Clark WH and Franz JR. Do triceps surae muscle dynamics govern non-uniform Achilles tendon tissue displacements? PeerJ (In press).

Abstract. The human Achilles tendon (AT) consists of sub-tendons arising from the gastrocnemius and soleus muscles that exhibit non-uniform tissue displacements thought to facilitate some independent actuation. However, the mechanisms governing non-uniform displacement patterns within the AT, and their relevance to triceps surae muscle contractile dynamics, have remained elusive. We used a dual-probe ultrasound imaging approach to investigate triceps surae muscle dynamics (i.e., medial gastrocnemius-GAS, soleus-SOL) as a determinant of non-uniform tendon tissue displacements in the human AT. We hypothesized that superficial versus deep differences in AT tissue displacements would be accompanied by and correlate with anatomically consistent differences in GAS versus SOL muscle shortening. Nine subjects performed ramped isometric contractions at each of five ankle joint angles spanning 10° dorsiflexion to 30° plantarflexion. For all conditions, SOL shortened by an average of 78% more than GAS during moment generation. This was accompanied by, on average, 51% more displacement in the deep versus superficial region of the AT. The magnitude of GAS and SOL muscle shortening positively correlated with displacement in their associated sub-tendons within the AT. Moreover, and as hypothesized, superficial versus deep differences in sub-tendon tissue displacements positively correlated with anatomically consistent differences in GAS versus SOL muscle shortening. We present the first in vivo evidence that triceps surae muscle dynamics may precipitate non-uniform displacement patterns in the architecturally complex AT.

Luc-Harkey BA, Franz JR, Losina E, Pietrosimone B. Association between Kinesiophobia and Walking Gait Characteristics in Individuals with Anterior Cruciate Ligament Reconstruction. Gait & Posture (In press).

Abstract:

Background: Individuals with anterior cruciate ligament reconstruction (ACLR) demonstrate persistent alterations in walking gait characteristics that contribute to poor long-term outcomes. Higher kinesiophobia, or fear of movement/re-injury, may result in the avoidance of movements that increase loading on the ACLR limb. Research Question: Determine the association between kinesiophobia and walking gait characteristics in physically active individuals with ACLR. Methods: We enrolled thirty participants with a history of unilateral ACLR (49.35±27.29 months following ACLR) into this cross-sectional study. We used the Tampa Scale for Kinesiophobia (TSK-11) to measure kinesiophobia. We collected walking gait characteristics during a 60-second walking trial, which included gait speed, peak vertical ground reaction force (vGRF), instantaneous vGRF loading rate, peak internal knee extension moment (KEM), and knee flexion excursion. We calculated lower extremity kinetic and kinematic measures on the ACLR limb, and limb symmetry indices between ACLR and contralateral limbs (LSI= [ACLR/contralateral]*100). We used linear regression models to determine the association between TSK-11 score and each walking gait characteristic. We determined the change in R2 (ΔR2) when adding TSK-11 scores into the linear regression model after accounting for demographic covariates (sex, Tegner activity score, graft type, time since reconstruction, history of concomitant meniscal procedure). Results: We did not find a significant association between kinesiophobia and self-selected gait speed (ΔR2 0.038, P=0.319). Kinesiophobia demonstrated weak, non-significant associations with kinetic and kinematic outcomes on the ACLR limb and all LSI outcomes (ΔR2 range = 0.001 to 0.098). Significance: These data do not support that kinesiophobia is a critical factor contributing to walking gait characteristics in physically active individuals with ACLR.

Rasske K and Franz JR. Aging effects on the Achilles tendon moment arm during walking. Journal of Biomechanics (In press).

