Publications by authors named "Jason R Franz"

84 Publications

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

J Appl Physiol (1985) 2021 09 2. Epub 2021 Sep 2.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, United States.

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.
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http://dx.doi.org/10.1152/japplphysiol.00262.2021DOI Listing
September 2021

The effects of triceps surae muscle stimulation on localized Achilles subtendon tissue displacements.

J Exp Biol 2021 Aug 5;224(15). Epub 2021 Aug 5.

Joint Department of Biomedical Engineering, UNC Chapel Hill & NC State University, Chapel Hill, NC 27599, USA.

The triceps surae muscle-tendon unit is composed 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 suggest that sliding between adjacent subtendons may facilitate independent muscle actuation. We aim to more clearly define the relationship 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. Ten young adults completed four experimental activations at three ankle angles (-20, 0 and 20 deg) with the knee flexed to approximately 20 deg: MG stimulation (STIMMG), SOL stimulation (STIMSOL), combined stimulation, and volitional contraction. At 20 deg plantarflexion, STIMSOL elicited 49% larger tendon non-uniformity (SOL-MG subtendon tissue displacement) than that of STIMMG (P=0.004). For STIMSOL, 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.
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http://dx.doi.org/10.1242/jeb.242135DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8353163PMC
August 2021

Quality of Life Associates With Moderate to Vigorous Physical Activity Following Anterior Cruciate Ligament Reconstruction.

J Athl Train 2021 Jul 30. Epub 2021 Jul 30.

2Human Movement Science Curriculum, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA.

Context: Higher knee function is linked to psychological readiness to return to sport following anterior cruciate ligament reconstruction (ACLR). Individuals with ACLR participate in less physical activity compared to matched uninjured controls, yet the association between knee function and physical activity following ACLR remains unclear.

Objective: To determine the association between patient-reported knee function measured with the Knee Injury and Osteoarthritis Outcomes Score Quality of Life (KOOS-QOL), daily steps, and minutes spent in moderate to vigorous physical activity (MVPA) in individuals with ACLR. Secondarily, we determined associations between KOOS-QOL, daily steps, and MVPA in individuals with ACLR who presented with (symptomatic) and without (asymptomatic) clinically meaningful knee related symptoms.

Design: Cross-sectional study.

Setting: Laboratory, Free-living conditions.

Patients Or Other Participants: Sixty-six individuals with primary unilateral ACLR (55% female, 22±4 years, 28±33 months post-ACLR, BMI: 24.2±2.9 kg/m2).

Outcome Measure(s): We collected KOOS and retrospectively stratified participants into those with (symptomatic [n=30]) or without (asymptomatic [n=36]) clinically meaningful knee related symptoms based on previously defined KOOS cutoffs. We assessed daily steps and MVPA from ActiGraph GT9X Link accelerometers which each participant wore on the right hip for 7 days. We conducted linear regressions to determine associations between KOOS-QOL, daily steps, and MVPA.

Results: No significant associations existed in the entire sample between KOOS-QOL and daily steps (ΔR2=0.01, P=0.50) or MVPA (ΔR2=0.01, P=0.36). In symptomatic individuals, greater KOOS-QOL associated with greater MVPA (ΔR2=0.12, P=0.05,). No significant associations existed between KOOS-QOL, daily steps, and MVPA in the asymptomatic group.

Conclusions: Symptomatic individuals with ACLR who spent more time in MVPA reported higher quality of life.
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http://dx.doi.org/10.4085/1062-6050-0670.20DOI Listing
July 2021

Reduced Achilles Tendon Stiffness Disrupts Calf Muscle Neuromechanics in Elderly Gait.

Gerontology 2021 Jul 16:1-11. Epub 2021 Jul 16.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, North Carolina, USA.

