With our work on dynamic balance control during walking in the presence of optical flow perturbations, we often wonder whether the visuomotor control of walking balance adapts to such perturbations over time. Such adaptation may arise from multisensory reweighting, the central process that determines the relative priority placed on somatosensory , visual, and vestibular feedback. Our most recent work investigating the propensity for visuomotor adaptation in human balance control was recently accepted for publication in the journal Human Movement Science.
Thompson JD and Franz JR. Do kinematic metrics of walking balance control adapt to perturbed optical flow? Human Movement Science (In Press).
Abstract. Visual (i.e., optical flow) perturbations can be used to study balance control and balance deficits. However, it remains unclear whether walking balance control adapts to such perturbations over time. Our purpose was to investigate the propensity for visuomotor adaptation in walking balance control using prolonged exposure to optical flow perturbations. Ten subjects (age: 25.4 ± 3.8 years) walked on a treadmill while watching a speed-matched virtual hallway with and without continuous mediolateral optical flow perturbations of three different amplitudes. Each of three perturbation trials consisted of 8 minutes of prolonged exposure followed by 1 min of unperturbed walking. Using 3D motion capture, we analyzed changes in foot placement kinematics and mediolateral sacrum motion. At their onset, perturbations elicited wider and shorter steps, alluding to a more cautious, general anticipatory balance control strategy. As perturbations continued, foot placement tended toward values seen during unperturbed walking while step width variability and mediolateral sacrum motion concurrently increased. Our findings suggest that subjects progressively shifted from a general anticipatory balance control strategy to a reactive, task-specific strategy using step-to-step adjustments. Prolonged exposure to optical flow perturbations may have clinical utility to reinforce reactive, task-specific balance control through training.
A major undertaking with our collaborators in the Department of Industrial and Systems Engineering at NC State, our systematic review of neuroimaging in the study of static and dynamic balance control has been accepted for publication in the journal Frontiers in Human Neuroscience.
Wittenberg E, Thompson J, Nam CS, Franz JR, Neuroimaging of human balance control: A systematic review. Frontiers in Human Neuroscience (In press).
Abstract. This review examined 83 articles using neuroimaging modalities to investigate the neural correlates underlying static and dynamic human balance control, with aims to support future mobile neuroimaging research in the balance control domain. Furthermore, this review analyzed the mobility of the neuroimaging hardware and research paradigms as well as the analytical methodology to identify and remove movement artifact in the acquired brain signal. We found that the majority of static balance control tasks utilized mechanical perturbations to invoke feet-in-place responses (27 out of 38 studies), while cognitive dual-task conditions were commonly used to challenge balance in dynamic balance control tasks (20 out of 32 studies). While frequency analysis and event related potential characteristics supported enhanced brain activation during static balance control, that in dynamic balance control studies was supported by spatial and frequency analysis. Twenty-three of the 50 studies utilizing EEG utilized independent component analysis to remove movement artifacts from the acquired brain signals. Lastly, only eight studies used truly mobile neuroimaging hardware systems. This review provides evidence to support an increase in brain activation in balance control tasks, regardless of mechanical, cognitive, or sensory challenges. Furthermore, the current body of literature demonstrates the use of advanced signal processing methodologies to analyze brain activity during movement. However, the static nature of neuroimaging hardware and conventional balance control paradigms prevent full mobility and limit our knowledge of neural mechanisms underlying balance control.
Our recent work into the neuromuscular origins of step-to-step corrective motor responses underlying balance control during human walking has been accepted for publication in Nature: Scientific Reports.
Stokes HE, Thompson, JD, and Franz JR. The neuromuscular origins of kinematic variability during perturbed walking. Nature: Scientific Reports (In Press)
Abstract: We investigated the neuromuscular contributions to kinematic variability and thus step to step adjustments in posture and foot placement across a range of walking speeds in response to optical flow perturbations of different amplitudes applied in a custom virtual environment. We found that perturbations significantly increased step width, decreased step length, and elicited larger trunk sway compared to normal walking. However, perturbation-induced effects on the corresponding variabilities of these measurements were much more profound. Consistent with our hypotheses, we found that: (1) perturbations increased EMG activity of the gluteus medius and postural control muscles during leg swing, and increase the coactivation of antagonistic leg muscles during limb loading in early stance, and (2) changes in the magnitude of step to step adjustments in postural sway and lateral foot placement positively correlated with those of postural control and gluteus medius muscle activities, respectively, in response to perturbations. However, (3) interactions between walking speed and susceptibility to perturbations, when present, were more complex than anticipated. Our study provides important mechanistic neuromuscular insight into walking balance control and important reference values for the emergence of balance impairment.
