Sc. Gandevia et al., VOLUNTARY ACTIVATION OF HUMAN ELBOW FLEXOR MUSCLES DURING MAXIMAL CONCENTRIC CONTRACTIONS, Journal of physiology, 512(2), 1998, pp. 595-602
1. To measure voluntary activation of human elbow flexor muscles durin
g maximal concentric contractions, the twitch interpolation method was
modified to enable detection of torque increments evoked by single st
imuli during contractions of up to 300 deg s(-1). Subjects flexed the
elbow to rotate a loaded beam 'as fast as possible' (load typically 23
-58 N m) from 70 deg below to 70 deg above the horizontal. Electrical
stimuli were delivered to biceps brachii when the beam passed through
the horizontal. Voluntary activation was estimated from the amplitude
of the interpolated twitch, which was expressed as a percentage of the
twitch produced by relaxed muscles shortening at the same velocity. 2
. In eleven subjects, the level of voluntary activation during repeate
d maximal concentric contractions (median 99.4%) did not differ signif
icantly from that during maximal isometric contractions (98.0%). Volun
tary activation during maximal contractions did not depend on shorteni
ng velocity and was the same when tested at two angles 30 deg apart. 3
. To induce fatigue, five subjects repeatedly lifted and lowered a hea
vy load at about 30 deg s(-1), and continued for ten to twelve contrac
tions after they needed assistance to continue lifting. All maintained
the capacity to attain maximal levels of activation. 4. It is conclud
ed that voluntary drive to elbow flexor muscles during maximal concent
ric contractions is usually maximal or near-maximal, and that this lev
el of drive can be maintained during development of peripheral fatigue
.