Dehydration has many deleterious effects on cognitive and physical performance as well as physiological function, in the context of sports, industrial work, clinical rehabilitation, and military applications. Because sweat loss and electrolyte loss vary across individuals, conventional sweat testing strategies using absorbent patch techniques are employed in laboratory settings to characterize sweat biomarkers; however, these techniques are not suitable for remote environments. Here, an updated wearable microfluidic sweat testing system targeted for recreational athletes is presented that includes a microfluidic patch accommodating a broad range of sweating rates, and a smartphone app incorporating digital image processing algorithms to enable real-time analysis under different lighting conditions and patch orientations. Expansive field trials (n = 148 subjects) show significant correlations between the microfluidic patch and standard absorbent patch in measuring sweating rate and sweat chloride concentration during recreational exercise. This validation study demonstrates the applicability of the microfluidic patch and software platform for field testing in recreational athletes.

The purpose of this study was to expand our previously published sweat normative data/analysis (n = 506) to establish sport-specific normative data for whole-body sweating rate (WBSR), sweat [Na+], and rate of sweat Na+ loss (RSSL). Data from 1303 athletes were compiled from observational testing (2000-2017) using a standardized absorbent sweat patch technique to determine local sweat [Na+] and normalized to whole-body sweat [Na+]. WBSR was determined from change in exercise body mass, corrected for food/fluid intake and urine/stool loss. RSSL was the product of sweat [Na+] and WBSR. There were significant differences between sports for WBSR, with highest losses in American football (1.51 +/- 0.70 L/h), then endurance (1.28 +/- 0.57 L/h), followed by basketball (0.95 +/- 0.42 L/h), soccer (0.94 +/- 0.38 L/h) and baseball (0.83 +/- 0.34 L/h). For RSSL, American football (55.9 +/- 36.8 mmol/h) and endurance (51.7 +/- 27.8 mmol/h) were greater than soccer (34.6 +/- 19.2 mmol/h), basketball (34.5 +/- 21.2 mmol/h), and baseball (27.2 +/- 14.7 mmol/h). After ANCOVA, significant between-sport differences in adjusted means for WBSR and RSSL remained. In summary, due to the significant sport-specific variation in WBSR and RSSL, American football and endurance have the greatest need for deliberate hydration strategies.

This study determined the relations between regional (REG) and whole body (WB) sweating rate (RSR and WBSR, respectively) as well as REG and WB sweat Na+ concentration ([Na+]) during exercise. Twenty-six recreational athletes (17 men, 9 women) cycled for 90 min while WB sweat [Na+] was measured using the washdown technique. RSR and REG sweat [Na+] were measured from nine regions using absorbent patches. RSR and REG sweat [Na+] from all regions were significantly ( P < 0.05) correlated with WBSR ( r = 0.58-0.83) and WB sweat [Na+] ( r = 0.74-0.88), respectively. However, the slope and y-intercept of the regression lines for most models were significantly different than 1 and 0, respectively. The coefficients of determination ( r2) were 0.44-0.69 for RSR predicting WBSR [best predictors: dorsal forearm ( r2 = 0.62) and triceps ( r2 = 0.69)] and 0.55-0.77 for REG predicting WB sweat [Na+] [best predictors: ventral forearm ( r2 = 0.73) and thigh ( r2 = 0.77)]. There was a significant ( P < 0.05) effect of day-to-day variability on the regression model predicting WBSR from RSR at most regions but no effect on predictions of WB sweat [Na+] from REG. Results suggest that REG cannot be used as a direct surrogate for WB sweating responses. Nonetheless, the use of regression equations to predict WB sweat [Na+] from REG can provide an estimation of WB sweat [Na+] with an acceptable level of accuracy, especially using the forearm or thigh. However, the best practice for measuring WBSR remains conventional WB mass balance calculations since prediction of WBSR from RSR using absorbent patches does not meet the accuracy or reliability required to inform fluid intake recommendations. NEW & NOTEWORTHY This study developed a body map of regional sweating rate and regional (REG) sweat electrolyte concentrations and determined the effect of within-subject (bilateral and day-to-day) and between-subject (sex) factors on the relations between REG and the whole body (WB). Regression equations can be used to predict WB sweat Na+ concentration from REG, especially using the forearm or thigh. However, prediction of WB sweating rate from REG sweating rate using absorbent patches does not reach the accuracy or reliability required to inform fluid intake recommendations.

