Wearable devices often use photoplethysmography to estimate heart rate (HR) by measuring light reflected from the cutaneous vascular bed. Notably, higher melanin content of the epidermis may reduce the amount of light transmitted through the skin. Previous studies examining the impact of skin tone on photoplethysmographic HR measurement accuracy have produced mixed results; however, none measured epidermal melanin content. To determine whether objectively measured skin pigmentation influences the accuracy of photoplethysmographic HR measurement during rest, exercise, and recovery. Skin pigmentation was quantified from colorimeter measures using Individual Typology Angle (ITA°), a strong correlate of epidermal melanin, in 28 healthy adults (White, n = 16; Black, n = 10; Middle Eastern, n = 1; Persian, n = 1). Chest-worn HR (criterion; Polar H10) and photoplethysmographic HR from three devices (Apple Watch Series 8, Garmin vivosmart 5, SlateSafety BAND V2) were recorded continuously during rest, cycling, and recovery; HR data were averaged into 30-s epochs for analysis. A linear mixed-effects model determined whether ITA° influenced mean absolute error of HR (MAEHR) for each device. ITA° predicted MAEHR for SlateSafety (β = − 0.011, P = 0.001), but not for Apple (P = 0.62) or Garmin (P = 0.29). Missing data were disproportionately attributed to participants with dark skin (ITA° < 10°; 36

The extent to which groundskeepers experience thermal and cardiovascular strain, dehydration, and accompanying declines in kidney function during work in hot-humid conditions is unknown. Demographics, thermal and cardiovascular strain, hydration, and kidney function were assessed in 20 groundskeepers (18 men; mean ± SD, age = 37.9 ± 8.4 years, body mass index = 31.5 ± 7.5 kg⋅m-2) during work on two summer days. Before (AM) and after (PM) the work shift, resting blood pressure (BP) and heart rate (HR) were measured and urine and blood samples were collected. During the work shift, fluid intake was recorded, gastrointestinal temperature (TGI) was recorded, and HR was recorded continuously. In 45% of participants, AM BP > 130/80 mm Hg on Day 1 and Day 2. Peak HR and TGI achieved across both days were 143 ± 15 b·min-1 and 37.7 ± 0.3 °C, respectively, and average peak wet bulb globe temperature was 39.1 ± 3.5 °C. On both days, urine specific gravity (USG = 1.021 ± 01) and urine color (UCOL = 6 ± 1) did not change across the work shift (all P > 0.28), but most subjects began with concentrated urine (62.5% USG ≥ 1.020 and 95% UCOL ≥ 4). Total fluid intake = 2.3 ± 1.6 L during work and consisted of 70% water, 25% sugar-sweetened beverages, 5% other. For six subjects, the average estimated glomerular filtration rate (eGFR) at AM was ≤ 60 mL·min-1·1.73 m-2. Five subjects had SCr increases from AM to PM ≥ 0.3 mg·dL-1, signifying an acute kidney injury (AKI). While hyperthermia was not prevalent, subjects began and ended the workday with concentrated urine. Hypertension, obesity, and low water intake may have contributed to the overall low kidney function and AKIs observed.

