Exercise in the heat is physically demanding, where endurance capacity is reduced compared with thermoneutral environments (13) because of an increase in cardiovascular strain (9), reductions in neuromuscular function (28), and central fatigue (36). Although physiological impairment has been clearly implicated, elevated psychological strain from thermal stress may impair performance via reduced dopamine levels or motivation (5), arousal (29), and increases in negative moods (27). A reduction in global and local neural network efficiency (32), as well as shifts in neural resources in the attention networks (23), contributes to an increased difficulty in neural processing relative to thermoneutral conditions (19,30). In addition, task-dependent cognitive changes occur in the heat (16,31), where higher-order functions such as executive function, vigilance, visual memory, and planning decreases with a passive rise in core temperature by 1.0°C, whereas simple tasks such as reaction time and working memory are less vulnerable (15,19,23). The evidence for cognitive changes with exercise in the heat is less clear, as light and moderate exercises have produced improvements or minimal changes in short-term memory, information processing, and executive function (7,22,40). However, potential limitations such as a learning effect of the cognitive batteries used and a lack of thermal clamping during cognitive tasks hamper full understanding. In addition, psychological perceptions are more vulnerable to thermal stress (16) and are proposed to alter cognitive function before any measurable physiological changes in the heat (21). For example, Gaoua et al. (14) found that, before any changes in core temperature, sensory displeasure from passive heat exposure significantly reduced working memory but not reaction time.
As there are decrements in performance because of psychological strain, psychological interventions may alter cognitive and endurance capacity in the heat. Simple interventions such as deception through lower than actual visual and experimenter feedback on ambient and core temperatures—before and during exercise in the heat—improves cycling performance compared with no deception (8). Furthermore, 4 h of broad-spectrum mental skills training improved the distance covered by trained runners for 90 min by 1.15 km (8%) in 30°C (3). Combined, these findings suggest that the plasticity of psychological perceptions before and during thermal stress plays an important role in altering exercise capacity. However, it is unknown if psychological skills training can affect both exercise and cognitive performance.
Motivational self-talk (MST), which is commonly used in broad-based psychological skills training (2,3), may be a key contributor to improved cognitive and endurance capacity in the heat. MST is a multidimensional top-down regulation strategy that focuses on an individual’s self-addressed verbalizations, reappraising negative thought patterns that arise during tasks with instructional and motivational statements (17). These statements influence an individual’s conscious attention and appraisal process, leading to a regulation of behavioral performance. In thermoneutral environments, a 2-wk MST intervention increased endurance capacity by 18% during a time to exhaustion (TTE) test (4), and a separate MST intervention significantly improved 10 km time trials time by 4% (1). MST has been proposed to improve cognitive performance, as qualitative analyses have demonstrated improvements in confidence, focus, attention, and a reduction in the perception of effort (4,17,18). However, the effect of MST on cognitive function, endurance capacity in the heat, or at an elevated core temperature (e.g., ~1.0°C) has yet to be systematically tested and quantified. Therefore, we tested an MST intervention that was specific to heat tolerance and cognitive function. We hypothesized that the MST would 1) increase endurance capacity by prolonging time to voluntary fatigue and 2) improve performance in higher-order cognitive tasks as they are more vulnerable to heat stress and thermal perception.
The experimental protocol and procedures were approved by the Bioscience Research Ethics Board at Brock University (REB no. 14-162) and conformed to the latest revision of the Declaration of Helsinki. All participants were screened using a modified Physical Activity Readiness Questionnaire, and a full explanation of procedures, discomforts, and risks was given before obtaining informed written consent.
Fourteen male and four female trained cyclists and triathletes (18–50 yr) received either a 2-wk MST or a 2-wk control (CON) regimen (seven males and two females per group). There were no differences in baseline age, height, body mass, peak oxygen consumption (V˙O2peak), and peak power output (PPO) between groups (Table 1). On the basis of the study of De Pauw et al. (11), participants were classified as performance level 3 (scale of 1–5).
