The Applied Biomechanics Lab, along with an investigative team spanning principal investigators in the UNC College of Arts & Sciences, UNC School of Medicine, and NC State College of Engineering has received a $772k National Science Foundation Grant to revolutionize our region’s scientific and technological infrastructure for the quantitative measurement of human movement. With additional financial support from the Dean’s offices in the College of Arts & Sciences and the School of Medicine, the departments of Biomedical Engineering and Exercise and Sport Science, and the Office of the Vice Chancellor for Research, the grant will support not only the acquisition of a state-of-the-art high-speed biplane fluoroscopy system, but also technical support staffing, creation of a Collaborative Fluoroscopy Research Core, support for instructional innovation in the classroom, and development of new community outreach programs.

This was a plan that started in concept nearly three years ago and was a true team effort, especially the Principal Investigative team: Dr. Jason Franz (UNC/NC State BME), Dr. Brian Pietrosimone (UNC EXSS), Dr. Troy Blackburn (UNC EXSS), Dr. Kate Saul (NC State MAE), and Dr. Helen Huang (UNC/NC State BME). The proposal also received enthusiastic support from a broader network of scientists and engineers spanning UNC Greensboro, High Point University, NC A&T, and Elon University. This acquisition is the first such instrument available to any of the students, faculty, and fellows at the 17 public UNC system campuses, and its availability has the potential to catalyze lasting new disciplinary, collaborative, and interdisciplinary research and educational impact across our region.

High-speed biplane fluoroscopy systems provide continuous multi-dimensional cine x-ray images at up to 1000 samples/s for the purpose of directly quantifying three-dimensional bone positions, orientations, and articulating surface mechanics that are impossible to capture with even the most sophisticated of comparable technologies (e.g., MRI). The highly competitive award will allow a broad network for researchers, as well as the diverse student bodies they serve, to measure with unparalleled resolution the precise complexities of bone motion critical to overcoming the pivotal scientific and technological challenges across many disciplines, including: (1) understanding how musculoskeletal mechanics and function are achieved and maintained over the mammalian lifespan, (2) developing mechanistic links between movement biomechanics and underlying biology, (3) identifying technological opportunities for surgical innovation, (4) advancing ergonomics and occupational science toward for a strong and vibrant workforce, (5) developing more sophisticated bioengineered materials and tissues, and (6) introducing the next generation of rehabilitation robotics.

Walking Speed Does Not Affect Net Vastus Lateralis Fascicle Length Change on Average During Weight Acceptance

Amanda E. Munsch, Brian Pietrosimone, Jason R. Franz

Abstract

Faster walking speeds increase the demand on quadriceps muscles to produce adequate force to decelerate body mass and control knee flexion. Quadriceps fascicle behavior (i.e., fascicle lengthen changes influences force generation, which in turn affects mechanical loading of the articular cartilage during walking and the biochemical environment of the knee joint. The fascicle behavior underlying different walking speeds remains unclear but should be characterized to better understand how the quadriceps muscles accommodate faster walking speeds, speeds that often associate with better cartilage health outcomes. Our purpose was to quantify quadriceps muscle net fascicle behavior during weight acceptance across a range of walking speeds in the context of more well-documented changes in muscle activity and knee joint moments. We hypothesized that vastus lateralis (VL) fascicles in healthy young adults would produce force with more overall lengthening in early stance at faster walking speeds with concomitant increases in muscle-tendon unit (MTU) lengthening, internal peak knee extensor moment (pKEM), vertical ground reaction force (GRF), and muscle activity. Participants walked for two-minute trials at their preferred speed and at 0.75 m/s and 1.75 m/s. We find that on average the VL accommodates the greater mechanical demands of walking at faster speeds with greater muscle activity and while resisting muscle lengthening behavior. We infer that tendon stretch accommodates MTU lengthening in healthy young adults across a range of speeds and suggest these results motivate additional studies aimed at evaluating VL fascicle behavior individuals with known quadriceps strength deficits, inhibition, or heightened risk for developing osteoarthritis.

The Applied Biomechanics Lab had a fantastic visit to the North American Congress on Biomechanics in Ottawa, Ontario. Our group had the pleasure of sharing our work, connecting with our colleagues, and engaging in great scientific discussions. We also got to explore the city of Ottawa and learn about its rich history.

