March 2010

Document Type


Degree Name



Dept. of Science & Engineering


Oregon Health & Science University


The human balance control system stabilizes an inherently unstable body through torque generation around the numerous joints of the body via mechanisms that include intrinsic musculoskeletal properties and neural activation of muscles based upon reflexes and sensory integration. It is largely unknown how these mechanisms interact and contribute to balance control during sway in the frontal plane. This dissertation identifies mechanisms of frontal plane balance control using systems identification techniques including frequency-response functions, impulse-response functions, and mathematical modeling. Chapters two and three identify frontal plane control mechanisms of the upper body (UB) while lateral sway of the lower body (LB) is prevented in healthy control and bilateral vestibular loss subjects. Continuous tilts of the pelvis and visual surround were used to evoke UB sway. Results suggest that the major contributions to UB system damping came through inter-segmental proprioceptive cues, and major contributions to UB system stiffness came through intrinsic mechanical properties and sensory integration of inter-segmental proprioceptive and pelvis-orienting proprioceptive cues. Vestibular cues contribute to spinal stability in controls but visual cues made only minor contributions in both subject groups. Chapters four and five identify frontal plane control mechanisms of both the UB and LB during freestanding sway in healthy control subjects using various frontal plane stance widths. Continuous rotations of a surface and visual surround were used to evoke body sway. Results showed that in narrower stance conditions, the LB and UB control system was nonlinear across stimulus amplitude in both eyes open and eyes closed conditions. This LB nonlinearity was consistent with a sensory reweighting mechanism whereby subjects shifted away from reliance on proprioceptive information and shifted toward reliance on visual/vestibular information to control their LB as stimulus amplitude increased. In contrast, the UB nonlinearity was primarily due to a decrease in stiffness contributions from all sensory systems as stimulus amplitude increased. In wider stances, intrinsic stiffness of the LB increased and active control of the LB became more linear (i.e., sensory and mechanical contributions to LB control remained relatively fixed across stimulus amplitude).




Div. of Biomedical Engineering


School of Medicine



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