Walking modality affects respiratory muscle action and contribution to respiratory effort

We hypothesized that walking at increased speed or increasing gradient might have different effects on chest wall kinematics and respiratory muscle power components, and contribute differently to respiratory effort sensation. We measured the volumes of chest wall compartments by optoelectronic plethysmography, esophageal, gastric and transdiaphragmatic (P-di) pressures, and the sensation of the respiratory effort by a Borg scale in five normal subjects walking both at ascending gradient with constant speed (AG) and at ascending speed with constant gradient (AS). Chest wall kinematics, evaluated by displacement of chest wall compartments, did not show any significant difference between AS and AG. Muscle power, calculated as the product of mean flow and mean pressure, increased similarly, but its partitioning into pressure and velocity of shortening differed in the two modes. A greater increase in the pressure developed by the abdominal muscles (P-abm) (4.06-fold), and in the velocity of shortening of both rib cage inspiratory muscles (v(rcm,i)) (2.01-fold) and the diaphragm (v(di)) (1.90-fold) was associated with a lower increase in the pressure developed by the rib cage inspiratory muscles (P-rcm,P-i) (1.24-fold) and P-di (0.99-fold) with AG. Instead, with AS, a lower increase in P-abm (2.12-fold), v(rcm,i) (1.66-fold) and v(di) (1.54-fold) was associated with a greater increase in P-rcm,P-i (1.56-fold) and P-di (1.97-fold). A combination of P-abm and v(di) during AG (Wald chi(2)=23.19, P<0.0000), with the addition of P-rcm,P-i during AS (Wald chi(2)=29.46, P<0.0000), was the best predictor of Borg score. In conclusion, the general strategy adopted by respiratory centers during different walking modes does not differ in terms of ventilation, chest wall kinematics, and respiratory muscle power production, whereas it does in terms of partitioning of power into pressure and velocity of shortening, and respiratory muscle contribution to respiratory effort sensation. Combinations of different patterns of flow and pressure generation made the respiratory effort sensation similar during AS and AG modes.