Journal of Applied Physiology, Vol 57, Issue 2 371-379, Copyright © 1984 by American Physiological Society
Selective activation of quadriceps muscle fibers according to bicycling rate
G. Citterio and E. Agostoni
Moving average electromyography (MA) of quadriceps muscle bellies has been
recorded during bicycling at different rates (30-70 cycles/min) or forces
(1-3 kg). For power increments (50-100%) achieved by increasing force at
constant rate, MA during pedal downstroke always increased. For similar
power increments achieved by increasing the rate at constant force, MA did
not increase (37% of cases), increased less (37%), or increased similarly
(26%). Investigations by others on the rat suggest that the lack of
increase of MA despite power increment was not compensated by other muscle
activity; hence it indicates a shift from slow to fast fibers, which
provide greater power per unit stimulus. Smaller increase of MA with
increasing rate rather than force at isopower could depend on this shift or
on muscle properties, if operating on ascending limb of power-velocity
curve. This, however, does not seem the case for slow fibers, which should
develop peak power at about 25 cycles/min. Hence, fibers of quadriceps
muscle of humans seem selectively activated according to movement speed, as
previously found in inspiratory muscles of rabbits.
