The fan cooling vest use reduces thermal and perceptual strain during outdoor exercise in the heat on a sunny summer day.

The fan cooling vest is coming into very common use by Japanese outdoor manual workers. We examined that to what extent using this vest reduces thermal strain and perception during outdoor exercise in the heat on a sunny summer day. Ten male baseball players in high school conducted two baseball training sessions for 2-h with (VEST) or without (CON) a commercially available fan cooling vest on a baseball uniform. These sessions commenced at 10 a.m. on separate days in early August. The fan airflow rate attached the vest was 62 L·s-1. Neither ambient temperature (Mean ± SD: VEST 31.9 ± 0.2°C; CON 31.8 ± 0.7°C), wet-bulb globe temperature (VEST 31.2 ± 0.4°C; CON 31.4 ± 0.5°C) nor solar radiation (VEST 1008 ± 136 W·m-2; CON 1042 ± 66 W·m-2) was different between trials. Mean skin temperature (VEST 34.5 ± 1.1°C; CON 35.1 ± 1.4°C), infrared tympanic temperature (VEST 38.9 ± 0.9°C; CON 39.2 ± 1.2°C), heart rate (VEST 127 ± 31 bpm; CON 139 ± 33 bpm), body heat storage (VEST 140 ± 34 W·m-2; CON 160 ± 22 W·m-2), thermal sensation (- 4-4: VEST 0 ± 2; CON 3 ± 1) and rating of perceived exertion (6-20: VEST 11 ± 2; CON 14 ± 2) were lower in VEST than CON (all P < 0.05). Total distance measured with a global positioning system (VEST 3704 ± 293 m; CON 3936 ± 501 m) and body fluid variables were not different between trials. This study indicates that the fan cooling vest use can reduce thermal strain and perception during outdoor exercise in the heat on a sunny summer day. Cooling with this vest would be effective to mitigate thermal risks and perceptual stress in athletes and sports participants under such settings.

Rising vs. falling phases of core temperature on endurance exercise capacity in the heat.

PURPOSE: Core temperature (Tc) shows rising (05:00-17:00 h) and falling (17:00-05:00 h) phases. This study examined the time-of-day effects on endurance exercise capacity and heat-loss responses to control Tc in the heat at around the midpoint of the rising and falling phases of Tc. METHODS: Ten male participants completed cycling exercise at 70% peak oxygen uptake until exhaustion in the heat (30 °C, 50% relative humidity). Participants commenced exercise in the late morning at 10:00 h (AM) or evening at 21:00 h (PM). RESULTS: Time to exhaustion was 28 ± 13% (mean ± SD) longer in PM (49.1 ± 16.3 min) than AM (38.7 ± 14.6 min; P < 0.001). Tc before and during exercise were higher in PM than AM (both P < 0.01) in accordance with the diurnal variation of Tc. The rates of rise in Tc, mean skin temperature, thermal sensation and rating of perceived exertion during exercise were slower in PM than AM (all P < 0.05). Dry and evaporative heat losses and skin blood flow during exercise were greater in PM than AM (all P < 0.05). During 30-min post-exercise recovery, the rates of fall in Tc and skin blood flow were faster and thermal sensation was lower in PM than AM (all P < 0.05). CONCLUSIONS: This study indicates that endurance exercise capacity is greater and heat-loss responses to control Tc during and following exercise in the heat are more effective in the late evening than morning. Moreover, perceived fatigue during exercise and thermal perception during and following exercise are lower in the late evening than morning.

The Running and Technical Performance of U13 to U18 Elite Japanese Soccer Players During Match Play.