Background: It is well-established that adults with elevated psychiatric symptoms or a psychiatric disorder (e.g. depression) engage in lower amounts of physical activity (PA) compared to adults with fewer symptoms/no diagnosis. However, less is known about the association between psychiatric symptoms and PA behaviour in trauma-exposed adults. Most prior investigations have focused on independent associations between overall depression or posttraumatic stress disorder (PTSD) symptoms in relation to PA and have neglected specific symptom domains (e.g. anhedonia). Therefore, we conducted secondary analyses on a parent dataset to examine whether PTSD symptom severity moderates the association between anhedonia and PA volume.Methods: Trauma-exposed adults (N = 107, 61% women, M ± SD age = 28 ± 9 y 54% White) completed questionnaires assessing demographic information, anhedonia, PTSD symptom severity (overall and symptom clusters), and PA volume (total MET-min/week). Main effects and interactions between anhedonia and PTSD symptom severity in relation to PA volume were examined with robust linear regression models.Results: We observed a significant anhedonia × PTSD symptom severity interaction. An inverse association between anhedonia and PA volume was observed among adults with lower-to-moderate PTSD symptom severity, and a positive association between anhedonia and PA volume was observed among adults with higher PTSD symptom severity. Significant anhedonia × PTSD symptom severity interactions for avoidance (Cluster C) and negative alterations in arousal/reactivity (Cluster E) symptoms were also observed, with associations in the same direction as the overall PTSD symptom severity model.Conclusion: These preliminary findings suggest that co-occurring anhedonia and PTSD symptom severity profiles interact to influence PA volume in trauma-exposed adults. Future research is needed to understand why trauma-exposed adults with greater co-occurring anhedonia and PTSD symptom severity profiles reported engaging in higher levels of PA compared to those with lower symptom severity profiles. Such knowledge could help healthcare practitioners tailor treatment plans to incorporate or adjust PA prescriptions.

IntroductionHigher work rates may be sustainable when maintaining target rating of perceived exertion (RPE) compared to maintaining target heart rate (THR) during high-intensity interval training (HIIT) exercise in hot conditions, but may also result in greater thermal strain and cardiovascular drift, as well as greater decrements in maximal oxygen uptake (V˙O2max).PurposeTo test the hypotheses that maintaining target RPE compared to THR during HIIT in the heat results in 1) smaller work rate adjustments, 2) greater thermal and cardiovascular strain, and 3) larger decreases in V˙O2max.MethodsEight adults (4 women) completed a graded exercise test on a cycle ergometer in 22°C and then 4 cycling trials in 35°C, consisting of an 8-min warm-up at 70% maximal heart rate (HRmax) or 12 RPE followed by 1 (15HR and 15RPE) or 5 (43HR and 43RPE) rounds of HIIT (1 round = 4 min work at 90% HRmax or 17 RPE and 3 min recovery at 70% HRmax or 12 RPE) totaling 15 min or 43 min of exercise, respectively. Each trial ended with a GXT to measure V˙O2max.ResultsIn the 43-min trials work rate decreased from the first to the fifth work interval in both conditions, but by a non-significant, yet moderately larger (ES = 0.53) amount during 43HR (46 ± 29 W) compared to 43RPE (30 ± 28 W). From the first to fifth work interval HR increased over time by 12 b⋅min–1 in 43RPE (p < 0.001), but did not increase during 43HR (p = 0.36). Rectal temperature increases were not different between conditions (43HR = 0.7°C, p < 0.001; 43RPE = 0.8°C, p < 0.001). V˙O2max decreased 15.6% (ES = 0.41) between 15RPE and 43RPE (p = 0.005), but it was not different over time during the HR-based trials [6.5%, ES = 0.16 (α adjusted for multiple comparisons = 0.0125) p = 0.03].ConclusionMaintaining target RPE and THR require considerable declines in work rate during HIIT in the heat, with ∼53% larger declines needed to maintain THR. The mitigation of cardiovascular drift in the THR trial may have contributed to the preservation of V˙O2max.

Resisted sprint (RS) training, such as sled or parachute towing, is commonly used for sprint training among field sport athletes. While RS training is frequently employed by athletes and coaches, there is little research on its benefits, especially compared to unresisted running (UR) training programs with similar training volumes. This systematic review and meta-analysis compared the effectiveness of RS training on acceleration compared to UR training. Potential sources were limited to peer-reviewed articles published in English prior to June 12, 2022, and gathered from the EBSCOhost, PubMed, and Web of Science online databases identified using combinations of the following terms: towing, sled, "resisted sprint," "sprint acceleration," "sprint performance," and "sprint speed." The search returned 1,159 sources, from which 15 were eligible for inclusion. Fifty effects were used to estimate the impact of RS training on initial sprint speed. Based on the cumulative results from these studies, RS training yielded a small improvement in acceleration but was not different from same volume of UR training (Hedges' d Effect Size=0.11, 95% CI: -0.01 to 0.23; p=0.08). These results do not support the use of RS training over UR training for improving initial sprint speed; however, further research should be conducted.

