Work and heat in a muscle twitch.

In a single twitch, when a muscle shortens against a load, doing work, the heat produced is independent of the work done, provided the amount of shortening is kept constant. The total energy liberated may be expressed as (A -jW + ax)9 where A is the heat of activation, W is the work and ax is the heat of shortening. This relation is true not merely for the whole contraction but for any part of it. The rate at which energy is liberated by a muscle during a twitch, in excess of the activation heat, is a decreasing linear function of the load P: dWIdt + adxjdt = 6(P0 — P). This relation is the basis of the characteristic equation connecting the speed of shortening to the load. These relations are the same as were previously found for tetanic contractions, with similar constants. The active state appears to be set up suddenly very soon after a shock. The physical basis of these conclusions is discussed.