Older adults overcome reduced triceps surae structural stiffness to preserve ankle joint quasi-stiffness during walking.

Krupenevich RL, Clark WH, Sawicki GS, Franz JR.

Abstract. Ankle joint quasi-stiffness is an aggregate measure of the interaction between triceps surae muscle stiffness and Achilles tendon stiffness. This interaction may be altered due to age-related changes in the structural properties and functional behavior of the Achilles tendon and triceps surae muscles. We hypothesized that, due to a more compliant Achilles’ tendon, older adults would exhibit lower ankle joint quasi-stiffness than young adults, during walking and during isolated contractions at matched triceps surae muscle activations. We also hypothesized that, independent of age, triceps surae muscle stiffness and ankle joint quasi-stiffness would increase with triceps surae muscle activation. We used conventional gait analysis in one experiment and, in another, electromyographic biofeedback and in vivo ultrasound imaging applied during isolated contractions. We found no difference in ankle joint quasi-stiffness between young and older adults during walking. Conversely, we found that: (i) young and older adults modulated ankle joint quasi-stiffness via activation-dependent changes in triceps surae muscle length-tension behavior, and (ii) at matched activation, older adults exhibited lower ankle joint quasi-stiffness than young adults. Despite age-related reductions during isolated contractions, ankle joint quasi-stiffness was maintained in older adults during walking – which may be governed via activation-mediated increases in muscle stiffness.

 

Can optical flow perturbations detect walking balance impairment in people with multiple sclerosis?

Brian P. Selgrade, Diane Meyer, Jacob J. Sosnoff, Jason R. Franz

Abstract. People with multiple sclerosis (PwMS) who exhibit minimal to no disability are still over twice as likely to fall as the general population and many of these falls occur during walking. There is a need for more effective ways to detect preclinical walking balance deficits in PwMS. Therefore, the purpose of this study was to investigate the effects of optical flow perturbations applied using virtual reality on walking balance in PwMS compared to age-matched controls. We hypothesized that susceptibility to perturbations – especially those in the mediolateral direction – would be larger in PwMS compared to controls. Fourteen PwMS and fourteen age-matched controls walked on a treadmill while viewing a virtual hallway with and without optical flow perturbations in the mediolateral or anterior-posterior directions. We quantified foot placement kinematics, gait variability, lateral margin of stability and, in a separate session, performance on the standing sensory organization test (SOT). We found only modest differences between groups during normal, unperturbed walking. These differences were larger and more pervasive in the presence of mediolateral perturbations, evidenced by higher variability in step width, sacrum position, and margin of stability at heel-strike in PwMS than controls. PwMS also performed worse than controls on the SOT, and there was a modest correlation between step width variability during perturbed gait and SOT visual score. In conclusion, mediolateral optical flow perturbations revealed differences in walking balance in PwMS that went undetected during normal, unperturbed walking. Targeting this difference may be a promising approach to more effectively detect preclinical walking balance deficits in PwMS.

Effects of age and target location on reaction time and accuracy of lateral precision stepping during walking

Brian P. Selgrade, Marcus E. Childs, and Jason R. Franz

Abstract. Older adults have poorer lateral balance and deficits in precision stepping accuracy, but the way these deficits manifest with lateral step distance is unclear. The purpose of this study was to investigate aging effects on lateral precision stepping performance in reaction to near and distant foot placement targets during treadmill walking. We hypothesized that older adults would step to targets later and less accurately than young adults, and that these difference would be more pronounced for distant targets. During the study, young and older adults stepped on lateral targets projected onto the surface of a treadmill one stride prior to their targeting step. We measured stepping accuracy to the target, the time when the swing foot diverged from its normal swing trajectory, and swing phase gluteus medius activity. Both groups had similar performance stepping to near targets, suggesting that giving older subjects a full stride to react to target location mitigates visuomotor processing delays that have contributed to deficits in stepping performance in prior studies. However, when stepping to distant targets, older adults had larger errors and later divergence times than young adults. This alludes to age-related deficits other than those in visuomotor processing contribute to poorer performance for more difficult stepping tasks. Furthermore, while young adults increased early swing gluteus medius activity with lateral target distance, older adults did not. This is the first study to show a potential neuromuscular basis for precision stepping deficits in older adults.

The Applied Biomechanics Laboratory, in collaboration with the UNC Division of Physical Therapy and the Department of Chemical and Biomedical Engineering at West Virginia University, has been awarded a pilot research grant from the NIH-funded North Carolina Translational and Clinical Sciences Institute for our project titled “The peripheral motor repertoire as a neuromuscular constraint on walking balance integrity in age-related falls risk”. The scientific premise of the project is that all individuals rely on a principal number of peripheral neuromuscular commands – a “peripheral motor repertoire” – to accomplish everyday walking tasks during which falls may occur. Our overall objective is to test the hypothesis that a reduced peripheral motor repertoire used for everyday walking tasks represents a neuromuscular constraint on older adults’ ability to successfully respond to walking balance perturbations and prevent falls in the community. Our long-term goal is to introduce a novel neuromuscular mechanism for age-associated balance impairment as a target for diagnostic testing and rehabilitation to prevent falls in older adults.

