Thermoregulatory responses and hydration practices in heat-acclimatized adolescents during preseason high school football

Abstract

Context: Previous researchers have not investigated the thermoregulatory responses to multiple consecutive days of American football in adolescents.

Objective: To examine the thermoregulatory and hydration responses of high school players during formal preseason football practices.

Design: Observational study.

Setting: Players practiced outdoors in late August once per day on days 1 through 5, twice per day on days 6 and 7, and once per day on days 8 through 10. Maximum wet bulb globe temperature averaged 23 +/- 4 degrees C.

Patients or other participants: Twenty-five heat-acclimatized adolescent boys (age = 15 +/- 1 years, height = 180 +/- 8 cm, mass = 81.4 +/- 15.8 kg, body fat = 12 +/- 5%, Tanner stage = 4 +/- 1).

Main outcome measure(s): We observed participants within and across preseason practices of football. Measures included gastrointestinal temperature (T(GI)), urine osmolality, sweat rate, forearm sweat composition, fluid consumption, testosterone to cortisol ratio, perceptual measures of thirst, perceptual measures of thermal sensation, a modified Environmental Symptoms Questionnaire, and knowledge questionnaires assessing the participants' understanding of heat illnesses and hydration. Results were analyzed for differences across time and were compared between younger (14-15 years, n = 13) and older (16-17 years, n = 12) participants.

Results: Maximum daily T(GI) values remained less than 40 degrees C and were correlated with maximum wet bulb globe temperature (r = 0.59, P = .009). Average urine osmolality indicated that participants generally experienced minimal to moderate hypohydration before (881 +/- 285 mOsmol/kg) and after (856 +/- 259 mOsmol/kg) each practice as a result of replacing approximately two-thirds of their sweat losses during exercise but inadequately rehydrating between practices. Age did not affect most variables; however, sweat rate was lower in younger participants (0.6 +/- 0.2 L/h) than in older participants (0.8 +/- 0.1 L/h) (F(1,18) = 8.774, P = .008).

Conclusions: Previously heat-acclimatized adolescent boys (T(GI) < 40 degrees C) can safely complete the initial days of preseason football practice in moderate environmental conditions using well-designed practice guidelines. Adolescent boys replaced most sweat lost during practice but remained mildly hypohydrated throughout data collection, indicating inadequate hydration habits when they were not at practice.

Figure 1

Schematic of the 10-day preseason practice sessions protocol. Δ Indicates hydration status, sweat rate, fluid consumed, gastrointestinal temperature, and perceptual measures. ^a Indicates sweat patches. ^b Indicates hormone responses. ^c Indicates full contact. For days not noted as full contact, contact was minimal or limited.

Figure 2

Maximum gastrointestinal and wet bulb temperature responses across days. ^a Indicates more than day 7AM. ^b Indicates more than day 6AM. ^c Indicates more than day 6PM. ^d Indicates more than day 7PM. All symbols, P < .05. Abbreviations: AM, before practice began each day; PM, after returning to the locker room once practice ended each day.

Figure 3

A, Sweat rate across days. ^a Indicates more than day 5. ^b Indicates more than day 7PM. ^c Indicates more than day 6AM and 6PM ^d Indicates more than days 3 and 4. B, Total sweat loss across days. ^e Indicates more than days 5 and 7PM. ^f Indicates more than day 6AM. ^g Indicates more than day 4. C, Fluid consumed across days. ^h Indicates more than days 5 and 6AM. ⁱ Indicates more than day 7PM. ^j Indicates more than day 4. ^k Indicates more than day 6PM. All symbols, P < .05. Abbreviations: AM, before practice began each day; PM, after returning to the locker room once practice ended each day.

Figure 4

Testosterone to cortisol ratio responses across days. ^a Indicates more than day 7 (P < .05).