Stress-relaxation in mammalian gastrocnemius muscle

¹ Department of Physiology, State University of Iowa, Iowa City, Iowa

Rat whole gastrocnemii (ML) and their medial (M) and lateral (L) heads separately, when stretched 5 mm over resting length at rates of 0.29, 1.16 and 4.71 mm/sec., reach slightly higher levels of passive tension with faster stretches, because the faster the stretch the less is ‘give’ able to keep pace with the stretch. Attained tension of L is less than that of M at all stretch speeds, because ‘give’ occurs more readily in L than in M. The faster the stretch the greater the amount of ensuing fast SR, because the less the amount of tension lost by ‘give’ during stretch the greater the amount of tension left to be lost during ensuing fast SR. The faster the stretch the more residual tension left at 180 seconds; the slower the stretch the less residual tension left at 180 seconds. Slow SR appears to be affected very little by speed of stretch. Fast stretch high attained tension more fast SR; slow stretch low attained tension less fast SR. There is some ‘give’ at fast stretches in both M and L, with M showing more than L. In attained tension and amount of fast SR, M's behavior is modified more by the presence of L than L's behavior is by the presence of M. It is suggested that the contractile component is responsible for fast SR and noncontractile component for slow SR.