INTRODUCTION/PURPOSE:The purpose of this study was to test the hypothesis that cardiovascular (CV) drift and associated decrements in maximal oxygen uptake (V̇O 2max ) are greater in high-fit compared with low-fit women during exercise at the same %V̇O 2max , but comparable at the same rate of metabolic heat production. METHODS:Six high-fit (HI) and six low-fit (LO) women cycled in 35°C for 15 or 45 min at the same relative intensity (60% V̇O 2max ; 15REL and 45REL) or fixed rate of heat production (500 W; 15FX and 45FX), immediately followed by a graded exercise test to measure V̇O 2max . The separate 15- and 45-min trials permitted measurements of V̇O 2max over the same time interval as CV drift. RESULTS:During 45REL, higher heat production in HI (496 ± 51 vs 364 ± 44 W in LO) resulted in greater end-exercise core temperature (38.7°C ± 0.4°C vs 38.2°C ± 0.1°C, P = 0.03), greater increases in HR (15 bpm (10%) vs 10 bpm (6%), P = 0.03) and decreases in stroke volume (11 mL per beat (16%) vs 5 mL per beat (8%), P = 0.001), and larger reductions in V̇O 2max (16% vs 5%, P = 0.04) compared with LO. During 45FX, temperature responses, CV drift, and decreased V̇O 2max were not different between groups (all P > 0.05), despite differences in %V̇O 2max (60% vs 75% for HI and LO, respectively). CONCLUSIONS:We conclude metabolic heat production modulates the CV drift-V̇O 2max relationship, independent of fitness level. These results support previous findings showing the magnitude of CV drift is proportional to reductions in V̇O 2max .

Heart rate drifts upward over time during interval exercise and during exercise in hot conditions. As such, work rate must be lowered to maintain target heart rate. The purpose was to characterize acute work rate adjustments during high-intensity interval training based on target heart rate. Seven humans (three females) completed five study visits: a graded exercise test on a cycle ergometer to measure maximal heart rate (HRmax) in ∼22 °C and four trials performed in ∼22 °C (TEMP) or ∼35 °C (HOT), consisting of an 8 min warm-up at 70% HRmax followed by one (15TEMP and 15HOT) or five (43TEMP and 43HOT) rounds of high-intensity interval training (one round = 4 min work at 90% HRmax and 3 min recovery at 70% HRmax) totaling 15 min or 43 min of exercise, respectively. Work rate was lowered 33 ± 20 W ( p = 0.005) in 43TEMP and 56 ± 30 W ( p = 0.003) in 43HOT between the first and fifth work intervals. Thermal strain (0.2 °C higher rectal temperature, p = 0.01) and cardiovascular strain (6 beats·min–1 larger increase in heart rate from first to fifth recovery interval, p = 0.01) were greater in 43HOT versus 43TEMP. Using target heart rate during high-intensity interval training may reduce the training stimulus, especially in hot environments, but it may also limit thermal strain and enable participants to complete the prescribed workout despite the heat.

