Chest wall kinematics and respiratory muscle action in walking healthy humans

We studied chest wall kinematics and respiratory muscle action in five untr ained healthy men walking on a motor-driven treadmill at 2 and 4 miles/h wi th constant grade (0%). The chest wall volume (Vcw), assessed by using the ELITE system, was modeled as the sum of the volumes of the lung-apposed rib cage (Vrc,p), diaphragm-apposed rib cage (Vrc,a), and abdomen (Vab). Esoph ageal and gastric pressures were measured simultaneously. Velocity of short ening (Vdi) and power [(W)over dot di = diaphragm pressure (Pdi) x V-di] of the diaphragm were also calculated. During walking, the progressive increa se in end-inspiratory Vcw (P < 0.05) resulted from an increase in end-inspi ratory Vrc,p and Vrc,a (P < 0.01). The progressive decrease (P < 0.05) in e nd-expiratory Vcw was entirely due to the decrease in end-expiratory Vab (P < 0.01). The increase in Vrc,a was proportionally slightly greater than th e increase in Vrc,p, consistent with minimal rib cage distortion (2.5 +/- 0 .2% at 4 miles/h). The Vcw end-inspiratory increase and end-expiratory decr ease were accounted for by inspiratory rib cage (RCM,i) and abdominal (ABM) muscle action, respectively. The pressure developed by RCM,i and ABM and P di progressively increased (P < 0.05) from rest to the highest workload. Th e increase in Vdi, more than the increase in the change in Pdi, accounted f or the increase in (W)over dot di. In conclusion, we found that, in walking healthy humans, the increase in ventilatory demand was met by the recruitm ent of the inspiratory and expiratory reserve volume. ABM action accounted for the expiratory reserve volume recruitment. We have also shown that the diaphragm acts mainly as a flow generator. The rib cage distortion, althoug h measurable, is minimized by the coordinated action of respiratory muscles .