Effect of Taurine Combined With Creatine on Repeated Sprinting Ability After Exhaustive Exercise Under Hot and Humid Conditions

Abstract

Background:

Taurine (TAU) and creatine (Cr) are common ergogenic aids used by athletes to enhance performance; however, the effect of their combined supplementation, and on recovery in high temperature and humidity environments, has not been studied.

Hypothesis:

Combined TUA and Cr will have greater effect on physiological indicators and repetitive sprint performance recovery after exhaustive exercise under hot and humid conditions than single supplementation or placebo.

Study Design:

Single-blind crossover randomized controlled study.

Level of Evidence:

Level 2.

Methods:

Participants (12 sports students) were assigned randomly to 1 of 4 supplementation intervention groups: placebo (P), taurine (T), creatine (C), or taurine + creatine (T+C). Exercise protocol included exhaustion tests and repeated sprinting exercises were conducted in a laboratory environment at 35 °C/65% relative humidity. Heartrate, blood lactate (BLa), tympanic temperature, thermal sensation, and rating of perceived exertion were monitored throughout. Heartrate variability, time to exhaustion (TTE), reaction time, and countermovement jump (CMJ) height were tracked before and after exhaustion exercise and before sprint exercise.

Results:

TTE was significantly higher in the T+C group than in the P group (P = 0.04). BLa and tympanic temperature increased rapidly in all 4 conditions, then decreased gradually, and T group peak values were higher than those of P group (P = 0.04; P < 0.01). CMJ decreased in the C and T+C groups (P = 0.04; P = 0.04) after exhaustive exercise, unlike other groups (P > 0.05). Indicators of repeated sprint exercise, peak power, mean power, and power decrement showed a decreasing trend within groups but no difference between groups (P > 0.05).

Conclusion:

In this small student group, under hot and humid conditions, T+C supplementation significantly enhanced TTE.

Clinical Relevance:

TAU, Cr, and their combined supplementation do not significantly improve repeated sprint performance after exhaustive exercise under hot and humid conditions.

Investigations have shown that exercise in hot and humid environments can increase the burden on physiological regulation, such as thermoregulation and fluid balance,^10,15,36 and adversely affect exercise performance and recovery. ¹⁹ Therefore, scientific supplementation with nutritional ergogenic aids is essential in training and competition so that athletes can better adapt to the sports environment and achieve high-quality training and recovery.

Taurine (TAU) - a sulfur-containing amino acid - is one of the primary ingredients in popular energy drinks. ²⁶ TAU plays a beneficial role in a variety of metabolic and physiological processes such as glucose and lipid regulation, ²⁷ energy metabolism, ³⁸ anti-inflammatory modulation, ²⁸ and antioxidant effects, ²⁹ which in turn enhance athletic performance. However, the effects of TAU supplementation for different intake methods and exercise metabolic patterns remain controversial. ²⁷ Liu et al ²⁸ showed that 7 days of continuous TAU supplementation at 6 g per day significantly increased the time to exhaustion (TTE) in maximum oxygen consumption (VO_{2 max}) test cycling. However, Milioni et al ³¹ showed that acute supplementation with 6 g of TAU 1.5 hours before exercise failed to significantly improve TTE in the 110% VO_{2 max} test. In addition, TAU supplementation also has thermoregulatory effects. It has been shown that acute supplementation with 50 mg/kg TAU promotes an elevated localized sweat rate in hot environments, which in turn improves thermoregulatory responses and endurance cycling performance. ³³ However, most of the current literature focuses on the effects of TAU supplementation only on physiological indices and exercise performance during exercise,^8,46 with fewer studies on recovery and re-exercise capacity after training rounds. ³⁸ Hence, this also needs to be studied further. In addition, the impact of high temperature and humidity on exercise capacity and recovery is a significant issue to be explored in this study.

