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!

Age-related differences in calf muscle recruitment strategies in the time-frequency domain during walking as a function of task demand

Hoon Kim and Jason R. Franz

ABSTRACT

Activation of the plantar flexors is critical in governing ankle push-off power during walking, which decreases due to age. However, electromyographic (EMG) signal amplitude alone are unable to fully characterize motor unit recruitment during functional activity.  Although not yet studied in walking, EMG frequency content may also vary due to age-related differences in muscle morphology and neural signaling. Our purpose was to quantify plantar flexor activation differences in the time-frequency domain between young and older adults during walking across a range of speeds and with and without horizontal aiding and impeding forces. Ten healthy young (24.0±3.4 years) and older adults (73.7±3.9 years) walked at three speeds and walked with horizontal aiding and impeding force while muscle activations of soleus (SOL) and gastrocnemius (GAS) were recorded. The EMG signals were decomposed in the time-frequency domain with wavelet transformation. Principal component analyses extracted principal components (PC) and PC scores. Compared to young adults, we observed that GAS activation in older adults: 1) was lower across all frequency ranges during midstance and in slow to middle frequency ranges during push-off, independent of walking speed, and 2) shifted to slower frequencies with earlier timing as walking speed increased. Our results implicate GAS time-frequency content, and its morphological and neural origins, as a potential determinant of hallmark ankle push-off deficits due to aging, particularly at faster walking speeds. Rehabilitation specialists may attempt to restore GAS intensity across all frequency ranges during mid to late stance while avoiding disproportionate increases in slower frequencies during early stance.

The Effects of Triceps Surae Muscle Stimulation on Localized Achilles Subtendon Tissue Displacements

Nathan L. Lehr, William H. Clark, Michael D. Lewek, Jason R. Franz

Abstract. The triceps surae muscle tendon unit is comprised of the lateral and medial gastrocnemius (MG) and soleus (SOL) muscles and three in series elastic “subtendons” that form the Achilles tendon. Comparative literature and our own in vivo evidence suggests that sliding between adjacent subtendons may facilitate independent muscle actuation. We aim to more clearly define the relation between individual muscle activation and subtendon tissue displacements. Here, during fixed-end contractions, electrical muscle stimulation controlled the magnitude of force transmitted via individual triceps surae muscles while ultrasound imaging recorded resultant subtendon tissue displacements. We hypothesized that MG and SOL stimulation would elicit larger displacements in their associated subtendon. 10 young adults completed 4 experimental activations at 3 ankle angles (-20°, 0°, 20°) with knee flexed to approximately 20°: MG stimulation (STIM_MG), SOL stimulation (STIM_SOL), combined stimulation, and volitional contraction. At 20° plantarflexion, STIM_SOL elicited 49% larger tendon non-uniformity (SOL – MG subtendon tissue displacement) than that of STIM_MG (p=0.004). For STIM_SOL, a one-way post-hoc ANOVA revealed a significant main effect of ankle angle (p=0.009) on Achilles tendon non-uniformity. However, peak tendon non-uniformity decreased by an average of 61% from plantarflexion to dorsiflexion, likely due to an increase in passive tension. Our results suggest that localized tissue displacements within the Achilles tendon respond in anatomically consistent ways to differential patterns of triceps surae muscle activation, but these relations are highly susceptible to ankle angle. This in vivo evidence points to at least some mechanical independence in actuation between the human triceps surae muscle-subtendon units.

Effects of age and locomotor demand on foot mechanics during walking.

Rebecca L. Krupenevich, Samuel F. Ray, Howard E. Kashefsky, Kota Z. Takahashi, Jason R. Franz

Abstract. Older adults exhibit reductions in trailing leg push-off power that are often attributed to deficits in plantarflexor force-generating capacity. However, growing evidence suggests that the foot may also contribute to push-off power during walking. Thus, age-related changes in foot structure and function may contribute to altered foot mechanics and ultimately reduced push-off power. The purpose of this paper was to quantify age-related differences in foot mechanical work during walking across a range of speeds and, at a single fixed speed with varied demands for trailing leg push-off.  9 young and 10 older adults walked at 1.0, 1.2, and 1.4 m/s, and at 1.2 m/s with an aiding or impeding horizontal pulling force equal to 5% BW. We calculated foot work in Visual3D using a unified deformable foot model, accounting for contributions of structures distal to the hindfoot’s center-of-mass. Older adults walked while performing less positive foot work and more negative net foot work (p<0.05). Further, we found that the effect of age on mechanical work performed by the foot and the ankle-foot complex increased with increased locomotor demand (p<0.05). Our findings suggest that during walking, age-related differences in foot mechanics may contribute to reduced push-off intensity via greater energy loss from distal foot structures, particularly during walking tasks with a greater demand for foot power generation. These findings are the first step in understanding the role of the foot in push-off power deficits in older adults and may serve as a roadmap for developing future low-cost mobility interventions.