The experiment implemented a PRE/POST-test design with four sessions in total. The first session consisted of collecting anthropometric data, baseline cognitive function, and determining V˙O2peak. The second session was a familiarization trial of the experiment protocol to reduce the learning effect of the cognitive tasks. The third (PRE) and the fourth (POST) sessions were the experimental trials, with the same testing protocols as the familiarization trial. Sessions 1–3 were each separated by a minimum of 1 wk to ensure recovery and to reduce the potential for heat acclimation. Sessions 3–4 were separated by a minimum of 14 d, during which time either MST or CON was performed. Female participants completed the experimental sessions during 7–10 d and 21–24 d into their self-reported menstrual cycle. All sessions were performed at the same time of day to control for circadian fluctuations in core temperature. Participants were asked to maintain a similar activity regime throughout the experiment’s duration, to follow similar meals and hydration practices 24 h before a trial, and to not consume caffeine 4 h before a trial.
Upon arrival to the laboratory, participants provided informed consent, height (cm) and mass (kg) were measured, and body fat was determined using the seven-site skinfold measurement (Harpenden, Baty International, West Sussex, UK) technique (20,38). Participants then completed the Cognitive Failure Questionnaire (CFQ), which is a 25-item questionnaire that is a self-evaluative measure of general fluid intelligence and is related to four factors of absentmindedness (memory, distractibility, blunders, and names) (6,39). Items were scored on a five-point Likert scale (from 0 = “never” to 4 = “very often”). CFQ scores can range from 0 to 100, where average CFQ scores are between 19 and 45 (35). Participants were excluded from the study if the CFQ score is >45, as this score indicates considerable difficulties in completing tasks that require vigilance. Upon the completion of the CFQ, an incremental test to exhaustion was performed in a thermoneutral environment (~22°C, 30% relative humidity [RH]) on a cycle ergometer (Velotron; RacerMate Inc., Seattle, WA) to determine PPO and V˙O2peak. The test began with a standardized 5-min warm-up at 100 W, followed by workload increase of 25 W (males) or 20 W (females) each minute until exhaustion. V˙O2peak was defined as the highest 30-s value measured breath by breath from expired gases collected through a soft silicone facemask connected to an online gas collection system, whereas PPO was the highest power output achieved during the last full 1-min stage.
To determine hydration status, participants voided their bladder to measure urine specific gravity (USG; PAL-10S, Atago, Tokyo, Japan) upon arrival for sessions 2–4. Participants were considered euhydrated if USG was ≤1.020, or else the test was rescheduled. Participants were dressed in cycling shorts and shoes then performed a baseline measure of cognitive function using a cognitive test battery (CTB; described in the CTB section) in a thermoneutral environment (22.0°C, ~30% RH). Upon the completion of the CTB, participants entered an environmental chamber set at 35.0°C, ~50% RH, with 3.0 m·s−1 of airflow, and were fitted with a soft silicone mask to collect expired gases.
The first exercise period (EX1) consisted of constant-load cycling with a 5-min warm-up at 100 W followed by 25 min of cycling at 60% of PPO. Participants were allowed to freely choose their cadence between 60 and 120 rpm. Upon the completion of EX1, participants completed a 30-min rest period (R1) and then performed the CTB. Participants sat in a chair inside the environmental chamber with airflow stopped and were fitted with a 100% vinyl poncho to minimize heat loss. To reduce the physical and perceptual strain of thirst, 250 mL of water was provided ad libitum.
Upon the completion of R1 and the CTB, participants performed a second constant-load exercise bout (EX2), consisting of a 5-min warm-up at 125 W followed by 80% PPO to voluntary exhaustion (4). TTE was determined as follows: 1) volitional fatigue, 2) cadence dropped less than 60 rpm for five consecutive seconds, or 3) a rectal temperature (Tre) of 40.0°C for 1 min. No verbal encouragement was given during the TTE to eliminate the superimposition of any extraneous verbal statements (4) as well as no external feedback (e.g., time, HR, and cadence), and TTE duration was not provided to any participants to minimize goal setting in future tests. After the completion of the TTE, participants performed a second rest period (R2) identical with R1.
