Can shank acceleration provide a clinically feasible surrogate for individual limb propulsion during walking?

Noah L. Pieper, Michael D. Lewek, Jason R. Franz

Abstract: Aging and many pathologies that affect gait are associated with reduced ankle power output and thus trailing limb propulsion during walking. However, quantifying trailing limb propulsion requires sophisticated measurement equipment at significant expense that fundamentally limits clinical translation for diagnostics or gait rehabilitation. As a component of joint power, our purpose was to determine if shank acceleration estimated via accelerometers during push-off can serve as a clinically feasible surrogate for ankle power output and peak anterior ground reaction forces (GRF) during walking. As hypothesized, we found that young adults modulated walking speed via changes in peak anterior GRF and peak ankle power output that correlated with proportional changes in shank acceleration during push-off, both at the individual subject (R²≥0.80, p<0.01) and group average (R²≥0.74, p<0.01) levels. In addition, we found that unilateral deficits in trailing limb propulsion induced via a leg bracing elicited unilateral and relatively proportional reductions in peak anterior GRF, peak ankle power, and peak shank acceleration. These unilateral leg bracing effects on peak shank acceleration correlated with those in peak ankle power (braced leg: R²=0.43, p=0.028) but those in both peak shank acceleration and peak ankle power were disassociated from those in peak anterior GRF. In conclusion, our findings in young adults provide an early benchmark for the development of affordable and clinically feasible alternatives for assessing and monitoring trailing limb propulsion during walking.

How age and surface inclination affect joint moment strategies to accelerate and decelerate individual leg joints during walking.

Jeroen B. Waanders, Alessio Murgia, Tibor Hortobágyi, Paul DeVita, Jason R. Franz

Abstract. A joint moment also causes motion at other joints of the body. This joint coupling-perspective allows more insight into two age-related phenomena during gait. First, whether increased hip kinetic output compensates for decreased ankle kinetic output during positive joint work. Second, whether preserved joint kinetic patterns during negative joint work in older age have any functional implication. Therefore, we examined how age and surface inclination affect joint moment strategies to accelerate and/or decelerate individual leg joints during walking. Healthy young (age: 22.5±4.1 years, n=18) and older (age: 76.0±5.7 years, n=22) adults walked at 1.4 m/s on a split-belt instrumented treadmill at three grades (0%, 10%, -10%). Lower-extremity moment-induced angular accelerations were calculated for the hip (0% and 10%) and knee (0% and -10%) joints. During level and uphill walking, both age groups showed comparable ankle moment-induced ipsilateral (p=0.774) and contralateral (p=0.047) hip accelerations, although older adults generated lower ankle moments in late stance. However, ankle moment-induced contralateral hip accelerations were smaller (p=0.001) in an older adult subgroup (n=13) who showed larger hip extension moments in early stance than young adults. During level and downhill walking, leg joint moment-induced knee accelerations were unaffected by age (all p>0.05). These findings suggest that during level and uphill walking increased hip flexor mechanical output in older adults does not arise from reduced ankle moments, contrary to increased hip extensor mechanical output. Additionally, results during level and downhill walking imply that preserved eccentric knee extensor function is important in maintaining knee stabilization in older age.

Visuomotor error augmentation affects mediolateral head and trunk stabilization during walking

Mu Qiao, Jackson T. Richards, and Jason R. Franz

Abstract. Prior work demonstrates that humans spontaneously synchronize their head and trunk kinematics to a broad range of driving frequencies of perceived mediolateral motion prescribed using optical flow. Using a closed-loop visuomotor error augmentation task in an immersive virtual environment, we sought to understand whether unifying visual with vestibular and somatosensory feedback is a control goal during human walking, at least in the context of head and trunk stabilization. We hypothesized that humans would minimize visual errors during walking – i.e., those between the visual perception of movement and actual movement of the trunk. We found that subjects did not minimize errors between the visual perception of movement and actual movement of the head and trunk. Rather, subjects increased mediolateral trunk range of motion in response to error-augmented optical flow with positive feedback gains. Our results are more consistent with our alternative hypothesis – that visual feedback can override other sensory modalities and independently compel adjustments in head and trunk position. Also, aftereffects following exposure to error-augmented optical flow included longer, narrower steps and reduced mediolateral postural sway, particularly in response to larger amplitude positive feedback gains. Our results allude to a recalibration of head and trunk stabilization toward more tightly regulated postural control following exposure to error-augmented visual feedback. Lasting reductions in mediolateral postural sway may have implications for using error-augmented optical flow to enhance the integrity of walking balance control through training, for example in older adults.

