Control of Voluntary Trunk Movements in Man. Mechanisms for Postural Equilibrium During Standing

The relatively large mass of the upper body and its elevated position in relation to the area of support during standing accentuate the importance of an accurate control of trunk movements for the maintenance of equilibrium. This fact has often been emphasized but never studied in detail. In this thesis the kinematics and motor patterns of simple voluntary trunk movements are investigated during standing. The analysis integrates neurophysiology and biomechanics using electromyographic (EMG) and optoelectronic techniques. Both the voluntary (primary movement) and the involuntary (associated postural adjustment) components of the movement are considered. The results demonstrate how the central nervous system (CNS) in its control of equilibrium utilizes biomechanical principles such as mechanical leverage of the different muscles and the interaction of active (muscle force) and passive forces (e.g. gravity and forces in stretched ligaments and/or muscles). Both primary and associated movements were found to be controlled by task specific and flexible muscle synergies which adapt to the mechanical demands of the situation. These task specific synergies were related to the amplitude, velocity and direction of the movement. Slow movements were often initiated through the action of gravity after a decrease or cessation of activity in postural muscles. Fast movements, however, were always initiated by a marked burst of activity in the agonist muscles. Significant relationships were observed between kinematical parameters (amplitude, duration and velocity) of fast trunk movements and temporal aspects of the EMG pattern. Multiple regression analysis indicated that the time to onset of muscle activity braking the ongoing trunk movement contained more information regarding the amplitude of the movement than did the duration of the initiating burst of activity in the prime mover. This supports the view that the initiating agonist burst is preprogrammed, whereas the braking antagonist burst may be influenced by peripheral feedback such as from muscle stretch receptors. In the early phase of a fast trunk flexion an unexpected flexion of the knees was observed. It is suggested that this knee flexion is a fast postural adjustment passively initiated as a mechanical consequence of the activation of muscles controlling the primary movement. This mechanism, which for anatomical reasons cannot act during an extension of the trunk, simplifies the feed-forward control of equilibrium during voluntary trunk flexion movements. For fast trunk extension movements a preactivation of ankle muscles occurred which resulted in a delay in the onset of the prime mover muscles when measured during a simple reaction time task.(ABSTRACT TRUNCATED AT 400 WORDS)