 

 

Effects of Gait Biofeedback on Cartilage Oligomeric Matrix Protein in Individuals with ACL Reconstruction

Brittney A. Luc-Harkey, Jason R. Franz, Anthony C. Hackney, J. Troy Blackburn, Darin A. Padua, Todd A. Schwartz, and Brian Pietrosimone

 

 

Context: Gait biomechanics are linked to biochemical changes that contribute to the development of posttraumatic knee osteoarthritis in individuals with anterior cruciate ligament reconstruction (ACLR). It remains unknown if modifying peak loading during gait using real-time biofeedback will result in acute biochemical changes related to cartilage metabolism.

Objective: Determine if acutely manipulating peak vertical ground reaction force (vGRF) during gait influences acute changes in serum Cartilage Oligomeric Matrix Protein (sCOMP) concentrations in individuals with an ACLR.

Design: Crossover Study

Patients or Other Participants: Thirty individuals with an unilateral ACLR participated (70% Female, 20.4±2.9 years old, 24.4±4.2 body mass index (BMI), 47.8±27 months post-ACLR). Additionally, a subgroup was identified as those participants who demonstrated an increase in sCOMP following the control or natural loading condition (sCOMP_CHANGE >0ng/mL; n=22, 70% Female, 20.3±3 years old, 24.7±4.3 BMI, 47.3±29.3 months post-ACLR).

Main Outcome Measure(s): Serum was collected before and immediately following each condition to determine sCOMP_CHANGE.

Intervention: All participants attended four sessions involving 20 minutes of walking on a force-measuring treadmill consisting of a control condition (natural loading) followed by a random ordering of 3 loading conditions prescribed using real-time biofeedback: 1) a 5% increase in vGRF (high-loading), 2) a 5% decrease in vGRF (low-loading), and 3) symmetric vGRF between limbs. A general linear mixed model was used to determine differences in sCOMP_CHANGE between each altered loading condition and control in the entire cohort and subgroup.

Results: sCOMP_CHANGE was not different across all loading conditions for the entire cohort (F_3,29=1.34, P=0.282). Within the subgroup, sCOMP_CHANGE was significantly less during high-loading (1.95±24.22ng/mL, t₂₁=-3.53, P=0.005) and symmetrical loading (9.93±21.45ng/mL; t₂₁=-2.86, P=0.025) compared to the control (25.79±21.40ng/mL).

Conclusions: Increasing peak vGRF during gait decreases sCOMP in ACLR individuals who naturally demonstrate an increase in sCOMP following 20 minutes of walking.

Shorter gastrocnemius fascicle lengths in older adults associate with worse capacity to enhance push-off intensity in walking

Katie A. Conway and Jason R. Franz

Background: Reduced push-off intensity during walking is thought to play an important role in age-related mobility impairment. We posit that an age-related shift toward shorter plantarflexor operating lengths during walking functionally limits force generation, and thereby the ability of those muscles to respond to increased propulsive demands during walking. Research Question: To determine whether gastrocnemius muscle fascicle lengths during normal walking: (1) are shorter in older than young adults, and (2) correlate with one’s capacity to increase the propulsive demands of walking to their maximum. Methods: We used in vivo cine B-mode ultrasound to measure gastrocnemius fascicle lengths in 9 older and 9 young adults walking at their preferred speed, their maximum speed, and with horizontal impeding forces that increased in a ramped design at 1%BW/s to their maximum. A repeated measures ANOVA tested for effects of age and walking condition, and Pearson correlations assessed the relation between fascicle outcomes and condition performance. Results: A tendency toward shorter medial gastrocnemius muscle fascicle lengths in older versus young adults was not statistically significant. However, older adults walked with reduced peak fascicle shortening during all conditions compared to young adults – an outcome not explained by reduced muscle-tendon unit shortening and exacerbated during tasks with greater than normal propulsive demand. As hypothesized, we found a strong and significant positive correlation in older subjects between gastrocnemius fascicle lengths during normal walking and performance on the ramped impeding force condition (p=0.005, r²=0.704), even after controlling for isometric strength (p=0.011, r²=0.792) and subject stature (p=0.010, r²=0.700). Significance: Our findings provide muscle-level insight to develop more effective rehabilitation techniques to improve push-off intensity in older adults and assistive technologies designed to steer plantarflexor muscle fascicle operating behavior during functional tasks.

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!