To estimate diaphragm fibre length from thoraco-abdominal configuration we measured axial motion of the right-sided area of apposition (Aap) by ultrasonography and volumes displaced by chest wall compartments, (pulmonary and abdominal rib cage (Vrc,p, Vrc,a) and abdomen (Vab)) by optoelectronic plethysmography in 4 normal men during quiet breathing and incremental exercise without and with expiratory flow limitation. Points at the cephalic Aap border were digitised from echo images, mapped into 3D space and the axial distance from the xyphoidal transverse plane (Dap) measured simultaneously with the volumes. Linear regression analysis between Dap and the measured volume changes under all conditions showed: 1)Dap was linearly related more to Vab than to Vrc,p and Vrc,a; 2) this was highly repeatable between measures. Multiple stepwise regression analysis showed that Vab accounted for 89%- 96% of the variability of Dap, while the rib cage compartments added <1%. We conclude that, under conditions of quiet breathing and exercise with and without EFL, instantaneous Dap can be estimated from Vab.