Capacity-Demand Relationships in One-Legged Balance: Biomechanical Analyses

Abstract

A common screening test of balance is the timed ability to stand on one leg for as long as possible. A balance time of 5 s or less is an indicator that the patient has an elevated risk for fall-related injuries. But why one has to put one’s foot down early is not always obvious. The first part of this dissertation is a mechanistic theoretical and experimental study of the physical capacities underlying the ability to balance on one leg. The theoretical study points to the importance of maintaining both hip abduction and ankle inversion and eversion muscle strengths in order to maintain a large enough ‘Quasistatic Feasible Balance Region’; the latter is shown to depend largely upon frontal plane ankle muscle strength. When the states get too close to the boundaries of that region, additional recovery strategies are required, including a hip strategy, corresponding to that used in maintaining bipedal balance, and also a different behavior that is termed a ‘heel-toe shuffle’ in this dissertation. The dynamics of one-legged balance in the frontal plane were modeled by the equations of motion for a double inverted pendulum. Using published anthropometric and maximum muscle strength data, the model suggests that the hip abduction moment required to stand on one leg is substantial, ranging from 50% of maximum voluntary muscle strength in healthy young men to 82% of the maximum in healthy older women. These results were corroborated by our experimental results from tests of healthy young and older adults. Our analyses suggest that this hip abduction moment demand is not resisted by the abductor muscles alone, but also by the iliotibial mechanism, even for a level pelvis. The second part of this dissertation concerns the heel-toe shuffle which was occasionally used by 14 of 38 subjects to extend the time they stood on one leg in eyes open or closed trials. We found that all 20 young subjects and the majority of older subjects (13 out of 18) succeeded in performing a heel-toe shuffle locomotion task. The inverted pendulum model suggests that amongst those subjects a single lateral shuffle step was, on average, 2 to 6 times more effective in changing the angular velocity of the body in the frontal plane than exerting maximum ankle inversion moment over the same time interval. So heel-toe shuffle strategy is also at least as effective as the ‘Ankle Strategy’ and the ‘Hip Strategy’ in recovering one-legged balance. Lateral falls onto the greater trochanter are a known cause of hip fracture. It is clear from this dissertation that if one is to avoid a lateral fall while standing on one leg, one needs to be able to use the ankle invertor muscles to rapidly move the center of pressure as far laterally as possible. In addition, one needs to raise the pelvis to medially accelerate the center of mass, and initiate one or more lateral shuffle steps. The clinical implications of this dissertation are that one-legged balance could be tested in two ways: first, the prescribed method in which no arm or foot movements are permitted versus a second, freestyle, method in which any recovery motions are permissible. A good freestyle performance provides confidence in the ability of the patient to recover their balance outside the clinic.