Self-paced exercise in hot and cool conditions is associated with the maintenance of %V̇o_2peak within a narrow range

Abstract

Périard JD, Racinais S. Self-paced exercise in hot and cool conditions is associated with the maintenance of %V O2peak within a narrow range. J Appl Physiol 118: 1258 -1265. First published March 26, 2015 doi:10.1152/japplphysiol.00084.2015.-This study examined the time course and extent of decrease in peak oxygen uptake (V O2peak) during self-paced exercise in HOT (35°C and 60% relative humidity) and COOL (18°C and 40% relative humidity) laboratory conditions. Ten well-trained cyclists completed four consecutive 16.5-min time trials (15-min self-paced effort with 1.5-min maximal end-spurt to determine V O2peak) interspersed by 5 min of recovery on a cycle ergometer in each condition. Rectal temperature increased significantly more in HOT (39.4 Ϯ 0.7°C) than COOL (38.6 Ϯ 0.3°C; P Ͻ 0.001). Power output was lower throughout HOT compared with COOL (P Ͻ 0.001). The decrease in power output from trial 1 to 4 was ϳ16% greater in HOT (P Ͻ 0.001). Oxygen uptake (V O2) was lower throughout HOT than COOL (P Ͻ 0.05), except at 5 min and during the end-spurt in trial 1. In HOT, V O2peak reached 97, 89, 85, and 85% of predetermined maximal V O2, whereas in COOL 97, 94, 93, and 92% were attained. Relative exercise intensity (%V O2peak) during trials 1 and 2 was lower in HOT (ϳ84%) than COOL (ϳ86%; P Ͻ 0.05), decreasing slightly during trials 3 and 4 (ϳ80 and ϳ85%, respectively; P Ͻ 0.05). However, heart rate was higher throughout HOT (P ϭ 0.002), and ratings of perceived exertion greater during trials 3 and 4 in HOT (P Ͻ 0.05). Consequently, the regulation of self-paced exercise appears to occur in conjunction with the maintenance of %V O2peak within a narrow range (80-85% V O2peak). This range widens under heat stress, however, when exercise becomes protracted and a disassociation develops between relative exercise intensity, heart rate, and ratings of perceived exertion. cardiovascular strain; cycling; fatigue; hyperthermia; pacing; thermoregulation; time trial; V O2max SELF-PACED (I.E., TIME TRIAL) exercise requires the completion of a known distance in the quickest possible time or the production of the greatest amount of work in a set duration (1, 11). When undertaken in the heat, prolonged self-paced exercise performance is significantly impaired relative to cool conditions (10,27,28,42,46). The impairment in performance is mediated by the complex interplay of multiple physiological systems (e.g., nervous, metabolic) (8,34,45), with the development of thermal strain potentially modulating a performance-limiting increase in cardiovascular strain (5,27,28,30). Indeed, a narrowing core-to-skin temperature gradient during exercise-heat stress increases the skin blood flow requirement for heat dissipation (5,35,36). The combination of an increase in cutaneous blood volume and a temperature-mediated rise in intrinsic heart rate decreases cardiac filling, which leads to reductions in stroke volume and maximal cardiac output when heart rate approaches maximum (6,13,15,36). During prolonged constant-load exercise to exhaustion...

Core temperature up to 41.5ºC during the UCI Road Cycling World Championships in the heat

“…The average heart rate during the last 10% of TTs was 185 bpm. These physiological responses are in line with previous field5 and laboratory11 studies and suggest that relative exercise intensity was likely near maximal 9. Thus the average heart rates reported in the current study are similar or slightly above those previously reported during an ITT (174 bpm)12 and TTT (165 bpm)13 in temperate environmental conditions.…”

Cerebrocortical activity during self‐paced exercise in temperate, hot and hypoxic conditions

“…According to this premise, self‐paced exercise is regulated via positive and negative affective responses associated with the tolerance of discomfort, which influences the desire to maintain a given exercise intensity. It has recently been shown that despite reduced external workloads in response to a progressive reduction in VO 2max under heat stress and an acutely decreased VO 2max at altitude, a similar relative exercise intensity (~85%) is maintained during prolonged (~60 min) self‐paced efforts in control, hot and hypoxic conditions. This is also the approximate intensity (i.e.…”

“…The concomitant development of thermal and cardiovascular strain during prolonged (i.e. >30 min) self‐paced exercise under heat stress has been shown to impair performance . Similarly, acute hypoxic exposure compromises the ability to perform time trial exercise .…”

“…Given that relative exercise intensity during prolonged self‐paced efforts (e.g. 45–60 min) is maintained at ~85%VO 2max, an evaluation of cerebrocortical activity in cool, hot and hypoxic conditions when performing a protracted self‐regulated task may offer insight as to a common mechanism of impairment.…”

Prolonged self-paced exercise in the heat – environmental factors affecting performance

“…58,59). The physiological factors and/or altered perceptual clues that mediate changes in pacing strategy in the heat and subsequent adjustments following acclimatization are not well understood, but may involve integration between afferent feedback from thermal receptors, cardiovascular changes or familiarization, 60,61 but independent of the underlying mechanisms, it appears that pacing strategies are intuitively adopted by athletes in individual sports such as cycling and for team sports such as soccer 12,62 …”