Heart rate (HR) drifts upward over time during interval exercise and during exercise in hot conditions. As such, work rate must be lowered to maintain target HR (THR). Additionally, prolonged exercise in the heat can result in reduced maximal oxygen uptake (V̇O2max). PURPOSE: To characterize work rate adjustments during high-intensity interval training (HIIT) in a temperate and hot environment based on THR and to test the hypotheses that in a hot versus temperate environment 1) work rate must be lowered to a greater extent to maintain THR, 2) there will be greater thermal and cardiovascular strain, 3) V̇O2max will decrease to a greater extent. METHODS: Four men and 3 women completed the study, which began with a control graded exercise test (GXT) on a cycle ergometer to measure maximal HR (HRmax) and V̇O2max in 22 °C. The remaining two 15-min and two 43-min trials were counterbalanced and separated by ≥48 h. Each consisted of an 8-min warm-up (WU) at 70% HRmax followed by 1 or 5 rounds of HIIT (4 min at 90% HRmax; 3 min at 70% HRmax) and then a GXT to measure V̇O2max, performed in ~22 °C (TEMP) or ~ 35 °C (HOT): 15TEMP and 15HOT = WU and 1 round of HIIT; 43TEMP and 43HOT = WU and 5 rounds of HIIT. The separate 15- and 43-min trials were intended to measure V̇O2max before and after, respectively, cardiovascular and thermal strain ensued. RESULTS: Work rate had to be lowered 33 ± 20 W (p = 0.005) in 43TEMP and 56 ± 30 W (p = 0.003) in 43HOT between the first and final work intervals to maintain THR (p = 0.008 for the interaction effect and paired samples t-test comparing change scores between 43TEMP and 43HOT). From the first to final recovery interval, thermal and cardiovascular strain were greater in 43HOT versus 43TEMP: 0.2 °C higher rectal temperature (p = 0.01 for t-test comparing change scores); 6 b/min larger increase in HR (11 b/min versus 5 b/min; p = 0.01 for interaction effect). V̇O2max was 0.2 L/min lower in the heat (p = 0.001 for main effect of condition) and 0.2 L/min lower over time (p = 0.02 for main effect of time), but the decrease was not different between conditions (p = 0.29 for interaction). CONCLUSIONS: Using THR during HIIT may reduce the training stimulus, especially in hot environments, but it may also limit thermal strain and enable participants to complete the prescribed workout.

The effects of aerobic exercise (AE) on vascular health and function are well established, hence it is considered the cornerstone therapy to prevent and treat cardiovascular diseases. Yet, most adults do not engage in the recommended levels of AE to realize these health benefits. Alternate therapeutic modalities, like passive heating (PH), may provide similar cardiovascular benefit, but findings and methodologies are inconsistent. PURPOSE: To evaluate the acute cardiovascular and thermodynamic effects of PH and AE. METHODS: Seven low-fit but healthy adults (3 women; M ± SD; age = 23 ± 3 y, peak oxygen uptake [VO2] = 22.8 ± 2.6 ml·kg-1·min-1) with normal systolic/diastolic blood pressure (SBP/DBP: 116 ± 16/75 ± 11 mm Hg) completed a familiarization visit and 2 counterbalanced 30-min experimental interventions separated by >48 h: AE (recumbent cycling at 40% VO2 reserve) and PH (recumbent rest wearing a water-perfused, tube-lined suit circulating 49 °C water). SBP, DBP, heart rate (HR), vascular stiffness (pulse wave velocity [PWV]), core (Tc) and skin (Tsk) temperature were measured pre-, immediately post-, and 60 min post-intervention. Statistical analyses included descriptive statistics and two-way repeated measure ANOVA. RESULTS: Changes in SBP, DBP, and PWV were not different from rest by time or intervention (all p > 0.05). HR was elevated from rest immediately following PH and AE (mean difference [MD] ± SD = 5 ± 3 beats/min, p = 0.04). Tc increased more from rest immediately post-AE vs. PH (0.4 ± 0.3 °C, p = 0.01), and remained higher 60 min post-intervention (0.3 ± 0.3 °C, p = 0.01). Tsk increased more from rest immediately post-PH vs. AE (3.6 ± 2.6 °C, p = 0.01), and remained higher 60 min post-intervention (2.8 ± 1.6 °C, p = 0.02). CONCLUSIONS: Thirty min of AE or PH did not elicit significant changes in BP or vascular stiffness outcomes immediately post- or 60 min post-intervention, nor did responses differ by intervention. Tc and Tsk increased compared to rest, but no consistent pattern emerged for AE or PH. Our findings suggest that the “dose” of AE and PH prescribed was suboptimal for inducing favorable cardiovascular changes in our sample of young, healthy adults. To determine whether PH can serve as a suitable alternative to AE, it should be further investigated among adults with higher BP who stand to benefit the most.