Participants were randomly (www.random.org) selected to receive a 2-wk control (CON) or MST intervention between the PRE and the POST sessions, and groups were matched for sex. Participants were not a priori matched for age, anthropometrics, performance, or aerobic fitness and were only notified of condition placement at the completion of the PRE trial. CON performed their normal aerobic training regimen during this period. The MST intervention was given in two stages by a skills training workbook (1,4) and was designed to contextualize self-talk cues through practice to the demands of performing in the heat.
The first stage consisted of two self-talk exercises that focused on endurance and cognitive performance. In the first exercise, participants determined a list of negative statements that occurred during the EX1 and EX2 periods in the PRE trial then compared their list of negative statements to a list of 12 motivational statements used in previous self-talk literature (e.g., “Keep pushing, you’re doing well”) (4). Participants then determined their own list of five MST statements, after which they selected two statements that were deemed helpful for EX1 and two statements for EX2. In the second exercise, participants determined a list of negative statements that occurred during baseline, R1, and R2 during the CTB in the PRE trial. Participants then compared their list of negative statements to a list of five motivational statements (e.g., “I am focused”). Participants also generated one statement that they deemed motivational to use before and during each cognitive test in the POST trial.
The second stage of the MST was a 2-wk practice phase designed to rephrase and self-contextualize the use of statements believed to optimize performance (4). Participants performed their normal training regimen and were required to perform a minimum of three sessions in which they practiced their MST statements. After each exercise session, they completed a workbook assessing the efficacy, frequency, and their comfort with the four chosen exercise self-talk statements. Effective statements were recorded and were used in future exercise sessions, whereas ineffective statements were recorded and rephrased or replaced with a more suitable statement chosen by the participant. An experimenter was available throughout the practice period if the participant needed assistance with their workbook. The extent of self-talk usage during practice periods was assessed using a purpose-built 11-point Likert scale (from 0 = “not at all” to 10 = “greatly”), and the number of changed statements was recorded.
After the POST trial, each participant completed a group-specific experiment questionnaire. The MST group was asked the extent and perceived effectiveness of self-talk usage during EX1 and EX2 on a purpose-built 11-point Likert scale (from 0 = “not at all” to 10 = “greatly”). The MST group was then asked to qualitatively detail the ways self-talk was found to be beneficial or nonbeneficial. After the POST trial, the CON group was asked to qualitatively list any psychological strategies used during the experiment (e.g., self-talk, imagery, and arousal regulation) and the purpose of their use.
Tre and HR were continuously sampled throughout the trial and converted to 1-min averages. Tre was measured (1 Hz) using a flexible thermistor (Mon-A-Therm Core; Mallinkrodt Medical, St Louis, MO) inserted 15 cm beyond the anal sphincter. HR was collected (10 Hz) using a telemetric HR monitor (RS800CX; Polar Electro Oy, Kempele, Finland). Expired gases were collected through a silicone facemask with the exhalation port connected to a metabolic cart (ML206 Gas Analyzer; ADInstruments Inc., Colorado Springs, CO) and was continuously sampled during EX1 and EX2 to determine oxygen uptake (V˙O2, L·min−1).
RPE was assessed using a 6–20 scale (Borg 1982) and was recorded at t = 0, 5, 10, 20, and 30 min during EX1 and taken at t = 0 and every 2 min during EX2. Because of the individual variation of TTE, data (Tre, HR, V˙O2, and RPE) were converted into t = 0, iso-50%, iso-75%, and iso-100% relative time points of EX2 to compare trials.
To measure progressive changes in cognitive function, a 15-min CTB (CogState, New Haven, CT) was performed at baseline, R1, and R2, which consisted of a Groton Maze Learning Task (GMLT), a detection task, and a two-back test. A familiarization trial was used to increase familiarity and to minimize the learning effect of multiple exposures to tasks.
The GMLT is a touch screen–based cognitive task that measures executive function through error detection and spatial memory. The test consists of a 10 × 10 grid of squares that cover a hidden 28-step pathway that includes 11 turns. A blue tile on the top left corner of the screen indicates the starting position, and a red circle on the bottom right corner indicates the finish location. The GMLT is performed six times (initial test sequence and five-block trials) per test. The GMLT test sequence required approximately 5 to 10 min to complete. To minimize a learning effect due to repeated exposure, each maze is randomized and well matched for difficulty. Performance was measured for the total duration (s) and total number of errors measured during the five-block period.