Goto, H and Saward, C. The running and technical performance of U13 to U18 elite Japanese soccer players during match play. J Strength Cond Res 34(6): 1564-1573, 2020-The aims of the current study were (a) to examine age-related differences in match running performance with 2 different approaches (speed vs. metabolic power) in U13-U18 Japanese elite soccer players and (b) to examine age-related differences in technical match performance in U13-U18 Japanese elite soccer players. Participants were 110 outfield players from academies of 2 professional soccer clubs in Japan. Forty-eight 11-a-side official league matches (13, 6, 9, 7, 6, and 7 matches for U13, U14, U15, U16, U17, and U18 age-groups, respectively) were analyzed (152 complete match files). Global positioning system (15 Hz) and video analysis were used to analyze running and technical performance during matches, respectively. Total distance covered in absolute terms (U13 < [U14 and U15] < [U16-U18]; p < 0.05 for all), high-intensity running distance ([U13-U15] < [U16-U18]; p < 0.05 for all), and distance covered during the metabolic power zone of ≥35 W·kg relative to match playing time ([U13 < U16], [U13-U15] < [U17 and U18]; p < 0.05 for all) increased with age. The speed zone based approach (high-intensity running distance, ≥4.0 m·s) underestimated high-intensity demands compared with the metabolic power zone based approach (≥20 W·kg) by ∼33 to ∼57% (p < 0.01 for all), with the underestimation declining with age (p < 0.001). Pass accuracy improved with age from 73% at U13 to 85% at U18 (p < 0.001). Therefore, distance covered at high speeds and at high metabolic powers, and pass accuracy increase with age. Moreover, the speed zone based approach underestimates the demands of match play in Japanese elite youth soccer players. The current results could support coaches to develop players, identify talent, and produce age-specific training programs.

High-Intensity Demands of 6-a-Side Small-Sided Games and 11-a-Side Matches in Youth Soccer Players.

PURPOSE: The purposes of the present study were to examine high-intensity running distance during 6-a-side small-sided games (SSGs) and 11-a-side matches (11M) in youth soccer players using speed and metabolic power approaches and the magnitude of difference between the high-intensity running distance calculated with the 2 approaches. METHOD: A total of 11 outfield players (age = 16.3 [0.6] y) performed SSGs with 3 pitch sizes (small SSG [SSGS], medium SSG, and large SSG [SSGL]) and 11M. A Global Positioning System (15 Hz) was employed to calculate total distance covered, distance covered at a speed ≥4.3 m·s-1 (TS), and metabolic power of ≥20 W·kg-1 (TP). RESULTS: The total distance covered increased from SSGS through to SSGL (P < .001) and was greater during 11M and SSGL compared with other SSGs (P < .01). TS and TP increased from SSGS (TS vs TP = 98 [55] vs 547 [181] m) through to SSGL (538 [167] vs 1050 [234] m; P < .001). TS and TP during 11M (370 [122] vs 869 [233] m) was greater than SSGS (P < .001 for both) and less than SSGL (P < .05 for both). The magnitude of difference between TS and TP (as a percentage) was lower with an increase in pitch size during SSGs and was greater in SSGS (615% [404%]; P < .001), medium SSG (195% [76%]; P < .05), and smaller in SSGL (102% [33%]; P < .01) compared with 11M (145% [53%]). CONCLUSION: SSGs can replicate the high-intensity demands of 11M and the speed approach underestimates the high-intensity demands of SSGs and 11M compared with the metabolic power approach.

Influence of Biological Maturity on the Match Performance of 8- to 16-Year-Old, Elite, Male, Youth Soccer Players.

