Modulation of neuromuscular excitability in response to acute noxious heat exposure has no additional effects on central and peripheral fatigability

Abstract

Background: Whole-body hyperthermia (WBH) has an adverse effect on the nervous system and neurophysiological performance. In the present study, we examined whether short-duration whole-body immersion in 45°C water (HWI-45°C), which produces a strong neural and temperature flux without inducing WBH, can increase or impair neurophysiological performance in humans.Methods: Fifteen men (aged 25 ± 6 years) were enrolled in this study and participated in three experiments: 1) a brief (5-min) immersion of the whole body in 37°C water (WI-37°C); 2) a brief (5-min) HWI-45°C; and 3) a control trial in a thermoneutral condition at an ambient temperature of 24°C and 60% relative humidity. Before and after the immersions, neuromuscular function (electromyographic activity, reflexes, electrically and voluntary induced torque production, voluntary muscle activation level) were tested. To provoke central inhibition, the participants performed a sustained 2-min maximal voluntary contraction (MVC).Results: Thermophysiological strain was greater after HWI-45°C than after WI-37°C. Electrophysiological modulations of motor drive transmission and peripheral modulations of muscle contractility properties in response to HWI-45°C seemed to have little effect on central activation of the exercising muscles and no effect on MVC production.Conclusion: Although exposure to acute noxious heat was effective in evoking neuromuscular excitability, the increases in core temperature (∼0.2°C) and muscle temperature (∼0.6°C) did not induce moderate or severe WBH. These changes did not seem to affect central structures; that is, there were no additional increases in central and/or peripheral fatigue during a sustained 2-min MVC.

Age‐related differences in the neuromuscular performance of fatigue‐provoking exercise under severe whole‐body hyperthermia conditions

“…Aging is associated with reduced and more variable synaptic inputs that drive motor neuron activation, 19,20 reduced volume and size of motor cortical cell bodies, 21 fewer and larger motor units, 22 less stable neuromuscular junctions, 23 lower and more variable motor unit action potential discharge rates, 24 and smaller and slower skeletal muscles. 25,26 As these neuromuscular structures are sensitive to both external and internal body temperature, 11,15,[27][28][29][30] it raises the question of whether these neuromuscular modifications evident during aging would make older subjects more vulnerable to effects of passively induced severe whole-body hyperthermia and whether this would lead to a greater neuromuscular (failure) fatigability during fatigue-provoking, sustained MVC than in younger counterparts.…”

Passive heating-induced changes in muscle contractile function are not further augmented by prolonged exposure in young males experiencing moderate thermal stress

“…Caution was taken to similarly position the electrodes during the four experimental trials. An electrical stimulator (Digitimer DS7A, Digitimer, Hertfordshire, United Kingdom) was connected to the electrodes to deliver 0.5-ms square wave pulses at a constant current set at 100 mA and constant voltage limit set at 200 V. This selected current ensures full contraction and activation of the quadriceps muscles ( Eimantas et al, 2022a ).…”

Moderate muscle cooling induced by single and intermittent/prolonged cold-water immersions differently affects muscle contractile function in young males

“…Neuromuscular testing, consisting of electrically evoked torque of the knee extensors, maximal voluntary isometric contraction (MVIC) of the knee extensors, and maximal isokinetic concentric contraction of the knee extensors (KE-isoK) and knee flexors (KF-isoK) is illustrated in Figure 1A . Involuntary contractions and voluntary contractions were previously performed in studies from our research group ( Skurvydas et al, 2011 ; Brazaitis et al, 2014b ; Eimantas et al, 2022 ). More precisely, the participants sat upright in an isokinetic dynamometer (System 4; Biodex Medical Systems, Shirley, NY, United States) calibrated according to the manufacturer’s recommendations, with a correction for gravity performed using the Biodex Advantage program (Version 4.…”

“…Caution was made to keep a similar position of the electrodes during the four experimental trials. An electrical stimulator (Digitimer DS7A, Digitimer, Hertfordshire, United Kingdom) was connected to the electrodes and delivered 0.5-ms square wave pulses at a constant current set at 100 mA and constant voltage limit set at 200 V. This selected current ensures full contraction and activation of the muscle ( Eimantas et al, 2022 ).…”

“…HRT was calculated as the time taken for the torque to decline from the peak value to 50% of the peak value. RTD and RTR were calculated by using Excel software and raw data exported from Biodex system (100Hz sampling rate) and were defined as the peak slope of torque per 10 ms (Δtorque/Δ10 ms) ( Eimantas et al, 2022 ).…”