All posts by Jason Franz

Paper Accepted (June 2017)

Orselli MI, Franz JR, Thelen DG. The effect of Achilles tendon compliance on triceps surae mechanics and energetics in walking. Journal of Biomechanics (In press).

Abstract. Achilles tendon (AT) compliance can affect the generation and transmission of triceps surae muscle forces, and thus has important biomechanical consequences for walking performance. However, the uniarticular soleus (SOL) and the biarticular (GAS) function differently during walking, with in vivo evidence suggesting that their associated fascicles and tendinous structures exhibit unique kinematics during walking. Given the strong association between muscle fiber length, velocity and force production, we conjectured that SOL and GAS mechanics and energetic behavior would respond differently to altered AT compliance. To test this, we characterized GAS and SOL muscle and tendon mechanics and energetics due to systematic changes in tendon compliance using musculoskeletal simulations of walking. Increased tendon compliance enlarged GAS and SOL tendon excursions, shortened fiber operation lengths and affected muscle excitation patterns. For both muscles, an optimal tendon compliance (tendon strains of approximately 5% with maximum isometric force) existed that minimized metabolic energy consumption. However, GAS muscle-tendon mechanics and energetics were significantly more sensitive to changes in tendon compliance than were those for SOL. In addition, GAS was not able to return stored tendon energy during push-off as effectively as SOL, particularly for larger values of tendon compliance. These fundamental differences between GAS and SOL sensitivity to altered tendon compliance seem to arise from the biarticular nature of GAS. These insights are potentially important for understanding the functional consequences of altered Achilles tendon compliance due to aging, injury, or disease.

Congratulations to Jessica Thompson (April 2017)

The Applied Biomechanics Laboratory would like to congratulate Jessica Thompson for successfully defending her Master’s thesis in the Joint Department of Biomedical Engineering. Her thesis was titled “The Kinematic and Electromyographic Response to Optical Flow Balance Perturbations in Walking: Visuomotor Adaptation and the Acute Effects of Age and Falls History”. Excellent work!

UNC Undergraduate Research Symposium

Ashish_Biol_20171.JPGCongratulations to Ashish Khanchandani on successfully presenting his research at the UNC Undegraduate Research Symposium. Ashish, in collaboration with other undergraduate and graduate students, is working on a project titled “The effects of physiological loading on dynamic variations in the Achilles tendon moment arm.” Nice work, Ashish!

ABL attends 2017 Regional ASB

The UNC/NCSU Applied Biomechanics Lab had a terrific showing at the 2017 Human Movement Science and Biomechanics Symposium this week! Congratulations to all our students and trainees!


Abstracts Presented:

Clark WH and Franz JR, Do triceps surae muscle dynamics govern non-uniform Achilles tendon deformations? Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Nuckols RW, Dick TJM, Franz JR, Sawicki GS, Using elastic ankle exoskeletons to counteract age-related structure-function deficits. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Qiao M, Feld JA, and Franz JR. Aging effects on leg joint variability during walking in the presence of optical flow perturbations. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Browne MG and Franz JR, Does dynamic stability govern propulsive force generation in human walking? Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Conway KA and Franz JR, The functional utilization of propulsive capacity during human walking. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Thompson JD and Franz JR, Age and falls history effects on antagonist leg muscle coactivation during walking with optical flow perturbations. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.



Paper Accepted (April 2017)

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.

Paper Accepted (March 2017)

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.


Paper Accepted (March 2017)

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.

Paper Accepted (February 2017)

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.

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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.

Welcome to Jeroen Waanders (Feb 2017)

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.