Effects of age and life-long endurance training on the passive mechanical properties of rat skeletal muscle.

The effects of age and life-long endurance training on the mechanical properties of both slow twitch (m. soleus) and fast twitch (m. rectus femoris) skeletal muscles in rats were investigated in a longitudinal study. Wistar-rats performed treadmill-running 5 days a week for 2 years. The ultimate measures of stress-strain together with the load-deformation measurements in the more physiological area of muscle function were performed. The ultimate strength and stiffness increased significantly with age, most prominently during the growth period, and significantly by training in the soleus muscle only. The results on the elastic efficiency and load-relaxation showed that the viscous and plastic properties decreased with age in both muscles. Training seemed to decrease these properties in m. soleus, whereas in m. rectus femoris the effect was reversed.