Corticospinal and peripheral responses to heat-induced hypo-hydration: potential physiological mechanisms and implications for neuromuscular function

Abstract

Heat-induced hypo-hydration (hyperosmotic hypovolemia) can reduce prolonged skeletal muscle performance; however, the mechanisms are less well understood and the reported effects on all aspects of neuromuscular function and brief maximal contractions are inconsistent. Historically, a 4–6% reduction of body mass has not been considered to impair muscle function in humans, as determined by muscle torque, membrane excitability and peak power production. With the development of magnetic resonance imaging and neurophysiological techniques, such as electromyography, peripheral nerve, and transcranial magnetic stimulation (TMS), the integrity of the brain-to-muscle pathway can be further investigated. The findings of this review demonstrate that heat-induced hypo-hydration impairs neuromuscular function, particularly during repeated and sustained contractions. Additionally, the mechanisms are separate to those of hyperthermia-induced fatigue and are likely a result of modulations to corticospinal inhibition, increased fibre conduction velocity, pain perception and impaired contractile function. This review also sheds light on the view that hypo-hydration has ‘no effect’ on neuromuscular function during brief maximal voluntary contractions. It is hypothesised that irrespective of unchanged force, compensatory reductions in cortical inhibition are likely to occur, in the attempt of achieving adequate force production. Studies using single-pulse TMS have shown that hypo-hydration can reduce maximal isometric and eccentric force, despite a reduction in cortical inhibition, but the cause of this is currently unclear. Future work should investigate the intracortical inhibitory and excitatory pathways within the brain, to elucidate the role of the central nervous system in force output, following heat-induced hypo-hydration.

Observing an expert's action swapped with an observer's face increases corticospinal excitability during combined action observation and motor imagery

“…From this perspective, advanced technology has developed video games in which the character's face resembles the player's, providing a more immersive experience. Several cortical regions are differently activated when observing self‐face and others' faces (Kircher et al, 2000; Uddin et al, 2022). Neuroimaging studies have examined brain activity during the observation of a transformed face created by merging the faces of the observer and others while adjusting their proportions (Kircher et al, 2000; Uddin et al, 2022).…”

“…Several cortical regions are differently activated when observing self‐face and others' faces (Kircher et al, 2000; Uddin et al, 2022). Neuroimaging studies have examined brain activity during the observation of a transformed face created by merging the faces of the observer and others while adjusting their proportions (Kircher et al, 2000; Uddin et al, 2022). As the proportion of the observer's face in the transformed images increases, neural activity in the inferior frontal gyrus, inferior occipital gyrus and inferior parietal lobule increases (Kircher et al, 2000; Uddin et al, 2022).…”

“…Neuroimaging studies have examined brain activity during the observation of a transformed face created by merging the faces of the observer and others while adjusting their proportions (Kircher et al, 2000; Uddin et al, 2022). As the proportion of the observer's face in the transformed images increases, neural activity in the inferior frontal gyrus, inferior occipital gyrus and inferior parietal lobule increases (Kircher et al, 2000; Uddin et al, 2022). The inferior parietal lobule has neural connections with motor‐related brain regions, such as the premotor cortex (Jeannerod, 1994; Pellegrino et al, 1992).…”