Passive-heat stress does not induce muscle fatigue, central activation failure or changes in intracortical properties of wrist flexors

This study evaluated the effect of passive-heat stress on the neuromuscular properties of the wrist flexor muscles, which are commonly used in manual labour hand tasks. A combination of techniques were utilised, involving nerve stimulation and paired-pulse transcranial magnetic stimulation to assess changes in muscle strength, contractile properties, fatigue-resistance and central activation as well as indices of intracortical excitability in 10 healthy humans who were exposed to a passive heat stress protocol as well as a normothermia control protocol. Passive-heat stress increased core body temperature ~1°C (37.2 ± 0.4 to 38.2 ± 0.4°C; p < 0.01), mean skin temperature (34.5 ± 0.7°C to 37.3 ± 1.1°C; p < 0.01), and heart rate (79.5 ± 20.0 to 110.0 ± 23.0 beats/min; p=0.04). No effect was observed on muscle strength, contractile properties, muscle fatigability, central activation or indices of intracortical excitability (p > 0.05). These data indicate that allowing internal temperatures of workers to increase ≤1.0°C does not affect neuromuscular properties of the wrist flexors. Statement of Relevance: Exercise-heat stress has been shown to reduce human performance and exacerbate muscle fatigue. However, less is known about passive-heat stress, especially during milder heat stress encountered in many occupational settings. Accordingly, the effect of occupationally relevant passive-heat stress on the neuromuscular properties of the wrist flexors was examined.