Creatine (Cr) is an amino acid derivative produced naturally in the human body. ⁷ Short-term Cr supplementation (eg, 20 g/day for 5-7 days) increases total Cr content by 10% to 30% and phosphocreatine by 10% to 40%. ³ Early studies have found that Cr supplementation enhances adaptations to power training (muscular power, endurance, and strength) and improves single-repetition strenuous and repeated sprint exercise performance.^25,45 In recent years, it has also been shown that Cr supplementation increases PCr and glycogen content, ⁴⁴ alters calcium handling and force production, ²¹ reduces oxidative stress and inflammatory responses, and enhances the transport of ATP from the mitochondria to the site of utilization, ¹⁶ which in turn may have a positive impact on enhancing endurance performance and recovery. ²⁴ However, for different exercise intensities, the current findings on the effect of Cr on exercise are mixed and still need to be investigated further. ¹⁶ In addition, studies have reported that Cr may modulate thermoregulation by maintaining erythrocyte pressure capacity, increasing sweating rates, and thus improving exercise performance in hot and/or humid conditions.^12,40 However, whether Cr supplementation in hot and humid environments has an effect on postexercise recovery and re-exercise capacity has not been studied.

Although there are fewer studies on the effects of TAU combined with Cr on athletic performance, these ingredients are found in high levels in popular sports drinks. ⁶ Therefore, it is necessary to explore the effect of the combination of the 2 supplements. Based on the above effects of TAU and Cr on exercise performance and thermoregulation, it may be reasonable to hypothesize that the combined use of TAU and Cr under hot and humid conditions will improve endurance exercise performance and re-exercise performance after recovery.

In summary, although previous studies have investigated the effects of TAU and Cr on exercise performance under hot and humid conditions, the effect of combined supplementation of both together has not yet been examined. In addition, the recovery effects and re-exercise capacity of both supplements under these conditions need to be further investigated. Therefore, the purpose of this study was to investigate the effect of TAU combined with Cr on repeated sprint performance after exhaustive exercise under hot and humid conditions.

Methods

Ethics

Through questionnaires and a rigorous medical examination, participants were voluntarily enrolled in this experiment and understood the procedure and requirements, as well as the possible uncomfortable reactions, signed the informed consent form, and guaranteed sufficient time to complete the experiment. This study employed a single-blind crossover randomized controlled experimental design, in which participants were blinded to the research hypotheses. Institutional ethical approval was also given in accordance with the 1964 Declaration of Helsinki.

Study Design

A crossover randomized controlled experimental design was applied in this study. Participants entered the laboratory 1 week before the start of experiments to familiarize themselves with the procedures and equipment. Then, under 4 nutritional intervention conditions, participants completed an incremental ramp test to volitional exhaustion, recovered for 40 minutes, and then performed a 6 × 10 second repeated sprint exercise on a cycle ergometer (Figure 1). The whole experimental environment was controlled by the combination of heating (SAWO, CON4) and humidification (BELIN, SC-G060ZS) equipment at temperature 35 °C ± 1 °C and humidity 65% ± 2% relative humidity (RH). All participants were evaluated at the same time of day (2.00 p.m. to 4.00 p.m.) for each test to avoid the effect of circadian rhythms on experiment. The experimental period began in August 2022 and ended in December 2022.

Figure 1.

Flowchart of experiment.

Procedure

On test days, participants collected baseline parameters (height, body mass, heartrate variability [HRV], reaction time [RT], countermovement jump [CMJ], heartrate [HR], blood lactate [BLa], tympanic temperature [Tty], rating of perceived exertion [RPE], and thermal sensation [TS]) 10 minutes after entering the experimental environment. Participants were subsequently asked to perform a 5-minute 100 W steady-state warm-up and completed the experiment according to a prearranged schedule (Figure 1). The participants then completed a ramp test, which started at 50 W and increased 50 W every 3 minutes until exhaustion. The tests were conducted on an ergometer fixed in isokinetic mode (LODE 906900), such that cadence was controlled to 60 rpm. The ramp test was terminated at volitional exhaustion or when the cadence dropped <60 rpm for >10 seconds. TTE was recorded immediately at the end of the exhaustion test. HR, BLa, Tty, TS, and RPE were monitored immediately or 1 minute after exercise and every 5 or 10 minutes for 40 minutes after exhaustive exercise. HRV, RT, and CMJ were performed again 40 minutes after the exhaustion test (before repeating the sprint exercise). Subsequently, participants performed a repeated sprint exercise protocol on a Monark cycle ergometer (Monark Classic Ergomedic 894E), which comprised a series of 6 10-second sprints at a resistance of 0.075 × body mass, with each sprint interspersed by a 10-second recovery. Peak power (PP), mean power (MP), and percentage decrement (PD) were recorded during each of the 6 repetitions of the repeated sprint exercise.

