Dr. Franz joined two colleagues from BME this month in attending an HHMI/NSF sponsored program called “Mobile Summer Institutes on Scientific Teaching”. Together with Drs. Hubbard and Hu, Dr. Franz was also named a Scientific Teaching Fellow of the National Academies. The Institute provided a rigorous week-long program providing techniques and hands-on coaching and feedback on scientific teaching and student-centered learning. More information can be found here:

http://www.summerinstitutes.org/mobile-institutes

 

The result of an exciting new collaboration with Vanderbilt University, our most recent paper advances our fundamental understand of the accuracy, precision, drawbacks, and assumptions of our measurement techniques to interpret the functional role of muscle-tendon dynamics during movement.

Zelik KE and Franz JR. It’s positive to be negative: Achilles tendon work loops during human locomotion. PLoS One (In press).

Abstract. Ultrasound imaging is increasingly used with motion and force data to quantify tendon dynamics during human movement. Frequently, tendon dynamics are estimated indirectly from muscle fascicle kinematics (by subtracting muscle from muscle-tendon unit length), but there is mounting evidence that this Indirect approach yields implausible tendon work loops. Since tendons are passive viscoelastic structures, when they undergo a loading-unloading cycle they must exhibit a negative work loop (i.e., perform net negative work). However, prior studies using this Indirect approach report large positive work loops, often estimating that tendons return 2-5 J of elastic energy for every 1 J of energy stored. More direct ultrasound estimates of tendon kinematics have emerged that quantify tendon elongations by tracking either the muscle-tendon junction or localized tendon tissue. However, it is unclear if these yield more plausible estimates of tendon dynamics. Our objective was to compute tendon work loops and hysteresis losses using these two Direct tendon kinematics estimates during human walking. We found that Direct estimates generally resulted in negative work loops, with average tendon hysteresis losses of 2-11% at 1.25 m/s and 33-49% at 0.75 m/s (N=8), alluding to 0.51-0.98 J of tendon energy returned for every 1 J stored. We interpret this finding to suggest that Direct approaches provide more plausible estimates than the Indirect approach, and may be preferable for understanding tendon energy storage and return. However, the Direct approaches did exhibit speed-dependent trends that are not consistent with isolated, in vitro tendon hysteresis losses of about 5-10%. These trends suggest that Direct estimates also contain some level of error, albeit much smaller than Indirect estimates. Overall, this study serves to highlight the complexity and difficulty of estimating tendon dynamics non-invasively, and the care that must be taken to interpret biological function from current ultrasound-based estimates.

 

Orselli MI, Franz JR, Thelen DG. The effect of Achilles tendon compliance on triceps surae mechanics and energetics in walking. Journal of Biomechanics (In press).

Abstract. Achilles tendon (AT) compliance can affect the generation and transmission of triceps surae muscle forces, and thus has important biomechanical consequences for walking performance. However, the uniarticular soleus (SOL) and the biarticular (GAS) function differently during walking, with in vivo evidence suggesting that their associated fascicles and tendinous structures exhibit unique kinematics during walking. Given the strong association between muscle fiber length, velocity and force production, we conjectured that SOL and GAS mechanics and energetic behavior would respond differently to altered AT compliance. To test this, we characterized GAS and SOL muscle and tendon mechanics and energetics due to systematic changes in tendon compliance using musculoskeletal simulations of walking. Increased tendon compliance enlarged GAS and SOL tendon excursions, shortened fiber operation lengths and affected muscle excitation patterns. For both muscles, an optimal tendon compliance (tendon strains of approximately 5% with maximum isometric force) existed that minimized metabolic energy consumption. However, GAS muscle-tendon mechanics and energetics were significantly more sensitive to changes in tendon compliance than were those for SOL. In addition, GAS was not able to return stored tendon energy during push-off as effectively as SOL, particularly for larger values of tendon compliance. These fundamental differences between GAS and SOL sensitivity to altered tendon compliance seem to arise from the biarticular nature of GAS. These insights are potentially important for understanding the functional consequences of altered Achilles tendon compliance due to aging, injury, or disease.

The Applied Biomechanics Laboratory would like to congratulate Jessica Thompson for successfully defending her Master’s thesis in the Joint Department of Biomedical Engineering. Her thesis was titled “The Kinematic and Electromyographic Response to Optical Flow Balance Perturbations in Walking: Visuomotor Adaptation and the Acute Effects of Age and Falls History”. Excellent work!

Can virtual reality help us prevent falls in the elderly and others?

For the elderly and people with neurodegenerative conditions, balance is not taken for granted. UNC and NC State biomedical engineers are using a new virtual reality system that might one day be used to reveal balance impairments currently undetectable during conventional testing or normal walking.

Congratulations to Ashish Khanchandani on successfully presenting his research at the UNC Undegraduate Research Symposium. Ashish, in collaboration with other undergraduate and graduate students, is working on a project titled “The effects of physiological loading on dynamic variations in the Achilles tendon moment arm.” Nice work, Ashish!

The UNC/NCSU Applied Biomechanics Lab had a terrific showing at the 2017 Human Movement Science and Biomechanics Symposium this week! Congratulations to all our students and trainees!

