title: A reduced core to skin temperature gradient, not a critical core temperature, affects aerobic capacity in the heat.
abstract: The purpose of this study was to determine the impact of the core to skin temperature gradient during incremental running to volitional fatigue across varying environmental conditions. A secondary aim was to determine if a "critical" core temperature would dictate volitional fatigue during running in the heat. 60 participants (n=49 male, n=11 female; 24±5 yrs, 177±11 cm, 75±13 kg) completed the study. Participants were uniformly stratified into a specific exercise temperature group (18 °C, 26 °C, 34 °C, or 42 °C) based on a 3-mile run performance. Participants were equipped with core and chest skin temperature sensors and a heart rate monitor, entered an environmental chamber (18 °C, 26 °C, 34 °C, or 42 °C), and rested in the seated position for 10 min before performing a walk/run to volitional exhaustion. Initial treadmill speed was 3.2 km h(-1) with a 0% grade. Every 3 min, starting with speed, speed and grade increased in an alternating pattern (speed increased by 0.805 km h(-1), grade increased by 0.5%). Time to volitional fatigue was longer for the 18 °C and 26 °C group compared to the 42 °C group, (58.1±9.3 and 62.6±6.5 min vs. 51.3±8.3 min, respectively, p<0.05). At the half-way point and finish, the core to skin gradient for the 18 °C and 26 °C groups was larger compared to 42 °C group (halfway: 2.6±0.7 and 2.0±0.6 vs. 1.3±0.5 for the 18 °C, 26 °C and 42 °C groups, respectively; finish: 3.3±0.7 and 3.5±1.1 vs. 2.1±0.9 for the 26 °C, 34 °C, and 42 °C groups, respectively, p<0.05). Sweat rate was lower in the 18 °C group compared to the 26 °C, 34 °C, and 42 °C groups, 3.6±1.3 vs. 7.2±3.0, 7.1±2.0, and 7.6±1.7 g m(-2) min(-1), respectively, p<0.05. There were no group differences in core temperature and heart rate response during the exercise trials. The current data demonstrate a 13% and 22% longer run time to exhaustion for the 18 °C and 26 °C group, respectively, compared to the 42 °C group despite no differences in beginning and ending core temperatures or baseline 3-mile run time. This capacity difference appears to result from a magnified core to skin gradient via an environmental temperature advantageous to convective heat loss, and in part from an increased sweat rate.
authors: Montana Center for Work Physiology and Exercise Metabolism, Department of Health and Human Performance, The University of Montana, 32 Campus Drive, Missoula, MT 59812-1825, USA. Electronic address: john.cuddy@mso.umt.edu.
authors: John S
authors: JS
authors: Cuddy
authors: Montana Center for Work Physiology and Exercise Metabolism, Department of Health and Human Performance, The University of Montana, 32 Campus Drive, Missoula, MT 59812-1825, USA. Electronic address: walter.hailes@mso.umt.edu.
authors: Walter S
authors: WS
authors: Hailes
authors: Montana Center for Work Physiology and Exercise Metabolism, Department of Health and Human Performance, The University of Montana, 32 Campus Drive, Missoula, MT 59812-1825, USA. Electronic address: brent.ruby@mso.umt.edu.
authors: Brent C
authors: BC
authors: Ruby
mesh_terms: D000328:Adult; D001831:Body Temperature; D001833:Body Temperature Regulation; D005221:Fatigue / Q000503:physiopathology*; D005260:Female; D006339:Heart Rate; D006358:Hot Temperature; D006801:Humans; D008297:Male; D012420:Running / Q000502:physiology*; D012879:Skin Physiological Phenomena; D013546:Sweating; D055815:Young Adult
keywords: Environmental physiology; Heat stress; Hyperthermia; Thermotolerance; Volitional fatigue