity (range, 40 degrees; velocity, 30-1 80 degrees/s) followed by an isokinetic shortening (velocity, 90 degre es/s). In both frog single fibers and human muscles, the work producti on increased with both the velocity of stretch and the peak of force a ttained before the release up to a certain level; thereafter it declin ed with the further increases of these variables. In human muscles, th e enhancement of work production was not associated with a significant increase in integrated electromyogram. This suggests that changes in intrinsic mechanical properties of muscle fibers play an important rol e in the stretch-induced enhancement of work production.