Relationship of Respiratory Effort Sensation to Expiratory Muscle Fatigue during Expiratory Threshold Loading

Abstract

Abstract We investigated whether fatigue of the expiratory muscle, that is, the abdominal muscle, may acount for a change in the respiratory effort sensation in normal subjects during expiratory threshold loading. The respiratory effort sensation was scored using a modified Borg scale. Expiratory muscle fatigue was assessed both from changes in the maximal static expiratory pressure and in the centroid frequency (fc) of the abdominal muscle electromyogram (EMG). Expiratory threshold loading (magnitude of threshold; 40 to 60% of the maximal expiratory pressure at FRC, breathing frequency = 15/min, and duty cycle = 0.5) was continued until exhaustion or for 30 min. Loading was repeated following a 15-mln recovery period after the end of the first expiratory loading. The maximal static expiratory pressure during loading (Pmmax) decreased initially and then remained decreased. Decreases were smaller with the 40% load (22 ± 6%, SEM) than with the 60% load (37 ± 3%) (p < 0.05). The decrease during the second run of the 60% load was greater than during the first (p < 0.01 by ANOVA). The maximal expiratory pressure at TLC before the second run of the 60% load was decreased by 9 ± 3% compared with the control (p < 0.02) but that with the 40% load was not. The fc with the 60% load decreased Initially by 8 ± 1% and then remained constant, although no change was observed with the 40% load. The sensory score at the 60% load rose with time, and the increasing rate of the sensory score with time in the second run exceeded that of the first (0.87 ± 0.25 versus 0.62 ± 0.16 min−1, p < 0.05). The increasing rate of the sensory score at the 40% load was lower than that seen with the 60% load (0.21 ± 0.67 versus 0.62 ± 0.16 min‒1, p < 0.02). The relationship between the sensory score and Pmmax was virtually linear at all loadings. These four curves traced the same line, although the 60% load shifted to a higher sensation score than the 40% load and the second run shifted in a similar way. We conclude that the increased respiratory effort sensation during expiratory threshold loading is closely related to expiratory muscle fatigue as indicated by the decrease in expiratory muscle force.