1. The relationship between myosin heavy chain (MyHC) isoforms and high energy phosphate content was studied in human muscle fibres at rest and following maximal dynamic exercise lasting 25 s.
2.Single fibre fragments were characterized as type I, type IIA, type IIX or type IIAX. These latter fibres were subdivided into five groups on the basis of the proportion of MyHC IIX isoform present.
3. Resting ATP concentration in type I fibres was 10% lower than in type II fibres (P < 0.05), but no differences were found amongst type IIA, IIX and IIAX fibre groups. Phosphocreatine (PCr) content was lower in type I than in type II fibres (P < 0.01) and, amongst type II fibres, increased progressively with the amounts of MyHC IIX expressed.
4. After 25 s of maximal dynamic exercise ATP concentration was reduced in all fibres. The decrease was approximately 25% in type I fibres and between 47 and 66% in the type II subgroups.
5. Post-exercise PCr content was low in all fibre types. Fibre groups with the lowest post-exercise PCr also had the lowest ATP and the highest inosine monophosphate contents. delta PCr (rest to post-exercise) was smallest in type I fibres and showed a progressive increase in the type II fibre groups as the proportion of the faster IIX myosin heavy chain isoform increased.