Diaphragm and sternocleidomastoid muscle activity with increasing inspiratory pressure loads in people after stroke

This study reports the concomitant contraction pattern of the diaphragm and sternocleidomastoid (SCM) muscles at various inspiratory pressure loads in patients after stroke. Thirty-six participants (stroke duration: 3.6 ± 2.9 months) performed in random order, sets of 10 breaths at inspiratory loads of 30, 40, 50, 60, 70 and 80%, maximum inspiratory pressure (MIP). Bilateral muscle activity of the SCMs and diaphragm thickness were recorded simultaneously using surface electromyography (sEMG) and ultrasonography, respectively. Diaphragmatic thickness was significantly lower on the hemiplegic side compared to the non-affected side. The magnitude of diaphragmatic contraction, reflected by the calculated thickening fraction (DTf) for both hemidiaphragms, increased with inspiratory load and peaked at 50% MIP, but then decreased with any further increase in inspiratory pressure. SCM recruitment continued to increase bilaterally with increasing inspiratory pressure and was highest at 80% MIP, with recruitment activity significantly higher on the hemiplegic side compared to the non-affected side. Our results suggest that inspiratory load demands above 50%MIP are primarily met by increased SCM activity without any increase in diaphragmatic contraction. Adopting training intensities greater than 50%MIP in clinical inspiratory muscle training (IMT) programs needs to be re-considered.