The effect of elevated human muscle temperature on energy turnover was investigated during cycling exercise (at 85 % of V-O2max) at a contraction frequency of 60revs min(-1). Muscle temperature was passively elevated prior to exercise by immersion of the legs in a hot water bath (42degreesC). During exercise at this low pedalling rate, total energy turnover was higher (P<0.05) when muscle temperature was elevated compared with normal temperature (70.4+/-3.7 versus 66.9+/-2.4kJ min(-1), respectively). Estimated net mechanical efficiency was found to be lower when muscle temperature was elevated. A second experiment was conducted in which the effect of elevated human muscle temperature on energy turnover was investigated during cycling exercise (at 85 % of V-O2max) at a contraction frequency of 120 revs min(-1). Under the conditions of a high pedalling frequency, an elevated muscle temperature resulted in a lower energy turnover (P<0.05) compared with the normal muscle temperature (64.9 +/- 3.7 versus 69.0 +/- 4.7 kJmin(-1), respectively). The estimated net mechanical efficiency was therefore higher when muscle temperature was elevated. We propose that, in these experiments, prior heating results in an inappropriately fast rate of cross-bridge cycling when exercising at 60 revs min(-1), leading to an increased energy turnover and decreased efficiency.; However, at the faster pedalling rate, the effect of heating the muscle shifts the efficiency/velocity relationship to the right so that crossbridge detachment is more appropriately matched to the contraction velocity and, hence, energy turnover is reduced.