TY - JOUR
T1 - Ventilatory responses to inspiratory threshold loading and role of muscle fatigue in task failure
AU - Eastwood, P. R.
AU - Hillman, D. R.
AU - Finucane, K. E.
PY - 1994/1/1
Y1 - 1994/1/1
N2 - To examine respiratory muscle recruitment pattern during inspiratory loading and role of fatigue in limiting endurance, we studied seven normal subjects on 17 ± 6 days during breathing against progressive inspiratory threshold load. Threshold pressure (Pth) was progressively increased 14 ± 5 cmH2O every 2 min until voluntary cessation (task failure). Subjects could adopt any breathing pattern. Tidal volume (VT), chest wall motion, end-tidal PCO2, and arterial O2 saturation were measured. At moderate loads [50-75% of maximum Pth (Pth(max)], inspiratory time (TI) decreased and VT/TI and expiratory time increased, increasing time for recovery of muscles between inspirations. At high loads (> 75% Pth(max)), VT/TI decreased, which, with progressive decrease in end-expiratory lung volume (EELV) throughout, increased potential for inspiratory force development. Progressive hypoxia and hypercapnia occurred at higher work loads. Immediately after task failure all subjects could recover at high loads and still reachieve initial Pth(max) on reimposition of progressive loading. Respiratory pressures were measured in subgroup of three subjects: transdiaphragmatic pressure response to 0.1- ms bilateral supramaximal phrenic nerve stimulation at end expiration initially increased with increasing load/decreasing EELV, consistent with increasing mechanical advantage of diaphragm, but decreased at highest loads, suggesting diaphragm fatigue. Full recovery had not occurred at 30 min after task failure. We demonstrated that progressive threshold loading is associated with systematic changes in breathing pattern that act to optimize muscle strength and increase endurance. Task failure occurred when these compensatory mechanisms were maximal. Inspiratory muscles appeared relatively resistant to fatigue, which was late but persistent.
AB - To examine respiratory muscle recruitment pattern during inspiratory loading and role of fatigue in limiting endurance, we studied seven normal subjects on 17 ± 6 days during breathing against progressive inspiratory threshold load. Threshold pressure (Pth) was progressively increased 14 ± 5 cmH2O every 2 min until voluntary cessation (task failure). Subjects could adopt any breathing pattern. Tidal volume (VT), chest wall motion, end-tidal PCO2, and arterial O2 saturation were measured. At moderate loads [50-75% of maximum Pth (Pth(max)], inspiratory time (TI) decreased and VT/TI and expiratory time increased, increasing time for recovery of muscles between inspirations. At high loads (> 75% Pth(max)), VT/TI decreased, which, with progressive decrease in end-expiratory lung volume (EELV) throughout, increased potential for inspiratory force development. Progressive hypoxia and hypercapnia occurred at higher work loads. Immediately after task failure all subjects could recover at high loads and still reachieve initial Pth(max) on reimposition of progressive loading. Respiratory pressures were measured in subgroup of three subjects: transdiaphragmatic pressure response to 0.1- ms bilateral supramaximal phrenic nerve stimulation at end expiration initially increased with increasing load/decreasing EELV, consistent with increasing mechanical advantage of diaphragm, but decreased at highest loads, suggesting diaphragm fatigue. Full recovery had not occurred at 30 min after task failure. We demonstrated that progressive threshold loading is associated with systematic changes in breathing pattern that act to optimize muscle strength and increase endurance. Task failure occurred when these compensatory mechanisms were maximal. Inspiratory muscles appeared relatively resistant to fatigue, which was late but persistent.
KW - breathing pattern
KW - diaphragm
KW - respiratory muscles
UR - http://www.scopus.com/inward/record.url?scp=0028058203&partnerID=8YFLogxK
M3 - Article
C2 - 8175504
AN - SCOPUS:0028058203
SN - 8750-7587
VL - 76
SP - 185
EP - 195
JO - Journal of Applied Physiology
JF - Journal of Applied Physiology
IS - 1
ER -