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1 Department of Physiology,
McGill University, Montreal, Quebec H3G 1Y6;
2 Pavillon Notre-Dame, Centre
Hospitalier de l'Université de Montréal, Montreal, Quebec
H2L 4M1; 3 Guy Bernier Research
Center, Maisoneuve-Rosemont Hospital, University of Montreal, Montreal,
Quebec, Canada H1T 2M4;
4 Institute for Clinical
Neuroscience,
The purpose of this study was to evaluate the influence of
velocity of shortening on the relationship between diaphragm activation and pressure generation in humans. This was achieved by relating the
root mean square (RMS) of the diaphragm electromyogram to the
transdiaphragmatic pressure (Pdi) generated during dynamic contractions
at different inspiratory flow rates. Five healthy subjects inspired
from functional residual capacity to total lung capacity at different
flow rates while reproducing identical Pdi and chest wall configuration
profiles. To change the inspiratory flow rate, subjects performed the
inspirations while breathing across two different inspiratory
resistances (10 and 100 cmH2O · l
1 · s),
at mouth pressure targets of 
10, 20, 40, and
60 cmH2O. The diaphragm
electromyogram was recorded and analyzed with control of signal
contamination and electrode positioning. RMS values obtained for
inspirations with identical Pdi and chest wall configuration profiles
were compared at the same percentage of inspiratory duration. At
inspiratory flows ranging between 0.1 and 1.4 l/s, there was no
difference in the RMS for the inspirations from functional residual
capacity to total lung capacity when Pdi and chest wall configuration
profiles were reproduced (n = 4). At
higher inspiratory flow rates, subjects were not able to reproduce
their chest wall displacements and adopted different recruitment
patterns. In conclusion, there was no evidence for increased demand of
diaphragm activation when healthy subjects breathe with similar chest
wall configuration and Pdi profiles, at increasing flow rates up to 1.4 l/s.
diaphragm; electromyogram; force-velocity relationship
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