Investigating effects of moderate hyperthermia at two phases of the circadian cycle for core temperature (heat gain and peak), on quadriceps maximal voluntary contraction force

Abstract

Athletes often perceive a performance disadvantage in the morning, in part, because of a recognised deficit in functional muscle force capacity. This diurnal variation in muscle force production has been attributed to higher rectal (Trec) and muscle (Tm) temperatures in the evening as well as motivational, peripheral, and central factors. A warm-up is an essential component of sporting performance, however moderate hyperthermia reduces sporting gross muscular performance although possibly to a lesser degree in the morning (raising phase) than the peak of the core temperature rhythm (~17:00 h). We investigated whether i) increasing morning Trec temperatures to evening resting values by an active warm-up leads to quadriceps muscle force production becoming equal to evening values. Or ii) raising Trec passively in the morning or evening to 38.5°C results in greater quadriceps muscle force production reductions in the evening. Eight active males (mean±SD: age, 25.5 ± 1.9 yrs; body mass, 71.0 ± 6.7 kg; height, 1.79 ± 0.06 m) volunteered and randomly completed five sessions (separated by > 48 h): control morning (M, 07:30 h) and evening (E, 17:30 h) sessions (both with an active 5-min warm-up) and three further trials – an active warm-up 07:30 h trial (ME, until resting evening temperatures were reached), a morning (M38.5) and an evening (E38.5) passive warm-up trial which continued until Trec values reached 38.5°C (immersed in a water-bath @ ~40°C, 45–50% Relative humidity). During each trial, 5-measures of maximal voluntary contraction (MVC) of the quadriceps on an isometric dynamometer (utilizing the twitch-interpolation technique) were performed with force (peak and mean of the 5-trials) and percentage activation recorded. Trec, ratings of perceived exertion (RPE) and thermal comfort (TC) were measured. Measurements were made after the participants had reclined for 30-min at the start of the protocol and after the warm-ups/passive heating and prior to the measures for isometric dynamometry. Trec and Tm (at 3, 2 and 1 cm depths) temperatures were taken at rest, after the passive warm-up, and immediately before the isometric MVC measurements. Data were analysed by general linear models with repeated measures. Isometric force for knee extension showed higher values in the evening than morning (peak Δ83.2 N, mean Δ67.8 N; p < 0.05). Trec and Tm (at 3 cm depth) values were higher at rest in the evening than the morning (by 0.47 and 0.85°C respectively; p < 0.05) increasing from rest by 0.54 and 2.2°C, 1.78 and 2.2°C, and 1.31 and 1.8°C, in the ME, M38.5 and E38.5 conditions, respectively; ratings of thermal comfort reflecting this (p < 0.05). There was no significant effect of active ME warm-up and moderate hyperthermia M38.5 compared to morning control peak (peak or mean) torque (M). E38.5 reduced “mean” but not “peak” torque in the evening (Δ61.9 N, p = 0.009; p = 0.051). In summary, active warm-up did not improve isometric MVC in the morning and moderate hyperthermia reduced isometric MVC “mean” force only during the peak of the core temperature rhythm (~17:00 h).