To explore 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.
The study protocol involved participants performing six sets of 10 breaths each, with inspiratory loads ranging from 30% to 80% of their maximum inspiratory pressure (MIP), increasing in 10% increments. This sequence was conducted in random order. The measurement process was then repeated, this time with an emphasis on diaphragmatic descent during inspiration. Throughout each targeted breath, surface electromyography was concurrently recorded for the right SCM muscle, and the thickness of the right hemi-diaphragm was measured using ultrasonography. The diaphragm thickening fraction (DTf) defined as (thickness of the diaphragm at end-inspiratory - thickness at end-expiratory) / thickness at end-expiratory) was computed.
Sixty-two adults (age: 23±3 years) participated in the study. Irrespective of breathing instructions, the 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. DTf increased while SCM muscle activity decreased during forced inspiration with diaphragmatic breathing instruction.
In a cohort of young healthy adults, contraction of diaphragm was greatest at 50% MIP but declined with an increased inspiratory load to 80% MIP. Forced inspiration combined with focused diaphragmatic breathing 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. Further studies are necessary to determine whether similar respiratory muscle recruitment patterns exist in patients with respiratory dysfunction.
The findings from our study indicate that an inspiratory load of 50% MIP elicited the highest level of diaphragmatic recruitment in young healthy adults. However, inspiratory loads exceeding 50% MIP resulted in decreased diaphragmatic contraction activity. In contrast, activity in the SCM muscle progressively increased with higher inspiratory pressures. Providing explicit instructions on diaphragmatic breathing enhanced diaphragm muscle activity while simultaneously reducing SCM muscle recruitment. These results offer valuable insights that could influence the design of future IMT protocols and potentially alter clinical physiotherapy practices to optimize respiratory muscle training and management.
diaphragmatic breathing
sternocleidomastoid
