Postural responses to unexpected multidirectional upper body perturbations

Abstract

The goal of this thesis was to document the individual joint kinetics during recovery from multidirectional upper body perturbations, with the objective of increasing the current understanding of the underlying motor mechanisms used in postural control . Ten adult males received pushes to the trunk or pelvis. The joint kinetic profiles indicated that balance control was achieved by an integrated response across all joints. In the medio-lateral direction, the observed joint moments served to move the centre of pressure (COP) in the appropriate direction and to control the lateral collapse of the trunk. The individual joints involved in controlling the COP contributed differing amounts to the total recovery response: the hip and spinal moments provided the majority of the recovery ( ~85% ), while the ankles contributed a small, but significant amount ( ~ 15% ). The differing contributions are based on the anatomical constraints and the functional requirements of the balance task. The postural kinematic responses to upper body sagittal perturbations were variable between participants. [n spite of a fairly wide range of perturbation impulses, the kinematic response was consistent within each participant. The kinetic response consisted of a combination of ankle, knee, hip and spinal control. Active knee flexion provided substantial horizontal COM control. The control at the knee and hip was found to covary. The spinal contribution indicates that a four segment model ( e.g. feet, shank, thigh, and the head, arms and trunk segment: Runge et al., 1995; 1998) is not adequate to describe the postural control. The onset of the joint moment for both frontal and sagittal perturbations was synchronous with the joint angle change, and occurred too early (~90 ms) to be a result of active muscle contraction. Therefore, the first line of defense was provided by muscle stiffness, not reflex-activated muscle activity.

Control of centre of mass (COM) by COP has been validated for multiple tasks, and a new model was developed and validated to document the degree of perturbation and response based on the relationship between the COM and COP. The perturbing and response moments are calculated about the ankle joint and include the perturbing moment due to the applied force and the destabilizing effect of gravity on the COM once it moves outside the vertical. The COP is the main variable used to calculate the recovery moment. These simple moments characterize the temporal evolution of the perturbation and the recovery. The results indicate that the COP and COM are controlled not only reactively, but also predictively. The nervous system demonstrated remarkable flexibility in the control of balance, always initiating an appropriate response. That response was modulated based on the anatomical configuration and the response at other joints.