All posts by Jason Franz

Paper Accepted (March 2018)

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Qiao M, Truong K, Franz JR (2018). Does local dynamic stability during unperturbed walking predict the response to balance perturbations? An examination across age and falls history. Gait & Posture. In Press.


Background: Older adults are at an exceptionally high risk of falls, and most falls occur during locomotor activities such as walking. Reduced local dynamic stability in old age is often interpreted to suggest a lessened capacity to respond to more significant balance challenges encountered during walking and future falls risk. However, it remains unclear whether local dynamic stability during normal, unperturbed walking predicts the response to larger external balance disturbances.

Research question: We tested the hypothesis that larger values of local dynamic instability during unperturbed walking would positively correlate with larger changes thereof due to optical flow balance perturbations.

Methods: We used trunk kinematics collected in subjects across a spectrum of walking balance integrity – young adults, older non-fallers, and older fallers – during walking with and without mediolateral optical flow perturbations of four different amplitudes.

Results: We first found evidence that optical flow perturbations of sufficient amplitude appear capable of revealing independent effects of aging and falls history that are not otherwise apparent during normal, unperturbed walking. We also reject our primary hypothesis; a significant negative correlation only in young adults indicated that individuals with more local dynamic instability during normal, unperturbed walking exhibited smallerresponses to optical flow perturbations. In contrast, most prominently in older fallers, the response to optical flow perturbations appeared independent of their baseline level of dynamic instability.

Significance: We propose that predicting the response to balance perturbations in older fallers, at least that measured using local dynamic stability, likely requires measuring that response directly.

Paper Accepted (February 2018)

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Qiao M and Franz JR (2018). Aging effects on leg joint variability during walking with balance perturbations. Gait & Posture. In press.


Background: Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations.

Research Question: We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults.

Methods: We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes.

Results: We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively.

Significance: Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.

NCSRR Research Grant (November 2017)

We are very excited to have our work recently recognized by the National Center for Simulation in Rehabilitation Research. The Applied Biomechanics Laboratory has received an Outstanding Research Grant for our project titled “Incorporating physiological moment arm dynamics into simulations of human movement.”

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Moment arms, the distance from a muscle-tendon line of action to the rotational joint center, are a critical functional component of the human musculoskeletal system, transforming muscle contractile forces into joint moments, enabling mechanical power generation to produce movement. In contrast to longstanding conventions, growing evidence suggests that muscle-tendon moment arms exhibit highly dynamic variations during human movement, reflecting combinatory effects of joint kinematics and muscle loading.

Support from this project will empower our early efforts to: (1) integrate a novel dynamic moment arm prediction framework into computational models of the lower extremity and (2) predict and validate the functional and age-associated consequences on in vivo triceps surae muscle-tendon behavior during walking.

Our sincere thanks to NCSRR and the OpenSim team.



Paper Accepted (November 2017)

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Browne MG and Franz JR, Does dynamic stability govern propulsive force generation in human walking? Royal Society Open Science (In Press).

Abstract. Before succumbing to slower speeds, older adults may walk with a diminished push-off to prioritize stability over mobility. However, direct evidence for tradeoffs between push-off intensity and balance control in human walking, independent of changes in speed, has remained elusive. As a critical first step, we conducted two experiments to investigate: (i) the independent effects of walking speed and propulsive force (FP) generation on dynamic stability in young adults, and (ii) the extent to which young adults prioritize dynamic stability in selecting their preferred combination of walking speed and FP generation. Subjects walked on a force-measuring treadmill across a range of speeds as well as at constant speeds while modulating their FP according to a visual biofeedback paradigm based on real-time force measurements.  In contrast to improvements when walking slower, walking with a diminished push-off worsened dynamic stability by up to 32%. Rather, we find that young adults adopt an FP at their preferred walking speed that maximizes dynamic stability. One implication of these findings is that the onset of a diminished push-off in old age may independently contribute to poorer balance control and precipitate slower walking speeds.

New NIH award! (September 2017)

We are very excited to share that the Applied Biomechanics Laboratory has been awarded a new research grant from the National Institute on Aging to fund our research on walking balance control and risk of falls in the elderly. The project will continue to support our ongoing collaboration with Drs. Erik Wikstrom (UNC EXSS) and Prue Plummer (UNC PT) as we continue toward new and more effective ways to detect and mitigate falls risk.

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Project Synopsis (“The sensorimotor locus of balance control in elderly gait”: Our premise is that optical flow perturbations, particularly when applied during walking, can effectively identify balance deficits due to aging and falls history and subsequently condition successful balance control strategies through training. Our approach tightly integrates visuomotor perturbations via an immersive virtual environment with advanced biomechanical analyses and a series of sensory, motor, and cognitive-motor correlates to gain unprecedented insight into aging and falls history effects on balance control and response to perturbations. This information has the potential to facilitate promising and transformative new approaches for identifying and mitigating the exceptionally high risk of falls in our aging population.”



