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Cohn BA, Valero-Cuevas FJ. Muscle redundancy is greatly reduced by the spatiotemporal nature of neuromuscular control. FRONTIERS IN REHABILITATION SCIENCES 2023; 4:1248269. [PMID: 38028155 PMCID: PMC10663283 DOI: 10.3389/fresc.2023.1248269] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 06/26/2023] [Accepted: 10/11/2023] [Indexed: 12/01/2023]
Abstract
Animals must control numerous muscles to produce forces and movements with their limbs. Current theories of motor optimization and synergistic control are predicated on the assumption that there are multiple highly diverse feasible activations for any motor task ("muscle redundancy"). Here, we demonstrate that the dimensionality of the neuromuscular control problem is greatly reduced when adding the temporal constraints inherent to any sequence of motor commands: the physiological time constants for muscle activation-contraction dynamics. We used a seven-muscle model of a human finger to fully characterize the seven-dimensional polytope of all possible motor commands that can produce fingertip force vector in any direction in 3D, in alignment with the core models of Feasibility Theory. For a given sequence of seven force vectors lasting 300 ms, a novel single-step extended linear program finds the 49-dimensional polytope of all possible motor commands that can produce the sequence of forces. We find that muscle redundancy is severely reduced when the temporal limits on muscle activation-contraction dynamics are added. For example, allowing a generous ± 12% change in muscle activation within 50 ms allows visiting only ∼ 7% of the feasible activation space in the next time step. By considering that every motor command conditions future commands, we find that the motor-control landscape is much more highly structured and spatially constrained than previously recognized. We discuss how this challenges traditional computational and conceptual theories of motor control and neurorehabilitation for which muscle redundancy is a foundational assumption.
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Affiliation(s)
- Brian A. Cohn
- Department of Computer Science, University of Southern California, Los Angeles, CA, United States
| | - Francisco J. Valero-Cuevas
- Department of Computer Science, University of Southern California, Los Angeles, CA, United States
- Department of Biomedical Engineering, Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA, United States
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Mulla DM, Keir PJ. Neuromuscular control: from a biomechanist's perspective. Front Sports Act Living 2023; 5:1217009. [PMID: 37476161 PMCID: PMC10355330 DOI: 10.3389/fspor.2023.1217009] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/04/2023] [Accepted: 06/21/2023] [Indexed: 07/22/2023] Open
Abstract
Understanding neural control of movement necessitates a collaborative approach between many disciplines, including biomechanics, neuroscience, and motor control. Biomechanics grounds us to the laws of physics that our musculoskeletal system must obey. Neuroscience reveals the inner workings of our nervous system that functions to control our body. Motor control investigates the coordinated motor behaviours we display when interacting with our environment. The combined efforts across the many disciplines aimed at understanding human movement has resulted in a rich and rapidly growing body of literature overflowing with theories, models, and experimental paradigms. As a result, gathering knowledge and drawing connections between the overlapping but seemingly disparate fields can be an overwhelming endeavour. This review paper evolved as a need for us to learn of the diverse perspectives underlying current understanding of neuromuscular control. The purpose of our review paper is to integrate ideas from biomechanics, neuroscience, and motor control to better understand how we voluntarily control our muscles. As biomechanists, we approach this paper starting from a biomechanical modelling framework. We first define the theoretical solutions (i.e., muscle activity patterns) that an individual could feasibly use to complete a motor task. The theoretical solutions will be compared to experimental findings and reveal that individuals display structured muscle activity patterns that do not span the entire theoretical solution space. Prevalent neuromuscular control theories will be discussed in length, highlighting optimality, probabilistic principles, and neuromechanical constraints, that may guide individuals to families of muscle activity solutions within what is theoretically possible. Our intention is for this paper to serve as a primer for the neuromuscular control scientific community by introducing and integrating many of the ideas common across disciplines today, as well as inspire future work to improve the representation of neural control in biomechanical models.
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Yeo SH, Verheul J, Herzog W, Sueda S. Numerical instability of Hill-type muscle models. J R Soc Interface 2023; 20:20220430. [PMID: 36722069 PMCID: PMC9890125 DOI: 10.1098/rsif.2022.0430] [Citation(s) in RCA: 6] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2022] [Accepted: 12/13/2022] [Indexed: 02/02/2023] Open
Abstract
Hill-type muscle models are highly preferred as phenomenological models for musculoskeletal simulation studies despite their introduction almost a century ago. The use of simple Hill-type models in simulations, instead of more recent cross-bridge models, is well justified since computationally 'light-weight'-although less accurate-Hill-type models have great value for large-scale simulations. However, this article aims to invite discussion on numerical instability issues of Hill-type muscle models in simulation studies, which can lead to computational failures and, therefore, cannot be simply dismissed as an inevitable but acceptable consequence of simplification. We will first revisit the basic premises and assumptions on the force-length and force-velocity relationships that Hill-type models are based upon, and their often overlooked but major theoretical limitations. We will then use several simple conceptual simulation studies to discuss how these numerical instability issues can manifest as practical computational problems. Lastly, we will review how such numerical instability issues are dealt with, mostly in an ad hoc fashion, in two main areas of application: musculoskeletal biomechanics and computer animation.
