Role of central respiratory muscle fatigue in endurance exercise in normal subjects

Abstract

The role of central respiratory muscle fatigue in determining endurance time (ET) of steady-state ergometry, ventilation (VE), and breathing pattern during exhaustive submaximal exercise is not known. Six normal subjects exercised on a cycle ergometer to exhaustion at 72-82% of maximal power output on three occasions. During the second test, inspiratory muscle load was reduced (approximately 50% of baseline load) for all but the last 3 min of exercise. ET was determined, and VE, tidal volume (VT), respiratory rate (f), and sense of breathing effort (Borg scale) were assessed at different points during the assisted exercise and compared with the values obtained at the same time in identical tests without assist, carried out before and after the assisted test (different days). Borg scale rating was less and there was a nonsignificant trend for VT and VE to be higher and for f to be lower when the assist was in place than at the same time during the unassisted runs. In the last 3 min of exercise, when the respiratory load was comparable (assist removed) but ventilatory work history was different, there were no significant differences in sense of respiratory effort, VE, VT, or f between the experimental and control tests, and ET was also similar. We conclude that central respiratory muscle fatigue plays no role in determining ET, sense of respiratory effort, or breathing pattern in normal subjects during exhaustive submaximal exercise.

Ventilation and Respiratory Mechanics

“…Using this approach, several studies have shown no benefit of inspiratory muscle unloading on constant-load exercise despite significant reductions (20%-40% control) in respiratory muscle loads during the exercise (172,283,338). In contrast, Harms et al (213) reported a statistically significant 14% increase in exercise capacity with inspiratory muscle unloading and a statistically significant 15% decrease with inspiratory muscle loading compared to control conditions in male cyclists exercising at a highintensity, constant work rate.…”

“…Differences in the O 2 cost of breathing between moderately fit (∼ 10%V o 2 max) and highly fit subjects (∼ 15%V o 2 max) during near maximal exercise suggest that the effect of reducing the WOB would be most noticeable in the highly fit subjects. Second, the exercise intensity used in previous studies was approximately 70% to 80%V o 2 max (172,283,338), whereas the subjects in the Harms et al (213) study began exercising at approximately 90%V o 2 max and reachedV o 2 max at end exercise. Blood flow redistribution between the respiratory muscles and the locomotor muscles occurs with loading and unloading at maximal exercise (210), but not during exercise at 50% or 75%V o 2 max (568).…”

Inspiratory muscles do not limit maximal incremental exercise performance in healthy subjects

“…Furthermore, there was a trend for the "slope" of the RPE/power output relationships to be lower with PAV versus placebo, although statistical significance was only achieved for limb discomfort. Although no other study has reported the effect of mechanical unloading on the perceptual responses to incremental exercise, several studies have shown that perceptual ratings are reduced during constant load exercise (Marciniuk et al 1994;Krishnan et al 1996;Harms et al 2000).…”

Exercise-induced respiratory muscle fatigue: implications for performance

“…1). Not all studies have shown a benefit of mechanical unloading on exercise tolerance (30,60,77), but these studies were conducted at relatively lower exercise intensities (ϳ70 -80% of V O 2max ) in less-fit subjects. A potential limitation of this approach to unloading is that the pressures delivered by the ventilator during maximal exercise can be substantial and hence disruptive to subjects, even after thorough familiarization (41,106).…”