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Original Research

Effects of a Short-Term Heat Acclimation Protocol in Elite Amateur Boxers

Stone, Brandon L.1,2,3; Ashley, John D.1; Skinner, Robert M.2; Polanco, Jose P.2; Walters, Mason T.2,4; Schilling, Brian K.5; Kellawan, J.M.1

Author Information
Journal of Strength and Conditioning Research: July 2022 - Volume 36 - Issue 7 - p 1966-1971
doi: 10.1519/JSC.0000000000004233
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Elite combat athletes (e.g., boxers) require a complex combination of physiological and cognitive attributes to be successful (2,10,17,22). Visual perception, cognitive processing, kinesthetic awareness, and the ability to coordinate explosive movements are all part of a boxer's technical and tactical skill set. Furthermore, boxing is a weight management sport which requires a high relative lean body mass, well-developed upper- and lower-body strength, and advanced levels of anaerobic and aerobic fitness (10,17). These attributes present a unique physiological challenge when training boxers because training needs to incorporate the combination of weight control with elements that prepare the athletes for the high cardiovascular demands experienced during competition, which involve both anaerobic and aerobic pathways (29). However, the ability to enhance physiological capacity and aerobic performance in elite athletes is difficult because their higher training background affects significant training adaptations (15,39). Therefore, determining training strategies that improve aerobic performance in elite boxers without significantly altering body mass is of significant interest to athletes and coaches in combat sports similar to boxing.

A possible training strategy that may promote improved aerobic and anaerobic capacity without altering body mass is heat acclimation (HA). An acute bout of exercise results in a host of responses, including elevated peripheral (skin) and central (brain) temperature, heart rate (HR), ventilation, and perceived exertion. Heat and humidity exacerbate these, which can limit acute exercise performance (2,23,27). Conversely, repeated exposure to hot/humid environments elicits several physiological adaptions that attenuate these acute deleterious physiological changes (2,23,29). These adaptations can occur as acclimation (response to a controlled environment setting) or acclimatization (response to a naturally occurring environment) (11). Partial HA occurs within 7 exposures (termed short-term HA [STHA]), whereas medium-term HA occurs between 8 and 14 HA exposures, and long-term HA (≥15 sessions) exhibits full acclimation (31,32,38).

Heat acclimation has been found to have performance benefits and to enhance anaerobic and aerobic capacity within team and endurance athletic populations (7,8,13,16,17,19,23,25), with evidence suggesting improvements may occur in as little as 5 days (16). Excitingly, these advantages have been found to extend to exercise in thermoneutral environments (11,23,27,28,31). For example, cyclists (27), and table tennis athletes (21), have shown improvements in a thermoneutral environment after STHA. However, little is known about the effects of STHA on any aspect of fitness and performance in elite-level combat athletes.

Mechanisms responsible for the potential performance enhancement in response to HA include a faster onset of sweating and an increased sweat rate (2,11). This enables the body to better regulate temperature during exercise through enhanced heat dissipation and widening the core to skin temperature gradient even in an elevated heat environment (26). For example, increases in sweat rate can improve maximal oxygen uptake, time trial, and intermittent sprint test performance (6,11,15,16,23,24,34).

Elite combat athletes travel and compete in a variety of challenging and thermoneutral environments within the same timeframe. Utilization of STHA to enhance aerobic capacity may optimize training time and could promote optimal adaptations within smaller time windows (21). The purpose of this study was to examine the effects of a 5-Day short-term heat acclimation protocol (5-DayHA) on aerobic performance, sweat loss, and hydration status in elite amateur boxers participating in the U.S. Olympic and Paralympic Committee (USOPC) Acrobat and Combat Sport—Heat and Humidity Acclimation Program.


Experimental Approach to the Problem

A retrospective analysis was used to understand the impact of a precompetition 5-DayHA on aerobic performance, body mass, and hydration status. All data used for this investigation were supplied to the investigators by the USOPC. The data provided were deidentified and coded before given to the study team for analysis. This retrospective data analysis was approved by the (HRPP) and Institutional Review Board at the University of Oklahoma.


