Assessing The Influence Of Work Of Breathing On Sex-differences In Inspiratory Muscle Activation During Exercise

BACKGROUND: During exercise, the activation of ‘extra-diaphragmatic’ inspiratory muscles (e.g., the scalenes and sternocleidomastoids) is higher in females than in males for a given absolute or relative minute ventilation (V̇E). The effect of sex on inspiratory muscle activation could be due to sex-differences in the work of breathing (WB), sex-differences in the shape of the ribcage, or both. Although altering ribcage geometry is not feasible, the WB can be manipulated acutely during exercise. PURPOSE: To determine the effect of WB and sex on inspiratory muscle activation during exercise in healthy males and females. METHODS: n = 11 healthy adults (n = 6 females, n = 5 males) completed two days of testing, each involving pulmonary function testing and an incremental exercise test to exhaustion while breathing either atmospheric air or normoxic helium-oxygen gas (i.e., 21% oxygen, 79% helium) to reduce the WB. At rest and throughout exercise, the WB was quantified using esophageal manometry and open-circuit spirometry, activation of the diaphragm (EMGDI) was measured using a multipair esophageal electrode catheter, and activation of the scalene (EMGSCA) and sternocleidomastoid (EMGSCM) muscle were measured using bipolar skin surface electrodes. RESULTS: During exercise while breathing atmospheric air, females had a higher WB for a given V̇E ≥ 60 l·min-1 than males (p < 0.01); however, breathing helium-oxygen gas during exercise reduced the WB in females to a similar level than their male counterparts breathing atmospheric air (p = 0.925). Despite having a similar WB at a V̇E of 70 ± 5 l·min-1 during exercise, females breathing helium-oxygen gas had a higher EMGSCA and EMGSCM (both p < 0.05), but a similar EMGDI (p = 0.910), than males breathing atmospheric air. CONCLUSION: The persistence of sex-differences in ‘extra-diaphragmatic’ inspiratory muscle activation during exercise after accounting for male-female differences in the WB suggests that morphological factors, such as ribcage geometry, may partially contribute to sex-differences in scalene and sternocleidomastoid activation during exercise. Natural Sciences and Engineering Research Council of Canada (RGPIN-2020-06564) and Research Manitoba (5409) operating grants