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Affiliation(s)
- Sang-Hoon Yeo
- School of Sport, Exercise & Rehabilitation Sciences, University of Birmingham, Birmingham, UK
| | - Jasper Verheul
- School of Sport, Exercise & Rehabilitation Sciences, University of Birmingham, Birmingham, UK
- Cardiff School of Sport and Health Sciences, Cardiff Metropolitan University, Cardiff, UK
| | - Walter Herzog
- Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada
| | - Shinjiro Sueda
- Department of Computer Science and Engineering, Texas A&M University, College Station, TX, USA
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Lu Y, Ni X, Yu J, Ni X, Sun Z, Wang J, Sun S, Wang J. Lower limb immobilization device induced small setup errors in the radiotherapy. Medicine (Baltimore) 2018; 97:e0384. [PMID: 29642197 PMCID: PMC5908616 DOI: 10.1097/md.0000000000010384] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
The aim of this study was to design a lower limb immobilization device and investigate its clinical application in the radiotherapy of the lower limbs.Around 38 patients who underwent lower limb radiotherapy using the designed immobilization device were included in this study. The setup errors were calculated by comparison of the portal images and the simulator films or digital reconstructed radiographs (DRRs).From all 38 patients accomplished the radiotherapy using this device, 178 anteroposterior portal images and 178 lateral portal images were used for the analysis of the positional accuracy. Significant differences were observed in the setup error of the head-foot direction compared with the left-right direction (t = 3.404, P = .002) and the anterior-posterior directions (t = 3.188, P = .003). No statistical differences were identified in the setup error in the left-right direction and anterior-posterior direction (t = 0.497, P = .622).The use of the in-house designed lower limb immobilization device allowed for relatively small setup errors. Furthermore, it showed satisfactory accuracy and repeatability.
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Laine CM, Valero-Cuevas FJ. Intermuscular coherence reflects functional coordination. J Neurophysiol 2017; 118:1775-1783. [PMID: 28659460 DOI: 10.1152/jn.00204.2017] [Citation(s) in RCA: 54] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2017] [Revised: 06/26/2017] [Accepted: 06/27/2017] [Indexed: 12/11/2022] Open
Abstract
Coherence analysis has the ability to identify the presence of common descending drive shared by motor unit pools and reveals its spectral properties. However, the link between spectral properties of shared neural drive and functional interactions among muscles remains unclear. We assessed shared neural drive between muscles of the thumb and index finger while participants executed two mechanically distinct precision pinch tasks, each requiring distinct functional coordination among muscles. We found that shared neural drive was systematically reduced or enhanced at specific frequencies of interest (~10 and ~40 Hz). While amplitude correlations between surface EMG signals also exhibited changes across tasks, only their coherence has strong physiological underpinnings indicative of neural binding. Our results support the use of intermuscular coherence as a tool to detect when coactivated muscles are members of a functional group or synergy of neural origin. Furthermore, our results demonstrate the advantages of considering neural binding at 10, ~20, and >30 Hz, as indicators of task-dependent neural coordination strategies.NEW & NOTEWORTHY It is often unclear whether correlated activity among muscles reflects their neural binding or simply reflects the constraints defining the task. Using the fact that high-frequency coherence between EMG signals (>6 Hz) is thought to reflect shared neural drive, we demonstrate that coherence analysis can reveal the neural origin of distinct muscle coordination patterns required by different tasks.