Seven elite amateur boxers, including 3 women (23.3 ± 4.1 years; 19−28, 178.1 ± 9.3 cm), took part in an STHA protocol (5-DayHA) to prepare for upcoming competitions (Table 1). All study procedures were completed at an elevation of ∼2,300 m. All subjects were acclimated to this altitude as resident athletes training and living at ∼2,300 m for a minimum of 14 days before participating in the 5-DayHA as recommended by the previous literature (5,14,25,42).

Table 1 - Subject characteristics.*
Variable Mean ± SD
Age (y) 23 ± 4
Height (cm) 178 ± 9
Body mass (kg) 72.0 ± 19.0
Maximal oxygen uptake (ml·kg−1·min−1) 47.6 ± 10.2
Maximal heart rate (bpm) 197 ± 4
*In preparation for upcoming competitions, 7 elite-level boxers participated in a 5-day STHA protocol after training and living at an elevation of 2300 m for 14 days. Maximal oxygen uptake was estimated (1), and maximal heart rate was obtained from that estimation.


5-Day Heat Acclimation

Subjects completed 60-minute HA sessions on 5 consecutive days. To maintain ecological validity, each training session occurred at the same time of day, at least 3 hours postprandial (Figure 1). Meals were individualized for each subject based on athlete needs by the USOPC and, therefore, were not standardized across the group. All subjects entered the environmental chamber (Hypoxico, New York) 120 minutes after the chamber had been set to 30 °C with 75% Relative Humidity. Each subject wore team-issued dri-FIT shorts, t-shirt, socks, and tennis shoes. All subjects performed a standard warm-up, exercise training session, cool-down, and stretch during each training session (Table 2). The dynamic warm-up comprised steady-state cycling for 3 minutes, followed by low-level plyometric activity (e.g., jump rope) followed by dynamic stretching and locomotive patterns (skipping and jogging). Intensity of the warm-up gradually increased over the duration of the warm-up (1,19). The cool-down comprised walking on a treadmill or cycling (of subject choice) at a self-selected pace for 2 minutes, focusing on breathing and lowering heart rate (19). The postactivity stretch comprised 5- to 30-second lower extremity and upper extremity stretches (3,9).

Figure 1.:
Study design. Each session consisted of a preweight and urine specific gravity (USG) measurement with a postsession weigh out. Pre-STHA and post-STHA protocol cardiovascular performance testing 24 hours before and after STHA, cardiovascular performance was assessed through the repeat sprint assessment test in normal environmental conditions (22 °C).
Table 2 - STHA protocol.*
Session Warm-up Protocol (work:rest) Intensity Cool-down Stretch
1 8 min 3 min:1 min × 10 repetitions 65% HRmax 10 min 10 min
2 8 min 1 min:3 min × 10 repetitions 70–75% HRmax 10 min 10 min
3 8 min 1 min:3 min × 10 repetitions 70–75% HRmax 10 min 10 min
4 8 min 1 min:3 min × 10 repetitions 70–75% HRmax 10 min 10 min
5 8 min 40-min steady state 65% HRmax 10 min 10 min
*STHA = short-term heat acclimation; HR = heart rate.
Protocol included 3 treadmills, 3 Airdyne cycle ergometers, and 3 jump rope stations. Groups alternated, so that each group underwent each modality per session and rotated starting modality in the following session.

Each exercise training session comprised 3 stations: (a) cycle ergometer (Wattbike, Waukesha, WI), (b) jump rope, and (c) treadmill (Woodway, Waukesha, WI). Placement for the starting rotation was randomized before session 1. The modality of the interval exercise alternated daily between the treadmill, jump rope, and cycling. Sessions 2–5 alternated, so that each athlete started on the following modality (Table 2). Individual heart rate corresponded to 65–75% of HRmax, so that regardless of modality executed, each session was completed at the prescribed work intensity. Fluids were given ad libitum up to 650 ml per athlete during each session.

