Mixed pre-cooling improves thermal strain but not running performance in female endurance athletes exercising in the heat across the menstrual cycle

This study investigated the effect of mixed pre-cooling (cooling vest and ice-slurry) on thermal and cardiovascular strain and performance in unacclimatized females exercising in the heat during menstrual cycle (MC) Phase-1 (low estrogen and progesterone) and Phase-4 (moderate estrogen and high progesterone). In a randomized crossover design, 11 runners completed two trials in each MC Phase: mixed pre-cooling (wearing a cooling vest around the torso for 30 min and ingesting 7.5 g.kg^-1 of body mass of ice slurry), and control (equivalent slurry volume at ~20°C without a cooling vest), prior to a 10-km running time trial (TT) in 34°C and 60% relative humidity. Rectal temperature (T_re), heart rate (HR), and perceptual responses were measured throughout the TT. Linear mixed models were fitted to estimate intervention and MC phase effects. Mixed pre-cooling had no effect on HR (1.06 [-0.83, 2.95] beats.min^-1; <i>p</i> = 0.272) or TT completion time (-0.23 [-1.32, 0.86] min; <i>p</i> = 0.677) versus control. T_re (-0.21 [-0.34, -0.07] °C; <i>p</i> = 0.003) and thermal sensation (-0.61 [-0.78, -0.44] au; <i>p</i> < 0.001) were significantly lower with mixed pre-cooling compared to control. Rise in T_re (-0.1 [-0.3, 0.0] °C) and thermal sensation were attenuated in MC phase-4 (-0.57 [-0.81, -0.34] au; all <i>p</i> < 0.05). Mixed pre-cooling was effective at attenuating the rise in T_re for a similar work rate, particularly in MC phase-4, where females may experience greater heat thermosensation. However, the magnitude of attenuated T_re does not improve 10-km running performance in the heat, suggesting additional cooling and/or heat mitigation strategies may be required to improve performance.