The detection task was used to test psychomotor function and reaction time. A playing card was presented on the screen in a face-down position, and participants were tasked with pressing a key when the card was turned over presenting the front of the card. This process continues until the task is completed. There was an interstimulus interval of 2 s between each presentation of 35 cards and continues until the task is complete. Performance was measured for speed (mean of the log10 transformed reaction times for correct responses) where a lower score represents a better performance. The detection task required approximately 2 min to complete.
The two-back test is a measure of attention and visual working memory. Participants were tasked with determining if the card presented is identical with the card presented two cards ago. There were a total of 48 cards presented, and participants could either answer “yes” or “no” for the card presented. Performance for this task was measured for speed of processing (ms) and total number of errors made. The two-back test took approximately 2 min to complete.
All continuous variable data are presented as the mean ± SD and were analyzed using separate group (MST vs CON) × trial (PRE vs POST) × time mixed-model repeated-measures ANOVA. A Bonferroni post hoc analysis was used to test significant main effects. Paired sample t-tests were used to identify significant main effects at specific time points within groups.
All ordinal data (RPE and qualitative feedback) are presented as the median (quartiles 1 and 3). RPE was analyzed using separate group (MST vs CON) × trial (PRE vs POST) × time mixed-model repeated-measures ANOVA, with a Wilcoxon signed-rank test used to compare within-group effects at specific time points. A Friedman’s ANOVA was used to analyze the amount of self-talk usage during the 2-wk practice phase in MST group. Statistical significance was set at P < 0.05. All analyses were performed using IBM SPSS Statistics for Windows (version 22.0; IBM Corp., Armonk, NY).
The MST intervention was successful in changing psychological skill usage. The MST group significantly (P = 0.003) increased their self-talk usage (0–10) in the third practice session (10 [9–10] perceived usage) compared with the first practice session (8 [5–8] perceived usage), with no differences (P = 0.137) compared with the second practice session (8 [7–9] perceived usage). Overall, the MST group changed approximately 2 (2–3) statements during the practice period from their initial first session. There were distinct differences in psychological strategies used in the POST trial between the MST and the CON. The MST group used two MST statements in EX1 and two statements in EX2. CON reported using some forms of self-talk, arousal regulation strategies (e.g., focus on breathing) and disassociation; however, these strategies were unstructured and unplanned.
There was no group–trial interaction (P = 0.579) at baseline for USG in between both MST (PRE, 1.012 ± 0.001; POST, 1.009 ± 0.004) and CON (PRE, 1.009 ± 0.003; POST, 1.009 ± 0.005). During EX1, there was a significant increase (all P < 0.05) in Tre and HR (Fig. 1) and V˙O2 and RPE (Fig. 2), with no trial–time–group (all P > 0.05) interaction between the CON and the MST groups for any of the variables. Tre change from baseline increased similarly and significantly for both MST (PRE, Δ +1.1°C; POST, Δ +1.0°C) and CON (PRE, Δ +1.1°C; POST, Δ +1.2°C) in both trials. Tre and HR significantly (all P < 0.05) decreased from the start to the end of R1, with no group–trial–time interaction (all P > 0.05) between CON and MST; Tre dropped ~0.1°C from the start of R1 to the end of the CTB (~20 min) in both trials.
In EX2, there was a significant group–trial interaction (F1,16 = 14.460, P = 0.002) (Fig. 3) for TTE duration. In the CON group, there was a nonsignificant (P = 0.280, β = 0.176) change in TTE from PRE (531 ± 178 s) to POST (510 ± 216 s). The MST group had a significant (P = 0.021, β = 0.886) increase in TTE from PRE (487 ± 173 s) to POST (679 ± 251 s), with all but one participant improving their TTE. Tre significantly increased in both groups (F3,33 = 3.766, P = 0.021), with no differences (all P > 0.05) for Tre at any iso–time point during PRE/POST or at iso-100% (PRE, 38.4°C ± 0.3°C, Δ +1.1°C; POST 38.4°C ± 0.2°C, Δ +1.2°C; P = 1.000; β = 0.238) for the CON group. The MST group finished with a significantly higher Tre by ~0.3°C at iso-100% of the TTE in POST (38.8°C ± 0.4°C, Δ +1.4°C) compared with the PRE trial (38.5°C ± 0.2°C, Δ +1.1°C, P = 0.023, β = 0.410), with no differences at iso-0%, iso-50%, and iso-75%. HR and V˙O2 significantly increased (all P < 0.05) over the TTE with no group–time–trial interaction (all P > 0.05) between CON and MST conditions.