Older adults walk slower and with a higher metabolic energy expenditure than younger adults. In this review, we explore the hypothesis that age-related declines in Achilles tendon stiffness increase the metabolic cost of walking due to less economical calf muscle contractions and increased proximal joint work. This viewpoint may motivate interventions to restore ankle muscle-tendon stiffness, improve walking mechanics, and reduce metabolic cost in older adults.
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http://dx.doi.org/10.1159/000516910DOI Listing
July 2021

Editorial: Tendon Structure-Function Relationship in Health, Ageing, and Injury.

Front Sports Act Living 2021 18;3:701815. Epub 2021 Jun 18.

University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, United States.

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http://dx.doi.org/10.3389/fspor.2021.701815DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8249574PMC
June 2021

Age does not affect the relationship between muscle activation and joint work during incline and decline walking.

J Biomech 2021 07 11;124:110555. Epub 2021 Jun 11.

University of Groningen, University Medical Center Groningen, Center for Human Movement Sciences, Groningen, the Netherlands; Institute of Sport Sciences and Physical Education, Faculty of Sciences, University of Pécs, Pécs, Hungary; Somogy County Kaposi Mór Teaching Hospital, Kaposvár, Hungary.

Older compared with younger adults walk with different configurations of mechanical joint work and greater muscle activation but it is unclear if age, walking speed, and slope would each affect the relationship between muscle activation and net joint work. We hypothesized that a unit increase in positive but not negative net joint work requires greater muscle activation in older compared with younger adults. Healthy younger (age: 22.1 yrs, n = 19) and older adults (age: 69.8 yrs, n = 16) ascended and descended a 7° ramp at slow (~1.20 m/s) and moderate (~1.50 m/s) walking speeds while lower-extremity marker positions, electromyography, and ground reaction force data were collected. Compared to younger adults, older adults took 11% (incline) and 8% (decline) shorter strides, and performed 21% less positive ankle plantarflexor work (incline) and 19% less negative knee extensor work (decline) (all p < .05). However, age did not affect (all p > .05) the regression coefficients between the muscle activation integral and positive hip extensor or ankle plantarflexor work during ascent, nor between that and negative knee extensor or ankle dorsiflexor work during descent. With increased walking speed, muscle activation tended to increase in younger but changed little in older adults across ascent (10 ± 12% vs. -1.0 ± 10%) and descent (3.6 ± 10.2% vs. -2.6 ± 7.7%) (p = .006, r = 0.47). Age does not affect the relationship between muscle activation and net joint work during incline and decline walking at freely-chosen step lengths. The electromechanical cost of joint work production does not underlie the age-related reconfiguration of joint work during walking.
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http://dx.doi.org/10.1016/j.jbiomech.2021.110555DOI Listing
July 2021

Cueing Changes in Peak Vertical Ground Reaction Force to Improve Coordination Dynamics in Walking.

J Mot Behav 2021 Jun 20:1-10. Epub 2021 Jun 20.

Department of Exercise and Sport Science, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.

AbstractsBiofeedback has been effectively implemented to improve the mediation and distribution of joint loads during gait, however, the inability to effectively coordinate lower limb movement by altering loading patterns may increase pathological stress and risk of injury and deleterious joint changes. This study examined the influence cueing an increase or decrease in lower extremity loading has on inter- and intralimb joint coordination during gait, applied herein for 12 persons following anterior cruciate ligament reconstruction across three loading conditions (control, high, and low). Visual biofeedback was presented on a screen via a force-measuring treadmill with targeted changes prescribed based on stride-to-stride peak vertical ground reaction forces bilaterally. The pattern and stability of coordination dynamics among each of the ankle, hip and knee joint pairs were assessed via discrete relative phase and cross-recurrence quantification analyses for each condition. High and low loading altered the pattern and stability of intralimb coordination; low loading led to decreased coordination stability (20° greater than control condition) and high loading resulted in a more tightly coupled coordination pattern (higher %CDET). With thoughtful consideration for movement control, biofeedback can be used to target mechanisms leading to long-term deleterious joint adaptations.
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http://dx.doi.org/10.1080/00222895.2021.1929810DOI Listing
June 2021

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

J Biomech 2021 06 7;123:110499. Epub 2021 May 7.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, USA.