Goto, H, Morris, JG, and Nevill, ME. Influence of biological maturity on the match performance of 8- to 16-year-old, elite, male, youth soccer players. J Strength Cond Res 33(11): 3078-3084, 2019-The aim of this study was to examine the influence of biological maturity on match performance in elite, youth, male soccer players. The participants were 80 Premier League Academy outfield players (8-16 years old). Biological maturity was determined by calculating estimated chronological age at peak height velocity. The U9 and U10 squads played 6-a-side and the U11-U16 squads played 11-a-side interacademy matches. All matches were analyzed using a 1-Hz global positioning system (SPI Elite™, GPSports, Canberra, Australia) with squad-specific speed zones, which were calculated based on 5-m flying sprint speed in the last 5 m of 10-m sprint test. In the U9/U10s, earlier maturers were given a longer pitch time by coaches (∼4 minutes per match; p = 0.029) and covered a greater total distance (∼9%; ∼400 m; p = 0.037) and a greater distance by walking (∼13%; ∼100 m; p = 0.024) and jogging (∼12%; ∼200 m; p = 0.014) during a match compared with later maturers. In the U13/U14s, earlier maturers covered a greater distance per hour of a match by high-speed running compared with later maturers (∼25%; ∼130 m; p = 0.028) and spent a longer percentage of time in high-speed running during a match compared with later maturers (3.4% vs. 2.7%, p = 0.034). Thus, coaches should take care to provide all players with a similar pitch time and should be aware in the talent identification and development process, particularly with the U13/U14 age group, because maturity can influence high-speed match running performance.

Solar Radiation Exposure Has Diurnal Effects on Thermoregulatory Responses During High-Intensity Exercise in the Heat Outdoors.

Otani, H, Goto, T, Goto, H, Hosokawa, Y, and Shirato, M. Solar radiation exposure has diurnal effects on thermoregulatory responses during high-intensity exercise in the heat outdoors. J Strength Cond Res 33(10): 2608-2615, 2019-This study investigated the diurnal effects of variations in solar radiation associated with changing solar elevation angle on thermoregulatory responses during high-intensity exercise in the heat outdoors. Ten male high school soccer players completed two 2-hour soccer training sessions under a clear sky in the heat of summer. These sessions were commenced at 0900 hours (AM) and 1600 hours (PM) on separate days. Solar radiation and elevation angle were higher in AM (820-1,070 W·m and 45-69°) than PM (620-110 W·m and 34-10°: both p < 0.001). Neither ambient temperature (AM 29-32° C; PM 31-31° C) nor wet-bulb globe temperature was different between trials. Although mean skin temperature was not different between trials, infrared tympanic temperature was higher at the end of exercise in AM than PM (p < 0.001). Heart rate (p < 0.01) and body heat gain from the sun (p < 0.001) were greater during exercise in AM than PM. Dry heat loss was smaller, but evaporative heat loss was greater in AM than PM (both p < 0.001). Thermal sensation and rating of perceived exertion were similar between trials, but GPS measurements showed a less total distance and distance covered by walking, jogging, and running in AM than PM (p < 0.01). This study demonstrates a greater thermoregulatory strain in AM than PM during 2-hour high-intensity soccer training in the heat under a clear sky. This observation is accompanied by a progressive increase in environmental heat stress with rising solar radiation and elevation angle in AM and a greater body heat gain from the sun in AM compared with PM.

Motion analysis of U11 to U16 elite English Premier League Academy players.

This study examined (1) the distances and speeds covered during match play for U11 to U16 English Premier League Academy players, (2) the differences in match performance between retained and released players and (3) the effect of calculating speed zones in different ways when analysing match performance. Eighty-one academy outfield players (10-16 years old), competing in 11-a-side matches, were analysed using a 1-Hz global positioning system with three speed zones (absolute, squad and individual). Pitch dimensions were 78.7 × 54.1 m (U11 and U12), 88.0 × 64.2 m (U13) and 100.8 × 68.2 m (U14-U16). Match durations were 20 min × 2 + 15 min × 2 or 25 min × 3 (U11), 25 min × 3 (U12 and U13) and 40 min × 2 (U14-U16). The matches were part of the regular series of inter-academy matches between Premier League Academies during a season. Completion of at least a half of the duration of a match in two separate matches (mean ± s = 3.1 ± 0.8 matches; range: 2-4 matches) was the criterion for inclusion in the study. Total match running distance increased with age from ~5700 (U11) to ~6700 m · h(-1) (U15) (P < 0.01). Using the absolute speed zones, it was possible to discern differences in high-intensity (>6.0 m · s(-1)) distance covered with age (U11: 29 m · h(-1), U16: 164 m · h(-1), P < 0.01). Using the squad speed zones, it was possible to discern differences between retained and released players in the U11/U12s (moderate-speed running) and in the U15/U16s (walking, jogging and low-speed running) (P < 0.01). Thus, total and high-intensity running distances increase with age, and walking, jogging, low-speed running and moderate-speed running distances are greater in retained than in released players and these differences are best identified through the use of absolute and squad speed zones, respectively.

