FROM UNC RESEARCH STORIES

The treadmill hums as bright lights from a wraparound virtual reality screen flicker on the walls, casting moving shadows across Elizabeth Christopher’s face. Small silver sensors dot her joints, measuring her movement. And behind a computer monitor nearby sits Jason Franz, a professor of biomedical engineering at UNC-Chapel Hill.

Franz is the director of the Applied Biomechanics Lab in the Lampe Joint Department of Biomedical Engineering at UNC-Chapel Hill and NC State University. His mission is simple: find solutions to help people age gracefully. In a world where nearly one in five Americans will be over 65 by 2030, that goal is more urgent than ever.

Franz has spent the past two decades investigating how age-related changes in muscle strength, joint flexibility, and balance impact mobility, especially as people enter their later years. Falls among older adults are the leading cause of injury and death, yet they are often preventable with the right interventions.

“I am 61, almost 62 years old,” Christopher, a participant in one of Franz’s studies, shares. “I’m much more aware of the fear of falling. I’m much more aware of trying to challenge myself in different ways to stay active both mentally and physically. It’s part of getting older and not having the flexibility [or] the stability we take for granted until it’s not there anymore.”

In Franz’s state-of-the-art lab, participants walk, move, and even slip — all under carefully controlled scenarios designed to simulate the unpredictable nature of daily life.

“Young people fall all the time,” he says. “The difference between a fall that you might see in a toddler and a fall that we view in our older community is the consequences. The risk of fracture, risk of injury, and the economic costs of a fall can be devastating.”

 

Mapping mechanics

Growing up, Franz spent his summers on the eastern shore of Maryland with his grandfather. Over time, he saw the toll that aging could take on a man that had once been active.

“I watched him go from being the most physically capable person that I knew to becoming frail,” Franz says.  “He had difficulty with walking and standing up out of a chair and eventually started falling. And it struck me that we could probably do better as engineers in coming up with solutions to meet the needs of those in our rapidly aging communities.”

His lab’s research has led to important findings about how aging impacts walking and mobility. Older adults experience changes in how the body moves and how the nervous system controls movement — changes that can make someone more likely to feel tired or to fall.

One key finding is that older adults often exhibit less vigor when pushing off the ground, which leads to shorter, shuffling steps and slower walking speeds. This change is primarily due to muscle redistribution. Older adults rely more on their hip and thigh muscles, which cost us more energy than the ankle muscles used by younger adults.

Franz’s team also found that the Achilles tendon, which plays a critical role in pushing off the ground during walking, loses its stiffness with age. As the tendon loosens, it doesn’t stretch and recoil as effectively. This change means the muscles attached to it, mainly those in the calf, must contract more to produce the same movement, leading to higher muscle activation, increased energy consumption, and more fatigue.

This makes older adults more vulnerable to losing their balance, especially during longer walks or on uneven terrain. The lab’s work is focused on developing interventions to mitigate this.

They use tools like wearable sensors, 3D motion capture cameras, force sensors on a treadmill, and video X-ray to provide real-time biofeedback or inform assistive technologies. That data can help individuals adjust their gait and walking patterns, making them more dynamic and less prone to falls.

 

Lasting balance

Franz meets with study participants, physical therapists, and members of the older population regularly to ensure his work is having the impact he intends and to address their needs directly.

“We want to have genuine impact when we’re doing a study, and the only way we can do that is by speaking the language of the individuals who might make use of the work we’re doing,” he explains.

“I find that I do kind of trip on my foot every now and then just walking outside,” Christopher says. “My mother has experienced a couple of falls, and she’s 90 now. And so I want to avoid that in my future.

Franz also prioritizes mentoring and teaching in his lab. The curiosity and tenacity that postdoctoral scholars, graduate students, and undergraduate researchers bring to his research gives him hope for the future of the field.

“As we advocate for the fields of rehabilitation engineering and biomechanics more broadly, I would love to see a greater understanding of the impact we’re having on the world,” he says.

 

 

We are thrilled to congratulate Aubrey Gray for successfully defending her PhD dissertation, titled “Mechanical leverage of the foot and ankle: aging, balance, and the future of super shoes”. We also thank the wonderful support of her committee members, Jacque Cole (UNC/NC State), Kate Saul (NC State), Eric Ryan (UNC), and Kota Takahashi (Utah). Fantastic job, Aubrey!

Congratulations to 4th-year PhD candidate Emily Eichenlaub for receiving a 2024/2025 Award for Research Achievement from the Lampe Joint Department of Biomedical Engineering. The awards committee noted Emily’s NIH Fellowship “The Proactive and Reactive Neuromechanics of Instability in Aging and Dementia with Lewy Bodies”, industry internship experience in wearable sensing and movement biomechanics, and impressive scholarship and productivity as reasons for her selection.