Older adults exhibit reductions in 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 push-off power. 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.
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http://dx.doi.org/10.1016/j.jbiomech.2021.110499DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8223147PMC
June 2021

Automated analysis of medial gastrocnemius muscle-tendon junction displacements in heathy young adults during isolated contractions and walking using deep neural networks.

Comput Methods Programs Biomed 2021 Jul 27;206:106120. Epub 2021 Apr 27.

Joint Department of Biomedical Engineering, UNC Chapel Hill & NC State University, Chapel Hill 27599, NC, USA.

Background And Objective: Direct measurement of muscle-tendon junction (MTJ) position is important for understanding dynamic tendon behavior and muscle-tendon interaction in healthy and pathological populations. Traditionally, obtaining MTJ position during functional activities is accomplished by manually tracking the position of the MTJ in cine B-mode ultrasound images - a laborious and time-consuming process. Recent advances in deep learning have facilitated the availability of user-friendly open-source software packages for automated tracking. However, these software packages were originally intended for animal pose estimation and have not been widely tested on ultrasound images. Therefore, the purpose of this paper was to evaluate the efficacy of deep neural networks to accurately track medial gastrocnemius MTJ positions in cine B-mode ultrasound images across tasks spanning controlled loading during isolated contractions to physiological loading during treadmill walking.

Methods: Cine B-mode ultrasound images of the medial gastrocnemius MTJ were collected from 15 subjects (6M/9F, 23 yr, 71.9 kg, 1.8 m) during treadmill walking at 1.25 m/s and during maximal voluntary isometric plantarflexor contractions (MVICs). Five deep neural networks were trained using 480 manually-labeled images collected during walking, defined as the ground truth, and were then used to predict MTJ position in images from novel subjects: 1) during walking (novel-subject) and 2) during MVICs (novel-condition).

Results: We found an average mean absolute error of 1.26±1.30 mm and 2.61±3.31 mm between the ground truth and predicted MTJ positions in the novel-subject and novel-condition evaluations, respectively.

Conclusions: Our results provide support for the use of open-source software for creating deep neural networks to reliably track MTJ positions in B-mode ultrasound images. We believe this approach to MTJ position tracking is an accessible and time-saving solution, with broad applications for many fields, such as rehabilitation or clinical diagnostics.
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http://dx.doi.org/10.1016/j.cmpb.2021.106120DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8223120PMC
July 2021

The metabolic and mechanical consequences of altered propulsive force generation in walking.

J Biomech 2021 06 18;122:110447. Epub 2021 Apr 18.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, USA. Electronic address:

Older adults walk with greater metabolic energy consumption than younger for reasons that are not well understood. We suspect that a distal-to-proximal redistribution of leg muscle demand, from muscles spanning the ankle to those spanning the hip, contributes to greater metabolic energy costs. Recently, we found that when younger adults using biofeedback target smaller than normal peak propulsive forces (F), they do so via a similar redistribution of leg muscle demand during walking. This alludes to an experimental paradigm that emulates characteristics of elderly gait independent of other age-related changes relevant to metabolic energy cost. Thus, our purpose was to quantify the metabolic and limb- and joint-level mechanical energy costs associated with modulating propulsive forces during walking in younger adults. Walking with larger F increased net metabolic power by 47% (main effect, p = 0.001), which was accompanied by small by relatively uniform increases in hip, knee, and ankle joint power and which correlated with total joint power (R = 0.151, p = 0.019). Walking with smaller F increased net metabolic power by 58% (main effect, p < 0.001), which was accompanied by higher step frequencies and increased total joint power due to disproportionate increases in hip joint power. Increases in hip joint power when targeting smaller than normal F accounted for more than 65% of the variance in the measured changes in net metabolic power. Our findings suggest that walking with a diminished push-off exacts a metabolic penalty because of higher step frequencies and more total limb work due to an increased demand on proximal leg muscles.
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http://dx.doi.org/10.1016/j.jbiomech.2021.110447DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8160453PMC
June 2021

Muscle metabolic energy costs while modifying propulsive force generation during walking.

