Decay of inspiratory muscle activity and breath timing in man.

Time required by moving average EMG of diaphragm and parasternal intercostal muscle to decay to 25% of its peak (T0.25) decreased of expiratory time (TE). Accordingly, relative decay rate (RDRI) of EMG increased of breathing frequency (f). This confirms conclusions based on decay of inspiratory muscle pressure under discontinuous inspiratory elastic load (DIL). Data were better fitted by power functions: only this combination of T0.25 vs. TE and RDRI vs. f equations provided a relationship fitting corresponding inspiratory time vs. TE data. For each timing, RDRI of EMG was higher under DIL than during unloaded breathing. Under DIL scattering of EMG data increased, coupling between diaphragm and parasternal muscle EMG decreased, and RDRI of EMG was higher than that of pressure, likely because of muscle intrinsic properties. This difference decreased with the increase of f: this could be due to recruitment of fast fibers at high f. Both during loaded and unloaded breathing T0.25 was proportional to time from peak to zero EMG, indicating that decay shape did not change with timing.