An interdisciplinary team spanning Biomedical Engineering, Exercise and Sports Science, Biostatistics and the Thurston Arthritis Research Center has received a new 5-year, $3M R01 Grant from the National Institutes of Health titled “Discovering the Mechanisms Linking Gait to Osteoarthritis Onset and Progression.”

The project will be led by Co-Principal Investigators Dr. Jason Franz (Associate Professor in BME) and Dr. Brian Pietrosimone (Professor in Exercise and Sport Science) and will see pivotal contributions from BME Associate Professors Dr. Brian Diekman and Dr. David Lalush as well as from Dr. Todd Schwartz (Professor of Biostatistics) and Dr. Lara Longobardi (Associate Professor of Medicine).

Together, the research team will investigate the underlying mechanistic pathway to explain how aberrant knee joint loading in walking alters the mechanical, biophysical and biological properties of tibiofemoral articular cartilage in individuals at risk for knee osteoarthritis. The researchers noted “Establishing this mechanistic pathway is the single most important milestone toward advancing precision gait retraining as an effective strategy for preventing knee osteoarthritis.”

The Applied Biomechanics had a fantastic time attending the 2024 American Society of Biomechanics in Madison, WI. Many thanks to the meeting and program chairs, the presenters, and attendees for facilitating and participating in great scientific discussions!

Congratulations to the members of the Applied Biomechanics Lab that presented their research.

Presenters:

Plantar Sensation Associates With Gait Instability In Older Adults, Andrew Shelton

Do Shoe Structural Features Matter For Agility And Stability During Walking?, Kayva Katugam-Dechene

The Effects Of Calf Muscle Length On Local Muscle Fatigability, Anh Nguyen

The Effects of Age and Anticipation on Proactive and Reactive Balance Responses to Treadmill Belt Perturbations During Walking, Emily Eichenlaub

The Role Of Tendon Stiffness In Governing Leg Muscle Responsiveness To Unanticipated Slips In Younger And Older Adults, Ross Smith

The Effects Of Anticipation On Distal Leg Muscle Excitations In Response To Surface Translations During Standing, Virginie Ruest

Foot Specific Determinants Of Habitual Walking Speed And Endurance In Young Adults, Ross Smith

Reduced Achilles Tendon Stiffness In Aging Associates With Higher Metabolic Cost Of Walking, Aubrey Gray

The Effects Of Gluteus Medius Fatigability On Gait Instability In Older Adults, Andrew Shelton

From the UNC/NC State Joint BME News announcements: https://bme.unc.edu/2024/01/dr-jason-franz-receives-two-awards-to-accelerate-wearable-sensing-to-optimize-knee-joint-health/

BME Associate Professor Dr. Jason Franz has established a highly productive and collaborative line of research that integrates wearable sensing and machine learning for precision rehabilitation of individuals with knee osteoarthritis. That research, in close partnership with Dr. Brian Pietrosimone from the UNC Department of Exercise and Sports Science, was recently recognized with two awards to accelerate their path from scientific discovery to commercialization and genuine translational impact.

The first, a 2-year $110k translational research grant from the North Carolina Biotechnology Center, will generate patient data to demonstrate proof-of-concept and feasibility of a novel wearable sensing and machine learning prediction technology for detecting, treating and monitoring aberrant forces during walking relevant to the onset and progression of knee osteoarthritis.

The second, a $50k commercialization grant from UNC Kickstart Venture Services, was awarded to VETTA Solutions – the start-up company inspired by these research discoveries and co-founded by Drs. Franz and Pietrosimone.

Commercialization and entrepreneurship are cornerstones of our mission here in BME, and we want to congratulate Dr. Franz and his entire team for their recent success.”

The effect of prolonged walking on leg muscle activity patterns and vulnerability to perturbations. Journal of Electromyography and Kinesiology.

Abstract:
Understanding the consequences and ecological relevance of muscle fatigue is important to guide the development of strategies to preserve independence. However, few studies have examined walking-related fatigue and the effects on walking instability. Our purpose was to investigate the effects of prolonged walking on leg muscle activity and vulnerability to balance perturbations. Eighteen healthy young adults completed a 30-min walking trial at their preferred walking speed while leg muscle activities were recorded. Before and after the 30-min walk, participants responded to five 5% body weight lateral force perturbations. Time-frequency analysis with wavelet transformation and principal component analyses assessed neuromuscular adaptations of muscles to prolonged walking. Following prolonged walking, we observed a time-dependent increase in EMG intensities at slower frequencies for the soleus and tibialis anterior and a decrease in mean amplitudes for the soleus, lateral gastrocnemius, and semitendinosus. Mean mediolateral CoM displacement following perturbations averaged 21% larger after the 30-min walk. Our results suggest that walking for 30 minutes at a comfortable speed elicits complex neuromuscular adaptations indicative of local muscle fatigue and an increased vulnerability to walking balance perturbations. These findings could inform fatigue monitoring systems or walking assistive devices aimed at reducing walking-related fatigue and maintaining independent mobility.

