Influence of muscle fiber type and pedal frequency on oxygen uptake
kinetics of heavy exercise.
Barstow, Thomas J., Andrew M. Jones, Paul H. Nguyen, Richard Casaburi.
Division of Respiratory and Critical Care Physiology and Medicine,
Department of Medicine, Harbor-UCLA Medical Center, Torrance, CA
90509
APStracts 3:0253A, 1996.
We tested the hypothesis that the amplitude of the additional, slow
component of o2 during heavy exercise is correlated with the
percentage of type II (fast twitch) fibers in the contracting
muscles. Ten subjects performed transitions to a work rate calculated
to require a o2 equal to 50% between the estimated lactate threshold
(LT) and o2max (50%_). Nine of the subjects also consented to a
muscle biopsy of the vastus lateralis. To potentially enhance the
influence of differences in fiber type among subjects, transitions
were made while pedaling at 45, 60, 75 and 90 rpm in different
trials. Baseline o2 was designed to be similar at the different pedal
rates by adjusting baseline work rate, while the absolute increase in
work rate above the baseline was the same. The o2 response following
the onset of exercise was described using a three-exponential model.
The relative magnitude of the slow component at the end of 8 min of
exercise was significantly negatively correlated with % type I fibers
at every pedal rate (r = 0.64 to 0.83, P<0.05 to P<0.01).
Further, the gain of the fast component for o2 (as ml/min/watt) was
positively correlated with the % type I fibers across pedal rates (r
= 0.69 to 0.83). Increasing pedal rate was associated with decreased
relative stress of the exercise (as _lactate, _o2 or relative
perceived exertion), but did not affect the relationships between %
fiber type and o2 parameters. The relative contribution of the slow
component was also significantly negatively correlated with o2max
(r=-0.65), while the gain for the fast component was positively
associated (r = 0.68 to 0.71 across rpm). The amplitude of the slow
component was significantly correlated with net end-exercise lactate
at all four pedal rates (r = 0.64 to 0.84), but lactate was not
correlated with % type I (P>0.05). We conclude that fiber type
distribution significantly affects both the fast and slow components
of o2 during heavy exercise. Further, fiber type and fitness may have
both codependent and independent influences on the metabolic and gas
exchange responses to heavy exercise.
Received 23 January 1996; accepted in final form 21 May 1996.
APS Manuscript Number A76-6.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 June 96