Two functional muscle groupings during postural equilibrium tasks in
Jacobs, Ron and Jane M. Macpherson.
R. S. Dow Neurological Sciences Institute.
APStracts 3:0141N, 1996.
SUMMARY AND CONCLUSIONS
1. This study examined the relation between EMG activation and the contact
force and joint torques of the left hindlimb during postural equilibrium tasks
in the standing cat. It is the appropriate application of force by the limbs
against the support surface that allows the animal to control its center of
mass and maintain equilibrium. 2. Cats were trained to stand quietly on a
moveable force platform. During quiet stance, the cat was perturbed by a
platform translation in each of 12 directions evenly spaced in the horizontal
plane. Electromyographic (EMG) activity of mono- and biarticular thigh
muscles, three-dimensional ground reaction force under the paw (contact
force), and kinematics of the hindlimb segments were recorded. Net joint
torques were computed using inverse dynamics. The analysis focused on the
functional organization of the rapid, automatic postural response in relation
to the sagittal plane contact force and joint torques. 3. The muscles of the
thigh were subdivided into two functional groups, based on the relationship of
the evoked response to the various components of the sagittal plane contact
force or joint torques. The first group, consisting of the monoarticular and
some biarticular muscles, was correlated with the vertical force component,
Fz. The second group, consisting of a separate group of biarticular muscles,
was correlated with the difference between knee and hip torque. This torque
difference is a function of both sagittal plane force components, Fz and Fy,
and is related to contact force direction. 4. It is suggested that this
subdivision of muscle activations reflects a neural strategy of parallel
control of the two muscle groups in relation to their influence on Fz and Fy.
Such a control mechanism could be a strategy for simplifying the control of
the multisegmented limb in contact force tasks such as maintaining postural
Received 24 January 1996; accepted in final form 4 June 1996.
APS Manuscript Number J45-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 4 July 96