Previous research has measured the amount of sweat absorbed in basketball uniforms during exercise, but data are limited in other sports. PURPOSE: To determine the amount of trapped sweat (TS) in various sports uniforms during sport-specific, laboratory-based exercise. METHODS: Eleven male (30 ± 5 years, 75.7 ± 5.2 kg) and 6 female (29 ± 4 years, 59.9 ± 9.9 kg) moderately-trained athletes completed 3 trials consisting of 120 min intermittent sport-specific exercise in standard uniforms for various sports, including football (n=9 men), basketball (n= 4 men, 5 women), soccer (n=4 men, 5 women), baseball/softball (n=4 men, 4 women), and/or endurance (n=5 men, 4 women) in a temperature- controlled laboratory (basketball: 25°C, 55% rh; all other sports: 30°C, 55% rh). Protocols were designed to simulate the demands of each sport (endurance: 82 ± 5% HRmax, RPE 13 ± 2; football: 75 ± 10% HRmax, RPE 13 ± 1; soccer: 77 ± 10% HRmax, RPE 12 ± 1; basketball: 66 ± 12% HRmax, RPE 10 ± 2; and baseball/softball: 59 ± 3% HRmax, RPE 9 ± 2). Sweat loss (SL) was determined from change in nude body mass corrected for fluid intake, urine loss, respiratory water loss, and metabolic mass loss. Nude and clothed body mass were measured pre- and post-exercise to determine TS. Analysis of variance followed by Tukey’s post hoc test was used to compare sports. Data are mean ± SD. RESULTS: There were significant differences in SL between sports (p<0.0001): football (2.61 ± 0.36 kg), endurance (2.18 ± 0.53 kg) and soccer (1.99 ± 0.81 kg) > basketball (1.24 ± 0.37 kg) and baseball/softball (1.19 ± 0.38 kg). There were also significant differences in TS (p<0.0001): football (0.58 ± 0.14 kg) > endurance (0.28 ± 0.16 kg) and soccer (0.24 ±0.18 kg) > basketball (0.11 ± 0.08 kg) and baseball/softball (0.15 ± 0.12 kg). TS as a percentage of SL was significantly (p<0.0001) higher in football (22.5 ± 3.8%) than endurance (12.2 ± 4.7%), soccer (10.9 ± 3.4%), basketball (9.2 ± 4.4%), and baseball/softball (10.8 ± 6.2%). CONCLUSION: Sports with higher SL were associated with higher volumes of TS in uniforms. The football uniform (including full pads) led to the most TS and greatest underestimations in SL. Such high volumes of TS are also likely to have ramifications for evaporative heat loss capacity and therefore warrant future research investigating the effects of TS on thermoregulation.