Match analysis of U9 and U10 english premier league academy soccer players using a global positioning system: relevance for talent identification and development.

The purpose of this study was to examine the match activity profile of U9 and U10 elite soccer players and to establish if there were any differences between players who were subsequently retained or released by their clubs. Such information should prove valuable in the design of training programs for these very young players and in the talent identification and development process. A Global Positioning System was used to analyze 2-4 interacademy 6-a-side matches of English Premier League Academy players (U9: N = 22 and U10: N = 12) who trained 3 times a week (4.5 hours). Speed zones were created based on 5 and 10-m sprint times, and an independent sample t-test was employed for a statistical analysis. Both squads covered ∼4,000 m in total or ∼4,700 m·h during a match (p = NS between squads), with the U10 squad tending to cover a greater distance at moderate (p = 0.10) and high speeds (p = 0.08) than the U9 squad. Retained group covered a greater distance than released group (retained vs. released: 4,478 ± 513 m vs. 4,091 ± 462 m, p < 0.05) during a match and covered a greater distance during low-speed running in absolute (1,226 ± 259 m vs. 1,005 ± 221 m, p < 0.05) and relative (1,325 ± 235 m·h vs. 1,132 ± 210 m·h, p < 0.05) terms. Thus, U9 and U10 players cover over 4000 m in match play, and those players who are retained by academies cover a greater distance in total and at low speeds (2.1-3.1 m·s). This information may support the preparation of squad training programs and the talent identification and development process.

Thermal strain is greater in the late afternoon than morning during exercise in the gym without airflow and air conditioning on a clear summer day.

Introduction: There are no reports examining the time-of-day effect on team training sessions in the gym without airflow and air conditioning on thermal strain in the summer heat. We investigated this effect during badminton training sessions on a clear summer day. Methods: Nine male high school badminton players (Mean ± SD; age 17.1 ± 0.6 y, height 171 ± 4 cm, body mass 59 ± 7 kg) completed two 2.5-h badminton training sessions in the gym without airflow and air conditioning. The training sessions were started at 0900 h (AM) and 1600 h (PM) on separate days in August. Skin temperatures (chest, triceps, thigh, calf), infrared tympanic temperature, heart rate, thermal sensation and rating of perceived exertion were recorded at rest and at regular intervals during the sessions. Results: Indoor and outdoor environmental heat stress progressively increased in AM and decreased in PM during the sessions. Ambient temperature (AM 30.1 ± 0.9°C; PM 33.2 ± 1.0°C: P < 0.001) and wet-bulb globe temperature (AM 28.1 ± 0.5°C; PM 30.0 ± 0.9°C: P = 0.001) during the sessions in the gym were higher in PM than AM. Mean skin temperature (AM 34.2 ± 1.0°C; PM 34.7 ± 0.7°C: P < 0.001), infrared tympanic temperature (AM 37.8 ± 0.5°C; PM 38.1 ± 0.4°C: P = 0.001) and thermal sensation (AM 2.7 ± 1.4; PM 3.3 ± 1.0: P < 0.001) during the sessions were higher in PM than AM. Body heat storage (AM 159 ± 30 W·m-2; PM 193 ± 30 W·m-2: P < 0.05) was greater in PM than AM. There were no time-of-day differences in the average heart rate (AM 75 ± 4% age-predicted maximal heart rate; PM 76 ± 5 age-predicted maximal heart rate: P = 0.534), body mass loss (AM 0.6 ± 0.3 kg; PM 0.8 ± 0.2°C: P = 0.079), the volume of water ingested (AM 1.5 ± 0.1 L; PM 1.6 ± 0.3 L: P = 0.447) and rating of perceived exertion (AM 16 ± 2; PM 16 ± 3: P = 0.281). Conclusions: This study indicates greater thermal strain in PM trial than in AM trial during team training sessions in the gym without airflow and air conditioning on a clear summer day. Therefore, athletes and coaches of indoor sports should perceive that athletes may be exposed to a greater risk for thermal strain in the late afternoon from 1600 h than in the morning from 0900 h during the sessions in the gym under these conditions.

