We are very proud to congratulate Billy Clark for successfully defending his PhD dissertation, titled “Interaction between triceps surae muscles and the Achilles tendon in young and older adults: mechanisms and functional consequences”. Outstanding job! We also thank the wonderful support of his committee members, Drs. Xiaogang Hu (UNC/NCSU BME), Mike Lewek (UNC PT), Kate Saul (NCSU MAE), and Eric Ryan (UNC EXSS). Billy will be starting a position as a post-doc in the Department of Ecology and Evolutionary Biology at Browne University working under the mentorship of Dr. Tom Roberts. Big things ahead!!

The effects of a 6-week horizontal impeding force gait training protocol on push-off intensity in older adults

Conway KA, Crudup KL, Lewek MD, Franz JR

Abstract. Aging and many gait pathologies are often characterized by deficits in push-off intensity (i.e., propulsive ground reaction forces and peak ankle moment and power output) during walking. Unfortunately, conventional interventions such as progressive resistance training, designed to enhance calf muscle mechanical output, generally fail to translate strength gains to functional improvements in habitual push-off intensity. Methods: Horizontal impeding forces applied to the body’s center of mass systematically augment the mechanical output required from muscle-tendon units spanning the ankle during the push-off phase of walking, which could convey long-term benefits via training. Therefore, the purpose of this study was to investigate the preliminary efficacy of a 6-week horizontal impeding force training paradigm on improving habitual push-off intensity in 11 healthy but not physically active older adults (age: 76±4 years, 6F/5M). Results: We found that older adults significantly (p<0.05) increased measures of isometric strength by 18%, maximum walking speed by 10%, and 6-minute walk test distance by 9% as a result of horizontal impeding force training. As a more clinically significant contribution of this work, we found that those subjects also increased habitual peak ankle moment and peak ankle power during push-off following training by a significant 10% and 15%, respectively (p≤0.036). Conclusions: We conclude that the use of horizontal impeding forces in older adults improves their maximum muscular and walking capacities while encouraging access to newfound strength gains, thereby improving habitual push-off intensity during walking.

Imaging and simulation of inter-muscular differences in triceps surae contributions to forward propulsion during walking.

Clark WH, Pimentel RE, Franz JR.

Abstract. Forward propulsion during the push-off phase of walking is largely governed at the ankle by differential neuromechanical contributions from the biarticular medial (MG) and lateral gastrocnemii (LG) and the uniarticular soleus (SOL). However, the relative contribution of these individual muscles to forward propulsion is equivocal, with important implications for the design and control of wearable assistive devices and for targeted therapeutics. The aim of this study was to evaluate the agreement between empirical and model-predicted triceps surae contributions to forward propulsion during walking using conditions that systematically manipulated both walking speed and the mechanical demand for forward propulsion at a fixed speed – through the use of aiding and impeding forces. Ten young adults (age: 24.1 ± 3.6 years, 6M/4F) participated. We found that muscle-specific responses derived from experimental measurements (i.e., activation and fascicle behavior) were consistent with those derived from musculoskeletal simulations (i.e., muscle force and positive mechanical work) within the same subjects. In vivo, compared to walking normally, only LG muscle activation was affected by both aiding and impeding forces. Similarly, increased propulsive demand elicited greater relative fascicle shortening in the MG but not the SOL. In silico, only MG and LG force and positive mechanical work increased significantly to meet the increased demands for forward propulsion. By combining electromyography, ultrasound imaging, and musculoskeletal modeling in the same subjects, our cumulative findings suggest that the biarticular gastrocnemius muscles play a more significant role than the uniarticular soleus in governing changes in forward propulsion during the mid to late stance phase of walking.

The effects of knee extensor moment biofeedback on gait biomechanics and quadriceps contractile behavior

Munsch AE, Pietrosimone B, Franz JR

Abstract. Individuals with knee joint pathologies exhibit quadriceps dysfunction that, during walking, manifests as smaller peak knee extensor moment (pKEM) and reduced knee flexion excursion. These changes persist despite muscle strengthening and may alter stance phase knee joint loading considered relevant to osteoarthritis risk. Novel rehabilitation strategies that more directly augment quadriceps mechanical output during functional movements are needed to reduce this risk. As an important first step, we tested the efficacy of real-time biofeedback during walking to prescribe changes of ±20% and ±40% of normal walking pKEM values in 11 uninjured young adults. We simultaneously recorded knee joint kinematics, ground reaction forces, and, via ultrasound, vastus lateralis (VL) fascicle length change behavior. Participants successfully responded to real-time biofeedback and averaged up to 55% larger and 51% smaller than normal pKEM values with concomitant and potentially favorable changes in knee flexion excursion. While the VL muscle-tendon unit (MTU) lengthened, VL fascicles accommodated weight acceptance during walking largely through isometric, or even slight concentric, rather than eccentric action as is commonly presumed. Targeted pKEM biofeedback may be a useful rehabilitative and/or scientific tool to elicit desirable changes in knee joint biomechanics considered relevant to the development of osteoarthritis.

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.