Abstract Ng, J and Wingo, JE. Effect of ice slurry beverages on voluntary fluid intake and exercise performance. J Strength Cond Res 37(6): e376–e383, 2023—Voluntary intake of cold fluid is greater than warm fluid as a result of more favorable palatability, resulting in better maintenance of hydration status and improved exercise performance. It remains unclear whether voluntary ingestion of ice slurry beverages compared with cold fluid during exercise yields superior results. Eight recreationally active subjects (mean ± SD; age = 24 ± 4 years, height = 175.2 ± 7.8 cm, mass = 79.6 ± 11.2 kg, body fat = 13.0 ± 5.2%) completed a pretest in 22° C to determine maximal workload (Wmax). Then, in 2, separate, counterbalanced trials, they cycled for 60 minutes at 50%Wmax in 35° C and 40% relative humidity with either ad libitum ice slurry (−1.3 ± 0.3° C) or cold fluid (11.1 ± 2.4° C) ingestion. This was immediately followed by a 15-minute cycling time trial. Subjects avoided 2% body mass loss in both conditions but ingested twice as much cold fluid as ice slurry (fluid: 1,074.7 ± 442.1 g, ice slurry: 526.9 ± 214.1 g; p = 0.001). Nonetheless, neither 15-minute performances (cold fluid: 119.5 ± 34.8 kJ, ice slurry: 114.6 ± 20.9 kJ; p = 0.59) nor whole-body sweat rates (fluid: 1,370 ± 311 ml·h−1, ice slurry: 1,242 ± 191 ml·h−1; p = 0.20) were different between the conditions. Despite ingesting half as much ice slurry as cold fluid, subjects experienced similar physiological responses and thus had similar performances under heat stress while avoiding excessive hypohydration. Under the conditions of this study, ice slurry ingestion was an effective alternative form of exercise hydration.

Cardiovascular drift—a progressive increase in heart rate (HR) and decrease in stroke volume (SV) during prolonged exercise—is exacerbated by heat stress and thermal strain, and often accompanied by a decrease in work capacity (indexed as maximal oxygen uptake [V.O2max]). To attenuate physiological strain during work in the heat, use of work:rest ratios is recommended by the National Institute for Occupational Safety and Health. The purpose of this study was to test the hypothesis that during moderate work in hot conditions, utilizing the recommended 45:15 min work:rest ratio would result in cardiovascular drift ‘accumulating’ over consecutive work:rest cycles and accompanying decrements in V.O2max. Eight people (5 women; (mean ± SD) age = 25 ± 5 y; body mass = 74.8 ± 11.6 kg; V.O2max = 42.9 ± 5.6 mL·kg−1·min−1) performed 120 min of simulated moderate work (201–300 kcal·h−1) in hot conditions (indoor wet-bulb globe temperature = 29.0 ± 0.6 °C). Participants completed two 45:15 min work:rest cycles. Cardiovascular drift was evaluated at 15 and 45 min of each work bout; V.O2max was measured after 120 min. On a separate day, V.O2max was measured after 15 min under identical conditions for comparison before and after cardiovascular drift occurred. HR increased 16.7% (18 ± 9 beats·min−1, p = 0.004) and SV decreased 16.9% (−12.3 ± 5.9 mL, p = 0.003) between 15 and 105 min, but V.O2max was unaffected after 120 min (p = 0.14). Core body temperature increased 0.5 ± 0.2 °C (p = 0.006) over 2 h. Recommended work:rest ratios preserved work capacity but did not prevent the accumulation of cardiovascular and thermal strain.