Abstract. The Achilles tendon (AT) moment arm transforms triceps surae muscle forces into a moment about the ankle which is critical for functional activities like walking. Moreover, the AT moment arm changes continuously during walking, as it depends on both ankle joint rotation and triceps surae muscle loading (presumably due to bulging of the muscle belly). Here, we posit that aging negatively effects the architecturally complex AT moment arm during walking, which thereby contributes to well-documented reductions in ankle moment generation during push-off. We used motion capture-guided ultrasound imaging to quantify instantaneous variations in the AT moment arms of young (23.9±4.3 years) and older (69.9±2.6 years) adults during walking, their dependence on triceps surae muscle loading, and their association with ankle moment generation during push-off. Older adults walked with 11% smaller AT moment arms and 11% smaller peak ankle moments during push-off than young adults. Moreover, as hypothesized, these unfavourable changes were significantly and positively correlated (r2=0.38, p<0.01). More surprisingly, aging attenuated load-dependent increases in AT moment arm (i.e., that between heel-strike and push-off at the same ankle angle); only young adults exhibited a significant increase in their AT moment arm due to triceps surae muscle-loading. Age-associated reductions in triceps surae volume or activation, and thus muscle bulging during force generation, may compromise the mechanical advantage of the AT during the critical push-off phase of walking in older adults. Thus, strategies to restore and/or improve locomotor performance in our aging population should consider these functionally important changes in musculoskeletal behavior.

Luc-Harkey BA, Franz JR, Blackburn JT, Padua DA, Hackney AC, Pietrosimone B. Real-time Biofeedback Can Increase and Decrease Vertical Ground Reaction Force, Knee Flexion Excursion, and Knee Extension Moment during Walking in Individuals with Anterior Cruciate Ligament Reconstruction. Journal of Biomechanics (In press).

Abstract: Individuals with anterior cruciate ligament reconstruction (ACLR) often exhibit a “stiffened knee strategy” or an excessively extended knee during gait, characterized by lesser knee flexion excursion and peak internal knee extension moment (KEM). The purpose of this study was to determine the effect of real-time biofeedback (RTBF) cuing an acute change in peak vertical ground reaction force (vGRF) during the first 50% of the stance phase of walking gait on: 1) root mean square error (RMSE) between actual vGRF and RTBF target vGRF; 2) perceived difficulty; and 3) knee biomechanics. Acquisition and short-term recall of these outcomes were evaluated. Thirty individuals with unilateral ACLR completed 4 separate walking sessions on a force-measuring treadmill that consisted of a control (no RTBF) and 3 experimental loading conditions using RTBF including: 1) 5% vGRF increase (high-loading), 2) 5% vGRF decrease (low-loading) and 3) symmetric vGRF between limbs. Bilateral biomechanical outcomes were analyzed during the first 50% of the stance phase, and included KEM, knee flexion excursion, peak vGRF, and instantaneous vGRF loading rate (vGRF-LR) for each loading condition. Peak vGRF significantly increased and decreased during high-loading and low-loading, respectively compared to control loading. Instantaneous vGRF-LR, peak KEM and knee flexion excursion significantly increased during the high-loading condition compared to low-loading. Perceived difficultly and RMSE were lower during the symmetrical loading condition compared to the low-loading condition. Cuing an increase in peak vGRF may be beneficial for increasing KEM, knee flexion excursion, peak vGRF, and vGRF-LR in individuals with ACLR. Clinical Trials Number:NCT03035994

Conway KA, Bissette RG, Franz JR (2018). The functional utilization of propulsive capacity during human walking. Journal of Applied Biomechanics (in press).

Abstract: Aging and many gait pathologies are characterized by reduced propulsive forces and ankle moment and power generation during trailing leg push-off in walking. Despite those changes, we posit that many individuals retain an underutilized reserve for enhancing push-off intensity during walking that may be missed using conventional dynamometry. By using a maximum ramped impeding force protocol and maximum speed walking, we gained mechanistic insight into the factors that govern push-off intensity and the available capacity thereof during walking in young subjects. We discovered in part that young subjects walking at their preferred speed retain a reserve capacity for exerting larger propulsive forces of 49%, peak ankle power of 43%, and peak ankle moment of 22% during push-off – the latter overlooked by maximum isometric dynamometry. We also provide evidence that these reserve capacities are governed at least in part by the neuromechanical behavior of the plantarflexor muscles, at least with regard to ankle moment generation. We envision that a similar paradigm used to quantify propulsive reserves in older adults or people with gait pathology would empower the more discriminate and personalized prescription of gait interventions seeking to improve push-off intensity and thus walking performance.

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.

Abstract.

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.

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

Abstract

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.

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.”

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.

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 (F_P) 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 F_P generation. Subjects walked on a force-measuring treadmill across a range of speeds as well as at constant speeds while modulating their F_P 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 F_P 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.