Author(s): IMAI, A., TERADA, K., SUMI, D., Institution: ASICS CORPORATION, Country: JAPAN, Abstract-ID: 1575

INTRODUCTION:
Marathon competitions are frequently held in hot environments (e.g., Summer Olympics, World Athletics Championships); consequently, performance is significantly impaired compared to that in thermoneutral environments. It has been suggested that higher environmental temperatures reduce oxygen saturation in the vastus lateralis [1]. The increase in respiratory workload associated with hyperventilation is thought to induce respiratory muscle fatigue, and respiratory muscle fatigue is one of the factors that limit oxygen delivery to locomotor muscles [2]. However, the effects of higher environmental temperature and humidity on the oxygenation of both “locomotor” and “respiratory” muscles during actual running remain unclear. The purpose of this study was to investigate the effects of different environmental temperatures and humidities on locomotor and respiratory muscle oxygenation during prolonged running.
METHODS:
Six male runners (age, 32.8 ± 3.6 years; height, 171.7 ± 1.9 cm; weight, 59.0 ± 1.2 kg) completed prolonged running (60 min of treadmill running at 75% of maximal oxygen uptake) under three environmental conditions that were close to actual races (Olympic Games, World Athletics Championships, World Marathon Majors): (1) 32℃ condition (70% relative humidity), (2) 20℃ condition (60%RH), and (3) 10°C condition (50%RH), in a single-blind, randomized crossover design. All trials were performed in a climate-controlled chamber. Muscle oxygenation changes in the vastus lateralis and intercostal muscle were evaluated using near-infrared spectroscopy. Respiratory gases, heart rate, cardiac output, and core temperature were measured continuously during running, and skin blood flow was evaluated before and after running.
RESULTS:
Muscle oxygen saturation in the intercostal muscle was significantly lower at 32℃ compared with both 20℃ and 10℃ (ΔStO2; 32℃: -24.8±4.3%, 20℃: -15.7±3.7%, 10℃: -12.8±3.3%, p < 0.05). In contrast, muscle oxygen saturation in the vastus lateralis did not differ significantly among all conditions (ΔStO2; 32℃: -13.5±1.1%, 20℃: -14.4±2.4%, 10℃: -10.2±1.7%, p > 0.05). Peak core temperature was significantly higher at 32℃ compared with both 20℃ and 10℃ (32℃: 39.8±0.2 degrees, 20℃: 38.7±0.2 degrees, 10℃: 38.3±0.1 degrees, p < 0.05). Minute ventilation and respiratory rate were also significantly higher at 32℃ compared with both 20℃ and 10℃ (p < 0.05). Furthermore, five out of six subjects failed to complete the 60-min run at 32℃.
CONCLUSION:
Prolonged running in the heat induces hyperventilation for thermoregulation, resulting in a significant decrease in oxygen saturation in the intercostal muscle. Surprisingly, oxygen saturation in the vastus lateralis was similarly maintained under all environmental conditions. These findings suggest that respiratory muscle function may be a critical limiting factor and potential target for heat strategies in marathon races.
[1] Periard et al. (2013) / [2] Harms et al. (1997)