When the tension of a muscle contracting isometrically is rapidly lowered, there is an immediate and proportional rise of temperature. This is not due to physiological shortening, which is a relatively slow process, but is directly connected with the fall of tension. A similar effect occurs in any material possessing a normal (positive) thermal coefficient of linear expansion. It is the opposite of what is observed in bodies with long-range rubber-like elasticity. The experimental relation, in active muscle, between the heat (∆
Q
) immediately produced and the rapid fall of tension (-∆
P
) is ∆
Q
= 0∙018
l
o
(-∆
P
), where
l
o
is the standard length of the muscle. The constant 0∙018 is considerably greater than for metals but about the same as for ebonite and wood. In resting muscle, in the range of moderate tensions, the constant is of the opposite sign, and its absolute size is five to ten times as great. Resting muscle, in this range, has rubber-like elastic properties. During active contraction, therefore, the contractile filaments possess normal and not long-range elasticity. The force exerted by active muscle is not of thermokinetic origin. Unlike resting muscle its entropy and its internal energy both decrease when its tension is rapidly lowered. The power of physiological shortening, at a rate depending on the tension, is not directly derived from elastic properties. In normal relaxation after an isometric contraction there is known to be a substantial production of heat. This is derived partly from elastic energy developed earlier during contraction, in the series elastic component: the balance is fully accounted for by the thermoelastic heat resulting from the fall of tension.
The viscous elastic properties of muscle
