Effect of sacrum–backrest distance on respiratory function during wheelchair sitting: implications for seating assessment

Purpose Wheelchair seating configuration may influence respiratory mechanics by altering pelvic alignment, trunk posture, and diaphragmatic excursion. Deep sitting is generally recommended to maintain pelvic neutrality; however, the quantitative impact of sacrum–backrest distance on ventilatory performance remains unclear. This study aimed to investigate the effect of sacrum–backrest distance on respiratory function and thoracic expansion during wheelchair sitting.Materials and methods Nineteen healthy young females were assessed under three wheelchair sitting conditions defined by sacrum–backrest distance (0, 5, and 10 cm). Respiratory parameters – forced vital capacity (FVC), vital capacity (VC), inspiratory capacity (IC), tidal volume (TV), expiratory reserve volume (ERV), inspiratory reserve volume (IRV), forced expiratory volume in one second (FEV1), the FEV1/FVC ratio, maximal inspiratory pressure (PImax), and maximal expiratory pressure (PEmax) – were measured using spirometry. Thoracic expansion was also evaluated. Linear mixed models with Holm-adjusted post-hoc comparisons were used to analyse the effect of the sitting condition.Results Sitting condition showed significant main effects on FVC, VC, TV, ERV, IRV, FEV1, PImax, and PEmax. Particularly, FVC and FEV1 decreased by approximately 6% in the 10-cm condition compared with the 0-cm condition. No significant differences were observed for IC, the FEV1/FVC ratio, or thoracic expansion.Conclusions Forward displacement from the backrest was associated with reductions in several measures of respiratory performance, particularly forced respiratory parameters and expiratory muscle strength, suggesting that seat depth and sacral positioning may influence respiratory efficiency during wheelchair sitting. Sacrum–backrest positioning may therefore be an additional consideration in wheelchair seating assessments.