Relation Between Inspiratory Muscle Strength and Recruitment Onset of Neck Inspiratory Muscles

Abstract

The diaphragm contributes approximately 70–90% to whole inspiratory muscle pressure generation. In contrast, the neck inspiratory muscles are not always active during quiet breathing in healthy people but frequently active in patients with respiratory disease who has lower inspiratory muscle strength, probably due to help performing inspiration for a certain lung volume. Taken together, the recruitment onset for the neck inspiratory muscles during an inspiration may depend on the inspiratory muscle strength. PURPOSE: To test whether inspiratory muscle strength is associated with the recruitment onset for the neck inspiratory muscles during an inspiration. METHODS: First, eight healthy young subjects measured maximal inspiratory mouth pressure (MIP) as inspiratory muscle strength, and peak inspiratory flow rate (PIFR). Then, subjects matched their flow rate to 20, 40, 60, 80, and 100% PIFR during volitional inspiration from residual volume (Flow rate-control task) to determine the index of recruitment onset for the neck inspiratory muscles. Flow rate-control task was performed with (W; 23cmH2O) and without (WO) inspiratory load. Mean %PIFR and EMG amplitude (aEMG) of the sternocleidomastoid (SCM) and scalene (SC) were calculated over the duration of every 10% of maximal lung volume (MLV) ranging from 20% to 60% of MLV. The index of recruitment onset for each muscle was determined by the processed %PIFR-aEMG curves at each %MLV. Finally, a linear regression analysis was performed between MIP normalized to body weight (MIP/BW) and index of recruitment onset for each muscle across subjects, for which the Pearson product-moment correlation coefficient (r) was calculated at each %MLV. RESULTS: MIP was ranged from 124 to 205 cmH2O across subjects. When collapsed across %MLV, there were highly negative correlations between MIP/BW and index of recruitment onset for both muscles under W (SCM: r = −0.866; SC: r = −0.877, P < 0.01) and WO (SCM: r = −0.789; SC: r = −0.735, P < 0.05). CONCLUSION: The current results indicate that the neck inspiratory muscles of a subject who has lower inspiratory muscle strength are recruited at lower flow rate with a certain lung volume, suggesting that MIP/BW becomes useful outcome for assessing not only inspiratory muscle strength but also recruitment onset of the neck inspiratory muscles.