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Affiliation(s)
- Christopher M Laine
- Brain-Body Dynamics Laboratory, Department of Biomedical Engineering, Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, California
| | - Francisco J Valero-Cuevas
- Brain-Body Dynamics Laboratory, Department of Biomedical Engineering, Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, California
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von Walden F, Jalaleddini K, Evertsson B, Friberg J, Valero-Cuevas FJ, Pontén E. Forearm Flexor Muscles in Children with Cerebral Palsy Are Weak, Thin and Stiff. Front Comput Neurosci 2017; 11:30. [PMID: 28487645 PMCID: PMC5403928 DOI: 10.3389/fncom.2017.00030] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2016] [Accepted: 04/07/2017] [Indexed: 11/26/2022] Open
Abstract
Children with cerebral palsy (CP) often develop reduced passive range of motion with age. The determining factor underlying this process is believed to be progressive development of contracture in skeletal muscle that likely changes the biomechanics of the joints. Consequently, to identify the underlying mechanisms, we modeled the mechanical characteristics of the forearm flexors acting across the wrist joint. We investigated skeletal muscle strength (Grippit®) and passive stiffness and viscosity of the forearm flexors in 15 typically developing (TD) children (10 boys/5 girls, mean age 12 years, range 8–18 yrs) and nine children with CP Nine children (6 boys/3 girls, mean age 11 ± 3 years (yrs), range 7–15 yrs) using the NeuroFlexor® apparatus. The muscle stiffness we estimate and report is the instantaneous mechanical response of the tissue that is independent of reflex activity. Furthermore, we assessed cross-sectional area of the flexor carpi radialis (FCR) muscle using ultrasound. Age and body weight did not differ significantly between the two groups. Children with CP had a significantly weaker (−65%, p < 0.01) grip and had smaller cross-sectional area (−43%, p < 0.01) of the FCR muscle. Passive stiffness of the forearm muscles in children with CP was increased 2-fold (p < 0.05) whereas viscosity did not differ significantly between CP and TD children. FCR cross-sectional area correlated to age (R2 = 0.58, p < 0.01), body weight (R2 = 0.92, p < 0.0001) and grip strength (R2 = 0.82, p < 0.0001) in TD children but only to grip strength (R2 = 0.60, p < 0.05) in children with CP. We conclude that children with CP have weaker, thinner, and stiffer forearm flexors as compared to typically developing children.
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Affiliation(s)
- Ferdinand von Walden
- Department of Women's and Children's Health, Karolinska InstituteStockholm, Sweden
| | - Kian Jalaleddini
- Division of Biokinesiology and Physical Therapy, University of Southern CaliforniaLos Angeles, CA, USA
| | - Björn Evertsson
- Department of Neurology, Karolinska Hospital HuddingeStockholm, Sweden.,Karolinska InstituteStockholm, Sweden
| | - Johanna Friberg
- Department of Women's and Children's Health, Karolinska InstituteStockholm, Sweden.,Karolinska InstituteStockholm, Sweden
| | - Francisco J Valero-Cuevas
- Division of Biokinesiology and Physical Therapy, University of Southern CaliforniaLos Angeles, CA, USA.,Department of Biomedical Engineering, University of Southern CaliforniaLos Angeles, CA, USA
| | - Eva Pontén
- Department of Women's and Children's Health, Karolinska InstituteStockholm, Sweden.,Department of Pediatric orthopedic Surgery, Astrid Lindgren's Children HospitalStockholm, Sweden
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Jalaleddini K, Minos Niu C, Chakravarthi Raja S, Joon Sohn W, Loeb GE, Sanger TD, Valero-Cuevas FJ. Neuromorphic meets neuromechanics, part II: the role of fusimotor drive. J Neural Eng 2017; 14:025002. [PMID: 28094764 DOI: 10.1088/1741-2552/aa59bd] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
OBJECTIVE We studied the fundamentals of muscle afferentation by building a Neuro-mechano-morphic system actuating a cadaveric finger. This system is a faithful implementation of the stretch reflex circuitry. It allowed the systematic exploration of the effects of different fusimotor drives to the muscle spindle on the closed-loop stretch reflex response. APPROACH As in Part I of this work, sensory neurons conveyed proprioceptive information from muscle spindles (with static and dynamic fusimotor drive) to populations of α-motor neurons (with recruitment and rate coding properties). The motor commands were transformed into tendon forces by a Hill-type muscle model (with activation-contraction dynamics) via brushless DC motors. Two independent afferented muscles emulated the forces of flexor digitorum profundus and the extensor indicis proprius muscles, forming an antagonist pair at the metacarpophalangeal joint of a cadaveric index finger. We measured the physical response to repetitions of bi-directional ramp-and-hold rotational perturbations for 81 combinations of static and dynamic fusimotor drives, across four ramp velocities, and three levels of constant cortical drive to the α-motor neuron pool. MAIN RESULTS We found that this system produced responses compatible with the physiological literature. Fusimotor and cortical drives had nonlinear effects on the reflex forces. In particular, only cortical drive affected the sensitivity of reflex forces to static fusimotor drive. In contrast, both static fusimotor and cortical drives reduced the sensitivity to dynamic fusimotor drive. Interestingly, realistic signal-dependent motor noise emerged naturally in our system without having been explicitly modeled. SIGNIFICANCE We demonstrate that these fundamental features of spinal afferentation sufficed to produce muscle function. As such, our Neuro-mechano-morphic system is a viable platform to study the spinal mechanisms for healthy muscle function-and its pathologies such as dystonia and spasticity. In addition, it is a working prototype of a robust biomorphic controller for compliant robotic limbs and exoskeletons.