Previously, we published sweating rate (SR) and sweat sodium concentration ([Na+]) normative data in 506 athletes. PURPOSE: The purpose of this study was to expand the data set and analyses to establish sport-specific normative data for SR and rate of sweat Na+ loss. METHODS: Data from 1303 athletes (1103 male, 200 female) were compiled from field and lab testing. SR was calculated from the difference in pre- to post-exercise body mass, correcting for food/fluid intake and urine/stool loss. A standardized absorbent sweat patch technique was used to determine local sweat [Na+] and normalized to whole body sweat [Na+] using published regression equations. Rate of sweat Na+ loss was determined from the product of whole body sweat [Na+] and SR. The sport-specific analysis included sports with n>100; endurance (n=255), soccer (n=268), basketball (n=196), American football (n=271), and baseball (n=161). RESULTS: Data are mean ± SD. SR differed significantly between sports (ANOVA, Tukey’s post hoc; p<0.05); American football displayed the highest SR (1.5 ± 0.7 L/h), followed by endurance (1.3 ± 0.6 L/h), basketball (1.0 ± 0.4 L/h), soccer (0.9 ± 0.4 L/h) and baseball (0.8 ± 0.3 L/h). The rate of sweat Na+ loss was higher in American football (55.9 ± 36.8 mmol/h) and endurance (51.7 ± 27.8 mmol/h) compared with soccer (34.6 ± 19.2 mmol/h), basketball (34.5 ± 21.2 mmol/h), and baseball (27.2 ± 14.7 mmol/h). The rate of sweat Na+ loss was higher in soccer than baseball. After accounting for the impact of covariates (age, sex, body mass, temperature, humidity, season, and intensity), there were still significant differences (ANCOVA, Tukey’s post hoc; p<0.05) in the adjusted means for SR and rate of sweat Na+ loss; endurance (1.2 L/h, 43.1 mmol/h), football (1.0 L/h, 38.2 mmol/h) and soccer (1.0 L/h, 35.4 mmol/h) were higher than baseball (0.8 L/h, 25.5 mmol/h), and endurance was higher than basketball (0.9 L/h, 32.0 mmol/h). CONCLUSION: This study suggests the potential for significant variation in the rate of sweat fluid and Na+ losses between sports, with highest values generally occurring in endurance and American football. There are already products targeted to meet the needs of endurance athletes to replace their higher sweat fluid and electrolyte losses; perhaps there is also a need for products and education specific to other sports.

The aims of this study were to determine: (1) trapped sweat (TS) in basketball uniforms and the effect on sweat loss (SL) estimates during a laboratory-based basketball simulation protocol; (2) the impact of exercise intensity, body mass, age, and SL on TS; and (3) TS during on-court training to assess the ecological validity of the laboratory-based results. Twenty-four recreational/competitive male basketball players (23 ± 10 years, 77.0 ± 16.7 kg) completed three randomized laboratory-based trials (Low, Moderate, and High intensity) consisting of 150-min intermittent exercise. Eighteen elite male players (23 ± 4 years, 92.0 ± 20.6 kg) were observed during coach-led, on-court training. Nude and clothed body mass were measured pre and postexercise to determine TS. Data are mean ± SD. There was a significant effect of intensity on SL and TS (P < 0.001, Low<Moderate<High, ANOVA). During Low, subjects lost 1.10 ± 0.59 kg sweat and TS was 0.11 ± 0.15 kg (8.0 ± 5.1% SL). During Moderate, subjects lost 1.60 ± 0.56 kg sweat and TS was 0.21 ± 0.21 kg (11.6 ± 6.3% SL). During High, subjects lost 2.12 ± 0.66 kg sweat and TS was 0.38 ± 0.28 kg (16.0 ± 7.4% SL). Multiple regression and partial correlation analysis suggested TS was significantly related to SL (P < 0.0001; partial r = 0.81-0.89), whereas the contributions of body mass (P = 0.22-0.92) and age (P = 0.29-0.44) were not significant. TS during on-court training was 0.35 ± 0.36 kg, which was associated with a 14.1 ± 11.5% underestimation in SL, and was not statistically different than laboratory-based results (P = 0.59). Clothed body mass measurements should be used with caution, as TS is highly variable and can cause a significant underestimation in SL in athletes with high sweating rates.

