The Effect of Head-to-Head Competition on Behavioural Thermoregulation, Thermophysiological Strain and Performance During Exercise in the Heat

Prolonged exercise (≥ 30 min) is impaired in hot environments (air temperature ≥ 30 °C) compared with cooler conditions (air temperature ≤ 20 °C) [1], although the effect of ambient temperature on prolonged exercise performance is not dichotomous, but is instead a continuum, with the fastest performances often achieved at a temperature of ~ 10 °C and an exponential slowing occurring as temperature increases beyond this optimum [2, 3]. Early studies using fixed-intensity, time-to-exhaustion models emphasised the role of a ‘critical’ (~ 40 °C) core temperature (T _C) in the aetiology of fatigue in the heat [4]. However, this ‘critical’ threshold has subsequently been challenged, with Ely et al. [5] demonstrating no difference in running velocity during a self-paced 8000-m run in the heat for the portion of the run when T _C was > 40 °C, compared with the portion of the trial where T _C was < 40 °C. Nevertheless, overall completion times were slower in the heat compared with when the run was undertaken in cool conditions [5]; during self-paced exercise in a hot environment, performance may be impaired with modest hyperthermia [6] and work rate is often reduced before a ‘critical’ T _C [7]. The mechanisms underpinning this effect are complex, resulting from an interplay of cardiovascular, peripheral (muscular), central nervous [8] and psychological factors [9]. Nevertheless, voluntary reductions in work rate are, at least in part, intentionally mediated [10] and result in lower metabolic heat production, reduced physiological strain and improved thermal compensability [11]. Thus, it has been suggested that pacing is a thermoregulatory behaviour for preventing excessive body-heat storage [10].

In laboratory studies in the heat, participants typically exercise alone, yet, as has been noted [8], the T _C recorded in competitive, non-laboratory situations often exceeds that typical during self-paced laboratory trials [1]. Indeed, ‘competition’ has been cited as a risk factor for exertional heat illness [8, 12]. Although laboratory evidence for this assertion is limited, heat-related collapse in athletic competition is well documented in the field [13, 14]. Laboratory studies in cooler environments have shown improved 2000-m cycling performance when athletes believed they were competing against another participant in a simulated race, but were actually competing against an avatar of their own solo performance [15]. Similarly, participants who believed they were ‘racing’ against a previous 4000-m cycling time trial (TT), but were actually racing against an avatar with a 2% higher power, matched the superior performance [16]. If pacing is a thermoregulatory behaviour, it is important to understand the effect of competition on pacing, performance and thermoregulation during exercise in hot conditions.

Understanding of the cognitive basis for the ergogenic effect of competition is evolving. The presence of a competitor reduces the rating of perceived exertion (RPE), possibly by reducing internal attentional focus [17]; RPE is regarded as the key psychophysiological cue for regulating work rate [18]. Presently, it is unclear if thermal sensation (TS) and thermal comfort (TC) are also influenced by attentional focus, although the extent to which they modulate pacing may depend on the magnitude of hyperthermia [10]. Emotions may also be important [19]. Renfree et al. [20] demonstrated that although RPE did not differ between ‘slow’ and ‘fast’ trials, fast trials had high levels of positive affect and low negative affect. Slovic et al. [21] suggested that affectivity and decision making are influenced by the perceived risks and benefits associated with a given behaviour and low-risk perception is associated with a faster initial exercise pace than that adopted by high-risk perceivers [22]. Beyond the influence of affectivity and risk perception, within high-risk sports where severe injury or death is possible, some individuals may purposefully increase exposure to danger by undertaking deliberate risk-taking (DRT) behaviours, or conversely, precautionary behaviours (PB) which minimise and control dangers [23]; the extent to which these behaviours are relevant for competitive performance in a hot environment, where the dangers may be less obvious, remains to be established. The possibility that individuals with certain trait-like characteristics might be more susceptible to the effect of competition is consistent with studies of the placebo effect, whereby individuals scoring high on resilience, altruism and straightforwardness, and low on angry hostility, were more susceptible to a placebo intervention [24].

Accordingly, we tested the following hypotheses. Firstly, solo exercise performance will be reduced in hot versus cool conditions (H₁). Secondly, head-to-head competition will influence pacing during exercise in the heat resulting in faster performance than during solo exercise in the heat (H₂). Thirdly, any performance improvement with head-to-head competition will increase thermophysiological strain (H₃). Finally, performance improvements with head-to-head competition will be related to certain psychological trait-like characteristics (e.g. risk-taking behaviour), or states (e.g. positive affect) (H₄).