Does the effect of walking balance perturbations generalize across contexts? Human Movement Science.

Abstract: Balance perturbations are used to study locomotor instability. However, these perturbations are designed to provoke a specific context of instability that may or may not generalize to a broader understanding of falls risk. The purpose of this study was to determine if the effect of balance perturbations on instability generalizes across contexts. 29 healthy younger adults and 28 older adults completed four experimental trials, including unperturbed walking and walking while responding to three perturbation contexts: mediolateral optical flow, treadmill-induced slips, and lateral waist-pulls. We quantified the effect of perturbations as an absolute change in margin of stability from unperturbed walking. We found significant changes in mediolateral and anteroposterior margin of stability for all perturbations compared to unperturbed walking in both cohorts (p<0.042). However, in older adults only mediolateral effects of waist-pull perturbations and optical flow perturbations and waist-pull perturbations and treadmill-induced slips correlated significantly (r ≥0.398, p-values≤0.036). In younger adults but not in older adults, we found positive and significant correlations between the anteroposterior effect of waist-pull perturbations and optical flow perturbations, and the anteroposterior and mediolateral effect of treadmill-induced slips (r≥0.428, p-values≤0.021). We found no “goldilocks” perturbation paradigm to endorse that would support universal interpretations about locomotor instability. Building the most accurate patient profiles of instability likely requires a series of perturbation paradigms designed to emulate the variety of environmental contexts in which falls may occur.

Sustained limb-level loading: A ground reaction force phenotype common to individuals at high-risk for and those with knee osteoarthritis. Arthritis & Rheumatology.

OBJECTIVE: To compare the vertical (vGRF), anterior-posterior (apGRF) and mediolateral (mlGRF) ground reaction force (GRF) profiles throughout the stance phase of gait: 1) between individuals 6-12 months post anterior cruciate ligament reconstruction (ACLR) and uninjured matched controls; and 2) between ACLR and individuals with differing radiographic severities of knee osteoarthritis (KOA) defined as Kellgren and Lawrence (KL) grades KL2, KL3, and KL4.METHOD: A total of 196 participants were included in this retrospective cross-sectional analysis. Gait biomechanics were collected from individuals 6-12 months post-ACLR (n=36), uninjured controls matched to the ACLR group (n=36), and individuals with KL2 (n=31), KL3 (n=67), and KL4 OA (n=26). Between-group differences in vGRF, apGRF, and mlGRF were assessed in reference to the ACLR group throughout each % of stance phase using a functional linear model.RESULTS: The ACLR group demonstrated lesser vGRF and apGRF in early and late stance compared to the uninjured controls, with large effects (d range: 1.35-1.66).
Conversely, the ACLR group exhibited greater vGRF (87-90%; 4.88%BW; d=0.75) and apGRF (84-94%; 2.41%BW; d=0.79) than the KL2 group in a small portion of late stance. No differences in mlGRF profiles were observed between the ACLR and either the uninjured controls or the KL2 group. The magnitude of difference in GRF profiles between the ACLR and OA groups increased with OA disease severity.CONCLUSION: Individuals 6-12 months post-ACLR exhibit strikingly similar GRF profiles as individuals with KL2 KOA, suggesting both patient groups may benefit from targeted interventions to address aberrant GRF profiles.

Simulations suggest walking with reduced propulsive force would not mitigate the energetic consequences of lower tendon stiffness. Ricky Pimentel, Greg Sawicki, and Jason R. Franz

Abstract. Aging elicits numerous effects that impact both musculoskeletal structure and walking function. Tendon stiffness (kT) and push-off propulsive force (FP) both impact the metabolic cost of walking and are diminished by age, yet their interaction has not been studied. We combined experimental and computational approaches to investigate whether age-related changes in function (adopting smaller FP) may be adopted to mitigate the metabolic consequences arising from changes in structure (reduced kT). We recruited 12 young adults and asked them to walk on a force-sensing treadmill while prompting them to change FP (±20% & ±40% of typical) using targeted biofeedback. In models driven by experimental data from each of those conditions, we altered the kT of personalized musculoskeletal models across a physiological range (2-8% strain) and simulated individual-muscle metabolic costs for each kT and FP combination. We found that kT and FP independently affect walking metabolic cost, increasing with higher kT or as participants deviated from their typical FP. Our results show no evidence for an interaction between kT and FP in younger adults walking at fixed speeds. Individual lower body muscles showed unique effects across the kT and FP landscape. Our simulations suggest that reducing FP during walking would not mitigate the metabolic consequences of lower kT. Wearable devices and rehabilitative strategies can focus on either kT or FP to reduce age-related increases in walking metabolic cost.