RPE increased (P > 0.05) during the TTE with no group–trial–time interactions (F3,39 = 1.196, P = 0.324) or differences at any iso–time point. For the eight MST participants that improved TTE, cycling time after an RPE of 19 was assessed as an indirect indicator of maximal effort. Participants cycled longer after an RPE rating of 19 in POST (274 ± 177 s) compared with PRE (138 ± 118 s). The MST group rated the overall usage of self-talk statements (0–10) significantly (P = 0.046) higher in EX2 (9 [9–10] perceived usage) compared with EX1 (8 [6–10] perceived usage). In addition, participants also rated the overall effectiveness of self-talk statements (0–10) significantly (P = 0.026) higher in EX2 (10 [9,10] perceived effectiveness) than in EX1 (8 [6–9] perceived effectiveness).
Rectal temperature and HR significantly (all P < 0.05) decreased from the start to the end of R2 (Fig. 1). There was a significant trial (P = 0.002) and group–trial (P = 0.013) interaction for Tre in R2. MST had a significantly (P = 0.023) higher Tre at the start of POST, which continued throughout R2. Compared with baseline at the start of each trial, USG significantly increased (both P < 0.05) during trials in both MST (PRE, 1.016 ± 0.004; POST, 1.016 ± 0.004) and CON (PRE, 1.012 ± 0.005; POST, 1.013 ± 0.006), with no group–trial–time interaction (P = 0.337) but remained below the pretrial dehydration threshold of 1.020.
Performance on the CTB was first analyzed using all participants (n = 18) from the familiarization (FAM) trial compared with the PRE trial to test for any potential learning effect changes because of multiple exposures to the CTB. There was a significant decrease (all P < 0.05) in duration (Baseline-FAM: 217.9 ± 65.5 s) and errors (Baseline-FAM: 56.0 ± 31.6 errors) for the GMLT and a significant improvement in speed (Baseline-FAM: 2.55 ± 0.11) for the detection task during FAM (Table 2). There were no changes in speed or errors for the two-back test during FAM. There were no differences in performance for any cognitive measure from R2 in FAM to baseline in PRE (all P > 0.05), indicating no further learning effect for the CTB during the PRE and POST trials.
There were no significant group–time–trial interactions for both duration (F2,30 = 0.295, P = 0.747, β = 0.080) and errors made (F2,30 = 0.765, P = 0.474, β = 0.163) in the GMLT (Table 2). However, there was a significant trial interaction for both duration (F1,16 = 11.798, P = 0.005, β = 0.421) and errors made (F1,16 = 5.972, P = 0.027, β = 0.334). The MST group had a significantly faster completion in the POST trial at baseline (P = 0.009) and in R2 (P = 0.039), but not during R1 (P = 0.164), as well as fewer errors made at baseline (P = 0.019) and R2 (P = 0.012), but not during R1 (P = 0.108), during the GMLT. There were no differences in duration or errors made at any time point in the CON group on the GMLT. There were no significant differences in speed or errors during the detection task or two-back test from PRE to POST for both MST and CON (Table 2).