PURPOSE:This study investigated the effect of beverage osmolalities, carbohydrate (CHO) type, and CHO concentration on gastric emptying in euhydrated subjects at rest. METHODS:The gastric emptying of water (W), four glucose beverages (2%, 4%, 6%, and 8% glucose: 2G, 4G, 6G, and 8G), and four sucrose beverages (2%, 4%, 6%, and 8% sucrose: 2S, 4S, 6S, and 8S) were determined in eight healthy subjects using the modified George double-sampling technique. Subjects ingested a beverage (7 mL·kg body weight) containing 25 ppm phenol red as quickly as possible (≤1.0 min), and subsequent gastric and blood samples were collected every 10 min for 40 min. A linear regression and a repeated-measures ANOVA were used for statistical analysis. RESULTS:The gastric secretion volume was not significantly different among beverages across time. Gastric residual beverage volume (GRBV) at each sampling time point was not different among 2S, 4S, 6S, 8S, and water (P > 0.05). The 8G resulted in a significantly greater GRBV compared with other beverages at 20, 30, and 40 min (P < 0.05). GRBV from 6G was significantly higher than 2G at 30 min, but no other statistical differences were found among W, 2G, 4G, and 6G. The 8S had a greater GRBV compared with W at 40 min (P < 0.05). Mean gastric osmolality positively correlated to mean GRBV (r = 0.93). Gastric emptying rate was negatively correlated to the calories emptied (r = 0.84) with a greater effect from glucose beverages compared with sucrose beverages. CONCLUSIONS:These data suggest that glucose exerts a stronger inhibitory stimulus compared with sucrose on gastric emptying and that a physiological threshold exists for the combined influence of glucose concentration and beverage osmolality to trigger the feedback inhibition of gastric emptying.

The absorbent patch method is often used to estimate whole body (WB) sweating responses in athletes. However, no study havs investigated the relation between results obtained with the local absorbent patch method (L) versus WB for both sweating rate (SR) and sweat sodium concentration ([Na+]). Therefore, the objective of this study was to determine the relation between LSR and WBSR as well as local and WB sweat [Na+] during moderate intensity exercise. Thirteen non‐heat acclimated, male, recreational athletes (age: 31 ± 6 y; BSA: 1.88 ± 0.18 m2; VO2max: 51±8 ml/kg/min) completed 90 min of moderate intensity cycling (75–85% maximal heart rate) in a warm environment (30°C, 41% relative humidity) to determine WB sweat [Na+] using the washdown technique. In addition, small sweat samples were collected from the dorsal and ventral forearm, tricep, chest, scapula, lower back, ventral thigh, calf, and forehead with absorbent patches (3M TegadermTM+Pad; pad size 10 cm2) and analyzed via ion chromatography to determine local sweat [Na+]. WBSR was determined from body mass change over time corrected for fluid intake, urine output, respiratory water loss, and metabolic mass loss. LSR was determined from the mass change in the absorbent pad and the duration of pad time on the skin. A linear regression analysis and Pearson product moment correlation were used to determine the relation between LSR versus WBSR and local versus WB sweat [Na+]. A repeated‐measures one‐way ANOVA with Dunnett post hoc test was used to determine differences between LSR and WBSR as well as local and WB sweat [Na+]. Data are shown as mean ± SD. WBSR and WB sweat [Na+] were 0.63 ± 19 mg/cm2/min and 40 ± 14 mmol/L, respectively. LSR and local sweat [Na+] from all nine anatomical sites were significantly correlated with WBSR (r2: 0.43–0.71, p < 0.05) and WB sweat [Na+] (r2: 0.59–0.82, p < 0.01), respectively. There was no difference between calf LSR (0.73 ± 31 mg/cm2/min) and WBSR. For all other anatomical sites LSR was significantly greater than WBSR (by +0.34 ± 0.25 mg/cm2/min (thigh) to +3.86 ± 2.86 mg/cm2/min (forehead), p < 0.001). With respect to sweat [Na+], ventral forearm, dorsal forearm, thigh, calf, and lower back were not different than WB (36 ± 20 mmol/L (calf) to 50 ± 24 mmol/L (lower back)). However, local sweat [Na+] from the scapula, tricep, forehead, and chest were significantly greater than WB sweat [Na+] (by +12 ± 11 mmol/L (tricep) to +31 ± 16 mmol/L (scapula), p < 0.001). In conclusion, while many local sites overestimated WB sweating responses, there were significant correlations between LSR and WBSR as well as local and WB sweat [Na+] at all nine anatomical sites tested. In general, it seems that the limbs (e.g., dorsal and ventral forearms, ventral thigh, and calf) are the most representative of WBSR and WB sweat [Na+]. These results can help inform sweat testing best practices in non‐heat acclimated, male, recreational athletes during moderate exercise in a warm environment.Support or Funding InformationThis study was funded by the Gatorade Sports Science Institute, a division of PepsiCo, Inc. The views expressed in this article are those of the authors and do not necessarily reflect the position or policy of PepsiCo, Inc.

