Submitted on June 20, 2006
Accepted on December 21, 2006
The influence of acute inspiratory loading upon diaphragm motor evoked potentials in healthy humans
Emma Zoe Ross1*, Alex Nowicky2, and Alison McConnell1
1 Centre for Sports Medicine & Human Performance, Brunel University, Middlesex, Uxbridge, United Kingdom
2 School of Health Sciences & Social Care, Brunel University, Middlesex, Uxbridge, United Kingdom
* To whom correspondence should be addressed. E-mail: emma.ross{at}brunel.ac.uk.
Acute prior activity of the inspiratory muscles can enhance inspiratory muscle strength and reduce effort perception during subsequent inspiratory efforts. However, the mechanisms subserving these changes are poorly understood. Responses to magnetic stimulation in ten subjects were studied after an acute bout of non-fatiguing inspiratory loading (IML), corresponding to 40% of subjects’ initial maximal inspiratory pressure (MIP), and after an acute bout of non-loaded, forced inspirations (NLF). Motor evoked potentials elicited by cortical stimulation (MEPC), and phrenic nerve stimulation (MEPP) were recorded transcutaneously from the diaphragm before, immediately after, and 15 min after two sets of 30 inspiratory efforts, at rest and during a MIP effort. After IML MIP increased to (±SEM) 113 ±3% of baseline and diaphragm MEPP (during MIP) significantly increased (129 ±10% of baseline). Diaphragmatic MEPC (during MIP), expressed as a percentage MEPPmax, decreased after IML (from 29 ±9% to 20 ±6%, P = 0.017) and after NLF (from 43 ±5% to 31 ±5%, P = 0.032). Observations from the biceps brachi demonstrated that changes after IML and NLF were specific to the inspiratory muscle, since no significant changes were observed in biceps force generation, MEPP or MEPC amplitudes. These data indicate that after IML increased global inspiratory strength is accompanied by increased peripheral excitability and by a dampening of corticospinal excitability of the diaphragm.