Comput Methods Biomech Biomed Engin 2021 Mar 22:1-14. Epub 2021 Mar 22.

Joint Department of Biomedical Engineering, UNC Chapel Hill and NC State University, Chapel Hill, NC, USA.

We pose that an age-related increase in the metabolic cost of walking arises in part from a redistribution of joint power where muscles spanning the hip compensate for insufficient ankle push-off and smaller peak propulsive forces (F). Young adults elicit a similar redistribution when walking with smaller F via biofeedback. We used targeted F biofeedback and musculoskeletal models to estimate the metabolic costs of operating lower limb muscles in young adults walking across a range of F. Our simulations support the theory of distal-to-proximal redistribution of joint power as a determinant of increased metabolic cost in older adults during walking.
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http://dx.doi.org/10.1080/10255842.2021.1900134DOI Listing
March 2021

Effects of Horizontal Impeding Force Gait Training on Older Adult Push-Off Intensity.

Med Sci Sports Exerc 2021 03;53(3):574-580

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC.

Introduction: 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-wk horizontal impeding force training paradigm on improving habitual push-off intensity in 11 healthy but not physically active older adults (age = 76 ± 4 yr, 6 females and 5 males).

Results: We found that older adults significantly (P < 0.05) increased measures of isometric strength by 18%, maximum walking speed by 10%, and 6-min 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 after 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.
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http://dx.doi.org/10.1249/MSS.0000000000002500DOI Listing
March 2021

Gradually learning to increase gait propulsion in young unimpaired adults.

Hum Mov Sci 2021 Feb 19;75:102745. Epub 2020 Dec 19.

Division of Physical Therapy, Department of Allied Health Sciences, University of North Carolina at Chapel Hill, USA. Electronic address:

Distorted visual feedback (DVF) may employ both implicit and explicit approaches to enhance motor learning. Our purpose was to test the effect of DVF of gait propulsion on the capacity to alter propulsive forces, and to determine the biomechanical determinants of propulsion. Seventeen young unimpaired individuals walked for three minutes of baseline (no feedback), then completed three randomly ordered, 10-minute Learning conditions: Real, 10DVF, and 20DVF. During the DVF conditions, we gradually decreased the feedback value without the participants' knowledge. For all Learning conditions, participants were instructed to maintain the propulsive force between two targets representing ±1 standard deviation as obtained from baseline. A one-minute retention trial without any feedback was performed immediately after Learning. Participants increased propulsive forces and trailing limb angle in both DVF conditions that persisted through retention; however, no change in ankle plantarflexion moment was noted. These findings offer promise of translation to clinical populations with propulsion deficits and require combined implicit and explicit learning components.
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http://dx.doi.org/10.1016/j.humov.2020.102745DOI Listing
February 2021

These legs were made for propulsion: advancing the diagnosis and treatment of post-stroke propulsion deficits.

J Neuroeng Rehabil 2020 10 21;17(1):139. Epub 2020 Oct 21.

Division of Physical Therapy, Medical University of South Carolina, Charleston, SC, USA.

