Comparison of mechanical properties between slow and fast mammalian muscles

Ranvier (1873) first suggested that certain skeletal muscles contained functionally distinct types of fibres. He observed in rabbits a slow persistent contraction which he associated with red muscles in contrast to brisk responses from pale muscles. Subsequent investigation, based on histological as well as physiological observations, showed that the slow fibres resisted fatigue better (Ranvier, 1880) and their tetanic responses fused at lower stimulus frequencies (Fischer, 1908; Cooper & Eccles, 1930). These differences were related to the contractile properties rather than to neuromuscular transmission since the responses reported by Ranvier were directly evoked from animals poisoned with curare. Following these more qualitative reports were those characterizing muscle activity in terms of physical systems. The non-linear relation between active tension production and the velocity of shortening was first emphasized by Fenn & Marsh (1935). Hill (1938) confirmed and expanded these mechanical studies to include thermal measurements. The result was a mathematical expression relating force and rate of energy release, the constants of which, determined by either mechanical or thermal methods, were similar. This description of the active contractile response of muscle states that the rate of total energy liberation in excess of the isometric rate is proportional to the reduction of external tension (P) below maximal isometric force (P0). The equation is usually written