Sample Size

A priori sample sizes were calculated using G*Power (Version No. 3.1.9.7. Franz Faul University). ¹⁴ Given the typical effect sizes (Cohen d = 0.5-1.0) reported using repeat sprint protocols with TAU and Cr,^32,41,43 a sample size of 7 was deemed sufficient to identify differences between groups with a statistical power of 0.80. However, 12 participants were selected for this study to prevent insufficient data after sample attrition.

Participants

Twelve male college students majoring in physical education were selected for this study (age, 23.8 ± 2.4 years; height, 178.0 ± 6.1 cm; body mass, 75.7 ± 7.5 kg). The inclusion criteria were as follows: (1) current physical education and training students; (2) PAR-Q answered “No” to all questions, no risk of exercise; (3) no caffeine or other exercise supplements were consumed in the last month and throughout the experiment; and (4) sufficient time to fit in the entire experiment.

Supplementation Interventions

TAU and Cr monohydrate were chosen for the nutritional intervention in this experiment. The dosages of TAU followed the recommendations of recent studies, TAU supplementation was given 60 minutes before measurements because TAU plasma concentrations peak at 1 hour. In addition, Cr supplementation was carried out using the most effective dosage method currently available in the literature, with 0.3 g/kg body weight Cr supplementation per day for the first 5 days of the experiment and divided into an average of 4 applications. Participant body mass was measured before each trial to determine the correct dose, and the supplements were balanced such that an equal number of capsules were ingested between conditions. The capsules contained one of the following: placebo (P), 0.3 g/kg maltodextrin + 50 mg/kg maltodextrin; TAU (T), 0.3 g/kg maltodextrin + 50 mg/kg TAU; Cr (C), 0.3 g/kg Cr + 50 mg/kg maltodextrin; TAU plus Cr (T+C), 0.3 g/kg Cr + 50 mg/kg TAU.

Measurements

Physiological parameters

Heartrate

Upon entering the laboratory, the subject wore an HR monitor (Polar H10; coefficient of variation [CV], 0.73%) and sat still for 10 minutes to obtain basal HR. Subsequently, HR was recorded immediately after the end of the exhaustion test and at 5, 10, 15, 20, 30, and 40 minutes after the exhaustion test (before repeating the sprint exercise).

Blood Lactate

BLa was measured at 8 timepoints: at rest (baseline), immediately after completion of exhaustion exercise, and at 5, 10, 15, 20, 30, and 40 minutes after exercise. BLa was obtained by testing capillary blood from the earlobe using the Lactate Scout blood lactate meter (Lactate Scout; CV, 1.70%) and following the manufacturer instructions.

Tympanic Temperature

An infrared ear temperature gun (Braun, IRT6520; CV, 0.54%) was used to measure tympanic membrane temperature as an estimate of core temperature. This method has been shown to be an accurate and true reflection of core temperature and is quick, safe, and time saving. ⁴² The Tty was monitored during the completion of predetermined exercise protocol and was taken immediately after exercise and at 5, 10, 15, 20, 30, and 40 minutes after exercise.