Advances in medical diagnosis and treatment have facilitated the emergence of precision medicine. In contrast, locomotor rehabilitation for individuals with acquired neuromotor injuries remains limited by the dearth of (i) diagnostic approaches that can identify the specific neuromuscular, biomechanical, and clinical deficits underlying impaired locomotion and (ii) evidence-based, targeted treatments. In particular, impaired propulsion by the paretic limb is a major contributor to walking-related disability after stroke; however, few interventions have been able to target deficits in propulsion effectively and in a manner that reduces walking disability. Indeed, the weakness and impaired control that is characteristic of post-stroke hemiparesis leads to heterogeneous deficits that impair paretic propulsion and contribute to a slow, metabolically-expensive, and unstable gait. Current rehabilitation paradigms emphasize the rapid attainment of walking independence, not the restoration of normal propulsion function. Although walking independence is an important goal for stroke survivors, independence achieved via compensatory strategies may prevent the recovery of propulsion needed for the fast, economical, and stable gait that is characteristic of healthy bipedal locomotion. We posit that post-stroke rehabilitation should aim to promote independent walking, in part, through the acquisition of enhanced propulsion. In this expert review, we present the biomechanical and functional consequences of post-stroke propulsion deficits, review advances in our understanding of the nature of post-stroke propulsion impairment, and discuss emerging diagnostic and treatment approaches that have the potential to facilitate new rehabilitation paradigms targeting propulsion restoration.
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http://dx.doi.org/10.1186/s12984-020-00747-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7579929PMC
October 2020

Biofeedback augmenting lower limb loading alters the underlying temporal structure of gait following anterior cruciate ligament reconstruction.

Hum Mov Sci 2020 Oct 25;73:102685. Epub 2020 Sep 25.

Department of Exercise and Sport Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA. Electronic address:

Biofeedback has recently been explored to target deviant lower extremity loading mechanics following anterior cruciate ligament reconstruction (ACLR) to mitigate the development of post traumatic osteoarthritis. The impact this feedback has on the structure of the stride interval dynamics-a barometer of gait system health-however, have yet to be examined. This study was designed to assess how feedback, used to alter lower-extremity loading during gait, affects the structure of stride interval variability by examining long-range stride-to-stride correlations during gait in those with unilateral ACLR. Twelve participants walked under three separate loading conditions: (1) control (i.e., no cue) (2) high loading, and (3) low loading. Baseline vertical ground reaction force (vGRF) data was used to calculate a target 5% change in vGRF for the appropriate loading condition (i.e., high loading was +5% vGRF, low loading was -5% vGRF). The target for the load condition was displayed on a screen along with real-time vGRF values, prescribing changes in stride-to-stride peak vertical ground reaction forces of each limb. From time-series of stride intervals (i.e., duration), we analyzed the mean and standard deviation of stride-to-stride variability and, via detrended fluctuation analysis (i.e., DFA α), temporal persistence for each feedback condition. Both the high and low loading conditions exhibited a change toward more temporally persistent stride intervals (high loading: α =0.92, low loading: α = 0.98) than walking under the control condition (α = 0.78; high vs. control: p = .026, low vs. control: p = .001). Overall, these results indicate that altering lower extremity load changes the temporal persistence of the stride internal dynamics in ACLR individuals, demonstrating the implications of the design of gait training interventions and the influence feedback has on movement strategies.
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http://dx.doi.org/10.1016/j.humov.2020.102685DOI Listing
October 2020

Imaging and Simulation of Inter-muscular Differences in Triceps Surae Contributions to Forward Propulsion During Walking.

Ann Biomed Eng 2021 Feb 8;49(2):703-715. Epub 2020 Sep 8.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, 10010 Mary Ellen Jones, Chapel Hill, NC, 27599, USA.

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 (i.e., MG, LG, and SOL) 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.
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http://dx.doi.org/10.1007/s10439-020-02594-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8020010PMC
February 2021

Cyclically producing the same average muscle-tendon force with a smaller duty increases metabolic rate.

Proc Biol Sci 2020 08 19;287(1933):20200431. Epub 2020 Aug 19.

The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, 455 Callaway Manufacturing Research Center Building, 813 Ferst Drive NW, Atlanta, GA 30332, USA.

