SENSORIMOTOR CONTROL OF BALANCE WHILE WALKING IN YOUNG AND OLDER ADULTS
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Abstract
Falls are a major health care system concern affecting over 25% of older adults over the age of 65, increasing morbidity and mortality and decreasing quality of life while accumulating medical costs of around $50 billion a year. The ability to maintain upright balance control is a sensorimotor process which includes taking in sensory information from the environment and the body to orient in space and make appropriate motor actions to keep the body upright. Sensory information relevant to balance includes information on head orientation from the vestibular system, muscle segment locations from the proprioceptive system, and head position relative to the environment via the visual system. These streams of sensory information are integrated by the central nervous system to provide an optimized estimate of body orientation relative to the environment. The nervous systems optimizes the sensory channels accordingly based on their salience, a process referred to as sensory reweighting.
Sensory reweighting has been investigated extensively in standing, however, falls mostly occur during walking. The processing of the visual, vestibular, and proprioceptive systems all change from standing to walking. Vision is expanding to include navigation and there is additional input of optic flow. The head moves to a larger degree adding noise from the vestibular system. The lower and upper limbs are now in motion and involve phases of single leg and double leg stance, changing contact information with the ground. Furthermore, motor strategies change from standing to walking. In standing, the base of support is stationary and much of the movement is restricted to the sagittal plane including joint movement at the ankle, knees, and hips. In walking, motor strategies for balance control increase as the base of support is always moving spatially through the gait cycle, and active control is mainly required in the medial/lateral direction.
Aim 1 of this dissertation work explores sensory reweighting while walking. We applied noisy visual stimuli to participants using virtual reality at various stimulus amplitudes while walking on a self-paced treadmill. We measured their sensitivity to the visual stimulus for medial/lateral balance control using a linear systems theory approach. We found that healthy individuals decreased their sensitivity to a visual stimulus for balance control as the stimulus increased in amplitude, supporting sensory reweighting as a mechanism for upright balance control while walking.
Aim 2 focuses on the aging population who are at an increased risk of falls. The vestibular, proprioceptive, and visual systems all degrade with age. Hair cell counts decline in all 5 sensory organs of the vestibular system, leading to increases in vestibular motion detection thresholds after age 40 for yaw and roll rotations as well as horizontal and vertical translations. The sensitivity, acuity, and integration of the proprioceptive signal of the lower limbs declines with age, with acuity of the ankle complex increasing throughout adolescence then declining around age 50. Anatomical changes of the eyes occur in which the lens becomes thicker and loses elasticity, making it more difficult to focus on near and far-sighted objects. Moreover, visual motion detection thresholds increase for position and speed discrimination with aging. Older adults are known to place more emphasis on vision for balance control while standing and walking. In standing, visual re-weighting has been shown to be slower in older adults which can hinder the ability to recover from sudden environmental disturbances. In Aim 2, we compared measures of visual sensitivity for balance control from Aim 1 to older adults. We found no significant difference between changes in visual sensitivity as the visual stimulus increased in amplitude between the two populations, indicating that older adults have an intact re-weighting mechanism for upright balance control while walking. We also found supporting evidence that older adults have higher sensitivities to vision for balance control compared to young adults for all conditions of visual stimulus amplitudes.
Aim 3 focuses on measuring visual function in relation to visual sensitivity for medial/lateral balance control. Evidence has been reported that a sensory threshold, specifically measured for the vestibular system, correlates to the ability to perform a balance task successfully. This logic is applied to the visual system in Aim 3. There is currently no measure of eye function that has been related to visual influences of balance. The literature on eye function and fall risk has focused on measures of visual acuity, which is important for navigating the environment and locating objects that may cause a trip or slip, but it is well established that motion in the environment causes autonomic responses to the body by giving the illusion of self-movement or, “vection.” Our goal for Aim 3 was to compare individuals’ visual motion detection thresholds to their measures of visual sensitivity to balance control while walking. We found a moderate relationship between the two measures according to a Spearman rank correlation coefficient, indicating that a lower motion detection threshold was related to a lower sensitivity to vision for medial/lateral balance control, and vice versa.