High-energy phosphate compounds during exercise in human slow-twitch and fast-twitch muscle fibres.

Concentrations of ATP and creatine phosphate, the high-energy phosphates, were investigated in slow-twitch (ST) and fast-twitch (FT) muscle fibres in sprinters and in long-distance runners at rest, during light sprinting exercise and during an extremely exhaustive running exercise. At rest the long-distance runners had significantly (P less than 0.005) more creatine phosphate in ST than in FT muscle fibres. Both the light and the exhaustive exercise induced breakdown of creatine phosphate to very low levels (10-15 mmol/kg; dry weight) in FT muscle fibres. In response to the exhaustive exercise, creatine phosphate in the ST fibres of the sprinters fell to a significantly (P less than 0.05) lower value than of the long-distance runners in the ST fibres. During the recovery period the synthesis of creatine phosphate seemed to be more rapid in FT than ST muscle fibres in light exercise (P less than 0.02) and in sprinters in exhaustive exercise (P less than 0.05). This study suggests that (1) during short-term exercise FT muscle fibres consume more of their creatine phosphate stores than ST muscle fibres, (2) in vigorous exercise athletes trained for sprinting are able to recruit not only the FT but also the ST muscle fibres, and (3) in such trained sprinters creatine phosphate is possibly resynthesized more rapidly in FT than in ST muscle fibres.