Diurnal variation in repeated sprint performance cannot be offset when rectal and muscle temperatures are at optimal levels (38.5°C).

The present study investigated whether increasing morning rectal temperatures (T-rec) to evening levels, or increasing morning and evening T-rec to an "optimal" level (38.5 degrees C), resulting in increased muscle temperatures (T-m), would offset diurnal variation in repeated sprint (RS) performance in a causal manner. Twelve trained males underwent five sessions [age (mean +/- SD) 21.0 +/- 2.3 years, maximal oxygen consumption ((V)over dot) O-2 max) 60.0 +/- 4.4 mL.kg(-1) min(-1), height 1.79 +/- 0.06 m, body mass 78.2 +/- 11.8 kg]. These included control morning (M, 07:30 h) and evening (E, 17:30 h) sessions (5-min warm-up), and three further sessions consisting of a warm-up morning trial (M-epsilon in 39-40 degrees C water) until T-rec reached evening levels; two "optimal" trials in the morning and evening (M-38.5 and E-38.5, in 39-40 degrees C water) respectively, until T-rec reached 38.5 degrees C. All sessions included 3 x 3-s task-specific warm-up sprints, thereafter 10 x 3-s RS with 30-s recoveries were performed a non-motorised treadmill. T-rec and T-m measurements were taken at the start of the protocol and following the warm-up periods. Values for T-rec and T-m at rest were higher in the evening compared to morning values (0.48 degrees C and 0.69 degrees C, p < 0.0005). RS performance was lower (7.8-8.3%) in the M for distance covered (DC; p = 0.002), average power (AP; p = 0.029) and average velocity (AV; p = 0.002). Increasing T-rec in the morning to evening values or optimal values (38.5 degrees C) did not increase RS performance to evening levels (p = 1.000). However, increasing T-rec in the evening to "optimal" level through a passive warm-up significantly reduced DC (p = 0.008), AP (p < 0.0005) and AV (p = 0.007) to values found in the M condition (6.0-6.9%). Diurnal variation in T-rec and T-m is not wholly accountable for time-of-day oscillations in RS performance on a non-motorised treadmill; the exact mechanism(s) for a causal link between central temperature and human performance are still unclear and require more research.