Abstracts Presented:

Clark WH and Franz JR, Do triceps surae muscle dynamics govern non-uniform Achilles tendon deformations? Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Nuckols RW, Dick TJM, Franz JR, Sawicki GS, Using elastic ankle exoskeletons to counteract age-related structure-function deficits. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Qiao M, Feld JA, and Franz JR. Aging effects on leg joint variability during walking in the presence of optical flow perturbations. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Browne MG and Franz JR, Does dynamic stability govern propulsive force generation in human walking? Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Conway KA and Franz JR, The functional utilization of propulsive capacity during human walking. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

Thompson JD and Franz JR, Age and falls history effects on antagonist leg muscle coactivation during walking with optical flow perturbations. Human Movement Science and Biomechanics Symposium. Chapel Hill, NC. March 2017.

 

 

With our work on dynamic balance control during walking in the presence of optical flow perturbations, we often wonder whether the visuomotor control of walking balance adapts to such perturbations over time. Such adaptation may arise from multisensory reweighting, the central process that determines the relative priority placed on somatosensory , visual, and vestibular feedback. Our most recent work investigating the propensity for visuomotor adaptation in human balance control was recently accepted for publication in the journal Human Movement Science.

Thompson JD and Franz JR. Do kinematic metrics of walking balance control adapt to perturbed optical flow? Human Movement Science (In Press).

Abstract. Visual (i.e., optical flow) perturbations can be used to study balance control and balance deficits. However, it remains unclear whether walking balance control adapts to such perturbations over time. Our purpose was to investigate the propensity for visuomotor adaptation in walking balance control using prolonged exposure to optical flow perturbations. Ten subjects (age: 25.4 ± 3.8 years) walked on a treadmill while watching a speed-matched virtual hallway with and without continuous mediolateral optical flow perturbations of three different amplitudes. Each of three perturbation trials consisted of 8 minutes of prolonged exposure followed by 1 min of unperturbed walking. Using 3D motion capture, we analyzed changes in foot placement kinematics and mediolateral sacrum motion. At their onset, perturbations elicited wider and shorter steps, alluding to a more cautious, general anticipatory balance control strategy. As perturbations continued, foot placement tended toward values seen during unperturbed walking while step width variability and mediolateral sacrum motion concurrently increased. Our findings suggest that subjects progressively shifted from a general anticipatory balance control strategy to a reactive, task-specific strategy using step-to-step adjustments. Prolonged exposure to optical flow perturbations may have clinical utility to reinforce reactive, task-specific balance control through training.

A major undertaking with our collaborators in the Department of Industrial and Systems Engineering at NC State, our systematic review of neuroimaging in the study of static and dynamic balance control has been accepted for publication in the journal Frontiers in Human Neuroscience.

Wittenberg E, Thompson J, Nam CS, Franz JR, Neuroimaging of human balance control: A systematic review. Frontiers in Human Neuroscience (In press).

Abstract. This review examined 83 articles using neuroimaging modalities to investigate the neural correlates underlying static and dynamic human balance control, with aims to support future mobile neuroimaging research in the balance control domain. Furthermore, this review analyzed the mobility of the neuroimaging hardware and research paradigms as well as the analytical methodology to identify and remove movement artifact in the acquired brain signal. We found that the majority of static balance control tasks utilized mechanical perturbations to invoke feet-in-place responses (27 out of 38 studies), while cognitive dual-task conditions were commonly used to challenge balance in dynamic balance control tasks (20 out of 32 studies). While frequency analysis and event related potential characteristics supported enhanced brain activation during static balance control, that in dynamic balance control studies was supported by spatial and frequency analysis. Twenty-three of the 50 studies utilizing EEG utilized independent component analysis to remove movement artifacts from the acquired brain signals. Lastly, only eight studies used truly mobile neuroimaging hardware systems. This review provides evidence to support an increase in brain activation in balance control tasks, regardless of mechanical, cognitive, or sensory challenges. Furthermore, the current body of literature demonstrates the use of advanced signal processing methodologies to analyze brain activity during movement. However, the static nature of neuroimaging hardware and conventional balance control paradigms prevent full mobility and limit our knowledge of neural mechanisms underlying balance control.

 

Our recent work into the neuromuscular origins of step-to-step corrective motor responses underlying balance control during human walking has been accepted for publication in Nature: Scientific Reports.

Stokes HE, Thompson, JD, and Franz JR. The neuromuscular origins of kinematic variability during perturbed walking. Nature: Scientific Reports (In Press)

 

Abstract: We investigated the neuromuscular contributions to kinematic variability and thus step to step adjustments in posture and foot placement across a range of walking speeds in response to optical flow perturbations of different amplitudes applied in a custom virtual environment. We found that perturbations significantly increased step width, decreased step length, and elicited larger trunk sway compared to normal walking. However, perturbation-induced effects on the corresponding variabilities of these measurements were much more profound. Consistent with our hypotheses, we found that: (1) perturbations increased EMG activity of the gluteus medius and postural control muscles during leg swing, and increase the coactivation of antagonistic leg muscles during limb loading in early stance, and (2) changes in the magnitude of step to step adjustments in postural sway and lateral foot placement positively correlated with those of postural control and gluteus medius muscle activities, respectively, in response to perturbations. However, (3) interactions between walking speed and susceptibility to perturbations, when present, were more complex than anticipated. Our study provides important mechanistic neuromuscular insight into walking balance control and important reference values for the emergence of balance impairment.