Ground contact duration and stride frequency each affect muscle metabolism and help scientists link walking and running biomechanics to metabolic energy expenditure. While these parameters are often used independently, the product of ground contact duration and stride frequency (i.e. duty factor) may affect muscle contractile mechanics. Here, we sought to separate the metabolic influence of the duration of active force production, cycle frequency and duty factor. Human participants produced cyclic contractions using their soleus (which has a relatively homogeneous fibre type composition) at prescribed cycle-average ankle moments on a fixed dynamometer. Participants produced these ankle moments over short, medium and long durations while maintaining a constant cycle frequency. Overall, decreased duty factor did not affect cycle-average fascicle force ( ≥ 0.252) but did increase net metabolic power ( ≤ 0.022). Mechanistically, smaller duty factors increased maximum muscle-tendon force ( < 0.001), further stretching in-series tendons and shifting soleus fascicles to shorter lengths and faster velocities, thereby increasing soleus total active muscle volume ( < 0.001). Participant soleus total active muscle volume well-explained net metabolic power ( = 0.845; < 0.001). Therefore, cyclically producing the same cycle-average muscle-tendon force using a decreased duty factor increases metabolic energy expenditure by eliciting less economical muscle contractile mechanics.
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http://dx.doi.org/10.1098/rspb.2020.0431DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7482283PMC
August 2020

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

PeerJ 2020 8;8:e9509. Epub 2020 Jul 8.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, United States of America.

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.
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http://dx.doi.org/10.7717/peerj.9509DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7353917PMC
July 2020

Older Adults Overcome Reduced Triceps Surae Structural Stiffness to Preserve Ankle Joint Quasi-Stiffness During Walking.

J Appl Biomech 2020 Jun 5:1-8. Epub 2020 Jun 5.

University of North Carolina at Chapel Hill and North Carolina State University.

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. The authors hypothesized that, due to a more compliant of 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. The authors also hypothesized that, independent of age, triceps surae muscle stiffness and ankle joint quasi-stiffness would increase with triceps surae muscle activation. The authors used conventional gait analysis in one experiment and, in another, electromyographic biofeedback and in vivo ultrasound imaging applied during isolated contractions. The authors found no difference in ankle joint quasi-stiffness between young and older adults during walking. Conversely, this study found that (1) young and older adults modulated ankle joint quasi-stiffness via activation-dependent changes in triceps surae muscle length-tension behavior and (2) 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.
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http://dx.doi.org/10.1123/jab.2019-0340DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8020011PMC
June 2020

Biomechanical effects of manipulating peak vertical ground reaction force throughout gait in individuals 6-12 months after anterior cruciate ligament reconstruction.

Clin Biomech (Bristol, Avon) 2020 06 22;76:105014. Epub 2020 Apr 22.

MOTION Science Institute, Department of Exercise and Sport Science, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States; Human Movement Science Curriculum, University of North Carolina at Chapel Hill, Chapel Hill, NC, United States.

Background: We aimed to determine the effect of cueing an increase or decrease in the vertical ground reaction force impact peak (peak in the first 50% of stance) on vertical ground reaction force, knee flexion angle, internal knee extension moment, and internal knee abduction moment waveforms throughout stance in individuals 6-12 months after an anterior cruciate ligament reconstruction.

Methods: Twelve individuals completed 3 conditions (High, Low, and Control) where High and Low Conditions cue a 5% body weight increase or decrease, respectively, in the vertical ground reaction force impact peak compared to usual walking. Biomechanics during High and Low Conditions were compared to the Control Condition throughout stance.

Findings: The High Condition resulted in: (a) increased vertical ground reaction forces at each peak and decreased during mid-stance, (b) greater knee excursion (i.e., greater knee flexion angle in early stance and a more extended knee in late stance), (c) greater internal extension moment for the majority of stance, and (d) lesser second internal knee abduction moment peak. The Low Condition resulted in: (a) vertical ground reaction forces decreased during early stance and increased during mid-stance, (b) decreased knee excursion, (c) increased internal extension moment throughout stance, and (d) decreased internal knee abduction moment peaks.