HRV Measurement

This experiment evaluated the effects of exhaustion exercise on autonomic function by testing changes in HRV before and after exercise. ¹¹ When collecting HRV indicators, the subject sat in a quiet and softly lit room for 5 minutes (35 °C/65% RH) while wearing an Omegawave autonomic assessment sensor (Omegewave, personal version; CV, 1.82%). Participants remained still, did not speak, and were not moving any part of the body during the test. The experiment focused on RMSSD (root mean squared differences of the standard deviation) - a time-domain indicator of HRV.

Subjective Parameters

Thermal Sensation

A 9-point standard heat sensation scale where −4 = “very cold”, 0 = “neutral,” and 4 = “very hot” was employed, where participants were asked to state their heat sensations according to the scale before and during recovery from exhaustive exercise. ³⁹

Rating of Perceived Exertion

RPE (Borg Scale 6-20) was a used to subjectively quantify a person’s perception of the physical demands of an activity. ³⁴ Participants were asked to state their perceived exertion according to the scale. RPE was recorded immediately after the end of the exhaustion test and at 5, 10, 15, 20, 30, and 40 minutes after the exhaustion test.

Sport Performance

Countermovement Jump

The CMJ test is a practical method for evaluating neuromuscular fatigue (NMF). ⁸ The vertical jump height was obtained by applying vertical jump mat (Omegewave; CV, 0.81%). Participants stand on a vertical jumping mat with feet shoulder-width apart and arms hanging down naturally. When hearing the command to start, the participant squatted in place until his knees were bent at 90°, and then jumped quickly upwards. The mean of 3 attempts was taken as the CMJ score.

Time to Exhaustion

TTE was used as a measure of endurance performance in the laboratory environment. The participants started cycling (LODE 906900) at 50 W and increased 50 W every 3 minutes until exhaustion. TTE was recorded immediately at the end of the exhaustion test.

Reaction Time

RT was measured as a test of the coordination and rapid reaction ability of the human nervous and muscular system. ³⁰ Before the test, participant personal information was entered into a computer (Omegewave; CV, 0.74%). Participants heard a warning beep and concentrated on a computer tone given 50 times in succession. The subjects were instructed to respond by pressing buttons with their dominant hand. The time taken by each subject to respond to the tone was monitored and the mean RT was recorded.

PP, MP, and PD

Repeated sprint exercise was completed on a Monark cycle ergometer (Monark Classic Ergomedic 894E). Before the test, the load on the weight basket was set at 7.5% of the participant’s body mass (in kg). ²⁰ Participants were asked to accelerate gradually before encountering resistance, and then to maintain full power for six 10-second sprints. The computer (Monark Anaerobic Test Software) automatically recorded PP, MP, and PD after each sprint bout.

Randomization

Participants were numbered and divided randomly into 4 groups of 3 each by a random function method. The experiment was then conducted according to the contents of the cross-tab in Figure 2 by entering the different intervention protocols in turn. The washout period between experimental trials was 28 days.

Figure 2.

Experimental random grouping chart. C, creatine (Cr) group; P, placebo group; T, taurine (TAU) group; T+C, TAU + Cr group.

Blinding

A single-blind experiment was chosen for this investigation. The blinded subjects were informed that the purpose of the study was to investigate the effect of different nutritional supplements on exercise performance. The experiment used capsules to blind the subjects to the taste and texture of the different nutritional supplements. The researcher also used a uniform experimental discourse during the experimental tests to avoid cueing the participants. To determine the success of the blinding, simple binary “yes/no” questions were administered to participants at the completion of the study: (1) Did participants perceive differences between intervention groups? (2) Did participants feel able to identify their intervention allocation? and (3) Did they correctly identify their intervention allocation?

Standardization

At 72 hours before the start of each experimental condition, participants were instructed to follow nutritional guidelines, which were monitored by a dietitian to ensure that similar nutrients were consumed before the 4 conditions. In addition, participants were asked to avoid alcohol and other nutritional supplements throughout the test period and to avoid strenuous exercise for 48 hours before the test.