WBGT did not differ (p = 0.646) between HotSolo [26.0 °C (0.4)] and HotH2H [26.1 °C (0.4)], but was lower in CoolSolo [11.8 °C (0.4), both p < 0.001]. Seventeen participants completed all trials. One participant was withdrawn from two trials (HotSolo and HotH2H) due to the T _re withdrawal criteria; in each instance they were nearing completion (> 18.75 km covered). Data from beyond 18.75 km in the other trials were excluded for this participant and thermophysiological data at this distance were taken as the terminal data for this individual given the close proximity to the trial end; terminal perceptual data were not available. Analyses were repeated with this participant excluded and were not different from those reported.

This study is the first to demonstrate that head-to-head competition alters behavioural thermoregulation (i.e. pacing) during exercise in the heat. Compared with solo exercise in the heat, head-to-head competition resulted in increased external work rate [+11.8 (12.3) W], metabolic heat production, and thermophysiological strain, and improved performance [−53 (57) s] such that participants matched their (solo) performance under cool conditions. Despite thermophysiological differences, perceptual responses were unchanged with head-to-head competition. Moreover, the increased end-exercise T _re (+0.18 (0.32) °C) with head-to-head competition was related to the anticipatory heart rate increase, and was predicted by participants’ attitudes towards DRT, whereas self-reported resilience predicted change in performance time between HotSolo and HotH2H. These findings are important because they are consistent with the suggestion that competition is a risk factor for exertional heat illness [8, 12], provide potential mechanistic insight into the underlying processes (disassociation between perceived and actual thermophysiological state) and demonstrate that simple psychophysiological and psychological measures may have utility in identifying susceptible individuals.

Increased ambient temperature impaired solo exercise, as has been demonstrated previously [1‐3]. Thus, H₁ (solo performance will be reduced in hot versus cool conditions) is accepted. Although initial pacing profiles were similar, power was lower in HotSolo versus CoolSolo from 12 km onwards, which is in accordance with a recent meta-analysis [7]. The reduced power occurred with a relatively modest T _re (~ 38.0 °C) and is consistent with the assertion that pacing is a thermoregulatory behaviour which prevents excessive body-heat storage in advance of a high T _C [10]. The driver(s) influencing this process are unclear, although thermophysiological (T _re [4], skin temperature and cardiovascular strain [38], heat storage [39]), and perceptual (TS, TC, [10], RPE [18]) factors have been implicated. Indeed, differences were evident between HotSolo and CoolSolo in some thermophysiological (increased \(\overline{T}_{re}\), \(\overline{T}_{sk}\), \(\overline{T}_{b}\), \(\dot{V}\) O ₂, heart rate, PSI) and perceptual indices (increased RPE, TS, PeSI, reduced TC). Although the onset of differences did not always coincide with the altered pacing, this does not preclude their involvement as behavioural cues and certain thresholds may need to be surpassed before influencing behaviour.

Head-to-head competition altered pacing in the heat such that the performance decrement in HotSolo versus CoolSolo was abolished and participants matched their CoolSolo performances. Indeed, 11 out of the 18 participants (61%) were able to record a faster time in HotH2H than in CoolSolo, such was the ergogenic effect of the head-to-head competition. Thus, H₂ (head-to-head competition will influence pacing during exercise in the heat resulting in faster performance times than during solo exercise in the heat) is accepted. The influence of head-to-head competition was greatest early on (0–4 km), where power was higher in HotH2H than HotSolo and CoolSolo; despite the high ambient temperature, participants exceeded their ‘competitor’s’ pace, generated (unknowingly) by their own cool-conditions performance. The early stages of exercise are most susceptible to manipulation because initial pace is primarily generated using feed-forward processes, incorporating knowledge of exercise endpoint, previous experience, environmental conditions, and motivational factors [18], and any afferent feedback cues influencing behaviour are less intense. Our data are similar to pacing in Olympic and World Championship distance-running races, where athletes initially adjust their speed to match their opponents, rather than adopting their usual pace [40]. According to decision-making theory, which has recently been applied to pacing [41], a potentially large payoff, such as beating a competitor, might encourage a riskier strategy and tolerance of greater physiological disruption (or harm). From a neuroscience perspective, increased motivation stimulates dopamine release from the ventral tegmental area and activation of the motivation/reward pathway via the dopaminergic mesolimbic pathway [42]; norepinephrine also plays a role [43]. Indeed, the dopamine/norepinephrine reuptake inhibitor, bupropion, improves exercise in the heat [44]. The release of norepinephrine and epinephrine is also consistent with a psychophysiological stress response, which is in keeping with the anticipatory heart-rate rise [45] in HotH2H, whereas cortisol, which is also released in stressful situations, modulates behaviour related to increased motivated decision making, where high-risk choices yield potentially big rewards [46]. Exogenous cortisol supplementation increases risk-taking behaviour [47], which is associated with a higher initial pace [22]. Although speculative, these responses could account for the faster early pace in HotH2H. Future studies should seek to investigate these putative mechanisms using appropriate neuro-imaging techniques and blood measures, including catecholamines, neurotransmitters and stress hormones.