Quantifying Relations Between Walking Speed, Propulsive Force, and Metabolic Cost

Richard E. Pimentel, Jordan N. Feldman, Michael D. Lewek, Jason R. Franz

Abstract

Walking speed is a useful surrogate for health status across the population. Walking speed appears to be governed in part by interlimb coordination between propulsive (FP) and braking (FB) forces generated during step-to-step transitions and is simultaneously optimized to minimize metabolic cost. Of those forces, FP generated during push-off has received more significant attention as a contributor to walking performance. Our goal was to first establish empirical relations between FP and walking speed and then to quantify their effects on metabolic cost in young adults. To specifically address any linkage between FP and walking speed, we used a self-paced treadmill controller and real-time biofeedback to independently prescribe walking speed or FP across a range of condition intensities.  Walking with larger and smaller FP led to instinctively faster and slower walking speeds, respectively, with about 80% of variance in walking speed explained by FP. We also found that comparable changes in either FP or walking speed elicited predictable and relatively uniform changes in metabolic cost, together explaining ~53% of the variance in net metabolic power and ~14% of the variance in cost of transport. These results provide empirical data in support of an interdependent relation between FP and walking speed, building confidence that interventions designed to increase FP will translate to improved walking speed. Repeating this protocol in other populations may identify other relations that could inform the time course of gait decline due to age and disease.

Quadriceps Muscle Action and Association with Knee Joint Biomechanics in Individuals with ACL Reconstruction

Amanda E. Munsch, Alyssa Evans, Hope C. Davis-Wilson, Brian Pietrosimone, Jason R. Franz

Abstract. Insufficient quadriceps force production and altered knee joint biomechanics after anterior cruciate ligament reconstruction (ACLR) may contribute to a heightened risk of osteoarthritis (OA). Quadriceps muscles lengthening dynamics affect force production and knee joint loading; however, no study to our knowledge has quantified in vivo quadriceps dynamics during walking in individuals with ACLR or examined correlations between quadriceps dynamics and joint biomechanics. Our purpose was to quantify bilateral vastus lateralis (VL) fascicle length change behavior and the association thereof with gait biomechanics during the weight acceptance phase of walking (i.e., between heel-strike and the instant of pKEM) in individuals with ACLR. We hypothesized that ACLR limbs would exhibit more fascicle lengthening than contralateral limbs. We also hypothesized that ACLR limbs would exhibit positive correlations between VL fascicle lengthening and knee joint biomechanics during weight acceptance in walking. We quantified bilateral VL contractile dynamics via cine B-mode ultrasound imaging in 18 individuals with ACLR who walked on an instrumented treadmill and compared outcomes between limbs. In partial support of our hypothesis, ACLR limb VL fascicles activated without length change on average during early stance while fascicle length on the contralateral limb decreased. We found a positive association between fascicle lengthening and increase in KEM in both limbs in individuals following ACLR. Together, our results suggest that examining quadriceps muscle dynamics may elucidate underlying mechanisms relevant to OA.

Slowing Down to Preserve Balance in the Presence of Optical Flow Perturbations

Andrew D. Shelton, Ellora M. Mctaggart, Jessica L. Allen, Vicki S. Mercer, Jason R. Franz

Background: The use of sensory and mechanical perturbations applied during walking has grown in popularity due to their ability to elicit instability relevant to falls. However, the vast majority of perturbation studies on walking balance are performed on a treadmill at a fixed speed. Research question: The aim of the study was to quantify the effects of mediolateral optical flow perturbations on walking speed and balance outcomes in young adults walking with fixed-speed and self-paced treadmill controllers. Methods: Fifteen healthy young adults (8 female, age: 23.1±4.6 yrs) completed four five-minute randomized walking trials in a speed-matched virtual reality hallway. In two of the trials, we added continuous mediolateral optical flow perturbations to the virtual hallway. Trials with and without optical flow perturbations were performed with either a fixed-speed or self-paced treadmill controller. We measured walking speed, balance outcomes (step width, margin of stability, local dynamic instability) and gait variability (step width variability and margin of stability variability). Results: We found significant increases in step width (+20%, p=0.004) and local dynamic instability (+11%, p = 0.008) of participants while responding to optical flow perturbations at a fixed treadmill speed. We found no significant differences in these outcome measures when perturbations were applied on a self-paced treadmill. Instead, participants walked 5.7% slower between the self-paced treadmill controller conditions when responding to optical flow perturbations (1.48±0.13 m/s vs. 1.57±0.16 m/s, p=0.005). Significance: Our findings suggest that during walking, when presented with a balance challenge, an individual will instinctively reduce their walking speed in order to better preserve stability. However, comparisons to prior literature suggest that this response may depend on environmental and/or perturbation context. Cumulatively, our results point to opportunities for leveraging self-paced treadmill controllers as a more ecologically-relevant option in balance research with potential clinical applications in diagnostics and rehabilitation.