Statistics and Analysis

The data are presented as means and standard deviations and were initially confirmed to be normally distributed by using Bartlett’s and Levene’s tests. Homogeneity of variance was tested before further statistical analyses. TTE was analyzed using 1-way analysis of variance (ANOVA) with Bonferroni correction after confirmation of normality via Shapiro-Wilk test. Wilcoxon test was applied if data were non-normally distributed. A 2-way repeated measures ANOVA (RM-ANOVA) was conducted, with condition (P, T, C, T+C) and time as the independent variables. Paired samples t test was used for HRV, CMJ, and RT. A supplementation by time ANOVA for repeated measurements (ANOVA-RM) was used for HR, BLa, Tty, TS, RPE, PP, MP, and PD. Bonferroni correction was used for performing post hoc analyses where appropriate. Statistical analyses were carried out using statistical package SPSS with significance level of P < 0.05 (Version 25.0; IBM Corp).

Results

Physiological Parameters

Heartrate

There was an effect of time (F_(7,34) = 1182.392; P < 0.01; η²_partial = 0.996), although the intervention method (F_(3,40) = 1.033; P = 0.39; η²_partial = 0.072) and intervention method × time interaction (F_(7,36) = 1.357; P = 0.25; η²_partial = 0.209) were both found to be nonsignificant. HR rose and peaked rapidly after exercise, and then decreased gradually, but did not return to pre-exercise levels even after 40 minutes of exercise (Figure 3a).

Figure 3.

Dynamic changes in (a) HR, (b) BLa, (c) Tty, (d) TS, and (e) RPE over the 40 minutes after the exhaustion exercise in the 4 intervention methods. Symbols indicate significance at P < 0.05 compared with 0 min: ^#P group; ^&T group; ^C group; *T+C group. Significance at P < 0.05 in (b): ^☥P vs T; ^%T vs C; ^@T vs T+C. Significance at P < 0.05 in (c): ^☩T vs T+C, ^♁T vs C, ^@T vs P.

Blood Lactate

There was an effect of time (F_(7,34) = 126.345; P <0.01; η²_partial = 0.963), intervention method (F_(3,40) = 5.223; P <0.01; η²_partial = 0.281); and an intervention method × time interaction effect, (F_(7,36) = 3.628; P <0.01; η²_partial = 0.414). BLa increased significantly in the immediate postexercise period and then decreased gradually. The T group (14.58 [2.42]) was higher than the P group (11.10 [3.21]) at 5 minutes (P <0.01); higher than the P (8.76[2.98]), C (9.45[2.05]) and T+C (10.95[2.31]) groups at 10 minutes (P < 0.05); and higher than the P (6.83[2.37]) and C (8.03[2.89]) groups at 15 minutes (P < 0.05) (Figure 3b).

Temperature Measurement

Tympanic Temperature

There was an effect of time (F_(7,34) = 57.397; P < 0.01; η²_partial = 0.922) and intervention method (F_(3,40) = 5.646; P = 0.003; η²_partial = 0.297). No significant intervention method × time interaction effect (F_(7,36) = 1.343; P = 0.26; η²partial = 0.207) was observed. Tty increased significantly in all 4 groups immediately after exercise and then decreased gradually. The T group (37.86[0.35]) was significantly higher than the T+C group (37.30[0.38]) in the immediate postexercise period (P = 0.02) and significantly higher than the P (37.43[0.43]) and C (37.34[0.46]) groups at 5 minutes (P < 0.01) (Figure 3c).

Subjective Parameters

Thermal Sensation

There was a significant effect of time (F_(7,34) = 46.39; P < 0.01; η²_partial = 0.905); but not intervention method (F_(3,40) = 0.307; P = 0.82; η²_partial = 0.022) or intervention method × time interaction (F_(7,36) = 1.199; P = 0.328; η²_partial = 0.189). TS rose significantly (P < 0.05) in the immediate postexercise period and then decreased significantly during recovery (Figure 3d).

