PROJECT SUMMARY
Impaired mobility is the leading cause of disability among older adults, impacting independence and quality of
life. The ability to maintain balance in response to unexpected perturbations such as a bump, trip, or uneven
terrain is a critical component of healthy mobility. Older adults, however, have a reduced capacity to do so,
increasing their risk of falling. Despite many training methods aimed at improving the ability to adapt to
perturbations, outcomes are highly variable. This may be due, in part, to improperly targeted training.
The ability to rapidly and appropriately generate a corrective torque to maintain balance in response to a
perturbation is mediated by neural and biomechanical factors. The ankle is critical in the response to whole-body
perturbations as it generates a substantial portion of the corrective torque. Ankle torques opposing a perturbation
arise from the impedance of the ankle, the relationship between imposed displacements and the resultant
torques, and neurally-mediated changes in muscle activation. Increased reaction times and altered
musculotendon properties in older adults have been associated with an impaired ability to respond. However,
the relative importance of these changes to the response to perturbations remains unknown.
This proposal will focus on determining the relative contributions of ankle impedance and reaction time to the
ability to resist perturbations of posture. Impedance is directly related to the mechanical properties of the
structures spanning the joint (i.e. muscle, tendon, ligaments). While age-dependent changes in musculotendon
properties should impact ankle impedance, the magnitude of their effect remains unknown. This is a critical
barrier to understanding their contributions to altered motor performance. Aim 1 will determine if there are age-
dependent changes in ankle impedance and quantify their magnitude. Ankle, muscle, and tendon contributions
to impedance will be quantified using a novel technique employing robotic assessments of ankle mechanics and
ultrasound imaging of muscle and tendon motions. Impedance of these structures will be quantified using system
identification. This simultaneous assessment is vital for determining how the mechanical properties of the muscle
and tendon contribute to the mechanics of the ankle. Aim 2 will determine the extent to which ankle impedance
and reaction time contribute to age-related differences in the response to postural perturbations. Ankle posture
will be unexpectedly perturbed using a robotic device as the subject balances a virtual inverted pendulum
simulating the ankle loads generated during standing. Performance will be quantified by the ability to balance
the pendulum. Different conditions will be used to differentiate the impact of ankle impedance and reaction time
on task performance. Together, the results from this work will provide insight to the mechanism impairing older
adults’ ability to respond to unexpected postural perturbations. This quantitative assessment is vital for the
subject-specific optimization of training protocols aimed at enhancing mobility in the elderly.