Combined effects of solar radiation and airflow on endurance exercise capacity in the heat.

This study investigated the combined effects of different levels of solar radiation and airflow on endurance exercise capacity and thermoregulatory responses during exercise-heat stress. Ten males cycled at 70% peak oxygen uptake until exhaustion in an environmental chamber (30°C, 50% relative humidity). Four combinations of solar radiation and airflow were tested (800 W⋅m-2 and 10 km⋅h-1 [High-Low], 800 W⋅m-2 and 25 km⋅h-1 [High-High], 0 W⋅m-2 and 10 km⋅h-1 [No-Low], and 0 W⋅m-2 and 25 km⋅h-1 [No-High]). Participants were exposed to solar radiation by a ceiling-mounted solar simulator (Metal halide lamps) and the headwind by two industrial fans. Time to exhaustion was shorter (p < 0.05) in High-Low (mean ± SD; 35 ± 7 min) than the other trials and in High-High (43 ± 6 min) and No-Low (46 ± 9 min) than No-High (61 ± 9 min). There was an interaction effect in total (dry + evaporative) heat exchange which was less in High-Low and High-High than No-Low and No-High, and in No-Low than No-High (all p < 0.001). Core temperature, heart rate and thermal sensation were higher in high (High-Low and High-High) than no (No-Low and No-High) solar radiation trials and in lower (High-Low and No-Low) than higher (High-High and No-High) airflow trials (p < 0.05). Mean skin temperature and rating of perceived exertion were higher in high than no solar radiation trials (p < 0.05). This study indicates that combining high solar radiation and lower airflow have negative effects on thermoregulatory and perceptual strain and endurance exercise capacity than when combining high solar radiation and higher airflow and combining no solar radiation and lower/higher airflow during exercise-heat stress.

Longitudinal Physical Development of Future Professional Male Soccer Players: Implications for Talent Identification and Development?

The present study examined if elite youth male association football (soccer) players aged 8-19 y (n = 2,875) from the English talent development system, who ultimately achieved professional status differed in stature, body mass, and physical performance (20-m sprint speed, slalom agility speed, vertical counter-movement jump with arm swing jump height, multistage fitness test distance) compared with their non-professional peers. The study also examined the longitudinal pattern of development of stature, body mass, and physical performance, and if this was different between future professionals and non-professionals, while considering the effects of playing position. Multilevel modeling of the 8,898 individual (player-occasion) data points suggested that from age 12.0, the future professionals performed better in a vertical counter-movement jump with arm swing test and slalom agility test than future non-professionals, and improved at a faster rate, so that by age 18.0 the differences in vertical counter-movement jump with arm swing and slalom agility performance were 1.7 cm (p < 0.001, d = 0.3) and 0.14 s (p < 0.001, d = 0.5), respectively. In addition, future professionals were faster (by 0.02-0.04 s on the 20-m sprint, p < 0.001, d = 0.2) and ran further in the multistage fitness test (by 47 m, p = 0.014, d = 0.2) than future non-professionals throughout their development, but there were no differences in stature or body mass during development between the groups. Whereas, multistage fitness test performance improved linearly with age, the development of all other physical characteristics was non-linear. There were inter-individual differences in the development of all characteristics, and there were differences between playing positions in the development of all characteristics. Thus, in summary, future professionals jump higher, are more agile, faster, and more endurance fit than future non-professionals as they age, and the pattern of development is different in professionals and non-professionals for vertical jumping and slalom agility performance.

