Fast dynamic voluntary contractions enhance corticospinal facilitation of the human diaphragm: A transcranial magnetic stimulation study

In limb muscles, corticospinal excitability is modulated by motor context, with greater facilitation during movement initiation and dynamic contractions than during sustained isometric activation. Whether this applies to the human diaphragm remains uncertain, given the hybrid automatic-voluntary control and continuous activity of respiratory motoneurones. To determine whether corticospinal excitability of the human diaphragm is influenced by the dynamics of voluntary inspiratory contraction at a given level of inspiratory mouth pressure, nine healthy participants (3 women, 6 men; age 23-39 years) performed inspiratory efforts against an occluded mouthpiece. Diaphragm motor evoked potentials (Di-MEPs) elicited by transcranial magnetic stimulation were recorded from validated chest surface sites at end-expiration with the airway occluded, at rest, during sustained static inspiratory efforts at graded fractions of maximal inspiratory mouth pressure (Pi,max), and during dynamic inspiratory efforts matched for pressure (20% Pi,max) but differing in rate of pressure development (slow vs. fast). Static efforts increased Di-MEP amplitude and shortened latency in a pressure-dependent manner. Slow dynamic efforts produced similar facilitation to static efforts. Fast dynamic efforts elicited greater facilitation, with increased amplitude and shortened latency. Corticospinal excitability of the diaphragm is modulated by contraction dynamics, with rapid efforts inducing additional facilitation beyond force alone.