Am. J. Respir. Crit. Care Med., Vol 152, No. 4, Oct 1995, 1275-1283.
Contraction, relaxation, and economy of force generation in isolated human diaphragm muscle
C Coirault, B Riou, M Bard, I Suard and Y Lecarpentier
INSERM U275-LOA-ENSTA-Ecole Polytechnique, Service de Physiologie, Centre Hospitalier, Le Kremlin-Bicetre, France.
Contraction, relaxation, and energetics of normal human diaphragm strips (n
= 10) were investigated over the whole load continuum in both twitch and
tetanus modes. For a given level of isotonic total force and over a large
part of the contraction phase, instantaneous velocity was shown to be a
unique function of instantaneous length, regardless of time and initial
length. In afterloaded tetanic contractions and over a wide range of loads,
the peak lengthening velocity (VL) linearly decreased when maximum extent
of muscle shortening (delta L) decreased. Stimulation mode modulated the VL
versus delta L relationship, the slope of this linear regression being
greater in tetanus than in twitch. Conversely, over a wide range of load,
an increase in load linearly accelerated the peak rate of force decay,
regardless of the stimulation mode. The energetics of human diaphragm
muscle was evaluated in terms of both mechanical activity and economy of
force generation. Maximum mechanical work (Wmax) differed significantly
according to the stimulation mode, and the relative force at which Wmax
occurred was higher in tetanus than in twitch (p < 0.05). The G
curvature of the P-V hyperbola and maximum mechanical efficiency were
significantly higher in tetanus than in twitch. This strongly suggests that
the economy of force generation is higher in tetanus contractions than in
twitch.
