Impaired exercise performance in the heat is associated with an anticipatory reduction in skeletal muscle recruitment

Abstract

Exercise in the heat causes "central fatigue", associated with reduced skeletal muscle recruitment during sustained isometric contractions. A similar mechanism may cause fatigue during prolonged dynamic exercise in the heat. The aim of this study was to determine whether centrally regulated skeletal muscle recruitment was altered during dynamic exercise in hot (35 degrees C) compared with cool (15 degrees C) environments. Ten male subjects performed two self-paced, 20-km cycling time-trials, one at 35 degrees C (HOT condition) and one at 15 degrees C (COOL condition). Rectal temperature rose significantly in both conditions, reaching maximum values at 20 km of 39.2+/-0.2 degrees C in HOT and 38.8+/-0.1 degrees C in COOL (P<0.005 HOT vs. COOL). Core temperatures at all other distances were not different between conditions. Power output and integrated electromyographic activity (iEMG) of the quadriceps muscle began to decrease early in the HOT trial, when core temperatures, heart rates and ratings of perceived exertion (RPE) were similar in both conditions. iEMG was significantly lower in HOT than in COOL at 10 and 20 km, while power output was significantly reduced in the period from 80% to 100% of the trial duration in the HOT compared with COOL condition. Thus, reduced power output and iEMG activity during self-paced exercise in the heat occurs before there is any abnormal increase in rectal temperature, heart rate or perception of effort. This adaptation appears to form part of an anticipatory response which adjusts muscle recruitment and power output to reduce heat production, thereby ensuring that thermal homeostasis is maintained during exercise in the heat.

Self-paced exercise in hot and cool conditions is associated with the maintenance of %V̇o2peak within a narrow range

“…In recent years, considerable focus has been placed on developing models to describe the regulation of pacing and performance during time trial efforts (1,8,33,34,41,45,46). These models have proposed that performance is modulated to various extents by 1) the conscious awareness of physiological disturbances (e.g., afferent feedback from cardiorespiratory responses), which evoke behavioral adjustments in work rate, and by 2) the subconscious anticipation of potentially harmful challenges to homeostasis (e.g., heat storage), which downregulate muscle recruitment in a feedforward manner to avoid catastrophic system failure.…”

“…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).…”

The Effects of Direct Current Stimulation on Exercise Performance, Pacing and Perception in Temperate and Hot Environments

“…To achieve this, athletes regulate their energetic resources to maintain a maximal sustainable intensity to avoid premature fatigue and exhaustion [17]; this is termed pacing. The role of the brain in pacing is not entirely clear although RPE, which has been shown to be modulated by tDCS, is involved as a key perceptual anchor for the regulation and distribution of effort [18,19]. Manipulating perceived exertion might therefore provide a potential mechanism for influencing exercise pacing and performance and this may also extend to other environments.…”

“…Hot conditions are known to accelerate the rate of rise in RPE during fixed intensity (FI) exercise when contrasted to cool conditions [20]. Theoretically raised deep body temperature (hyperthermia) causes an increased demand of the central nervous system to recruit motor units to generate muscular force [19]. When power output is fixed at a high intensity, such as during a TTE, an increased level of effort is required compared to that of cool conditions [20].…”

Point:Counterpoint: Maximal oxygen uptake is/is not limited by a central nervous system governor

“…But more salient is the evidence showing that exercise is regulated "in anticipation" by an intelligent, complex system (14,15,20,(27)(28)(29) and not by a single component such as the cardiovascular system.…”