Changing size, composition, and contraction strength of gastrocnemius muscle

¹ Institute for Psychosomatic and Psychiatric Research and Training, Michael Reese Hospital, Chicago, Illinois

Gastrocnemius-soleus muscle weight, length, water and nitrogen content, active and passive length-tension curves, and twitch and tetanic tensions were measured in normal male rats 24–131 days old. Mathematical analyses of the data were performed in order to determine appropriate factors to correct these variables for increasing body size and also to detect changing relationships among variables within the course of normal growth. Muscle weight is an increasing proportion of body weight, whereas muscle length grows at an ever-decreasing rate. Water is a decreasing, and nitrogen an increasing, proportion of muscle weight during growth. Length-tension relationships are quite unchanging over the measured span. Contraction strength is in constant ratio to wet muscle weight throughout the age range, but is higher on the basis of dry weight or nitrogen content in younger than in older rats.