Our most recent work leveraging real-time visual biofeedback of propulsive force measurements during walking has been accepted for publication in the Journal of Biomechanics.
Browne MG and Franz JR. The independent effects of speed and propulsive force on joint power generation in walking (In press).
Abstract. Walking speed is modulated using propulsive forces (FP) during push-off and both preferred speed and FP decrease with aging. However, even prior to walking slower, reduced FP may be accompanied by potentially unfavorable changes in joint power generation. For example, compared to young adults, older adults exhibit a redistribution of mechanical power generation from the propulsive plantarflexor muscles to more proximal muscles acting across the knee and hip. Here, we used visual biofeedback based on real-time FP measurements to decouple and investigate the interaction between joint-level coordination, whole-body FP, and walking speed. 12 healthy young subjects walked on a dual-belt instrumented treadmill at a range of speeds (0.9 – 1.3 m/s). We immediately calculated the average FP from each speed. Subjects then walked at 1.3 m/s while completing a series of biofeedback trials with instructions to match their instantaneous FP to their average FP from slower speeds. Walking slower decreased FP and total positive joint work with little effect on relative joint-level contributions. Conversely, subjects walked at a constant speed with reduced FP, not by reducing total positive joint work, but by redistributing the mechanical demands of each step from the plantarflexor muscles during push-off to more proximal leg muscles during single support. Interestingly, these naturally emergent joint- and limb-level biomechanical changes, in the absence of neuromuscular constraints, resemble those due to aging. Our findings provide important reference data to understand the presumably complex interactions between joint power generation, whole-body FP, and walking speed in our aging population.
The Applied Biomechanics Lab is very happy to welcome Jeroen Waanders as a Visiting Scholar from the University of Groningen (The Netherlands). Jeroen, a Ph.D. candidate under the mentorship of Dr. Tibor Hortobagyi, will be with the lab through the end of 2017, and will lead a project investigating the role of eccentric muscle function in governing age-related gait changes.
The Applied Biomechanics Laboratory would like to congratulate Michael Browne for passing his written and oral qualifying examinations in the Joint Department of Biomedical Engineering!
The Applied Biomechanics Lab would like to congratulate Heather Stokes for being awarded the inaugural UNC/NCSU BME Undergraduate Researcher of the Month Award. Heather received the award at the Department seminar in recognition of outstanding scientific contributions to understanding the neuromuscular origins of step-to-step corrective motor responses underlying balance control during human walking.
Through an exciting collaboration with colleagues at the University of Sassary (Italy) and University of Applied Sciences and Arts of Italian Switzerland, the Applied Biomechanics Laboratory is very pleased to assist in conducting a newly-funded, three year study titled “A virtual reality based platform for the concurrent measurement of gaze and gait.”
Our ongoing work combining virtual reality and optical flow perturbations to investigate mechanisms governing walking balance control has led to a recently accepted publication in the journal IEEE Transactions on Neural Systems & Rehabilitation Engineering.
Franz JR, Francis CA, Allen MS, Thelen DG (In Press). Visuomotor entrainment and the frequency-dependent response of walking balance to perturbations.
Abstract. Visuomotor entrainment, or the synchronization of motor responses to visual stimuli, is a naturally emergent phenomenon in human standing. Our purpose was to investigate the prevalence and resolution of visuomotor entrainment in walking and the frequency-dependent response of walking balance to perturbations. We used a virtual reality environment to manipulate optical flow in ten healthy young adults during treadmill walking. A motion capture system recorded trunk, sacrum, and heel marker trajectories during a series of 3-min conditions in which we perturbed a virtual hallway mediolaterally with systematic changes in the driving frequencies of perceived motion. We quantified visuomotor entrainment using spectral analyses and changes in balance control using trunk sway, gait variability, and detrended fluctuation analyses (DFA). ML kinematics were highly sensitive to visual perturbations, and instinctively synchronized (i.e., entrained) to a broad range of driving frequencies of perceived ML motion. However, the influence of visual perturbations on metrics of walking balance was frequency-dependent and governed by their proximity to stride frequency. Specifically, we found that a driving frequency nearest to subjects’ average stride frequency uniquely compromised trunk sway, gait variability, and step-to-step correlations. We conclude that visuomotor entrainment is a robust and naturally emerging phenomenon during human walking, involving coordinated and frequency-dependent adjustments in trunk sway and foot placement to maintain balance at the whole-body level. These findings provide mechanistic insight into how the visuomotor control of walking balance is disrupted by visual perturbations and important reference values for the emergence of balance deficits due to age, injury, or disease.
The UNC/NCSU Applied Biomechanics Lab had a highly successful week presenting at the 40th annual meeting of the American Society of Biomechanics. Congratulations to all our students and trainees!