Heart rate (HR) and rating of perceived exertion (RPE) drift upward over time during exercise in hot conditions. As such, work rate must be lowered to maintain target intensity. PURPOSE: To characterize work rate adjustments during high-intensity interval exercise (HIIE) in the heat based on target HR and target RPE and to test the hypotheses that 1) work rate must be lowered to a greater extent to maintain target HR than to maintain target RPE and 2) greater thermal and cardiovascular strain will result from maintaining target RPE compared to target HR during HIIE in hot conditions. METHODS: 5 adults [3 men, (mean ± SD) age = 26 ± 8 y] completed two 43-min trials on a cycle ergometer in 35 °C. Exercise intensity was prescribed based on a target HR (HR-based) or target RPE (RPE-based). After an 8-min warm-up at 70% HRmax or an RPE of 12, subjects completed 5 rounds of HIIE (4 min of work at 90% HRmax or an RPE of 17 and 3 min of recovery at 70% HRmax or an RPE of 12). Three more subjects are anticipated to participate in the study. RESULTS: Percent change in power output from the first to last work bout was greater during HR-based trials (-31% ± 8%; -52 ± 35 W) compared to RPE-based trials (-14% ± 22%; -25 ± 29 W) (p = 0.045). By design, HR remained constant from the first to final work bout during the HR-based trial [4% ± 4% (6 ± 6 beats/min) increase, p = 0.073], but it increased 12% ± 5% (18 ± 8 beats/min; p = 0.006) during the RPE-based trial. Although work rate and cardiovascular responses were different between trials, the increases in rectal temperature over time (1.1 ± 0.5 °C for HR-based and 1.1 ± 0.3 °C for RPE-based) were not different (p = 0.29). CONCLUSIONS: These data show that method of exercise prescription affected work rate adjustments and cardiovascular strain, but not thermal strain during HIIE in hot conditions. Work rate had to be lowered ~27 W (108%) more to maintain target HR than to maintain target RPE, which may have practical significance in terms of the metabolic stimulus of the exercise and potentially compromised training adaptations. On the other hand, maintenance of a higher metabolic intensity during the RPE-based trial resulted in a 3 times greater increase in HR over time which may have implications for reduced maximal oxygen uptake associated with cardiovascular drift.

OBJECTIVES:Certain occupations are associated with greater risk of triggering a sudden cardiac event because of high levels of physical exertion and extreme thermal environments in which they occur. The extent to which sports officials--particularly high school (HS) American football referees--experience these conditions is unknown. Forty-six male HS officials (72% White/Caucasian; age = 48 ± 12 years, body mass index = 31.7 ± 6.6 kg·m-2) were studied to quantify the physiological strain and physical demands of officiating. METHODS:Referee demographics (e.g., experience, habitual exercise), pre-game urine specific gravity (USG), thermal (peak core temperature [Tcore]) and cardiovascular (average heart rate [HR]) strain, kinematic activity (e.g., total distance, speed, mechanical intensity), and environmental conditions were measured during 10 regular season varsity HS football games (≈2.5 h each) in the Southeastern United States (average wet bulb globe temperature and relative humidity: 18.9 ± 6.0 °C and 78.2% ± 12.1%). Analyses included descriptive statistics, bivariate correlations, and linear regression. RESULTS:Referees covered 5.2 ± 1.2 km per game, eliciting average HR and peak Tcore of 71.5% ± 8.0% HRmax and 38.3 ± 0.5 °C, respectively; 38% began games dehydrated (USG = 1.026 ± 0.004). Multiple regression analyses revealed that obesity (β = 0.34), not participating in regular exercise (β = -0.36), and officiating at lower mechanical intensity (β = -0.33) predicted greater cardiovascular strain (all p ≤ 0.03). White/Caucasian race/ethnicity (β = 0.59), younger age (β = -0.46), and obesity (β = 0.28) predicted greater thermal strain (all p ≤ 0.01). CONCLUSION:HS football referees experienced elevated levels of physiological strain while officiating, with individual factors modulating the magnitude of strain. Strategies aimed at reducing obesity, increasing exercise participation, and improving cardiovascular health should be emphasized to mitigate strain and prevent cardiac events.