Exercise sweat loss is often measured by an athlete’s change in body mass (BM). Nude BM is the most accurate method, but is impractical in many field situations. While clothed BM is more practical, it is confounded by trapped sweat (TS) in clothing. PURPOSE: To determine the amount of cumulative TS in basketball uniforms during a laboratory-based 2.5 h intermittent exercise protocol and the associated impact of BM, age, sweating rate, and exercise intensity. METHODS: 24 male basketball players [12 youth (15 ± 2 years, 71 ± 19 kg), 12 adults (32 ± 6 years, 84 ± 11 kg)] completed 3 randomized trials [low (L), moderate (M), and high-intensity (H)], each consisting of 5 X 30-min bouts of intermittent exercise (variable speeds on a treadmill interspersed with basketball drills on the floor) in a temperature-controlled room (23°C and 62% rh). The L, M, and H trials elicited progressively higher heart rate (HR) responses (L: 64 ± 6%, M: 75 ± 6%, and H: 82 ± 6% HRmax). Nude and clothed (jersey top and shorts; compression tank, shorts, arm, and knee sleeves; headband; socks; sneakers) BM were measured 25 min into each bout to determine TS. A split-plot ANOVA followed by Fisher’s LSD was used to determine between-subjects (BM, age, sweating rate) and within-subject (exercise intensity) effects on TS. Data are expressed as mean ± SD (range). RESULTS: Exercise intensity had a significant effect on TS (p < 0.001). At the end of the 2.5 h protocol, cumulative TS in the L, M, and H trials was 0.10 ± 0.15 kg (0.00-0.75 kg), 0.21 ± 0.21 kg (0.00-1.02 kg), and 0.38 ± 0.28 kg (0.03-1.33 kg), respectively. Sweating rate also had a significant effect on TS (p < 0.01), such that high sweating rates were associated with greater cumulative TS [0.33 ± 0.28 kg (0.05-1.33 kg)] than low sweating rates [0.12 ± 0.13 kg (0.00-0.53 kg)] after the 2.5 h protocol. BM (p = 0.57) and age (p = 0.29) had no significant effect on TS. CONCLUSION: TS varies considerably during simulated basketball activity; sweating rate and exercise intensity were the most important factors determining inter- and intra-individual differences, respectively, in the amount of TS in basketball uniforms. Future research is needed to determine if similar results are found during on-court basketball practices/games and to investigate trapped sweat in outdoor sports played in variable environmental conditions.

The purpose of this study was to establish normative data for regional sweat sodium concentration ([ Na+]) and wholebody sweating rate in athletes. Data from 506 athletes ( 367 adults, 139 youth; 404 male, 102 female) were compiled from observational athlete testing for a retrospective analysis. The participants were skill/ team- sport ( including American football, baseball, basketball, soccer and tennis) and endurance ( including cycling, running and triathlon) athletes exercising in cool to hot environmental conditions ( 15- 50 degrees C) during training or competition in the laboratory or field. A standardised regional absorbent patch technique was used to determine sweat [ Na+] on the dorsal mid-forearm. Whole- body sweat [ Na+] was predicted using a published regression equation ( y = 0.57x+ 11.05). Whole- body sweating rate was calculated from pre- to post- exercise change in body mass, corrected for fluid/ food intake ( ad libitum) and urine output. Data are expressed as mean +/- SD ( range). Forearm sweat [ Na+] and predicted wholebody sweat [ Na+] were 43.6 +/- 18.2 ( 12.6- 104.8) mmol center dot L- 1 and 35.9 +/- 10.4 ( 18.2- 70.8) mmol center dot L- 1, respectively. Absolute and relative whole- body sweating rates were 1.21 +/- 0.68 ( 0.26- 5.73) L center dot h- 1 and 15.3 +/- 6.8 ( 3.3- 69.7) ml center dot kg- 1 center dot h- 1, respectively. This retrospective analysis provides normative data for athletes' forearm and predicted whole- body sweat [ Na+] as well as absolute and relative whole- body sweating rate across a range of sports and environmental conditions.

