Heat acclimation does not improve running resilience in female athletes

Resilience, the ability to maintain a speed despite prior fatigue, is an important determinant of exercise performance. There is a need to explore how training stimuli can be leveraged to improve resilience. The purpose was to assess the use of a novel heat acclimation intervention on running resilience in endurance-trained female runners. We hypothesized that a three week mixed method heat acclimation intervention would improve resilience in female runners. 24 endurance-trained female runners were block-randomized based on VO 2 max into either the control (n = 12; CON) or heat acclimation (n = 12; HA) group. A heat tolerance test and resilience test were administered before (pre) and after (post) the intervention period. The heat tolerance test consisted of a 30-minute run in hot conditions (38°C & 40% RH) at 70% VO 2 max. The resilience test consisted of an estimation of critical speed (CS) by evaluating oxygen supply-demand balance via near-infrared spectroscopy, a time-to-exhaustion trial at 105% of predicted CS, and a second estimation of CS following the time to exhaustion. The heat acclimation intervention consisted of two active (running at 70% VO 2 max in progressive temperature increases of 38°C, 40°C and 42°C, 40% RH each week) and two passive (30 min; 41°C hot water immersion) heat exposures per week for three weeks. The CON group performed two thermoneutral exercise sessions per week for three weeks to match an increased training volume of the HA group. Resilience was defined as the change in predicted CS from before to after fatiguing exercise (expressed as a % change). The heat acclimation intervention effectively acclimated participants, indicated by a reduction in peak body core temperature (-0.3°C (-0.1, -0.5); p < 0.01; expressed as mean (95% CI)), peak heart rate (-5.9 (0.1, -11.9) beats per minute; p =0.03), and increased sweat rate (0.1L (0.1, 0.2); p < 0.01) between the pre and post heat tolerance test; with no differences in the CON group between pre and post. Resilience was similar between CON and HA groups before (-6.8% (-2.6, -10.9), -4.0% (-1.2, -6.8), p = 0.59; respectively) and after (-6.0% (0.78, -12.9), -0.8% (3.0, -4.5), p = 0.16; respectively) the intervention. These data indicate that our three week, mixed method heat acclimation improves thermoregulation in hot environments but failed to improve resilience in female runners. Supported by the Wu Tsai Human Performance Alliance and The Joe and Clara Tsai Foundation, and the Eugene and Clarissa Evonuk Memorial Graduate Fellowship. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.