Third year Ph.D. Candidate Billy Clark has been awarded an NIH National Research Service Award (F31) from the National Institute on Aging for his proposal titled “The role of muscle dynamics in governing Achilles subtendon behavior across the lifespan.” Billy will be co-advised on the project by Dr. Silvia Blemker, a Professor of Biomedical Engineering at the University of Virginia, and a mentoring committee spanning Engineering, Orthopedics, Geriatrics, and Exercise Science. Anticipated outcomes from his research will have immediate impact on our understanding of musculoskeletal mechanisms underlying age-related mobility impairment toward improving the health and welfare of our aging population. Moreover, his technological advancements in musculoskeletal imaging will revolutionize the use of in vivo ultrasound during functional locomotor behavior. More broadly, the knowledge gained from this study has the potential to accelerate the development of engineered tissues, regenerative medicine approaches and therapies, and orthopaedic surgical intervention.

Congratulations on this wonderful accomplishment!

Third year Ph.D. Candidate Katie Conway was recently awarded a Dissertation Completion Fellowship for the 2019-2020 academic year. Katie’s research in the lab focuses on the biomechanics of elderly gait and mobility impairment in our aging population. Currently, she is developing and using novel approaches for the functional assessment and restoration of push-off intensity during walking. Faculty reviewers on the Fellowship Committee of The Graduate School were impressed with the quality of her research and with the exceptional progress she is making toward completion of her degree and dissertation. Congratulations, Katie!!

Richards JT, Selgrade BP, Qiao M, Plummer P, Wikstrom EA, Franz JR. Time-dependent tuning of balance control and aftereffects following optical flow perturbation training in older adults. Journal of NeuroEngineering and Rehabilitation.

Abstract

Background: Walking balance in older adults is disproportionately susceptible to lateral instability provoked by optical flow perturbations. The prolonged exposure to these perturbations could promote reactive balance control and increased balance confidence in older adults, but this scientific premise has yet to be investigated. This proof of concept study was designed to investigate the propensity for time-dependent tuning of walking balance control and the presence of aftereffects in older adults following a single session of optical flow perturbation training.

Methods: 13 older adults participated in a randomized, crossover design performed on different days that included 10 minutes of treadmill walking with (experimental session) and without (control session) optical flow perturbations. We used electromyographic recordings of leg muscle activity and 3D motion capture to quantify foot placement kinematics, lateral margin of stability, and antagonist coactivation during normal walking (baseline), early (min 1) and late (min 10) responses to perturbations, and aftereffects immediately following perturbation cessation (post).

Results: At their onset, perturbations elicited 17% wider and 7% shorter steps, higher step width and length variability (+171% and +132%, respectively), larger and more variable margins of stability (MoS), and roughly twice the antagonist leg muscle coactivation (p-values<0.05). Despite continued perturbations, most outcomes returned to values observed during normal, unperturbed walking by the end of prolonged exposure. After 10-min of perturbation training and their subsequent cessation, older adults walked with longer and more narrow steps, modest increases in foot placement variability, and roughly half the MoS variability and antagonist lower leg muscle coactivation as they did before training.

Conclusions: Findings suggest that older adults: (i) respond to the onset of perturbations using generalized anticipatory balance control, (ii) deprioritize that strategy following prolonged exposure to perturbations, and (iii) upon removal of perturbations, exhibit short-term aftereffects that indicate a lessening of anticipatory control, an increase in reactive control, and/or increased balance confidence. We consider this an early, proof-of-concept study into the clinical utility of prolonged exposure to optical flow perturbations as a training tool for corrective motor adjustments relevant to walking balance integrity toward reinforcing task-specific, reactive control and/or improving balance confidence in older adults.

Clark WH and Franz JR, Triceps surae muscle-subtendon interaction differs between young and older adults. Connective Tissue Research. *Invited, Special Issue on Aging.

