"No matter what the workout intensity,
slow-twitch motor units are recruited first. If the workout intensity is low,
these motor units may be the only ones that are recruited. If the workout
intensity is high, such as when lifting heavy weights or per- forming intervals
on the track, slow- twitch motor units are recruited first, followed by
fast-twitch A and fast- twitch B, if needed.
There is some
evidence to suggest that the size principle could be altered or even reversed
during certain types of movements-specifically those that contain an eccentric
(muscle lengthening) component-such that fast-twitch motor units are recruited
before slow- twitch motor units (Denier van der Gon, et al., 1985; Grimby &
Hannerz, 1977; Nardone, et al., 1989; Smith, et al., 1980; Ter Haar Romeny, et
al., 1982). It is possible that a preferential recruitment of fast-twitch motor
units, if it exists, is influenced by the speed of the eccentric contraction,
and can only occur using moderate to fast speeds (Karp, 1997; Nardone, et al.,
1989)."
In 1992-93 I was running downhill miles @ maximum speed, ( an eccentric contraction ) in Mississauga, and this explains the gains in speed,
(ie., probable recruitment of all fibers.) I had constant knee pain.