Rating of Perceived Exertion

There was a significant effect of time (F_(7,34) = 90.768; P < 0.01; η²_partial = 0.949), and an intervention method × time interaction effect (F_(7,36) = 3.126; P = 0.01; η²_partial = 0.378). No intervention method effect (F_(3,40) = 1.229; P = 0.31; η²_partial = 0.084) was detected. The RPE rose significantly (P < 0.05) in the immediate postexercise period and then decreased significantly during recovery (Figure 3e).

Autonomic Function Measurement

HRV Measurement

Time (first, second) was used as the within-subjects factor and intervention methods (P, T, C, T+C) as between-subjects factor. RMSSD decreased in all 4 groups, but there was no significant difference between the P (D% = 59.68%; t = 4.295; P < 0.05; d = 1.295; 95% CI [11.857, 37.416]), T (D% = 64.36%; t = 6.701; P < 0.01; d = 1.934; 95% CI [15.557, 30.776]), C (D% = 57.15%; t = 3.295; P < 0.05; d = 0.993; 95% CI [6.591, 34.136]), and T+C (D% = 57.79%; t = 4.778; P < 0.05; d = 1.511, 95% CI [10.742, 30.058]) groups (Figure 4).

Figure 4.

Dynamic changes of HRV in 4 groups. C, creatine (Cr) group; HRV, heartrate variation; P, placebo group; RMSSD, root mean squared differences of the standard deviation; T, taurine (TAU) group; T+C, TAU + Cr group.

Sports Performance Measurement

Time to Exhaustion

TTE was different for 4 interventions (F_(3,40) = 3.127; P = 0.04). TTE was significantly higher in the T+C (814.90 [101.32]) group than in the P (697.73 [73.05]) group (P = 0.04), an increased TTE of 16.8%. There were no significant differences between the P and T (787.58 [94.71]) groups (Figure 5).

Figure 5.

TTE for the 4 interventions. ***P < 0.05. C, creatine (Cr) group; P, placebo group; T, taurine (TAU) group; T+C, TAU + Cr group; TTE, time to exhaustion.

Countermovement Jump

Time (first, second) was used as the within-subjects factor and intervention methods (P, T, C, T+C) as the between-subjects factor. There was no significant decrease in CMJ in the P (t = 1.698; P > 0.05; d = 0.512; 95% CI [–0.23, 1.703]) and T (t = 0.764; P > 0.05; d = 0.221; 95% CI [–0.52, 1.07]) groups, but there was a significant decrease in the C (D% = 1.84%; t = 2.488; P < 0.05; d = 0.75; 95% CI [0.152, 2.76]) and TC (D% = 2.6%; t = 3.541; P < 0.05; d = 1.12; 95% CI [0.574, 2.61]) groups (Figure 6).

Figure 6.

Change in CMJ before the 2 exercises. C, creatine (Cr) group; CMJ, countermovement jump; P, placebo group; T, taurine (TAU) group; T+C, TAU + Cr group.

Reaction Time

Time (first, second) was used as the within-subjects factor and intervention methods (P, T, C, T+C) as the between-subjects factor. There was no significant decrease in RT in the P (t = 0.949; P > 0.05; d = 0.029; 95% CI [–0.012, 0.005]), T (t = 1.689; P > 0.05, d = 0.487; 95% CI [–0.002, 0.016]), C(t = 1.543; P > 0.05; d = 0.465; 95% CI [–0.002, 0.012]), and T+C (t = 1.572; P > 0.05; d = 0.497; 95% CI [–0.002, 0.013]) groups, and no significant difference between the groups (Figure 7).

Figure 7.

Change in RT before 2 exercises. C, creatine (Cr) group; P, placebo group; RT, reaction time; T, taurine (TAU) group; T+C, TAU + Cr group.

PP, MP, and PD

The PP results showed a significant main effect of time (F_(5,36) = 16.725; P < 0.01; η²_partial = 0.699), but no intervention method (F_(3,40) = 0.042; P = 0.99; η²_partial = 0.003), or intervention method × time interaction effect (F_(5,38) = 0.938; P = 0.47; η²_partial = 0.110) was observed. PP decreased gradually as the number of sprints increased (Figure 8a).

Figure 8.