Despite 14 deaths of sport officials reported between 2003 and 2018, little is known about the physiological and environmental risk factors for injury or death during sport officiating. PURPOSE: To establish a comprehensive database for cases of illness, injury, and death of sport officials and to identify differences by sport or environment. METHODS: Relevant key words were utilized in a popular search engine to systematically retrieve online media sources from 2000-2019 for a referee, sporting official, umpire, or similar who experienced a work-related illness, injury, or death. Chi-square tests were used to determine if fatality is independent from type of injury, sport, and location (indoor, outdoor), and if type of injury is independent from sport or environmental conditions. RESULTS: Among 134 persons, 146 events of injury, illness, and death were identified. Sport officials were predominantly male (n = 129; female, n = 2; unknown, n = 3), ranging in age from 20-80 y (mean ± SD; 52 ± 13 y) with 2-48 y (23 ± 12 y) of officiating experience. Events were primarily musculoskeletal (n = 55), cardiovascular (CV) (n = 43), or head (n = 30) injury. Sixteen events (7 fatalities, all during soccer games) were intentional injury by another person; all others were presumed accidental. Events occurred most frequently during baseball (n = 30), soccer (n = 26), American football (n = 22), and ice hockey (n = 22) games, primarily at the professional (n = 70) and high school (n = 27) level. CV events occurred primarily while officiating basketball (n = 14) and American football (n = 12); head injuries occurred most often while officiating baseball (n = 11; P < 0.001). Fatalities (n = 36) occurred more frequently during soccer games (n = 13; P < 0.001) and almost half of all fatalities were CV events (n = 17; P < 0.001). Fatal events occurred at similar rates in indoor (16.0%) and outdoor (23.3%) settings (P = 0.32). Outdoor CV events occurred independent of dry-bulb temperature and relative humidity (P = 0.92). CONCLUSION: Soccer officials experience the highest frequency of violent injury and death. CV events occur most frequently while officiating basketball and American football; further investigation is warranted to determine if the high incidence of CV events correlate with health status and the high physiological demand of officiating these sports.

PURPOSE: We sought to systematically review and quantitatively synthesize the available literature to estimate the effect of glycerol supplementation on exercise performance. METHODS: Six electronic databases were searched for studies that included the following: human subjects ≥18 y, glycerol and placebo or control conditions/groups, and a measure of performance. RESULTS: Twenty-one studies yielded 33 effects (k). Subjects (n = 166) were young adults ([M ± SD] 25 ± 4 y), fit (maximal oxygen uptake = 58.3 ± 4.1 mL·kg-1·min-1), and mostly men (98%). The majority of the sample (k = 25, 76%) had subjects exercise in hot and/or humid environmental conditions (32.6 ± 2.6 °C), and more than half (k = 17, 52%) used a measure of total work (e.g., distance covered, time to exhaustion) to gauge performance. On average, studies achieved a moderate quality rating (satisfied 61.6 ± 9.2% of quality items), although the quality scores ranged widely from 38.3% to 86.4%. Overall, glycerol supplementation had a negligible effect on performance compared to a placebo or control treatment (standardized mean difference [SMD] = 0.12; 95% CI: -0.01, 0.26), and this SMD effect size lacked heterogeneity (Q32 = 10.25, p = 0.99; I2 = 0%). A priori subgroup analyses failed to identify any significant effect modifiers related to sample characteristics (e.g., training status, baseline hydration status), performance test and environmental conditions (e.g., anaerobic vs. aerobic test, hot vs. temperate environment), or glycerol supplementation (e.g., dosage, mixture vs. bolus consumption method) (p > 0.05 for all). CONCLUSIONS: Glycerol supplementation was found to have a negligible effect on exercise performance; however, the practical significance of its use should also be considered when interpreting these results. The lack of heterogeneity observed limits the external validity of our findings (i.e., ecological fallacy). Hence, the lack of improvement in performance may be attributable to the specific characteristics of our sample and not glycerol supplementation per se. Future studies should investigate the potential performance benefits of glycerol supplementation among unique populations or exercise modes in conditions that elicit greater and prolonged heat stress.