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 (F_P) during push-off and both preferred speed and F_P decrease with aging. However, even prior to walking slower, reduced F_P 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 F_P measurements to decouple and investigate the interaction between joint-level coordination, whole-body F_P, 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 F_P from each speed. Subjects then walked at 1.3 m/s while completing a series of biofeedback trials with instructions to match their instantaneous F_P to their average F_P from slower speeds. Walking slower decreased F_P and total positive joint work with little effect on relative joint-level contributions. Conversely, subjects walked at a constant speed with reduced F_P, 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 F_P, 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!

Our work using motion-capture guided ultrasound imaging to measure dynamic variation in the Achilles tendon moment arm in vivo during walking has been accepted for publication in the journal Computer Methods in Biomechanics and Biomedical Engineering. This work was performed in collaboration with our colleagues at the University of Wisconsin-Madison.

Rasske K, Thelen DG, Franz JR. Variation in the Human Achilles Tendon Moment Arm during Walking (In Press).

Abstract. The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.

Our review paper investigating the prevalence of and mechanisms governing age-associated reductions in propulsive power generation during walking has been accepted for publication in Exercise and Sports Sciences Reviews.

Franz JR. The age-associated reduction in propulsive power generation in walking (In Press).

Abstract. Propulsive power generation during push-off in walking decreases with advancing age. A common explanation is an accommodation for sarcopenia and muscle weakness. Yet, muscle strengthening often yields disappointing outcomes for walking performance. We examine the hypothesis that declines in force or power generating capacity of propulsive leg muscles cannot fully explain the age-related reduction in propulsive power generation during walking.

The Applied Biomechanics Laboratory and collaborators from the University of Wisconsin-Madison and the University of Virginia have been awarded a five-year NIH R01 award to study the role of age-related changes in tendon on motor performance. Dr. Franz’ lab will combine dynamic ultrasound imaging, quantitative motion capture, and biofeedback to investigate changes in localized Achilles tendon mechanics across the lifespan, as well as the effects on leg muscle contractile behavior and motor coordination during walking. Collaborators at the University of Wisconsin-Madison will employ dynamic MRI and shear wave elastography to characterize triceps surae muscle-tendon architecture and elasticity, and both sites will contribute their imaging work to the development and validation of multi-scale computational models of 3D muscle and tendon tissue mechanics at the University of Virginia.