PURPOSE:  To assess the effects of two heat mitigation strategies (sodium hyperhydration and combined pre-cooling) and menstrual cycle phase on thermal strain (rectal temperature) and exercise performance of female endurance athletes exercising in the heat.

METHODS: Fourteen endurance-trained females (34±6 years) participated across two studies that included two cross-over interventions during phase 1 (low hormones) and phase 4 (high hormones) of the menstrual cycle, confirmed via serum [oestrogen, progesterone]. Study 1 (n=8): Double blinded hyperhydration interventions consisted of 30 mL/kg fat free mass of fluid with either sodium chloride (NaCl, 7.5 g/L) or placebo (matched number of pills containing sugar) delivered two hours prior to 75 min of steady state cycling (SS) at 60% VO_2max followed by a 200-kJ time trial (TT). Study 2 (n=6): Combined pre-cooling interventions consisted of wearing an ice vest for 30min and ingestion of an ice‐slurry (7.5 g/kg of body mass at ~‐4°C, 12% carbohydrates) or control (matched to ice slurry at ~20°C and no vest) for 20min before exercise, followed by a 10 km treadmill running TT. Environmental conditions in both studies were 34°C, 60% relative humidity, 4.5 m/s airflow. Rectal temperature was measured at baseline, every 5 min during SS and every 50 kJ of cycling TT (Study 1); every 5min at rest and during warm-up, and every km of the running TT (Study 2). Data were analysed using linear mixed models and estimates are reported with 95% confidence intervals.

 

RESULTS: Study 1: There was no effect of hyperhydration with NaCl or menstrual cycle phase on the change in rectal temperature during SS or TT (p>0.05). Cycling TT performance improved with NaCl compared to placebo (-1.97 [-3.17,0.77] min; p=0.001), with a greater improvement in phase 4 of the menstrual cycle (Phase 4: -2.70 [-4.36,-1.04] min; p=0.001; Phase 1: -1.24 [-2.97,0.49] min; p=0.159). Study 2 (preliminary results): Change in rectal temperature from baseline improved with combined pre-cooling compared to control (-0.43 [-0.56,-0.31]°C; p<0.001), with a greater improvement in phase 4 (-0.91 [-1.08,-0.73]°C; p<0.001) but not phase 1 (0.04 [-0.13,0.22] °C; p=0.637) of the menstrual cycle. Running TT performance did not improve with combined pre-cooling compared to control (-0.23 [-1.30, 0.83] min; p=0.668), and completion time was longer in phase 4 (1.47 [0.41,2.54] min; p=0.007) relative to phase 1 of the menstrual cycle.

CONCLUSIONS: NaCl hyperhydration improved endurance cycling performance in the heat during phase 4 of the menstrual cycle, allowing a higher workload without changes in thermal strain. Combined pre-cooling attenuated thermal strain but did not improve 10 km running performance in the heat. Female endurance athletes should consider NaCl hyperhydration, especially during phase 4 of the menstrual cycle, when exercising and competing for prolonged durations in the heat with insufficient fluid access. 

 

Disclosure(s): The authors have nothing to disclose.