@article{robertson_vadakkeveedu_sawicki_2017, title={A benchtop biorobotic platform for in vitro observation of muscle-tendon dynamics with parallel mechanical assistance from an elastic exoskeleton}, volume={57}, ISSN={["1873-2380"]}, DOI={10.1016/j.jbiomech.2017.03.009}, abstractNote={We present a novel biorobotic framework comprised of a biological muscle-tendon unit (MTU) mechanically coupled to a feedback controlled robotic environment simulation that mimics in vivo inertial/gravitational loading and mechanical assistance from a parallel elastic exoskeleton. Using this system, we applied select combinations of biological muscle activation (modulated with rate-coded direct neural stimulation) and parallel elastic assistance (applied via closed-loop mechanical environment simulation) hypothesized to mimic human behavior based on previously published modeling studies. These conditions resulted in constant system-level force-length dynamics (i.e., stiffness), reduced biological loads, increased muscle excursion, and constant muscle average positive power output—all consistent with laboratory experiments on intact humans during exoskeleton assisted hopping. Mechanical assistance led to reduced estimated metabolic cost and MTU apparent efficiency, but increased apparent efficiency for the MTU + Exo system as a whole. Findings from this study suggest that the increased natural resonant frequency of the artificially stiffened MTU + Exo system, along with invariant movement frequencies, may underlie observed limits on the benefits of exoskeleton assistance. Our novel approach demonstrates that it is possible to capture the salient features of human locomotion with exoskeleton assistance in an isolated muscle-tendon preparation, and introduces a powerful new tool for detailed, direct examination of how assistive devices affect muscle-level neuromechanics and energetics.}, journal={JOURNAL OF BIOMECHANICS}, author={Robertson, Benjamin D. and Vadakkeveedu, Siddarth and Sawicki, Gregory S.}, year={2017}, month={May}, pages={8–17} }
 @article{robertson_farris_sawicki_2015, title={More is not always better: Modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics (vol 9, 046018, 2014)}, volume={10}, number={1}, journal={Bioinspiration & Biomimetics}, author={Robertson, B. D. and Farris, D. J. and Sawicki, G. S.}, year={2015} }
 @article{sawicki_robertson_azizi_roberts_2015, title={Timing matters: tuning the mechanics of a muscle-tendon unit by adjusting stimulation phase during cyclic contractions}, volume={218}, ISSN={["1477-9145"]}, DOI={10.1242/jeb.121673}, abstractNote={A growing body of research on the mechanics and energetics of terrestrial locomotion has demonstrated that elastic elements acting in series with contracting muscle are critical components of sustained, stable, and efficient gait. Far fewer studies have examined how the nervous system modulates muscle-tendon interaction dynamics to optimize ‘tuning’ or meet varying locomotor demands. To explore the fundamental neuromechanical rules that govern the interactions between series elastic (SEE) and contractile (CE) elements within a compliant muscle-tendon unit (MTU), we used a novel work loop approach that included implanted sonomicrometry crystals along muscle fascicles. This enabled us to de-couple CE and SEE length trajectories when cyclic strain patterns were applied to an isolated plantaris MTU from the bullfrog (Lithobates catesbeianus). Using this approach, we demonstrate that the onset timing of muscle stimulation (i.e., stimulation phase) that involves a symmetrical MTU stretch-shorten cycle during active force production, results in net zero mechanical power output, and maximal decoupling of CE and MTU length trajectories. We found it difficult to ‘tune’ the muscle-tendon system for strut-like isometric force production by adjusting stimulation phase only, as the zero power output condition involved significant positive and negative mechanical work by the CE. A simple neural mechanism- adjusting muscle stimulation phase- could shift a MTU from performing net zero to net positive (energy producing) or net negative (energy absorbing) mechanical work under conditions of changing locomotor demand. Finally, we show that modifications to the classical work loop paradigm better represent in vivo muscle-tendon function during locomotion.}, number={19}, journal={JOURNAL OF EXPERIMENTAL BIOLOGY}, author={Sawicki, Gregory S. and Robertson, Benjamin D. and Azizi, Emanuel and Roberts, Thomas J.}, year={2015}, month={Oct}, pages={3150–3159} }
 @article{robertson_sawicki_2015, title={Unconstrained muscle-tendon workloops indicate resonance tuning as a mechanism for elastic limb behavior during terrestrial locomotion}, volume={112}, number={43}, journal={Proceedings of the National Academy of Sciences of the United States of America}, author={Robertson, B. D. and Sawicki, G. S.}, year={2015}, pages={E5891–5898} }
 @article{robertson_sawicki_2014, title={Exploiting elasticity: Modeling the influence of neural control on mechanics and energetics of ankle muscle-tendons during human hopping}, volume={353}, ISSN={["1095-8541"]}, DOI={10.1016/j.jtbi.2014.03.010}, abstractNote={We present a simplified Hill-type model of the human triceps surae-Achilles tendon complex working on a gravitational-inertial load during cyclic contractions (i.e. vertical hopping). Our goal was to determine the role that neural control plays in governing muscle, or contractile element (CE), and tendon, or series elastic element (SEE), mechanics and energetics within a compliant muscle-tendon unit (MTU). We constructed a 2D parameter space consisting of many combinations of stimulation frequency and magnitude (i.e. neural control strategies). We compared the performance of each control strategy by evaluating peak force and average positive mechanical power output for the system (MTU) and its respective components (CE, SEE), force-length (F-L) and -velocity (F-V) operating point of the CE during active force production, average metabolic rate for the CE, and both MTU and CE apparent efficiency. Our results suggest that frequency of stimulation plays a primary role in governing whole-MTU mechanics. These include the phasing of both activation and peak force relative to minimum MTU length, average positive power, and apparent efficiency. Stimulation amplitude was primarily responsible for governing average metabolic rate and within MTU mechanics, including peak force generation and elastic energy storage and return in the SEE. Frequency and amplitude of stimulation both played integral roles in determining CE F-L operating point, with both higher frequency and amplitude generally corresponding to lower CE strains, reduced injury risk, and elimination of the need for passive force generation in the CE parallel elastic element (PEE).}, journal={JOURNAL OF THEORETICAL BIOLOGY}, author={Robertson, Benjamin D. and Sawicki, Gregory S.}, year={2014}, month={Jul}, pages={121–132} }
