Shortening velocity, contractile and relaxation properties and EMG changes of inspiratory ribcage muscles after fatigue in health subjects

Abstract

Aim: To investigate whether shortening velocity index of inspiratory ribcage (Velrcm) and diaphragm (Veldi) muscles, inspiratory muscle activity [sternocleidomastoid (SCM), scalene (SCL) and parasternal intercostal (PARA)], nasal inspiratory peak pressure and contractile and relaxation properties of inspiratory muscles during non-invasive sniff nasal inspiratory pressure (SNIP) maneuvers change after an inspiratory task failure (TF) protocol.

Methods: Twenty-six healthy subjects were assessed before and immediately after TF (10 consecutive SNIPs). Optoelectronic plethysmography was used to assess Velrcm and Veldi as volume variation of ribcage and abdomen divided by inspiratory time, respectively, and surface electromyography to assess inspiratory muscle activity. A threshold valve set at 70% of the individual’s maximal inspiratory pressure was used to evoke fatigue.

Results: TF induced fatigue of SCM, ESC and PARA (mean median frequency slopes of -0.763, -0.515 and -0.394, respectively; all p<.001). When compared with pre-fatigue values, SCM, ESC and PARA activities and peak pressure were lower during all 10 maneuvers post TF (p<.001). Maximal relaxation rate and maximal rate of pressure development were lower (p<.001) from the 1st until the 9th maneuver, while half-relaxation time and time constant of pressure decay until the 4th maneuver (p<.001). Velrcm was lower until the 9th maneuver (p<.01) with no changes in Veldi.

Conclusion: TF induces inspiratory ribcage muscle fatigue expressed as reduction of Velrcm, peak pressure and contractile and relaxation properties of inspiratory muscles in healthy even with low inspiratory muscle activation.