Controlling rectal and muscle temperatures: Can we offset diurnal variation in repeated sprint performance?

The present study investigated whether increasing morning rectal temperatures (T_rec) to resting.evening levels, or decreasing evening T_rec or muscle (T_m) temperatures to morning values, would influence repeated sprint (RS) performance in a causal manner. Twelve trained males underwent five sessions [age (mean ± SD) 21.8 ± 2.6 yr, peak oxygen uptake ([Formula: see text] peak) 60.6 ± 4.6 mL kg min^-1, stature 1.78 ± 0.07 m and body mass 76.0 ± 6.3 kg]. These included a control morning (M, 07:30 h) and evening (E, 17:30 h) session (5-min warm-up), and three further sessions consisting of a warm-up morning trial (M_E, on a motorised treadmill) until T_rec reached evening levels; and two cool-down evening trials (in 16-17°C water) until T_rec (E_Mrec) or T_m (E_Mmuscle) values reached morning temperatures, respectively. All sessions included a 3 × 3-s task-specific warm-up followed by 10 × 3-s RS with 30-s recoveries performed on a non-motorised treadmill. T_rec and T_m measurements were taken at the start of the protocol and following the warm-up or cool-down period. Values for T_rec and T_m were higher in the evening compared to morning values (0.45°C and 0.57°C, P < 0.05). RS performance was lower in the M for distance covered (DC), average power (AP) and average velocity (AV) (9-10%, P < 0.05). Pre-cooling T_rec and T_m in the evening reduced RS performance to levels observed in the morning (P < 0.05). However, an active warm-up resulted in no changes in morning RS performance. 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.