Recruitment of the diaphragm and sternocleidomastoid muscle during increasing inspiratory pressure loads in healthy young adults

Background: Inspiratory muscle training (IMT) is often employed to enhance improvement in inspiratory muscle strength. However, the relationship between inspiratory muscle recruitment patterns and increasing inspiratory load remains unclear. Furthermore, the effect of breathing instructions on diaphragm activity at various inspiratory loads is unknown. Objective: To investigate the recruitment activity of the diaphragm and sternocleidomastoid (SCM) muscle under increased inspiratory pressure loading, with and without diaphragmatic breathing instructions, in healthy young adults. Methods: Participants were invited to breathe, in random order, 6 sets of 10 breaths at inspiratory loads of 30 % to 80 % maximal inspiratory pressure (MIP), in 10 % increments. The measurement process was repeated with a specific focus of diaphragmatic descent during inspiration. During each targeted breath, recordings of surface electromyography of the right SCM muscle and thickness of the right hemi-diaphragm measured by ultrasonography were undertaken concurrently. Results: Sixty-two adults (31 males, age: 23±3 years) participated in the study. Irrespective of breathing instructions, computed diaphragm thickening fraction (DTf) increased with increasing inspiratory load but peaked at 50 % MIP and declined with increasing load to 80 % MIP. SCM recruitment increased with increasing inspiratory load to 80 % MIP. Forced inspiration with diaphragmatic breathing was associated with a higher DTf and lower SCM muscle activity. The above pattern was similar for both male and female participants. Female participants produced higher SCM activity compared with males but only at inspiration loads below 50 % MIP. Conclusions: In a cohort of young healthy adults, magnitude of diaphragmatic effort was greatest at 50 % MIP but declined with increasing inspiratory load to 80 % MIP. Forced inspiration combined with focused diaphragmatic breathing accentuated diaphragm contraction and reduced SCM muscle contribution. Our findings show that diaphragmatic muscle recruitment is non-linear and that the generic clinical prescription of an IMT program may need to be reconfigured.