The energetics of relaxation in a muscle twitch

Abstract When a muscle relaxes under a load heat is produced which is equivalent, or nearly equiva­lent, to the mechanical energy which disappears. The appearance of this heat is simultaneous with the disappearance of the mechanical energy. When a muscle is without load, or under zero tension, during the interval normally occupied by relaxation, there is no measureable heat after shortening is complete. This is true whatever the initial load, or the work done. If chemical changes occur during relaxation their net thermal effect is negligibly small. The heat produced in an isometric contraction is of a very complex nature, involving internal shortening, the transfer of work from one part to another and the dissipation of mechanical energy as heat. The nature of relaxation is discussed, and an analogy described with the process of ʻcold­ drawing’ of a crystalline long-chain polymer.