Hidenori Otani

Time-of-day effects of exposure to solar radiation on thermoregulation during outdoor exercise in the heat

ABSTRACT High solar radiation has been recognised as a contributing factor to exertional heat-related illness in individuals exercising outdoors in the heat. Although solar radiation intensity has been known to have similar time-of-day variation as body temperature, the relationship between fluctuations in solar radiation associated with diurnal change in the angle of sunlight and thermoregulatory responses in individuals exercising outdoors in a hot environment remains largely unknown. The present study therefore investigated the time-of-day effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during moderate-intensity outdoor exercise in the heat of summer. Eight healthy, high school baseball players, heat-acclimatised male volunteers completed a 3-h outdoor baseball trainings under the clear sky in the heat. The trainings were commenced at 0900 h in AM trial and at 1600 h in PM trial each on a separate day. Solar radiation and solar elevation angle during exercise continued to increase in AM (672–1107 W/m2 and 44–69°) and decrease in PM (717–0 W/m2 and 34–0°) and were higher on AM than on PM (both P < 0.001). Although ambient temperature (AM 32–36°C, PM 36–30°C) and wet-bulb globe temperature (AM 31–33°C, PM 34–27°C) also continued to increase in AM and decrease in PM, there were no differences between trials in these (both P > 0.05). Tympanic temperature measured by an infrared tympanic thermometer and mean skin temperature were higher in AM than PM at 120 and 180 min (P < 0.05). Skin temperature was higher in AM than PM at the upper arm and thigh at 120 min (P < 0.05) and at the calf at 120 and 180 min (both P < 0.05). Body heat gain from the sun was greater during exercise in AM than PM (P < 0.0001), at 0–60 min in PM than AM (P < 0.0001) and at 120–180 min in AM than PM (P < 0.0001). Dry heat loss during exercise was greater at 0–60 min (P < 0.0001), and lower at 60–120 min (P < 0.05) and 120–180 min (P < 0.0001) in AM than PM. Evaporative heat loss during exercise was greater in PM than AM at 120–180 min (P < 0.0001). Total (dry + evaporation) heat loss at the skin was greater during exercise in PM than AM (P < 0.0001), at 0–60 min in AM than PM (P < 0.0001) and at 60–120 and 120–180 min in PM than AM (P < 0.05 and 0.0001). Heart rate at 120–150 min was also higher in AM than PM (P < 0.05). Neither perceived thermal sensation nor rating of perceived exertion was different between trials (both P > 0.05). The current study demonstrates a greater thermoregulatory strain in the morning than in the afternoon resulting from a higher body temperature and heart rate in relation to an increase in environmental heat stress with rising solar radiation and solar elevation angle during moderate-intensity outdoor exercise in the heat. This response is associated with a lesser net heat loss at the skin and a greater body heat gain from the sun in the morning compared with the afternoon.

Comparison of tympanic membrane temperatures measured by contact and noncontact tympanic thermometers during prolonged exercise in the heat

Abstract We examined the agreement between the tympanic membrane temperature (Tty) measured by a contact tympanic thermometer (Contact-Tty) and the Tty measured by a noncontact tympanic thermometer (Infrared-Tty). In addition, we also evaluated the usefulness of an assessment of core body temperature using a noncontact tympanic thermometer during prolonged exercise in the heat. Seven healthy male subjects cycled for the same four experimental trials at 50% peak oxygen uptake for 90 min in the heat (32°C ambient temperature, 50% relative humidity and 26.6°C wet bulb globe temeprature). The correlation coefficient between both temperatures was strong, 0.89 ( p

Diurnal effects of prior heat stress exposure on sprint and endurance exercise capacity in the heat

ABSTRACT Active individuals often perform exercises in the heat following heat stress exposure (HSE) regardless of the time-of-day and its variation in body temperature. However, there is no information concerning the diurnal effects of a rise in body temperature after HSE on subsequent exercise performance in a hot environnment. This study therefore investigated the diurnal effects of prior HSE on both sprint and endurance exercise capacity in the heat. Eight male volunteers completed four trials which included sprint and endurance cycling tests at 30 °C and 50% relative humidity. At first, volunteers completed a 30-min pre-exercise routine (30-PR): a seated rest in a temperate environment in AM (AmR) or PM (PmR) (Rest trials); and a warm water immersion at 40 °C to induce a 1 °C increase in core temperature in AM (AmW) or PM (PmW) (HSE trials). Volunteers subsequently commenced exercise at 0800 h in AmR/AmW and at 1700 h in PmR/PmW. The sprint test determined a 10-sec maximal sprint power at 5 kp. Then, the endurance test was conducted to measure time to exhaustion at 60% peak oxygen uptake. Maximal sprint power was similar between trials (p = 0.787). Time to exhaustion in AmW (mean±SD; 15 ± 8 min) was less than AmR (38 ± 16 min; p < 0.01) and PmR (43 ± 24 min; p < 0.01) but similar with PmW (24 ± 9 min). Core temperature was higher from post 30-PR to 6 min into the endurance test in AmW and PmW than AmR and PmR (p < 0.05) and at post 30-PR and the start of the endurance test in PmR than AmR (p < 0.05). The rate of rise in core temperature during the endurance test was greater in AmR than AmW and PmW (p < 0.05). Mean skin temperature was higher from post 30-PR to 6 min into the endurance test in HSE trials than Rest trials (p < 0.05). Mean body temperature was higher from post 30-PR to 6 min into the endurance test in AmW and PmW than AmR and PmR (p < 0.05) and the start to 6 min into the endurance test in PmR than AmR (p < 0.05). Convective, radiant, dry and evaporative heat losses were greater on HSE trials than on Rest trials (p < 0.001). Heart rate and cutaneous vascular conductance were higher at post 30-PR in HSE trials than Rest trials (p < 0.05). Thermal sensation was higher from post 30-PR to the start of the endurance test in AmW and PmW than AmR and PmR (p < 0.05). Perceived exertion from the start to 6 min into the endurance test was higher in HSE trials than Rest trials (p < 0.05). This study demonstrates that an approximately 1 °C increase in core temperature by prior HSE has the diurnal effects on endurance exercise capacity but not on sprint exercise capacity in the heat. Moreover, prior HSE reduces endurance exercise capacity in AM, but not in PM. This reduction is associated with a large difference in pre-exercise core temperature between AM trials which is caused by a relatively lower body temperature in the morning due to the time-of-day variation and contributes to lengthening the attainment of high core temperature during exercise in AmR.

