From previous inverse dynamic analyses of human leg extensions, it was
hypothesized that the underlying processes for the activation of mono
- and biarticular muscles are different; the mono-articular muscles be
ing activated when they shortened, whereas the biarticular muscles app
eared responsible for the control of the external force direction. In
the present study, experiments were performed on a dynamometer which w
as especially developed to test this hypothesis. Subjects had to exert
different prescribed force vectors on a moving force-plate during leg
extension, which they had intensively practised prior to the actual e
xperiments. Of each trial, position, force and EMG activity were recor
ded. Net joint torques were calculated by the method of inverse dynami
cs and related to the EMG-patterns of the mono- and biarticular upper
leg muscles to reveal whether the previously observed different roles
in contact tasks might constitute a general principle in motor control
. The results showed that although the action of the biarticular m. re
ctus femoris and hamstrings muscles was consistent in controlling the
direction of the external force, the actions of the mono-articular mus
cles did not agree with their hypothesized role as simple work generat
ors. The generalizability of a different control for mono-and biarticu
lar muscles could thus not be confirmed for these tasks. They might ra
ther reflect one out of more available strategies the CNS can use to c
ontrol different contact control tasks.