The effects of dry-bulb temperature on self-paced exercise performance, along with thermal, cardiovascular and perceptual responses, were investigated by minimizing differences in the skin-to-air vapor pressure gradient (Psk,sat-Pa) between temperatures. Fourteen trained male cyclists performed 30-km time trials in 13˚C and 44% relative humidity (RH), 20˚C and 70% RH, 28˚C and 78% RH, and 36˚C and 72% RH. Power output was similar in 13˚C (275±31 W; mean and SD) and 20˚C (272±28 W; P=1.00), lower in 36˚C (228±36 W) than 13˚C, 20˚C and 28˚C (262±27 W; P<0.001) and lower in 28˚C than 13˚C and 20˚C (P<0.001). Peak rectal temperature was higher in 36˚C (39.6±0.4˚C) than all conditions (P<0.001) and higher in 28˚C (39.1±0.4˚C) than 13˚C (38.7±0.3˚C; P<0.001) and 20˚C (38.8˚C±0.3˚C; P<0.01). Heart rate was higher in 36˚C (163±14 beats·min-1) than all conditions (P<0.001) and higher in 20˚C (156±11 beats·min-1; P=0.009) and 28˚C (159±11 beats·min-1; P<0.001) than 13˚C (153±11 beats·min-1). Cardiac output was lower in 36˚C (16.8±2.5 l·min-1) than all conditions (P<0.001) and lower in 28˚C (18.6±1.6 l·min-1) than 20˚C (19.4±2.0 l·min-1; P=0.004). Ratings of perceived exertion were higher in 36˚C than all conditions (P<0.001) and higher in 28˚C than 20˚C (P<0.04). Self-paced exercise performance was maintained in 13˚C and 20˚C at a matched evaporative potential, impaired in 28˚C and further compromised in 36˚C in association with a moderately lower evaporative potential and marked elevations in thermal, cardiovascular and perceptual strain.
PurposeThis study aimed to determine the effect of different air velocities on heat exchange and performance during prolonged self-paced exercise in the heat.MethodsTwelve male cyclists performed a 700-kJ time trial in four different air velocity conditions (still air, 16, 30, and 44 km·h−1) in 32°C and 40% relative humidity. Performance, thermal, cardiovascular, and perceptual responses were measured, and heat balance parameters were estimated using partitional calorimetry, including the maximum potential for sweat evaporation (Emax).ResultsMean power output was lower in still air (232 ± 42 W) than 16 (247 ± 30 W), 30 (250 ± 32 W), and 44 km·h−1 (248 ± 32 W; all P < 0.001), but similar between the 16-, 30-, and 44-km·h−1 air velocity conditions (P ≥ 0.275). Emax was lower in still air (160 ± 13 W·m−2) than 16 (298 ± 25 W·m−2), 30 (313 ± 23 W·m−2), and 44 km·h−1 (324 ± 31 W·m−2) and lower in 16 than 44 km·h−1 (all P < 0.001). Peak core temperature was higher in still air (39.4°C ± 0.7°C) than 16 (39.0°C ± 0.45°C), 30 (38.8°C ± 0.3°C), and 44 km·h−1 (38.8°C ± 0.5°C; all P ≤ 0.002). Mean skin temperature was lower with greater airflow (P < 0.001) but similar in 30 and 40 km·h−1 (P = 1.00). Mean heart rate was ~2 bpm higher in still air than 44 km·h−1 (P = 0.035). RPE was greater in still air than 44 km·h−1 (P = 0.017).ConclusionsSelf-paced cycling in still air was associated with a lower Emax and subsequently higher thermal strain, along with a similar or greater cardiovascular strain, despite work rate being lower than in conditions with airflow. The similarity in performance between the 16-, 30-, and 44-km·h−1 air velocity conditions suggests that airflow ≥16 km·h−1 does not further benefit self-paced exercise performance in the heat because of modest improvements in evaporative efficiency.
New Findings What is the central question of this study?Hyperoxia enhances endurance performance by increasing O2 availability to locomotor muscles. We investigated whether hyperoxia can also improve prolonged self‐paced exercise in conditions of elevated thermal and cardiovascular strain. What is the main finding and its importance?Hyperoxia improved self‐paced exercise performance in hot and cool conditions. However, the extent of the improvement (increased work rate relative to normoxia) was greater in cool conditions. This suggests that the development of thermal and cardiovascular strain during prolonged self‐paced exercise under heat stress might attenuate the hyperoxia‐mediated increase in O2 delivery to locomotor muscles. Abstract The aim of this study was to determine whether breathing hyperoxic gas when self‐paced exercise performance is impaired under heat stress enhances power output. Nine well‐trained male cyclists performed four 40 min cycling time trials: two at 18°C (COOL) and two at 35°C (HOT). For the first 30 min, participants breathed ambient air, and for the remaining 10 min normoxic (fraction of inspired O2 0.21; NOR) or hyperoxic (fraction of inspired O2 0.45; HYPER) air. During the first 30 min of the time trials, power output was lower in the HOT (∼250 W) compared with COOL (∼273 W) conditions (P < 0.05). In the final 10 min, power output was higher in HOT‐HYPER (264 ± 25 W) than in HOT‐NOR (244 ± 31 W; P = 0.008) and in COOL‐HYPER (315 ± 28 W) than in COOL‐NOR (284 ± 25 W; P < 0.001). The increase in absolute power output in COOL‐HYPER was greater than in HOT‐HYPER (∼12 W; P = 0.057), as was normalized power output (∼30%; P < 0.001). The peripheral capillary percentage oxygen saturation increased in HOT‐HYPER and COOL‐HYPER (P < 0.05), with COOL‐HYPER being higher than HOT‐HYPER (P < 0.01). Heart rate was higher during the HOT compared with COOL trials (P < 0.01), as were mean skin temperature (P < 0.001) and peak rectal temperature (HOT, ∼39.5°C and COOL, ∼38.9°C; P < 0.01). Thermal discomfort was also higher in the HOT compared with COOL (P < 0.01), whereas ratings of perceived exertion were similar (P > 0.05). Hyperoxia enhanced performance during the final 25% of a 40 min time trial in both HOT and COOL conditions compared with normoxia. However, the attenuated increase in absolute and normalized power output noted in the HOT condition suggests that heat stress might mitigate the influence of hyperoxia.
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