Beyond the initial exercise period, participants matched the ‘competitor’ performance, such that power was higher in HotH2H than HotSolo (from 8 km) and not different from CoolSolo (from 4 km). Consequently, metabolic heat production increased, leading to increased thermophysiological strain (higher T _re, \(\overline{T}_{b}\), heart rate and PSI in HotH2H versus HotSolo) and enabling acceptance of H₃ (performance improvements with competition will increase thermophysiological strain). The implications of this finding are significant; high body temperatures are associated with exertional heat illnesses and prolonged high body temperatures (T _re >40 °C) can lead to exertional heat stroke [13]. Thus, our data are consistent with the hypothesis that competition increases heat-illness potential [8, 12]. Importantly, unlike during solo exercise, where thermophysiological differences were paralleled by perceptual differences, there were no between-conditions differences in TS, TC, RPE or PeSI between HotH2H and HotSolo.

It is possible that the magnitude of thermophysiological differences between HotH2H and HotSolo was below that influencing perception, or that key perceptual drivers were unaffected. For example, \({\overline{T}}_{sk}\), which influences TS [10], was not different between these conditions. Alternatively, head-to-head competition may alter the relationship between perceived and actual thermophysiological state. Previous research has shown some ‘centrally acting’ interventions enable athletes to ignore cues which normally regulate exercise in the heat. Methylphenidate (a dopamine reuptake inhibitor) and bupropion both improved performance and resulted in higher deep body temperatures during exercise at 30 °C, ~ 55% RH [44, 48], whereas psychological skills training enabled participants to run further during 90 min of exercise at 30 °C, 40% RH [9]; in each case there was no change in RPE or perceived thermal stress. Similarly, in temperate conditions, the presence of a competitor reduces internal attentional focus and limits attentional resources directed to afferent sensory feedback, thereby enabling a higher power for a given RPE [17]. Thus, head-to-head competition may have reduced internal attentional focus, limiting attentional resources for afferent sensory feedback and enabling a higher power, cardiovascular and thermal strain for a given perceptual state.

A further important finding was that the heart-rate rise with head-to-head competition was related to the change in T _re between HotH2H and HotSolo. Similarly, participants’ perceptions of their own resilience influenced their performance in a competition scenario, whereas DRT predicted the increased T _re with competition. Collectively, these findings enable acceptance of H₄ (performance improvements with head-to-head competition will be related to certain trait-like and state psychological characteristics). These findings suggest that it may be possible to identify individuals most influenced by head-to-head competition in a hot environment by simple psychological and psychophysiological measures. That is, those with high self-rated resilience, a high propensity for risk-taking behaviour and/or a pronounced anticipatory heart rate response, may be more susceptible to the effects of head-to-head competition.

The present study was not without limitation. For ethical and safety reasons, we could not induce exertional heat illness per se and whilst high deep-body temperatures are associated with exertional heat illness, the aetiology is complex and many individuals achieve high deep-body temperatures without developing heat illness [1]. Moreover, the average terminal T _re achieved in HotH2H was below that associated with heat stroke [13], although many of our participants (44%) achieved a T _re ≥ 39 °C and the lag inherent within this measurement site makes it likely that the ‘true’ terminal temperature was somewhat higher, whereas inter-individual variation exists in the deep-body temperatures of individuals with exertional heat illnesses [13]. Finally, given that an ergogenic effect of competition has previously been demonstrated under cooler conditions [15], an ergogenic effect of competition in the heat was perhaps not unexpected. However, the key point is that an increased ambient temperature elicited a behavioural change (altered pacing) during solo exercise that regulated the thermal strain experienced by the participant, but in the competitive situation this behaviour was altered with the consequence that thermal strain was increased. Although further work is required to clarify the extent to which this increases susceptibility to exertional heat illness, recent research demonstrated that group-paced activities, which are similar to the head-to-head competitive situation, account for 70% of exertional heat illness cases and 78.5% of subsequent hospitalizations among UK military personnel, the remainder occurring during self-paced exercise [14].

Compared with solo exercise in the heat, head-to-head competition in a hot environment alters behavioural thermoregulation (i.e. pacing), resulting in faster performances. Consequently, metabolic heat production and thermophysiological strain are increased, but this is not reflected in perceptual measures. These novel data are consistent with the hypothesis that competition is a risk factor for heat illness. We suggest that this effect may result from neurochemical changes due to a psychophysiological stress response or motivational effects, whereas reduced internal focus might alter the relationship between perceived and actual thermophysiological state. Finally, individuals with a propensity for DRT, or high levels of resilience, may be more sensitive to the effects of competition, indicating that certain trait-like characteristics might help identify those at increased risk of heat illness.