Dimensional and dynamic features of mammalian gastrocnemius muscle

Attempts were made to determine how behavior of the separate heads of the rat gastrocnemius muscle are related to give responses characteristic of the whole muscle. The lateral head is heavier and shows greater isometric twitch tension than the medial head; the sum of the twitch tensions approximates values for whole muscle. Isometric tetanus tensions of the two heads are of equal magnitude; their sum is only 68% of the value for whole muscle, the deficit being due to loss of mechanical advantage by being separated. Maximum rates of isometric twitch and tetanus tension development and decay are slower in the medial than in the lateral head. The sum of the rates of twitch development approximates the value for whole muscle; the sum of rates of tetanic tension development is 65% of the value for whole muscle, this deficit also being due to loss of mechanical advantage. The medial head has a longer latent period and twitch contraction time and a greater degree of twitch fusion than the lateral head. Load-velocity curves of separate heads are similar and resemble the responses of two halves of a single muscle split lengthwise.