Evolution of inspiratory and expiratory muscle pressures during endurance exercise

We investigated the relationship between minute ventilation ((V)over dot E) and net respiratory muscle pressure (Pmus) throughout the breathing cycle [Total Pmus = mean Pmus,I (inspiratory) + mean Pmus,E (expiratory)] in six normal subjects performing constant-work heavy exercise (CWHE, at similar t o 80% maximum) to exhaustion on a cycle ergometer. Pmus was calculated as t he sum of chest wall. pressure (elastic + resistive) and pleural pressure, and all mean Pmus variables were averaged over the total breath duration. P mus,I was also expressed as a fraction of volume-matched, flow-corrected dy namic capacity of the inspiratory muscles (P-cap,P-1). VE increased signifi cantly from 3 min to the end of CWHE and was the result of a significantly linear increase in Total Pmus (Delta = 43 +/- 9% from 3 min to end exercise , P < 0.005) in all subjects (r = 0.81-0.99). Although mean Pmus,I during i nspiratory flow increased significantly (Delta = 35 +/- 10%), postinspirato ry Pmus,I fell (Delta = -54 +/- 10%) and postexpiratory expiratory activity was negligible or absent throughout CWHE. There was a greater increase in mean Pmus,E (Delta = 168 +/- 48%), which served to increase (V)over dot E t hroughout CWHE. In five of six subjects, there were significant linear rela tionships between (V)over dot E and mean Pmus,I (r = 0.50-0.97) and mean Pm us,E (r = 0.82-0.93) during CWHE. The subjects generated a wide range of Pm us,I/P-cap,P-1 values (25-80%), and mean Pmus,I/P-cap,P-1 increased signifi cantly (Delta = 42 +/- 168) and in a linear fashion (I = 0.69-0.99) with VE throughout CWHE. The progressive increase in (V)over dot E during CWHE is due to 1) a linear increase in Total Pmus, 2) a linear increase in inspirat ory muscle load, and 3) a progressive fall in postinspiratory inspiratory a ctivity. We conclude that the relationship between respiratory muscle press ure and (V)over dot E during exercise is linear and not curvilinear.