Interpretation: Cueing a 5% body weight increase in vertical ground reaction force impact peak resulted in a more dynamic vertical ground reaction force loading pattern, increased knee excursion, and a greater internal extension moment during stance which may be useful in restoring gait patterns following anterior cruciate ligament reconstruction.
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http://dx.doi.org/10.1016/j.clinbiomech.2020.105014DOI Listing
June 2020

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

J Biomech 2020 05 28;104:109710. Epub 2020 Feb 28.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, USA. Electronic address:

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 suggests that 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.
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http://dx.doi.org/10.1016/j.jbiomech.2020.109710DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7480460PMC
May 2020

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

PLoS One 2020 10;15(3):e0230202. Epub 2020 Mar 10.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, United States of America.

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.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0230202PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7064213PMC
June 2020

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

Gait Posture 2020 03 21;77:89-94. Epub 2020 Jan 21.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, USA. Electronic address:

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.
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http://dx.doi.org/10.1016/j.gaitpost.2020.01.018DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7479307PMC
March 2020

Visuomotor error augmentation affects mediolateral head and trunk stabilization during walking.

Hum Mov Sci 2019 Dec 13;68:102525. Epub 2019 Nov 13.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill, Chapel Hill, NC and North Carolina State University, Raleigh, NC, USA.

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.
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http://dx.doi.org/10.1016/j.humov.2019.102525DOI Listing
December 2019

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

J Biomech 2020 01 22;98:109440. Epub 2019 Oct 22.

University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, United States.

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.
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http://dx.doi.org/10.1016/j.jbiomech.2019.109440DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7245140PMC
January 2020

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

J Biomech 2020 01 23;98:109449. Epub 2019 Oct 23.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, USA. Electronic address:

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 (R ≥ 0.80, p < 0.01) and group average (R ≥ 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: R = 0.43, p = 0.028) but those effects 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.
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http://dx.doi.org/10.1016/j.jbiomech.2019.109449DOI Listing
January 2020

Time-dependent tuning of balance control and aftereffects following optical flow perturbation training in older adults.

J Neuroeng Rehabil 2019 07 1;16(1):81. Epub 2019 Jul 1.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, 10206C Mary Ellen Jones Building, CB 7575, Chapel Hill, NC, 27599, USA.

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: Thirteen older adults participated in a randomized, crossover design performed on different days that included 10 min 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.

Trial Registration: clinicaltrials.gov ( NCT03341728 ). Registered 14 November 2017.
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http://dx.doi.org/10.1186/s12984-019-0555-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6604156PMC
July 2019

Increasing the Propulsive Demands of Walking to their Maximum Elucidates Functionally Limiting Impairments in Older Adult Gait.

J Aging Phys Act 2020 01 1;28(1):1-8. Epub 2020 Jan 1.

1 Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC.

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 biomechanical changes that do not directly limit 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.
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http://dx.doi.org/10.1123/japa.2018-0327DOI Listing
January 2020

Triceps surae muscle-subtendon interaction differs between young and older adults.

Connect Tissue Res 2020 01 22;61(1):104-113. Epub 2019 May 22.

Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, USA.

: Mechanical power generated via triceps surae muscle-tendon interaction during walking is important for walking performance. This interaction is made complex by distinct "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. However, its unclear if and how these changes affect muscle contractile behavior.: We investigated aging effects on triceps surae muscle-subtendon interaction using dual-probe ultrasound imaging during isolated muscle 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.: 9 younger subjects (age: 25.1 ± 5.6 years) and 10 older adult subjects (age: 74.3 ± 3.4 years) completed a series of ramped maximum isometric voluntary contractions at ankle angles spanning 0° (neutral) to 30° plantarflexion. Two linear array ultrasound transducers simultaneously recorded GAS and SOL fascicle kinematics and tissue displacements in their associated tendinous structures.: We revealed that older adults have more uniform subtendon tissue displacements that extend to anatomically consistent and potentially unfavorable changes in muscle contractile behavior - evidenced by smaller differences between gastrocnemius and soleus 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.
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http://dx.doi.org/10.1080/03008207.2019.1612384DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6872920PMC
January 2020
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