The relation between force developed and energy liberated in an isometric twitch

The development of force in muscular contraction requires a proportional expenditure of energy, and any quantitative theory of contraction must involve the constant of the propor­tion. In an isometric twitch the energy liberated, as well as the force developed, varies with muscle length; but over a wide range of lengths the ratio Pl0/H is nearly constant (P = force developed, l0 = standard resting length, H = energy liberated). Its actual value depends on the extensibility of the arrangements for recording force; when these were made as inextensible as possible, a value of about 10·3 was obtained. This is still rather too small, because of the compliance of the muscle itself, tendons, etc.; if this compliance could be eliminated the value would be about 13. In striated muscle the ultimate unit of length is the sarcomere. In order to develop a force of 1 dyne in the length of one sarcomere the energy required is about 2 × 10‒5 erg; or, assuming that the energy is derived from the splitting and neutralization of ATP and/or creatine phosphate, the number of molecules split is about 2·4 × 107.