The energetics of tortoise muscle.

1. A study has been made of the mechanical behaviour and the heat production of tortoise skeletal muscle during tetanic contractions.2. The relation between force (P) and velocity (v) is more curved than that of frog muscle. It can be fitted by Hill's equation (v/v(max) = (1 - P/P(0))/(1 + P/a)) using a value of P(0)/a considerably less than for frog muscle.3. The value of the shortening heat constant is less than in frog muscle. It is about a half of a. No evidence was found that the value depended on the load.4. The maintenance heat rate is much less than in frog muscle and, as in frog muscle, is roughly equal to a.b (where b = v(max).a/P(0)). The maintenance heat rate does not diminish during a long tetanus as it does in frog muscle.5. The ratio of work/(work + initial heat) is greater in tortoise muscle than in frog muscle. As in frog muscle, the recovery heat is about equal to (work + initial heat). It follows that tortoise muscle is more efficient than frog muscle at converting free energy into work.6. Possible theoretical bases for connexions between the shape of the force-velocity curve of a muscle and its efficiency are discussed.