Purpose <p>Inspiratory muscle training (IMT) is typically conducted with the patient seated on a stable surface. Considering the dual role of diaphragm in respiration and balance control, this study aims to investigate how different seating surfaces affect diaphragmatic and trunk muscle activity during loaded-inspiratory efforts in people after stroke.</p> Methods <p>Forty-one patients randomly performed 10 breaths with inspiratory resistance set at 50% of maximal inspiratory pressure in 2 sitting conditions on a chair (with and without a balance pad). Bilateral activity of the diaphragm (recorded by ultrasonography), sternocleidomastoid (SCM), abdominal muscles and erector spinae (ES), monitored by surface electromyography, were simultaneously recorded and analyzed using repeated-measures ANOVA.</p> Results <p>Compared with sitting on a stable surface, loaded-inspiratory efforts on an unstable surface significantly increased diaphragmatic contraction, reflected by diaphragm thickening fraction, by 20% and 16% on the non-affected and hemiplegic side, respectively (<i>p</i> &lt; 0.001). Meanwhile, SCM activity significantly decreased by nearly 10% bilaterally, ES activity increased by 7% on both sides, while abdominal muscle activity showed no significant change.</p> Conclusion <p>Incorporating postural challenges into inspiratory training appeared to enhance diaphragmatic and ES contraction and reduce SCM activation. The long-term effects of this approach on inspiratory function warrant further investigation.</p> Trial registration <p>Registration number: NCT06877338; 11 March 2025.</p>