Abstract. Mechanical power generated via triceps surae muscle-tendon interaction during walking is largely responsible for the total power needed to walk. This interaction is made complex by the biological architecture of the Achilles tendon, which consists of distinct bundles of tendon fascicles, known as “subtendons”, arising from the lateral and medial gastrocnemius (GAS) and soleus (SOL) muscles. Comparative data and our own in vivo evidence allude to a reduced capacity for sliding between adjacent subtendons compromising the Achilles tendon in old age. This is functionally important, as subtendon sliding could facilitate independent actuation between individual triceps surae muscles, perhaps augmenting contributions to trunk support and forward propulsion. Recently, we revealed that length change differences between the GAS and SOL of young adults positively correlated with non-uniform subtendon tissue displacement patterns. Here, we investigated aging effects on triceps surae muscle-subtendon interaction using dual-probe ultrasound imaging during a series of ramped isometric contractions. We hypothesized that, compared to young adults, older adults would have more uniform subtendon tissue displacements that are accompanied by anatomically consistent differences in GAS versus SOL muscle length change behavior. Our findings fully supported our hypotheses. Older adults had more uniform subtendon tissue displacements that extended to anatomically consistent and potentially unfavorable changes in muscle contractile behavior – evidenced by smaller differences between GAS and SOL peak shortening during isometric force generation. These findings provide an important biomechanical basis for previously reported correlations between more uniform Achilles subtendon behavior and reduced ankle moment generation during waking in older adults.

Conway KA and Franz JR. Increasing the propulsive demands of walking to their maximum elucidates functionally limiting impairments in older adult gait. Journal of Aging and Physical Activity (In press).

Abstract. We elucidated functional limitations in older adult gait by increasing horizontal impeding forces and walking speed to their maximums compared to dynamometry and to data from their young counterparts. Specifically, we investigated which determinants of push-off intensity represent genuine functionally limiting impairments in older adult gait versus locomotor changes that simply present as age-related deficits in walking performance. We found that older adults walked at their preferred speed with hallmark deficits in push-off intensity. These subjects were fully capable of overcoming deficits in propulsive ground reaction force, trailing limb positive work, trailing leg and hip extension, and ankle power generation when the propulsive demands of walking were increased to maximum. Of the outcomes tested, age-related deficits in ankle moment emerged as the lone genuine functionally limiting impairment in older adults. Distinguishing genuine functional limitations from age-related differences masquerading as limitations represents a critical step toward the development and prescription of effective interventions.

Browne MG and Franz JR. Ankle power biofeedback attenuates the distal-to-proximal redistribution in older adults. Gait & Posture (In press).

Abstract:

Background: Compared to young adults, older adults walk slower, with shorter strides, and with a characteristic decrease in ankle power output. Seemingly in response, older adults rely more than young on hip power output, a phenomenon known as a distal-to-proximal redistribution. Nevertheless, older adults can increase ankle power to walk faster or uphill, revealing a translationally important gap in our understanding.

Research Question: Our purpose was to implement a novel ankle power biofeedback paradigm to encourage favorable biomechanical adaptations (i.e. reverse the distal-redistribution) during habitual speed walking in older adults.

Methods: 10 healthy older adults walked at their preferred speeds while real-time visual biofeedback provided target increases and decreases of 10 and 20% different from preferred ankle power. We evaluated the effect of changes in ankle power on joint kinetics, kinematics, and propulsive ground reaction forces. Pre and post overground walking speed assessments evaluated the effect of increased ankle power recall on walking speed.

Results: Biofeedback systematically elicited changes in ankle power; increasing and decreasing ankle power by 14% and 17% when targeting ±20% different from preferred, respectively. We observed a significant negative correlation between ankle power and hip extensor work. Older adults relied more heavily on changes in ankle angular velocity than ankle moment to modulate ankle power. Lastly, older adults walked almost 11% faster when recalling increased ankle power overground.

Significance: Older adults are capable of increasing ankle power through targeted ankle power biofeedback – effects that are accompanied by diminished hip power output and a reversal of the distal-to-proximal redistribution. The associated increase in preferred walking speed during recall suggests a functional benefit to increased ankle power output via transfer to overground walking. Further, our mechanistic insights allude to translational success using ankle angular velocity as a surrogate to modulate ankle power through biofeedback.

Congratulations to Michael Browne on successfully defending his dissertation, titled “Biofeedback to modulate push-off intensity in older adults: implications at the muscle, joint, limb, and whole-body levels.” Nice work, Dr. Browne!!