The objective of this thesis was to test the hypothesis that the marked age-related decline in the ability of humans to stand on one leg was caused by age changes in biomechanical parameters. A servo-controlled platform was used to demonstrate that the threshold for detecting inversion/eversion ankle rotations at a 75% confidence level (TH\sb{75}) increased significantly with age from 0.1\sp\circ in healthy young adults (YA: 22 years) to 0.3\sp\circ in healthy elderly (OA: 72 years), and to 1.4\sp\circ in patients with peripheral neuropathy. Significant (50%) reductions in voluntary frontal plane (FP) ankle strengths and rates of developing moment (t\sb{r}) were found with age. Because of low foot-ground interface compliance, no significant changes in active or passive ankle FP rotational stiffness (ranging from 25 to 57 Nm/rad) were found with age. Three joint, FP, multi-link, direct and indirect dynamic biomechanical models were used to study the three movement phases and that constitute the transfer from bi- to unipedal stance. The 30% rate of the OA premature loss of balance following the Phase II transition can be ascribed to a failure to control the contralateral foot ground reaction impulse ("push-off") within 6 and 19% body weight. A linear auto-regressive model with external input (ARX) was used to estimate the closed loop control law used for regulating Phase III unipedal balance. The model was used to investigate tandem force place findings in 38 YA and 32 OA showing a 60% decrease on maximum time of unipedal balance and a 20% increase in the already significant FP ankle moments required for Phase III unipedal with age. Model sensitivity results showed these latter effects can be explained by the measured age-related increase in t\sb{r},\ TH\sb{75}, system dead time and decreased FP strengths, alone, or in concert with one another; they may lead to shortened OA unipedal balance times by advancing the onset of ankle muscle fatigue. Because system oscillations were found to be sensitive to position and velocity feedback gains, the OA difficult in balancing unipedally with eyes closed seems to be due to inadequate readjustment of these gains.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103804/1/9409750.pdfDescription of 9409750.pdf : Restricted to UM users only
