Contractile properties of hindlimb muscles in rat during surgical overload.

Structural and dynamic properties of the extensor digitorum longus (EDL) and soleus muscles of the rat under pentobarbital sodium anesthesia have been determined 4, 28, and 400 days after extirpation of the tibialis anterior and compared with the properties of muscles from the contralateral limb of the same animal. At 4 days the mass of the EDL increased by about 17%, but the maximum tetanic tension and tension per unit cross-sectional area decreased. The soleus showed a slight decrease in mass of about 6%. At 28 days the mass of the EDL had increased to 126% of the control value. Maximum isometric twitch and tetanic tensions increased, whereas tetanic tension per unit cross-sectional area returned to normal values; the number of sarcomeres in series increased, which led to an increase in the whole muscle speed of shortening. It is concluded that the synergistic EDL undergoes compensatory hypertrophy by the addition of contractile material in series and in parallel but without any change in the intrinsic properties of the muscle. The fibers of the antagonistic soleus shorten slightly but are otherwise unchanged.