We have explored the extent to which the maximal velocity of unloaded shortening (V(max)), the force generated per unit cross-sectional area (P(0)) and the curvature of the force-velocity relationship (a/P(0) in the Hill equation) contribute to differences in peak power of chemically skinned single fibres from the quadriceps muscle of healthy young male subjects. The analysis was restricted to type I and IIA fibres that contained a single type of myosin heavy chain on electrophoretic separation. Force-velocity relationships were determined from isotonic contractions of maximally activated fibres at 15 degrees C. Mean (+/- s.d.) peak powers were 1.99 +/- 0.72 watts per litre (W L(-1)) for type I fibres and 6.92 +/- 2.41 W L(-1), for type IIA fibres. The most notable feature, however, was the very large, sevenfold, range of power outputs within a single fibre type. This wide range was a consequence of variations in each of the three components determining power: P(0), V(max) and a/P(0). Within a single fibre type, P(0) varied threefold, and V(max) and a/P(0) two- to threefold. There were no obvious relationships between P(0) and V(max) or between P(0) and a/P(0). However, there was a suggestion of an inverse relationship between a/P(0) and V(max), the effect being to reduce, somewhat, the impact of differences in V(max) on peak power. In searching for the causes of variation in peak power of fibres of the same type, it appears likely that there are two factors, one that affects P(0) and another that leads to variation in both V(max) and a/P(0).