Repeat Sprint Test

The Repeat Sprint Test (RST) was conducted 24 hours before and after the 5-DayHA protocol (6,35). All subjects performed the test quarterly as part of their USOPC Acrobat and Combat Sport program, and it has been previously validated as a measure of maximal aerobic and anaerobic fitness (4,33,35). The RST comprises 6, 22-m sprints completed in under 30 seconds, with 30 seconds of active recovery between runs, until the subject can no longer finish 6 lengths in the required 30-second time. Both preintervention RST and postintervention RST were performed in a thermoneutral environment (22 °C, 20% RH), and performance was evaluated by the total number of sprints completed.

Sweat Rate and Hydration Status

Body mass (electric scale, Seca 896, Chino, CA) was assessed before and after each acclimation session, and the difference was used as noninvasive estimation of sweat rate (Figure 1) (4). Body mass is specifically important for weight-class sports, where fluctuations in body mass close to competition may put athletes outside weight-class limits (4) Before and after each session, hydration status was measured through urine specific gravity (USG) using a calibrated refractometer (Uricon-N, urine specific gravity refractometer, Atago Co., Tokyo, Japan).

Data Reduction and Statistical Analyses

Student's t-tests compared subjects' preintervention and postintervention repeat sprint performance, pre-exercise body mass, and hydration status. A repeated-measures analysis of variance (ANOVA) with Bonferroni post hoc analysis compared differences in sweat rate (through body mass) and hydration status across each acclimation session. The alpha level was set at p ≤ 0.05, and data are presented as mean ± SD. Cohen's d effect size calculations measured the magnitude of pairwise differences, interpreted as trivial (0.0), small (0.2), moderate (0.6), large (1.2), very large (2.0), nearly perfect (4.0), and perfect (infinite). Analysis was completed with JASP Team (2019) (Version 0.11.1).


Performance Measures

A 5-DayHA significantly improved RST performance by ∼46% (13 ± 7 to 19 ± 6, d = 0.92, p = 0.03, Figure 2) with a moderate-to-large effect size.

Figure 2.:
Repeat sprint test (RST) results in elite amateur boxers before and after STHA protocol (STHA). *Indicates significant increase from pre-STHA (p = 0.03, d = 0.92, n = 7).

Hydration Status and Sweat Rate (Body Mass)

Pre-exercise body mass did not substantially change over the 5-DayHA protocol (d = −0.15, p = 0.20), suggesting no changes in estimated sweat loss. Urine specific gravity did not significantly decrease over the same 5 days (1.017 ± 0.010 to 1.012 ± 0.007, d = 0.81, p = 0.07; Figure 3). Body mass significantly decreased during each session (d = −1.28, p < 0.05, Figure 4), whereas post hoc analysis exhibited no interday significant body mass change (p > 0.05).

Figure 3.:
Urine specific gravity (USG) before and after 5-day STHA protocol. (n = 7, d = 0.81, p = 0.07).
Figure 4.:
Sweat rate through within-session changes in body mass. Body mass significantly decreased during each session (d = −0.15 to −1.28, p < 0.05), whereas interday session exhibited no significant differences (p > 0.05).


The present findings suggest that a 5-DayHA protocol elicited positive physiological adaptations in elite boxers, leading to enhanced Repeat Sprint Test performance (RST, d = 0.92, p = 0.03) in a thermoneutral environment. Although sweat rate increased intrasession body mass (d = −0.15 to −1.28, p < 0.05), no intersession body mass changes occurred (p > 0.05). In addition, although changes in hydration status showed a large effect size (d = 0.81), there were no significant changes (p = 0.07).

Although previous investigations have examined the effects of STHA on exercise performance within both thermoneutral and hot/humid environments, only a few have examined HA in elite athletes. Buchheit et al. (7) examined the effects of HA in professional soccer players (8). Their findings showed that 7 days of heat exposure improved aerobic capacity (+7% increase in the Yo-Yo Intermittent Recovery Test, p < 0.01) in a thermoneutral environment (22 ± 1 °C, p < 0.05) (7). Although their investigation used a moderate HA protocol, their findings agree with that of this study, also exhibiting significant improvement in thermoneutral aerobic performance through a repeat sprint assessment.