Change in PP, MP, and PD in 4 groups during 6 repetitions of sprint. Symbols indicate significance at P < 0.05 compared with sprint 1: #T group, ^C group, *P group; ^&T+C group. C, creatine (Cr) group; MP, mean power; P, placebo group; PD, power decrement; PP, peak power; T, taurine (TAU) group; T+C, TAU + Cr group.

The MP results showed a significant main effect of time (F_(5,36) = 21.03; P < 0.01; η²_partial = 0.745 ], but no intervention method (F_(3,40) = 0.217; P = 0.88; η²_partial = 0.016), or intervention method × time interaction effect (F_(5,38) = 1.545; P = 0.20; η²_partial = 0.169) were observed. MP gradually decreased as the number of sprints increased (Figure 8b).

The PD results showed a significant main effect of time (F_(5,36) = 4.365; P = 0.003; η²_partial = 0.377 ], but not intervention method (F_(3,40) = 0.086; P = 0.967; η²_partial = 0.006) or intervention method × time interaction (F_(5,38) = 1.443; P = 0.231; η²_partial = 0.160) (Figure 8c).

Discussion

High temperature and high humidity pose thermoregulatory and performance challenges for athletes. ³³ However, the effects of TAU and Cr supplementation on athletic performance in such environments are less well studied, and the effects of combined supplementation of both ergogenic aids have not been investigated. In addition, controversy remains regarding their effects on exercise performance in different modes of energy metabolism.^2,5,27,41 This study was conducted to investigate this issue.

The present study found that supplementation with TAU and Cr in a hot and humid environment improved TTE by 11.8% to 12.9%, but there was no significant effect compared with the P group. These findings are in conflict with the results of others,^13,39 who found that TAU or Cr supplementation improved TTE. The reason for the discrepant findings may be due to the different intensity of the exercise. In addition, the relatively small sample used in this study may have been inadequate to detect a significant effect, and further research may be required. Interestingly, we found that the T+C group significantly improved TTE under hot and humid conditions. It has been shown that acute TAU supplementation improves thermoregulatory and endurance cycling performance at high temperatures by increasing local sweating rates. ³³ Also, it has been suggested that Cr supplementation may aid the thermoregulatory response to exercise in heat by maintaining plasma volume. ³⁵ There are 2 possible reasons to explain the results of this study. First, cumulative supplementation with exogenous supplements may act as a stacking effect. ⁴³ Second, the interaction of TAU and Cr elicits a thermoregulatory response in the body, which in turn promotes endurance cycling performance. At this stage, these are only speculative explanations, and further exploration of the potential mechanisms by which the combination of the 2 supplements promote athletic performance is required.

High temperature or high humidity environments have different effects on the physiological recovery of exercisers after high-intensity exercise. ¹⁹ We found that there were no significant differences in HR changes between the 4 groups after exhaustive exercise under hot and humid conditions, and the HR did not return to basal values after 40 minutes of exhaustive exercise. Despite studies showing that oral TAU was effective in reducing postexercise HR, ⁴³ and having positive effects on recovery after exercise, ³⁸ HR did not return to baseline after 40 minutes of recovery in the present study. Exercise stimulates sympathetic excitation, causing coronary artery constriction and accelerating the HR under hot, humid conditions. ¹ Meanwhile, studies have shown that the intensity of exercise stimulates parasympathetic activity to varying degrees. ²³ Therefore, the exercise environment and intensity may explain the present results to some extent. In addition, although the literature suggests that short-term Cr supplementation reduces HR during exercise,^22,32 there are fewer studies on HR recovery after exercise. Our study did not find any effect of TAU and Cr on HR recovery in the high temperature and humidity environment.

