Abstract This study examined the effect of sitting posture on regional chest wall shape in three dimensions, chest wall motion (measured with electromagnetic motion analysis system), and relative contributions of the ribcage and abdomen to tidal volume (%RC/ V t ) (measured with inductance plethysmography) in 7 healthy volunteers. In seven seated postures, increased dead space breathing automatically increased V t (to 1.5 V t ) to match volume between conditions and study the effects of posture independent of volume changes. %RC/ V t ( p < 0.05), chest wall shape ( p < 0.05) and motion during breathing differed between postures. Compared to a reference posture, movement at the 9th rib lateral diameter increased in the thoracolumbar extension posture ( p < 0.008). In slumped posture movement at the AP diameters at T1 and axilla increased ( p < 0.00001). Rotation postures decreased movement in the lateral diameter at the axilla ( p < 0.0007). The data show that single plane changes in sitting posture alter three-dimensional ribcage configuration and chest wall kinematics during breathing, while maintaining constant respiratory function.