MST is a top-down regulation strategy requiring participants to continuously reappraise negative self-talk and bottom-up feedback with self-contextualized motivational statements. The MST intervention used in the current study was designed to address endurance capacity and cognitive function in the heat. Participants increased usage of MST over the 2-wk practice period and self-reported significant increases in usage and effectiveness in the POST trial. CON had a slight but nonsignificant decrement (~4%) in TTE in the POST trial, with no differences in any physiological or perceptual response. MST resulted in ~29% improvement in TTE—concurrent with a longer duration near or at maximal intensity (RPE ≥ 19)— significantly high use of MST, and higher terminating Tre. In addition, MST improved the speed and the accuracy for an executive function task (GMLT) after the 2-wk MST program during both baseline testing in thermoneutral temperatures and after EX2 in the heat. To our knowledge, this is the first study to quantify the use of a psychological skills training intervention to improve executive function in either a thermoneutral or a hot environment. Overall, these findings demonstrate that the internal psychophysiological control of exercise and fatigue plays an important role in improving endurance capacity and higher-order cognitive function in the heat. This extends previous reports that MST is beneficial in improving endurance capacity (4,26) in thermoneutral environments, as well as the beneficial use of psychological skills training interventions on exercise performance in the heat (3).
The improved TTE for MST participants in the POST compared with PRE trials was concurrent with a higher terminal Tre with no differences in HR or V˙O2, suggesting that a primary response to MST was a greater psychological tolerance during thermophysiological strain. Psychological tolerance of thermal discomfort was previously proposed as a major determinant of exercise-heat capacity across aerobic fitness groups (9,37). In these studies, despite similar rates of heat storage, aerobically fit individuals could voluntarily tolerate exercise-heat stress much longer than nonfit individuals regardless of hydration or heat adaptation status, with the primary differences being a reduced perceptual discomfort at a set Tre partnered with a much higher terminal Tre. Similarly, Morrison et al. (28) observed a reduced tolerance to passive hyperthermia in nonaerobically fit individuals compared with highly fit, yet no differences in neuromuscular activation in the few nonfit individuals who could tolerate passive heating to 39.0°C compared with high-fit. The present data thus extend prior work by suggesting that even well-trained individuals remain pliable and trainable in their psychological tolerance to thermal or exercise discomfort. Future research is needed to determine the influence of psychological skills training on nonaerobically fit individual’s ability to voluntarily tolerate exercise-heat stress and performance, as a greater potential improvement may be possible due to the relatively lower baseline level of performance.
The conscious perception of exertion and fatigue is theorized to be derived from bottom-up afferent input from the cardiorespiratory, metabolic, and musculoskeletal systems, as well as the affective appraisal of exercise (12). Endurance performance is improved by interventions that reduce an individual’s sense of perceived exertion, whereas performance is impaired with increased perception of effort (24,25). MST has been demonstrated to reduce RPE at 50% iso–time point during a similar TTE test in thermoneutral conditions (4), and it has also been shown to have no effect on RPE during a 10-km time trial despite a significant improvement in completion time of ~4% (1). Because of the constant-load exercise used in the present study, participants experienced the same level of bottom-up afferent feedback and difficulty throughout the task, which led to a similar level of perceived exertion. RPE was similar between trials during EX1 and during all iso–time points in EX2 for both groups. However, the duration that the MST group could tolerate at near-maximal (RPE ≥ 19) exercise was ~100% greater during POST, suggesting that the MST group likely improved endurance capacity by continuously reappraising their desire to voluntarily terminate exercise and perception of fatigue. Combined with the increased MST usage during the practice period and high usage and perceived effectiveness of MST during the TTE, it is unlikely that MST directly improved performance through a lower perceived exertion throughout the task, but rather the learned ability to counteract the psychological context-specific demands throughout the tasks (16) and environmental conditions (1,2).
The MST intervention was used by participants during the CTB to maintain “focus” or increase “concentration” during the tasks, with no reported anxiolytic benefits, whereas the CON group reported no psychological strategies used in postexperiment questionnaires. The MST intervention led to a significant improvement in both the speed and the accuracy of the GMLT at baseline before entering the heat, and in R2 despite the 29% increase in TTE time. There were no changes in performance for the detection task and two-back test in either the MST or CON groups, which may reflect these tests’ relative simplicity compared with the GMLT, as simple tasks are less vulnerable to hyperthermia (Tre = 38.7°C) compared with complex tasks (16,31,33). These findings suggest that MST potentially leads to improvements in executive function but may have minimal to no performance changes on simple cognitive task performance in either thermoneutral or hot environments. Although MST requires mental effort, it does not appear to add a significant cognitive load or deplete attentional resources, as there were no recorded decrements with MST usage in the POST trial. Future research is needed to determine how MST specifically affects resource allocation and its role on cognitive function, as well as its effects at a higher thermal load (e.g., >39.0°C) than was induced in the current study.
