Changes of energy in a muscle during very slow stretches

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Abstract
When a muscle, during a maintained isometric contraction, is suddenly subjected to a load between 1.2 and 1.5 times its full isometric tension, the immediate effect is a brief stretch, expressing readjustment of the elastic parts of the muscle and lever system to the new tension. This is followed by a gradual fall of the load, progressively decreasing in speed, accompanying a slow lengthening of the contractile part of the muscle. Although a substantial amount of work is done by the load in falling, the heat production of the muscle is actually diminished; thus during the slow lengthening the whole of the work disappears. During a single stretch the heat deficit is proportional throughout to the amount of lengthening; the 'heat of lengthening' is negative. The constant of proportion is less, in general, the greater the load. The reason for this is considered. The tension during a rapid stretch at constant speed (as described by Abbott, Aubert & Hill 1951) rises much higher than the full isometric tension. As soon as the stretch ends the tension begins to fall, the elastic component shortening and stretching the contractile component. If the stimulus is continued this 'after-stretch' takes place very slowly; as in isotonic stretches work is absorbed and a heat deficit occurs. Whenever any part of a stretch occurs during relaxation, all the work performed during that period reappears as heat. These results are consistent with reversal of the physical and chemical events associated with shortening, and the conditions necessary for thermodynamic reversibility are considered. An analogy with the cold-drawing of an extruded thread of a long chain polymer is discussed. Other possible hypotheses described in the preceding paper are referred to.

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