Phosphocreatine and ATP content in human single muscle fibres before and after maximum dynamic exercise

The recovery of high-energy phosphate levels in single human skeletal muscl e fibres following shortterm maximal tall-out) exercise was investigated. T hree male volunteers exercised maximally for 25 s on an isokinetic cycling ergometer. Muscle biopsy samples from the vastus lateralis were collected a t rest, immediately postexercise and at 1.5 min of recovery. The subjects a lso per formed a second exercise bout 1.5 min after the first, on a separat e occasion. Single muscle fibres were dissected, characterized and assigned to one of four groups according to their myosin heavy chain (MyHC) isoform content; namely, type I, IIA, IIAx and IIXa (the latter two groups contain ing either less or more than 50% IIX MyHC). Fibres were analysed for adenos ine 5 ' -triphosphate (ATP), inosine-5 ' -monophosphate (IMP), phosphocreat ine (PCr) and creatine (Cr) levels. Type I fibres had a lower Clr content t han type II fibres (P <0.01). Within type II fibres resting [PCr] increased with increasing MyHC IIX isoform content (r=0.59, P <0.01). Post-exercise [PCr] was very low in all fibre groups (P <0.01 versus rest) while great re ductions in ATP were also observed (P <0.01 versus rest), especially in the type II fibre groups. [PCr] at 1.5 min of recovery was still lower compare d to rest for all fibre groups (P <0.01) especially in the IIAx and IIXa fi bres.