The Metabolic Cost of Walking Balance Control and Adaptation in Young Adults.

Shawn Ahuja and Jason R. Franz

Background: Our aim was to quantify the role of metabolic energy cost in governing neuromuscular adaptation to prolonged exposure to optical flow walking balance perturbations in young adults. Research Question: We hypothesized that metabolic cost would increase at the onset of balance perturbations in a manner consistent with wider and shorter steps and increased step-to-step variability. We also hypothesized that metabolic cost would decrease with prolonged exposure in a manner consistent with a return of step width and step length to values seen during normal, unperturbed walking. Methods: Healthy young adults (n=18) walked on a treadmill while viewing a virtual hallway. Optical flow balance perturbations were introduced over a 10-minute interval during a 20-minute walking bout while measuring step kinematics and metabolic energy cost. For all outcome measures, we computed average values during the following four time periods of interest: Pre (minutes 3-5), Early Perturbation (minutes 5-7), Late Perturbation (minutes 13-15), and Post (minutes 18-20). A repeated-measures ANOVA tested for main effects of time, following by post-hoc pairwise comparisons. Results: With the onset of perturbations, participants walked with 3% shorter, 17% wider, and 53-73% more variable steps. These changes were accompanied by a significant 12% increase in net metabolic power compared to walking normally. With prolonged exposure to perturbations, step width and step length tended toward values seen during normal, unperturbed walking – changes accompanied by a 5% reduction in metabolic power (p-values≤0.05). Significance: Our study reveals that the adoption of generalized anticipatory control at the onset of optical flow balance perturbations and the subsequent shift to task-specific reactive control following prolonged exposure have meaningful metabolic consequences. Moreover, our findings suggest that metabolic energy cost may shape the strategies we use to adapt walking balance in response to perturbations.

Our lab had a wonderful time attending our regional ASB meeting here at UNC Chapel Hill. Really inclusive meeting with especially great opportunities for undergraduate biomechanists. Our triangle-area ASB student chapter was well represented as well! Congratulations to Ricky Pimentel, Andy Shelton, Mandy Munsch, Jordan Feldman, and Ellie McTaggart for their terrific presentations.

Our laboratory was recently awarded two pilot grants from the UNC Thurston Arthritis Research Center (TARC). The awards will accelerate new interdisciplinary lines of research into: (i) the association between muscle action, inflammatory biomarkers, and cartilage loading during walking in people with osteoarthritis, and (ii) precision medicine approaches for individualized gait retraining to mitigate osteoarthritis. Our team science approach formally integrates biomedical engineering, exercise and sport science, orthopaedics, and rheumatology, allergy and immunology. Thanks for the support!

Our Applied Biomechanics Laboratory was notified this summer that two laboratory members were selected as the two finalists for the 2021 Journal of Biomechanics Award at this year’s American Society of Biomechanics (ASB) meeting. Both finalists, BME Senior Callum J. Funk (Exploring the functional boundaries and metabolism of triceps surae force-length relations during walking) and UNC Medical Student Shawn Ahuja (The metabolic cost of walking balance control and adaptation in young adults) were invited to deliver a podium presentation at the annual virtual meeting in August. In the conference’s closing ceremony, it was announced that Funk had won the prestigious Award.

The Journal of Biomechanics Award recognizes substantive and conceptually novel mechanics approaches explaining how biological systems function. It is one of the highest honors at the ASB conference. Many Congratulations to Funk and his coauthors (BME post-doc Rebecca Krupenevich, Georgia Tech faculty Gregory Sawicki, and senior author Jason Franz), as well as to finalist Shawn Ahuja for delivering exceptional presentations!