The long-term goal of this research is to understand the neural control of postural equilibrium and orientation. Adequate control of posture is vital for the performance of functional motor tasks, yet a firm understanding of how and where equilibrium is controlled is lacking. How: The maintenance of equilibrium is an example of a contact force control task in which the center of mass is controlled by the precise application of force by the limbs against the support surface. When the support surface under a standing cat is translated in many different directions in the horizontal plane, the cat exerts force against the surface in a characteristic pattern, with each muscle being recruited for a specific set of translation directions. Exciting new results suggest that the control of contact force by the hindlimb is decoupled into two parallel components, i) force amplitude, and ii) force direction. In the response to translation, the thigh muscles subdivide into two functional groups according to the correlation of their evoked activity with these components of the contact force. One group of muscles is correlated with the amplitude of the contact force exerted by the limb against the support. A second and distinct group of muscles is correlated with the difference in joint torque at the knee and hip. This function, knee torque minus hip torque, is directly related to the direction of the contact force vector exerted by the limb These results suggest that the nervous system may simplify the control of the multijointed limb by the use of two, parallel control systems for force amplitude and force direction, the decoupling strategy. Where: It has been suggested that postural control, like locomotion, could be controlled by circuits within the spinal cord. Knowledge of the organization of the postural circuits for equilibrium control is essential for determining the capacity of spinal cord injured patients to regain useful balance control. If the essential structures are relatively intact in a patient, then it is reasonable to develop strategies for the retraining of balance control. If the essential structures are severely damaged, then rehabilitative strategies should be directed toward developing external assistive devices for balance, including use of the upper extremities.
Three specific aims focus on how and where the nervous system controls the contact forces during postural responses.
Specific aim 1 will test whether the decoupled control of contact force amplitude and direction is a generalized neural strategy for many postural tasks and whether the composition of the two muscle groups is fixed or flexible.
Specific aim 2 will determine the role of sensory receptors in the foot in the control of the contact force. We hypothesize that pedal afferents are important for the control of contact force direction, and not force amplitude.
Specific aim 3 examines the role of the isolated lumbosacral spinal cord in controlling the contact force to maintain postural orientation and equilibrium.
