The influence of passive ankle joint power on balance recovery

Over one–third of Americans over the age of 65 fall each year, costing more than $19 billion in health care costs in 2000. Many adults 65+ who have not experienced a fall still fear falling, and fear can decrease quality of life and increase the likelihood of falls. Several factors such as muscle strength, power, stiffness and tendon properties change in the human body with age affecting balance, which has been tagged as a fall risk predictor. Additionally, balance recovery strategies also differ between young and older adults, with young adults primarily utilizing their ankle joint and older adults utilizing their hip. The role of passive ankle joint power in balance recovery is unknown. Therefore, we conducted three studies. In Study 1, we investigated the role of passive ankle joint power in balance recovery of young subjects and tested if the contribution of passive power to net ankle joint power changed with perturbation speed. In Study 2, we explored the factor of age in the contribution of passive ankle joint power to net ankle joint power. In Study 3, we searched for a link between the contribution of passive ankle joint power to net ankle joint power and balance recovery strategy. Passive joint torque through the full range of motion was collected for each subject. Each subject performed 5 stepping tasks at two speeds, fast and slow. Joint kinematics and kinetics were collected for each trial. Inverse dynamics were performed and net ankle joint torque and net ankle joint work were computed. Passive ankle joint torque models were optimized for each subject, and passive ankle joint powers were determined. In Study 1, there appeared to be no difference in net or passive joint powers with respect to perturbation speed. In Study 2, age affected net ankle joint powers and passive uniarticular plantar- and dorsiflexor powers. In Study 3, we noted a change in balance recovery strategy between young and older adults. We were unable to predict balance recovery strategy index based off of the percent contribution of passive ankle joint work to net ankle joint work. These studies bring greater clarity to the role of passive ankle joint power with respect to balance recovery