Greater thermoregulatory strain in the morning than late afternoon during judo training in the heat of summer.

PURPOSE: The time-of-day variations in environmental heat stress have been known to affect thermoregulatory responses and the risk of exertional heat-related illness during outdoor exercise in the heat. However, such effect and risk are still needed to be examined during indoor sports/exercises. The current study investigated the diurnal relationships between thermoregulatory strain and environmental heat stress during regular judo training in a judo training facility without air conditioning on a clear day in the heat of summer. METHODS: Eight male high school judokas completed two 2.5-h indoor judo training sessions. The sessions were commenced at 09:00 h (AM) and 16:00 h (PM) on separate days. RESULTS: During the sessions, indoor and outdoor heat stress progressively increased in AM but decreased in PM, and indoor heat stress was less in AM than PM (mean ambient temperature: AM 32.7±0.4°C; PM 34.4±1.0°C, P<0.01). Mean skin temperature was higher in AM than PM (P<0.05), despite greater dry and evaporative heat losses in AM than PM (P<0.001). Infrared tympanic temperature, heart rate and thermal sensation demonstrated a trial by time interaction (P<0.001) with no differences at any time point between trials, showing relatively higher responses in these variables in PM compared to AM during the early stages of training and in AM compared to PM during the later stages of training. There were no differences between trials in body mass loss and rating of perceived exertion. CONCLUSIONS: This study indicates a greater thermoregulatory strain in the morning from 09:00 h than the late afternoon from 16:00 h during 2.5-h regular judo training in no air conditioning facility on a clear day in the heat of summer. This observation is associated with a progressive increase in indoor and outdoor heat stress in the morning, despite a less indoor heat stress in the morning than the afternoon.

Exposure to high solar radiation reduces self-regulated exercise intensity in the heat outdoors.

High radiant heat load reduces endurance exercise performance in the heat indoors, but this remains unconfirmed in outdoor exercise. The current study investigated the effects of variations in solar radiation on self-regulated exercise intensity and thermoregulatory responses in the heat outdoors at a fixed rating of perceived exertion (RPE). Ten male participants completed 45-min cycling exercise in hot outdoor environments (about 31°C) at a freely chosen resistance and cadence at an RPE of 13 (somewhat hard). Participants were blinded to resistance, pedal cadence, distance and elapsed time and exercised at three sunlight exposure conditions: clear sky (mean ±â€¯SD: 1072 ±â€¯91 W·m-2; HIGH); thin cloud (592 ±â€¯32 W·m-2; MID); and thick cloud (306 ±â€¯52 W·m-2; LOW). Power output (HIGH 96 ±â€¯22 W; MID 103 ±â€¯20 W; LOW 108 ±â€¯20 W) and resistance were lower in HIGH than MID and LOW (P < .001). Pedal cadence was lower, the core-to-skin temperature gradient was narrower, body heat gain from the sun (SHG) was greater and thermal sensation was higher with increasing solar radiation and all variables were different between trials (P < .01). Mean skin temperature was higher in HIGH than MID and LOW (P < .01), but core temperature was similar between trials (P = .485). We conclude that self-regulated exercise intensity in the heat outdoors at a fixed RPE of somewhat hard is reduced with increasing solar radiation because of greater thermoregulatory strain, perceived thermal stress and SHG. This suggests that reduced self-selected exercise intensity during high solar radiation exposure in the heat may prevent excessive core temperature rise.

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

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.

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

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.