Effects of sprint and endurance training on passive stress-strain relationof fast- and slow-twitch skeletal muscle in Wistar rat

We investigated the effects of endurance and sprint training on the passive mechanical properties of fast-twitch (FT) and slow-twitch (ST) skeletal mu scles. Eight-week-old male Wistar rats (n = 18) were divided into three gro ups: control (C), sprint-trained (S) and endurance-trained (E). The trained animals exercised for 10 weeks on a treadmill. Under anaesthesia, Plantari s,and Soleus muscles were deformed cyclically in vivo at 0.33 mm s(-1) with length increments of 1 mm in successive cycles until rupture. The rupture of muscle occurs at belly. Stress-strain relation were constructed using th e maximum stress and maximum strain in each cycle. The data were fitted to an S-shaped curve. The curve-fitting parameters for trained and untrained m uscles showed significant statistical differences. Stress and strain at rup ture and maximum deformation energy were statistically greater for trained ST muscles (both groups) than for the controls. The changes induced by the present training protocols were not significant in Plantaris. The above res ults suggest the plasticity of passive structure caused by activity-demands .