Effects of Incremental Inspiratory Load on Inspiratory Muscle Activity in Subjects with Chronic Stroke in Comparison to Healthy Controls: A Case–Control Study

Background: Inspiratory training after stroke has been used to improve muscle function and strength; however, its acute effects under different inspiratory loads are not well understood. Aim: To assess inspiratory muscle activation under different incremental inspiratory loads in individuals with hemiparetic stroke and healthy controls and to compare muscle activation between hemibodies and sexes in post-stroke subjects using surface electromyography. Methods: Individuals with stroke and healthy controls were recruited. An incremental inspiratory load test was performed using the Threshold® IMT device, with progressive increases of 15%, 30%, 45%, and 60% of maximal inspiratory pressure (MIP) or up to the maximal device load based. Muscle activation of the scalene, sternocleidomastoid, and lowest intercostal space was recorded bilaterally using surface electromyography. Results: The sample consisted of 32 stroke participants (11 females and 21 males) and 14 healthy controls (six females and eight males). Muscle activity was decreased in stroke subjects in comparison to healthy individuals (p < 0.05). The paretic side showed decreased muscle activity when compared to the healthy side in all muscles during different incremental loads and maximal inspiratory pressure (p < 0.05), mainly in males. Females with stroke showed decreased activity in sternocleidomastoid (15% of MIP) and scalene (15% of MIP and MIP) (p < 0.05) compared to controls. Stroke males showed decreased diaphragm activity during 60% or maximal device load of MIP compared to controls (p < 0.05). Conclusions: Incremental inspiratory loads increased muscle activity in both stroke subjects and controls of both sexes, although in different patterns. Reduced inspiratory muscle activity on the paretic side suggests an influence of stroke on muscular function and should be considered during rehabilitation.