Hyperthermia and central fatigue during prolonged exercise in humans

Abstract

The present study investigated the effects of hyperthermia on the contributions of central and peripheral factors to the development of neuromuscular fatigue. Fourteen men exercised at 60% maximal oxygen consumption on a cycle ergometer in hot (40 degrees C; hyperthermia) and thermoneutral (18 degrees C; control) environments. In hyperthermia, the core temperature increased throughout the exercise period and reached a peak value of 40.0 +/- 0.1 degrees C (mean +/- SE) at exhaustion after 50 +/- 3 min of exercise. In control, core temperature stabilized at approximately 38.0 +/- 0.1 degrees C, and exercise was maintained for 1 h without exhausting the subjects. Immediately after the cycle trials, subjects performed 2 min of sustained maximal voluntary contraction (MVC) either with the exercised legs (knee extension) or with a "nonexercised" muscle group (handgrip). The degree of voluntary activation during sustained maximal knee extensions was assessed by superimposing electrical stimulation (EL) to nervus femoralis. Voluntary knee extensor force was similar during the first 5 s of contraction in hyperthermia and control. Thereafter, force declined in both trials, but the reduction in maximal voluntary force was more pronounced in the hyperthermic trial, and, from 30 to 120 s, the force was significantly lower in hyperthermia compared with control. Calculation of the voluntary activation percentage (MVC/MVC + EL) revealed that the degree of central activation was significantly lower in hyperthermia (54 +/- 7%) compared with control (82 +/- 6%). In contrast, total force of the knee extensors (MVC + force from EL) was not different in the two trials. Force development during handgrip contraction followed the same pattern of response as was observed for the knee extensors. In conclusion, these data demonstrate that the ability to generate force during a prolonged MVC is attenuated with hyperthermia, and the impaired performance is associated with a reduction in the voluntary activation percentage.

Effect of passive whole body heating on central conduction and cortical excitability in multiple sclerosis patients and healthy controls

“…In controls, there were no heat-related differences in peak force or force decline, which is in contrast to Nybo and Nielsen's work showing that, in healthy individuals, sustained maximal voluntary force declined significantly faster during hyperthermia (19). There are several possible explanations for this discrepancy.…”

“…Recruitment curve characteristics paralleled fatigue perception and decreases in force production in that cortical excitability, indicated by recruitment curve slope, decreased in MS patients during HS. Proposed mechanisms to explain the effect of HS on cortical excitability include alterations in output from thermoregulatory centers in the hypothalamus or other structures, including the motor cortex (19,23,34). In the context of the present study, these explanations are somewhat problematic because the HS condition did not reduce cortical excitability in healthy controls (Fig.…”

“…As mentioned earlier, HS in combination with sustained contractions results in even greater decrements in central activation (4,17,19). It would be useful to examine measures of cortical excitability at a range of temperatures produced both passively and during active conditions to examine the additional effect of peripheral signaling from muscle afferents on central responses.…”

The Effects of Acute Dopamine Reuptake Inhibition on Performance

“…Previous work conducted by Gonzalez-Alonso et al (6) concluded that a high internal body temperature (Tcore) per se causes fatigue in trained subjects during prolonged exercise in uncompensable hot environments. Hyperthermia has been demonstrate to exert a profound effect on the CNS, with a reduction in maximal muscle activity (17), altered EEG brain activity (16) and increased perceived exertion (18) reported when body temperature is elevated. These factors could all contribute to the reduction in performance seen when exercise is undertaken in the heat.…”

The effect of exercise induced hyperthermia on muscle fibre conduction velocity during sustained isometric contraction

“…It has also been suggested that this 5 fatigue acts as a protective mechanism to protect the body from extreme 6 damage which could occur with excessive heat retention. In support of this it 7 has been shown that voluntary muscular activation is reduced during 8 hyperthermic conditions and a greater rate of fatigue is observed during a 9 sustained isometric contraction (Nybo and Nielsen 2001). This increased fatigue 10 was suggested to be caused by a reduction in global neural activation as shown 11 by a decline in EMG amplitude.…”