Elevating muscle temperature results in an acute transformation in contractile characteristics towards a faster phenotype and higher power output (Sargeant, 1987), whilst whole-body hyperthermia has been shown to result in decreased sustained maximal isometric force production attributed to central inhibition (Nybo and Nielsen, 2001). The aim of the present study was to examine the combined and independent effects of an elevated core and muscle temperature on maximal isometric force production. With Grampian Ethics Board approval, 7 healthy males (age, 23±3 years, mass, 72.4±7.1 kg) volunteered to participate in the study. Each subject completed 4 trials with, normal body temperature (control), hot core and hot leg (HCL), hot core normal leg (HCNL) or normal core hot leg (HL). Under each experimental condition, subjects were instructed to exert maximum isometric knee extension force for 2 minutes. Central activation ratio was determined using maximal percutaneous twitch superimposed at 30s, 60s, 90s and 120s of exercise. Data were analysed using 2-way ANOVA and post-hoc T-Test with Bonferroni correction. The rate of change in force production was determined using a 3^rd-order polynomial. Application of the thermal clamp resulted in elevations in core temperature (38.7±0.2^oC HCL and HCNL) and leg temperature (38.8±0.2^oC, HCL, 37.9±0.3^oC HL). Peak torque production was significantly lower (177±49 Nm; P<=0.017) with HCL than the other three trials (194±51 Nm). The central activation ratio decreased with exercise duration, however, the decline was greatest (P<=0.013) with HCL and the time to reach the greatest rate of torque decline was fastest with HCL. Conclusions: These data demonstrate that whole body hyperthermia reduces muscle activation leading to a decline in sustained voluntary force production. References Sargeant, A.J. 1987. European Journal of Applied Physiology 56, 693-698 Nybo, L. & Nielsen, B. 2001. Journal of Applied Physiology 91, 1055-1060