 @article{robertson_farris_sawicki_2014, title={More is not always better: modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics}, volume={9}, ISSN={["1748-3190"]}, DOI={10.1088/1748-3182/9/4/046018}, abstractNote={Development of robotic exoskeletons to assist/enhance human locomotor performance involves lengthy prototyping, testing, and analysis. This process is further convoluted by variability in limb/body morphology and preferred gait patterns between individuals. In an attempt to expedite this process, and establish a physiological basis for actuator prescription, we developed a simple, predictive model of human neuromechanical adaptation to a passive elastic exoskeleton applied at the ankle joint during a functional task. We modeled the human triceps surae–Achilles tendon muscle tendon unit (MTU) as a single Hill-type muscle, or contractile element (CE), and series tendon, or series elastic element (SEE). This modeled system was placed under gravitational load and underwent cyclic stimulation at a regular frequency (i.e. hopping) with and without exoskeleton (Exo) assistance. We explored the effect that both Exo stiffness k Exo ?> and muscle activation A stim ?> had on combined MTU and Exo (MTU + Exo), MTU, and CE/SEE mechanics and energetics. Model accuracy was verified via qualitative and quantitative comparisons between modeled and prior experimental outcomes. We demonstrated that reduced A stim ?> can be traded for increased k Exo ?> to maintain consistent MTU + Exo mechanics (i.e. average positive power P ¯ mech + ?> output) from an unassisted condition (i.e. k Exo = 0 kN · m − 1 ?> ). For these regions of parameter space, our model predicted a reduction in MTU force, SEE energy cycling, and metabolic rate P ¯ met ?> , as well as constant CE P ¯ mech + ?> output compared to unassisted conditions. This agreed with previous experimental observations, demonstrating our model’s predictive ability. Model predictions also provided insight into mechanisms of metabolic cost minimization, and/or enhanced mechanical performance, and we concluded that both of these outcomes cannot be achieved simultaneously, and that one must come at the detriment of the other in a spring-assisted compliant MTU.}, number={4}, journal={BIOINSPIRATION & BIOMIMETICS}, author={Robertson, Benjamin D. and Farris, Dominic J. and Sawicki, Gregory S.}, year={2014}, month={Dec} }
 @article{farris_robertson_sawicki_2013, title={Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping}, volume={115}, ISSN={["1522-1601"]}, DOI={10.1152/japplphysiol.00253.2013}, abstractNote={Inspired by elastic energy storage and return in tendons of human leg muscle-tendon units (MTU), exoskeletons often place a spring in parallel with an MTU to assist the MTU. However, this might perturb the normally efficient MTU mechanics and actually increase active muscle mechanical work. This study tested the effects of elastic parallel assistance on MTU mechanics. Participants hopped with and without spring-loaded ankle exoskeletons that assisted plantar flexion. An inverse dynamics analysis, combined with in vivo ultrasound imaging of soleus fascicles and surface electromyography, was used to determine muscle-tendon mechanics and activations. Whole body net metabolic power was obtained from indirect calorimetry. When hopping with spring-loaded exoskeletons, soleus activation was reduced (30–70%) and so was the magnitude of soleus force (peak force reduced by 30%) and the average rate of soleus force generation (by 50%). Although forces were lower, average positive fascicle power remained unchanged, owing to increased fascicle excursion (+4–5 mm). Net metabolic power was reduced with exoskeleton assistance (19%). These findings highlighted that parallel assistance to a muscle with appreciable series elasticity may have some negative consequences, and that the metabolic cost associated with generating force may be more pronounced than the cost of doing work for these muscles.}, number={5}, journal={JOURNAL OF APPLIED PHYSIOLOGY}, author={Farris, Dominic James and Robertson, Benjamin D. and Sawicki, Gregory S.}, year={2013}, month={Sep}, pages={579–585} }
 @inproceedings{robertson_sawicki_2011, title={Influence of parallel spring-loaded exoskeleton on ankle muscle-tendon dynamics during simulated human hopping}, DOI={10.1109/iembs.2011.6090109}, abstractNote={Robotic assistance for rehabilitation and enhancement of human locomotion has become a major goal of biomedical engineers in recent years. While significant progress to this end has been made in the fields of neural interfacing and control systems, little has been done to examine the effects of mechanical assistance on the biomechanics of underlying muscle-tendon systems. Here, we model the effects of mechanical assistance via a passive spring acting in parallel with the triceps surae-Achilles tendon complex during cyclic hopping in humans. We examine system dynamics over a range of biological muscle activation and exoskeleton spring stiffness. We find that, in most cases, uniform cyclic mechanical power production of the coupled system is achieved. Furthermore, unassisted power production can be reproduced throughout parameter space by trading off decreases in muscle activation with increases in ankle exoskeleton spring stiffness. In addition, we show that as mechanical assistance increases the biological muscle-tendon unit becomes less ‘tuned’ resulting in higher mechanical power output from active components of muscle despite large reductions in required force output.}, booktitle={2011 Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC)}, author={Robertson, B. D. and Sawicki, G. S.}, year={2011}, pages={583–586} }