Numerous studies have reported on the thermoregulation and hydration challenges athletes face in team and individual sports during exercise in the heat. Comparatively less research, however, has been conducted on the American Football player. Therefore, the purpose of this article is to review data collected in laboratory and field studies and discuss the thermoregulation, fluid balance, and sweat losses of American Football players. American Football presents a unique challenge to thermoregulation compared with other sports because of the encapsulating nature of the required protective equipment, large body size of players, and preseason practice occurring during the hottest time of year. Epidemiological studies report disproportionately higher rates of exertional heat illness and heat stroke in American Football compared with other sports. Specifically, larger players (e.g., linemen) are at increased risk for heat ailments compared with smaller players (e.g., backs) because of greater body mass index, increased body fat, lower surface area to body mass ratio, lower aerobic capacity, and the stationary nature of the position, which can reduce heat dissipation. A consistent finding across studies is that larger players exhibit higher sweating rates than smaller players. Mean sweating rates from 1.0 to 2.9 L/h have been reported for college and professional American Football players, with several studies reporting 3.0 L/h or more in some larger players. Sweat sodium concentration of American Football players does not seem to differ from that of athletes in other sports; however, given the high volume of sweat loss, the potential for sodium loss is higher in American Football than in other sports. Despite high sweating rates with American Football players, the observed disturbances in fluid balance have generally been mild (mean body mass loss ≤2 %). The majority of field-based studies have been conducted in the northeastern part of the United States, with limited studies in different geographical regions (i.e., southeast) of the United States. Further, there have been a limited number of studies examining body core temperature of American Football players during preseason practice, especially at the high school level. Future field-based research in American Football with various levels of competition in hotter geographical regions of the United States is warranted.

Abstract This study compared a field versus reference laboratory technique for extracting (syringe vs. centrifuge) and analyzing sweat [Na(+)] and [K(+)] (compact Horiba B-722 and B-731, HORIBA vs. ion chromatography, HPLC) collected with regional absorbent patches during exercise in a hot-humid environment. Sweat samples were collected from seven anatomical sites on 30 athletes during 1-h cycling in a heat chamber (33°C, 67% rh). Ten minutes into exercise, skin was cleaned/dried and two sweat patches were applied per anatomical site. After removal, one patch per site was centrifuged and sweat was analyzed with HORIBA in the heat chamber (CENTRIFUGE HORIBA) versus HPLC (CENTRIFUGE HPLC). Sweat from the second patch per site was extracted using a 5-mL syringe and analyzed with HORIBA in the heat chamber (SYRINGE HORIBA) versus HPLC (SYRINGE HPLC). CENTRIFUGE HORIBA, SYRINGE HPLC, and SYRINGE HORIBA were highly related to CENTRIFUGE HPLC ([Na(+)]: ICC = 0.96, 0.94, and 0.93, respectively; [K(+)]: ICC = 0.87, 0.92, and 0.84, respectively), while mean differences from CENTRIFUGE HPLC were small but usually significant ([Na(+)]: 4.7 ± 7.9 mEql/L, -2.5 ± 9.3 mEq/L, 4.0 ± 10.9 mEq/L (all P < 0.001), respectively; [K(+)]: 0.44 ± 0.52 mEq/L (P < 0.001), 0.01 ± 0.49 mEq/L (P = 0.77), 0.50 ± 0.48 mEq/L (P < 0.001), respectively). On the basis of typical error of the measurement results, sweat [Na(+)] and [K(+)] obtained with SYRINGE HORIBA falls within ±15.4 mEq/L and ±0.68 mEq/L, respectively, of CENTRIFUGE HPLC 95% of the time. The field (SYRINGE HORIBA) method of extracting and analyzing sweat from regional absorbent patches may be useful in obtaining sweat [Na(+)] when rapid estimates in a hot-humid field setting are needed.