ABL attends ASB2017 in Boulder, CO (August 2017)

Image result for american society of biomechanicsThe Applied Biomechanics Lab and our collaborators had an excellent time attending and presenting our research at the 41st annual meeting of the American Society of Biomechanics in beautiful Boulder, CO. Congratulations to all our students and trainees!















Franz JR and Zelik KE, Too much work: revisiting ultrasound-based estimates of Achilles tendon energy storage and return.

Clark WH and Franz JR, Do triceps surae muscle dynamics govern non-uniform Achilles tendon displacements?

Thompson JD and Franz JR, Age and falls history effects on antagonist leg muscle coactivation during walking with optical flow perturbations.

Conway KA and Franz JR, The functional utilization of propulsive capacity during human walking.

Browne MG and Franz JR, Does dynamic stability govern propulsive force generation in human walking?

Browne MG and Franz JR, More push from your push-off: joint-level modifications to modulate propulsive forces in old age.

Qiao M, Feld JA, and Franz JR, Aging effects on leg joint variability during walking in the presence of optical flow perturbations.

Nuckols RW, Dick TJM, Franz JR, Sawicki GS, Using elastic ankle exoskeletons to counteract age-related structure-function deficits.

Allen JL, Thompson JD and Franz JR, Age and falls history effects on the modular control of walking with optical flow perturbations.

New Funding from the National MS Society (August 2017)

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The Applied Biomechanics Laboratory, in collaboration with UNC Healthcare and researchers at the University of Illinois at Urbana-Champaign, has been awarded an exciting new Pilot Research Grant from the National Multiple Sclerosis (MS) Society. The project will leverage our virtual reality infrastructure to quantify standing and walking balance control and response to perturbations in people with MS vs. age-matched controls. Our long-term translational goal is to develop novel and more effective markers of balance and mobility impairment for the detection of disease onset and progression.


ABL attends ISB2017 in Brisbane, Australia! (July 2017)

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Members of the Applied Biomechanics Lab traveled around the world to Brisbane, Australia for the 2017 Congress of the International Society of Biomechanics. There were koalas, kangaroos, sharks, and plenty of science and opportunities for new research collaborations. It was incredible well organized and we were so pleased to present the following two research abstracts:

Franz JR and Zelik KE. “Too much work: revisiting ultrasound-based estimates of Achilles tendon energy storage and return.”

Clark WH and Franz JR. “Do triceps surae muscle dynamics govern non-uniform Achilles tendon displacements?”

Mobile Summer Institutes on Scientific Teaching (July 2017)


Dr. Franz joined two colleagues from BME this month in attending an HHMI/NSF sponsored program called “Mobile Summer Institutes on Scientific Teaching”. Together with Drs. Hubbard and Hu, Dr. Franz was also named a Scientific Teaching Fellow of the National Academies. The Institute provided a rigorous week-long program providing techniques and hands-on coaching and feedback on scientific teaching and student-centered learning. More information can be found here:


Paper Accepted (June 2017)

The result of an exciting new collaboration with Vanderbilt University, our most recent paper advances our fundamental understand of the accuracy, precision, drawbacks, and assumptions of our measurement techniques to interpret the functional role of muscle-tendon dynamics during movement.

Zelik KE and Franz JR. It’s positive to be negative: Achilles tendon work loops during human locomotion. PLoS One (In press).

Abstract. Ultrasound imaging is increasingly used with motion and force data to quantify tendon dynamics during human movement. Frequently, tendon dynamics are estimated indirectly from muscle fascicle kinematics (by subtracting muscle from muscle-tendon unit length), but there is mounting evidence that this Indirect approach yields implausible tendon work loops. Since tendons are passive viscoelastic structures, when they undergo a loading-unloading cycle they must exhibit a negative work loop (i.e., perform net negative work). However, prior studies using this Indirect approach report large positive work loops, often estimating that tendons return 2-5 J of elastic energy for every 1 J of energy stored. More direct ultrasound estimates of tendon kinematics have emerged that quantify tendon elongations by tracking either the muscle-tendon junction or localized tendon tissue. However, it is unclear if these yield more plausible estimates of tendon dynamics. Our objective was to compute tendon work loops and hysteresis losses using these two Direct tendon kinematics estimates during human walking. We found that Direct estimates generally resulted in negative work loops, with average tendon hysteresis losses of 2-11% at 1.25 m/s and 33-49% at 0.75 m/s (N=8), alluding to 0.51-0.98 J of tendon energy returned for every 1 J stored. We interpret this finding to suggest that Direct approaches provide more plausible estimates than the Indirect approach, and may be preferable for understanding tendon energy storage and return. However, the Direct approaches did exhibit speed-dependent trends that are not consistent with isolated, in vitro tendon hysteresis losses of about 5-10%. These trends suggest that Direct estimates also contain some level of error, albeit much smaller than Indirect estimates. Overall, this study serves to highlight the complexity and difficulty of estimating tendon dynamics non-invasively, and the care that must be taken to interpret biological function from current ultrasound-based estimates.