Limited effects of short-term heat acclimation during a rugby league training week

We investigated the effects of five consecutive days of short-term heat acclimation (STHA), with a 48-h recovery, on simulated rugby league performance, peak oxygen uptake (V ̇O2peak), physiological and perceptual responses in a temperate environment. Twelve male rugby league players took part in a matched-pairs design, cycling 60-min/day, for five days at 40% V ̇O2peak, in a control (19 ± 1 °C; 50 ± 3 % RH; n = 6) or STHA group (33 ± 0.5 °C; 70 ± 4 % RH; n = 6). Subjects completed a V ̇O2peak test and rugby simulation, followed by a time-to-exhaustion, pre- and post-intervention, with physiological and perceptual responses measured during the interventions. Despite no effect on simulated performance (P > 0.05), there were increases in time-to-exhaustion among both groups (P = 0.016), without group effects (P = 0.802). The STHA group adapted across the intervention, with lower tympanic temperature, perceptual responses and heart rate over time (P < 0.05), which were typically higher than the control group (P < 0.05). Plasma volume did not increase over time or change between groups (P = 0.290) and V ̇O2peak reduced pre-to-post (P < 0.001). There was a relationship between body mass losses and plasma volume expansion (r = -0.79, P = 0.022). STHA improved tolerance to the heat but not performance in simulated matches or exhaustive tests in temperate conditions, compared to thermoneutral exercise. The limited effects of STHA might relate to the brief post-intervention recovery period or the lack of fluid ingestion control during acclimation.