Simultaneous whole-body wash-down (WBW) and regional skin surface sweat collections were completed to compare regional patch and WBW sweat calcium (Ca), magnesium (Mg), copper (Cu), manganese (Mn), iron (Fe), and zinc (Zn) concentrations. Athletes (4 men, 4 women) cycled in a plastic open-air chamber for 90 min in the heat. Before exercise, the subjects and cycle ergometer (covered in plastic) were washed with deionized water. After the onset of sweating, sterile patches were attached to the forearm, back, chest, forehead, and thigh and removed on saturation. After exercise, the subjects and cycle ergometer were washed with 5 L of 15-mM ammonium sulfate solution to collect all sweat minerals and determine the volume of unevaporated sweat. Control trials were performed to measure mineral contamination in regional and WBW methods. Because background contamination in the collection system was high for WBW Mn, Fe, and Zn, method comparisons were not made for these minerals. After correction for minimal background contamination, WBW sweat [Ca], [Mg], and [Cu] were 44.6 ± 20.0, 9.8 ± 4.8, and 0.125 ± 0.069 mg/L, respectively, and 5-site regional (weighted for local sweat rate and body surface area) sweat [Ca], [Mg], and [Cu] were 59.0 ± 15.9, 14.5 ± 4.8, and 0.166 ± 0.031 mg/L, respectively. Five-site regional [Ca], [Mg], and [Cu] overestimated WBW by 32%, 48%, and 33%, respectively. No individual regional patch site or 5-site regional was significantly correlated with WBW sweat [Ca] (r = -.21, p = .65), [Mg] (r = .49, p = .33), or [Cu] (r = .17, p = .74). In conclusion, regional sweat [Ca], [Mg], and [Cu] are not accurate surrogates for or significantly correlated with WBW sweat composition.

CONTEXT:Tennis is often played in hot, humid environments, intensifying the thermoregulatory strain placed on the athletes. As a safety measure, some tennis organizations allow for a 10-minute break in play between the second and third sets when environmental conditions are extreme. However, the actual effect of these breaks in reducing core temperature is unknown. OBJECTIVE:To determine change in core temperature after a 10-minute break in play and assess fluid balance in professional female tennis players during tournament matches in the heat. DESIGN:Cross-sectional study. SETTING:A Women's Tennis Association Tour-sanctioned outdoor tournament on hard courts under hot conditions (30.3°C ± 2.3°C). PATIENTS OR OTHER PARTICIPANTS:Seven professional tennis players. MAIN OUTCOME MEASURE(S):Change in core temperature after a 10-minute break in tournament play, fluid intake, and sweat losses during match play. RESULTS:Core temperature was reduced from 38.92°C to 38.67°C (change of -0.25°C ± 0.20°C) when a break was taken (P  =  .02). Mean sweat rate during match play was 2.0 ± 0.5 L/h. During that time, mean fluid intake was 1.5 ± 0.5 L/h, resulting in a 1.2% ± 1.0% reduction in body mass. CONCLUSIONS:Female professional tennis players are subjected to high heat loads during match play in hot environments. However, a 10-minute break in play decreased core temperature in 6 of 7 players by an average of 0.25°C, indicating that the break provides practical benefits in the field. Furthermore, although mean sweat rate in this group of female tennis players was high, most athletes were still able to minimize mass loss to less than 2% of their prematch weight.