Does resistive loading decrease diaphragmatic contractility before task failure?
While sustaining a load that leads to task failure, it is unclear whether diaphragmatic fatigue develops progressively or occurs only at task failure. We hypothesized that incremental loading produces a progressive decrease in diaphragmatic contractility ever before task failure. Ten subjects generated 60% of maximal transdiaphragmatic pressure (Pdimax) for 2 min, 4 min, and until task failure. Before loading, 20 min after each period of loading, and approximately 20 h after the last period of loading, Pdimax, nonpotentiated and potentiated Pdi twitch pressure (Pditw), and the pattern of respiratory muscle recruitment during a CO2 challenge were recorded. Sensation of inspiratory effort at the 4th min of the task-failure protocol was greater than at the same time in the preceding 4-min protocol. Surprisingly, potentiated Pditw and Pdimax were reduced after 2 min of loading and decreased further after 4 min of loading and after task failure; nonpotentiated Pditw was reduced after 4 min of loading and after task failure. The gastric pressure contribution to tidal breathing during a CO2 challenge decreased progressively in relation to duration of the preceding loading period, whereas expiratory muscle recruitment progressively increased. A rest period of approximately 20 h after task failure was not sufficient to normalize these alterations in respiratory muscle recruitment or fatigue-induced changes in diaphragmatic contractility. In conclusion, while sustaining a mechanical load, the diaphragm progressively fatigued, ever before task failure, and when challenged the rib cage-to-diaphragmatic contribution to tidal breathing and recruitment of the expiratory muscles increased pari passu with duration of the preceding loading.
Laghi
F
F
Division of Pulmonary and Critical Care Medicine, Edward Hines, Jr., Veterans Administration Hospital, and Loyola University of Chicago Stritch School of Medicine, Hines, Illinois 60141, USA.
Topeli
A
A
Tobin
M J
MJ
D000328:Adult; D000403:Airway Resistance / Q000502:physiology; D002245:Carbon Dioxide / Q000378:metabolism; D003964:Diaphragm / Q000033:anatomy & histology / Q000502:physiology*; D004576:Electromyography; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology*; D015656:Respiratory Mechanics / Q000502:physiology; D013990:Tidal Volume / Q000502:physiology; D014939:Work of Breathing / Q000502:physiology
D002245:Carbon Dioxide