The combined improvements in executive function and endurance capacity may be due to neurobiological changes that occur with exercise to fatigue and heat stress. Prefrontal cortex (PFC), lateral PFC, orbitofrontal cortex, anterior insular cortex (AIC), and anterior cingulate cortex (ACC) integrate afferent feedback and indirectly communicate with motor output to regulate voluntary exercise performance (34,35). The AIC and the ACC appraise afferent homeostatic signals to determine the perception of the bodily state (including thermal perception and fatigue) and emotions, predicts future perturbations in homeostasis to determine behavior, and are part of the executive attention network (10,30). These neural components become increasingly important during exercise in the heat, as hyperthermia shifts neural resources from and reduces activation in the ACC with a 1.0°C in core temperature (23), reduces dopamine and motivation (5), and decreases in arousal (through an elevated α/β index) in the PFC that is strongly correlated (r = 0.98) to exercising Tre (29). Paulus et al. (30) proposed that interventions working directly on the PFC, ACC, and AIC will improve performance in adverse environments. MST may work directly on the ACC (motivational component and task evaluation) and the AIC (through continuous affective reappraisal) to prolong the state of voluntary fatigue (34). As these paralimbic structures are involved in the executive function network, this may be why cognitive improvements are primarily seen in executive function tasks. MST appears to be more beneficial for endurance capacity in hot compared with thermoneutral environments (4) and may be due to the increased physiological and neurological strain. This is consistent with research using pharmaceutical manipulations of dopamine, where administration of dopamine reuptake inhibitors increased the time to voluntary fatigue (5) and increased exercise performance in hot but not in thermoneutral conditions (36). Overall this would indicate that the brain contributes to exercise tolerance through a top-down regulation of performance. However, future research is needed to determine potential mechanisms on how psychological strategies such as MST specifically affect neural structures/activation during exercise and with various task-dependent cognitive skills to optimize neural function and performance in the heat.
An experimental consideration of the study was not using a sham-control group using an intervention such as neutral self-talk (1). Experimenters spent more time with participants (~45 min longer), which may have improved performance because of social facilitation. Because of the study design, we cannot fully account for these potential confounding variables; however, great effort was used to reduce social facilitation/external motivation through no verbal feedback or encouragement during tests, use of same experimenters between trials, and no knowledge of results given between trials. Barwood et al. (1) found that the use of a sham-control neutral self-talk and spending the same amount of time with participants does not influence cycling performance, whereas the MST intervention had a beneficial effect. No volunteers were screened out during recruitment because of an overly high CFQ of >45, but future research should determine whether MST may have similar effects on individuals with high levels of absentmindedness. In addition, as improvements in executive function were not uniform (Baseline, R2 vs R1) throughout the POST trial, future research is needed to determine whether the extent or perceived effectiveness of MST usage, core temperature, or exercise task affects executive function.
In summary, a 2-wk MST intervention increased usage and self-contextualization of MST statements, likely improving voluntary endurance capacity and executive function in the heat by improving psychological tolerance of high physiological strain. These findings demonstrate that the internal psychophysiological control of exercise and fatigue plays an important role in improving endurance capacity in the heat and that trained athletes can benefit from psychological skills training interventions. As the MST intervention also improved executive function in both neutral and hot environments, future research should test the specific neural changes and adaptations that occur with MST and psychological skills training to determine the underlying mechanisms of the top-down regulation of exercise and fatigue.
The authors express their gratitude to the participants for their efforts throughout the study. The study was supported by the Natural Science and Engineering Research Council (NSERC) of Canada through a Discovery Grant (no. 227912-12, S. S. Cheung). S. S. Cheung was supported by a Canada Research Chair, and J. I. Vlaar was supported by an NSERC Undergraduate Student Research Award. The authors have no conflicts of interest to declare. The authors declare that the results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation and do not constitute endorsement by the American College of Sports Medicine.
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