Air velocity influences thermoregulation and endurance exercise capacity in the heat

This study examined the effects of variations in air velocity on time to exhaustion and thermoregulatory and perceptual responses to exercise in a hot environment. Eight male volunteers completed stationary cycle exercise trials at 70% peak oxygen uptake until exhaustion in an environmental chamber maintained at 30 °C and 50% relative humidity. Four air velocity conditions, 30, 20, 10, and 0 km/h, were tested, and the headwind was directed at the frontal aspect of the subject by 2 industrial fans, with blade diameters of 1 m and 0.5 m, set in series and positioned 3 m from the subjects chest. Mean ± SD time to exhaustion was 90 ± 17, 73 ± 16, 58 ± 13, and 41 ± 10 min in 30-, 20-, 10-, and 0-km/h trials, respectively, and was different between all trials (P < 0.05). There were progressive elevations in the rate of core temperature rise, mean skin temperature, and perceived thermal sensation as airflow decreases (P < 0.05). Core temperature, heart rate, cutaneous vascular conductance, and perceived exertion were higher and evaporative heat loss was lower without airflow than at any given airflow (P < 0.05). Dry heat loss and plasma volume were similar between trials (P > 0.05). The present study demonstrated a progressive reduction in time to exhaustion as air velocity decreases. This response is associated with a faster rate of core temperature rise and a higher skin temperature and perceived thermal stress with decreasing airflow. Moreover, airflow greater than 10 km/h (2.8 m/s) might contribute to enhancing endurance exercise capacity and reducing thermoregulatory, cardiovascular, and perceptual strain during exercise in a hot environment.

Influence of passive hyperthermia and diurnal variation on exercise performance and cognitive function in the heat

Both aerobic and anaerobic exercise performances have a diurnal variation. As commonly reported in previous studies, circadian rhythm in exercise performance is low in the morning and peaks in the evening. It has been demonstrated that hyperthermia before exercise attenuates subsequent exercise performance in the heat. However, combined effects of passive hyperthermia and the time-of-day on both aerobic and anaerobic exercise capacity and cognitive function in the heat have not been evaluated. Therefore, the aim of this study was to examine the effects of passive hyperthermia and circadian rhythm on aerobic and anaerobic exercise performances and cognitive function after exercise in the heat.

Relationship between the amount of fluid ingestion and renal concentrating ability during heavy exercise in the heat

This study examined whether the prevention of body mass loss during exercise in the heat by fluid ingestion would attenuate the decline in renal concentrating ability. Seven untrained subjects performed 105 min of intermittent exercise (15 min of exercise alternating with 3 min of rest) on a cycle ergometer at 70% VO2max (32°C and 60% RH). Subjects were tested under four conditions: (1) no fluid ingestion (NF), (2) ad libitum fluid ingestion (AF), (3) fluid ingestion equal to 1% of body mass (1% FL), (4) fluid ingestion equal to 2% of body mass (2% FL). Fluid was ingested immediately before exercise and at 15, 33, 51, 69 and 87 min of exercise in AF, 1% FL and 2% FL. Urine and blood samples were taken before and after exercise. During NF, AF, 1% FL and 2% FL, body mass loss was 2.2±0.1%, 1.1±0.2%, 1.1±0.1%, 0.1±0.1%, respectively. Total sweat loss was approximately 2% of body mass in all trials. Urine flow rate during exercise was 0.3±0.0, 0.4±0.1, 0.4±0.1, 0.5±0.1 ml/min during NF, AF, 1% FL and 2% FL, respectively, and was lower (p<0.05) than the pre-exercise level in NF and AF. Urine to serum osmolality ratio in 2% FL was higher (p<0.05) during exercise than the pre-exercise level. Creatinine clearance decreased (p<0.05) in all trials, and was higher (p<0.05) in 2% FL than in NF. Osmolar clearance was lower (p<0.05) during exercise than the pre-exercise level in NF, AF and 1% FL, and was higher (p<0.05) in 2% FL than in NF. Free water clearance increased (p<0.05) during exercise than the pre-exercise level in NF, AF and 1% FL, and was lower (p<0.05) in 2% FL than in NF. During exercise, 2% FL resulted in a concentrated urine production. Fluid ingestion in amounts equal to body mass loss at approximately 2% reduction in body mass is capable of attenuating the decline in renal concentrating ability during heavy exercise in the heat.