Another study by Buchheit et al. (6) examined the effect of HA in thermoneutral conditions, using the live high-train low model in the heat. In that study. the intervention occurred at sea level while subjects slept in hypobaric chambers at night to simulate high-altitude “living” conditions (6). Subjects from this study lived at an altitude of 2,300 m for a minimum of 14 days with the heat and humidity acclimation exposure conducted in an environmental chamber. Although different study designs were used, the findings from this study agree with Buchheit et al. (6), supporting aerobic performance improvements in a thermoneutral environment after an HA training protocol. Previous research has identified that by living (either naturally or through artificial means) and training at sea level may enhance exercise performance at both altitude and sea level (41). These enhancements may come as an increase in hemodynamic function (30,31), and that, acclimation may occur for 14 days (42). However, the present findings may suggest that aerobic performance may be positively influenced by 5-DayHA while living at altitude. For further reading, we introduce readers to literature surrounding altitude acclimation and performance enhancement (5,8,18,23,22,30,31,36,37,41,42).

Heat acclimation mechanisms associated with increased aerobic performance have been related to cardiovascular economy during the exercise test (31), including stroke volume, myocardial efficiency, and ventricular compliance (24). In aggregate, these cardiovascular changes may allow for greater oxygen delivery and metabolic byproduct removal at a lower relative heart rate during the same work performed before and after 5-DayHA. There are several possible adaptations caused by 5-DayHA used in this study that could explain the improvement in RST performance. Beneficial metabolic flux adaptations may lead to an increase in the amount of work completed (number of sprints). Furthermore, the increase in number of sprints may be owed to better oxygenation of the working muscle or increase in mitochondrial volume and density (13). Although this study did not directly assess aspects of cardiovascular or muscle metabolic function during exercise, the increase in RST performance observed after an STHA justifies further investigation into the physiological mechanisms of HA in elite combat athletes.

This study is not without limitations, which are important to acknowledge. First, this investigation was a retrospective analysis on data routinely collected as part of combat sports training at the USOPC as part of the Acrobat and Combat Sport Heat and Humidity Acclimation Program. Therefore, there was no control group that may limit the interpretation of the present findings. Second, the investigation had a small sample size (n = 7), which may have affected the statistical outcome. A large effect size and significant improvement in RST were observed (d = 0.92, p = 0.03), suggesting the investigation was adequately powered for the main outcome variable, which gives insight into the impact of STHA on aerobic performance. Conversely, hydration status exhibited a large effect but not a statistically significant change (d = 0.81, p = 0.07), which may indicate a low sample size for that variable. Previous single-group investigations have found increased hydration status (23) and physical performance (15,27), which would strengthen the present findings. Furthermore, this study did not measure core and skin temperature as previous studies have (7,15), which limits the ability to extrapolate potential mechanisms of STHA within combat athlete populations. Future investigations should aim to examine these physiological aspects to better understand the implications for training in these populations. Although the basis for this study offers insight into often used training strategies, it could not control for diet across subjects because each athlete received individual meal plans. Likewise, hydration status was of ecological ad libitum use. Although these may be limitations from a physiological perspective, weight-class sports and individual morphological characteristics offer individual approaches, and thus, the authors would contend this offers a more ecologically valid approach.

Practical Applications

To the best of our knowledge, the present findings are the first study to assess the effects of STHA in elite boxers. The present findings show a 5-day HA protocol yielded a ∼46% increase in RST performance performed in a thermoneutral environment while also exhibiting a large effect in hydration status. These data support previous findings that participation in an STHA can improve aerobic performance in thermoneutral temperatures. Collectively, the present findings suggest STHA can readily fit into the complex training and competition schedules of elite combat athletes, who manage complex energy system development coupled with weight management during precompetition training cycles.


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boxing; repeat sprint; performance

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