BLa is a sensitive indicator of exercise intensity and duration. ⁴ In this experiment, it was a surprising finding that BLa was significantly higher in the T group than in the P group 5 to 10 minutes after exhaustion exercise. TAU supplementation in hot environments has been reported to reduce BLa in the immediate postexercise period, ⁴¹ but few studies have been conducted on changes in BLa during the postexercise recovery period. High temperatures and humidity accelerate the body’s glycolytic metabolism, ¹⁵ suggesting that TAU supplementation might increase the rate of glycolytic flux. ³³ Meanwhile, TAU supplementation in hot and humid environments may exacerbate the fatigue response after exhaustive exercise, thereby causing an increase in BLa. ⁴¹ It has been indicated that short-term TAU supplementation is effective in reducing BLa accumulation after exercise in thermoneutral environments. ⁹ In contrast to previous investigations, our study used acute TAU supplementation in a high temperature and high humidity environment. Therefore, the differences in results between the present and previous studies might reflect different exercise environments or the form of TAU supplementation that was employed. It is worth noting that the T+C group in the hot and humid environment increased TTE significantly, but did not influence the recovery of BLa after exhaustion exercise.

One study has shown that acute TAU supplementation increases local sweating rates and decreases core temperature late in exercise, thereby improving thermoregulatory and endurance cycling performance in the heat (35°C, 40% RH). ³³ However, most studies have focused on thermoregulatory responses during exercise, with less research on postexercise changes in core temperature. The results of this experiment indicate that the increase in tympanic temperature after exhaustive exercise was indeed a delayed compensation phenomenon, although studies have shown that Cr supplementation may aid the thermoregulatory response to exercise in heat by maintaining plasma volume. ⁴⁰ However, the postexercise physiological response varies depending on the specific intensity of the exercise. ³⁵

Fatigue is defined as the decline in ability to create power. ¹³ The CMJ test is a method for evaluating NMF, ¹⁷ and recovery status in athletes. ¹⁸ The results of our study showed that CMJ in the C, T+C groups had a significant decrease 40 minutes after forceful exercise, compared with pre-exercise. This finding most likely reflects a decrease in muscle contractility. The underlying cause of the decrease in muscle contractility may be due to impaired excitation-contraction coupling of the muscle after high intensity exercise as well as a decrease in calcium ions released from the action potential. ³⁷ In addition, there was a decrease in CMJ of only 0.68% 40 minutes after exhaustive exercise in the T group compared with before exhaustive exercise. A study showed that TAU assists sarcoplasmic reticulum Ca²⁺ processing once it enters the muscle, and that improved muscle performance was attributed to TAU facilitating Ca²⁺ processing in cardiac and skeletal muscle cells. ²⁷ Therefore, whether TAU supplementation in hot and humid environments can contribute to the recovery of muscle fatigue still requires further research.

The results of this study found that there was a significant decrease in PP and MP with the increase in the number of sprints after repeated sprinting exercise in all 4 groups, and that there was no significant difference between the 4 groups. There are at least two possible explanations for this phenomenon. First, TAU and Cr supplementation does not sustain exercise for longer periods of time, resulting in an unaffected ability to repeat sprints after exhaustion exercise. Thus, supplementation dose and exercise intensity were the main influences on the experimental results. Second, the effects of high temperature and humidity on re-exercise performance may counteract the effects of TAU and Cr supplementation on exercise. At this stage, this reasoning is speculative and remains to be explored in vivo.

Limitations

There were significant limitations of this study. First, the experiments were based on a very small sample size (12). Although the differences in the results of some experiments were not significant, the effect sizes were large enough to expand the sample sizes for further study. Second, venous blood samples were not taken to test for plasma Cr or TAU concentrations. It is not possible to know accurately the degree of absorption of TAU and Cr in participants. Third, a double-blind research design was not followed. Finally, due to the limitations of the experimental conditions, this study did not use rectal or esophagal temperatures, but instead used Tty to measure core temperature. Although Tty measurement is a relatively accurate and reliable method under normal conditions, there are some errors due to the condition of the ear canal, operational methods, etc.

Conclusion

Under hot and humid conditions, TAU combined with Cr supplementation significantly enhances TTE. However, TAU, Cr, and combined supplementation did not significantly improve repeated sprint performance after exhaustive exercise.