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Affiliation(s)
- Kian Jalaleddini
- Division of Biokinesiology and Physical Therapy, University of Southern California, CA, United States of America
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Mueller S, Martin S, Schwarz M, Grunwald M. Temporal Modulations of Contact Force during Haptic Surface Exploration. PLoS One 2016; 11:e0152897. [PMID: 27073843 PMCID: PMC4830569 DOI: 10.1371/journal.pone.0152897] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2015] [Accepted: 03/21/2016] [Indexed: 11/18/2022] Open
Abstract
Individuals constantly modulate their exploratory movements and adapt their internal hypotheses to incoming sensory information to achieve a thorough and realistic percept. Perception depends on the exploratory movements as well as influencing them. While this seems to be common sense, scientifically we know very little about the temporal dynamics during haptic exploration. To address this, we investigated the exploratory force modulations of two groups of healthy young adults during the exploration of grated surfaces with differing detection difficulty during successive (n = 20) and random stimulus presentation (n = 20). Results showed that exploratory force depended on stimulus properties and increased with increasing detection difficulty. Both experiments yielded the same direction of results with slightly smaller effects in the random stimulus presentation group. Across exploration time average fingertip force also increased. The biggest increase occurred systematically at the beginning (within the first 40 percent) of exploration time per stimulus indicating that most critical information is received during the initial contact phase and is directly transformed into the exploration procedure and force application. Furthermore, video-analyses and comparisons to our high temporal resolution data revealed strong dynamic changes in pressure application during test stimulus exploration with differences in the force dynamics and exploration strategies of simple and difficult stimuli.
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Affiliation(s)
- Stephanie Mueller
- Haptic Research Laboratory, Paul-Flechsig-Institute for Brain Research, Medical Faculty, University of Leipzig, Leipzig, Germany
| | - Sven Martin
- Haptic Research Laboratory, Paul-Flechsig-Institute for Brain Research, Medical Faculty, University of Leipzig, Leipzig, Germany
| | - Michael Schwarz
- Haptic Research Laboratory, Paul-Flechsig-Institute for Brain Research, Medical Faculty, University of Leipzig, Leipzig, Germany
| | - Martin Grunwald
- Haptic Research Laboratory, Paul-Flechsig-Institute for Brain Research, Medical Faculty, University of Leipzig, Leipzig, Germany
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9
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Joshi MN, Keenan KG. Force fluctuations while pressing and moving against high- and low-friction touch screen surfaces. Exp Brain Res 2016; 234:1893-1901. [PMID: 26898313 DOI: 10.1007/s00221-016-4581-0] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2015] [Accepted: 01/30/2016] [Indexed: 11/25/2022]
Abstract
The purpose of this study was to identify the influence of a high- and low-friction surface on the ability to maintain a steady downward force during an index finger pressing and moving task. Fifteen right-handed subjects (24-48 years) performed a static force pressing task and a hybrid pressing and moving task on the surface of an iPad mini while holding a steady 2-N force on high- and low-friction surfaces. Variability of force was quantified as the standard deviation (SD) of normal force (F z) and shear force (F xy) across friction conditions and tasks. The SD of F z was 227 % greater during the hybrid task as compared to the static task (p < .001) and was 19 % greater for the high- versus low-friction condition (p = .033). There were positive correlations between SD of F z and F xy during the hybrid force/motion tasks on the high- and low-friction conditions (r (2) = 0.5 and 0.86, respectively), suggesting significant associations between normal and shear forces for this hybrid task. The correlation between the SD of F z for static and hybrid tasks was r (2) = 0.44, indicating that the common practice of examining the control of static tasks may not sufficiently explain performance during hybrid tasks, at least for the young subjects tested in the current study. As activities of daily living frequently require hybrid force/motion tasks (e.g., writing, doing the dishes, and cleaning counters), the results of this study emphasize the need to study motor performance during hybrid tasks in addition to static force tasks.
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Affiliation(s)
- Mukta N Joshi
- Department of Kinesiology, University of Wisconsin-Milwaukee, Milwaukee, WI, 53201, USA
- Center for Aging and Translational Research, University of Wisconsin-Milwaukee, Milwaukee, WI, USA
| | - Kevin G Keenan
- Department of Kinesiology, University of Wisconsin-Milwaukee, Milwaukee, WI, 53201, USA.
- Center for Aging and Translational Research, University of Wisconsin-Milwaukee, Milwaukee, WI, USA.
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10
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Inouye JM, Valero-Cuevas FJ. Muscle Synergies Heavily Influence the Neural Control of Arm Endpoint Stiffness and Energy Consumption. PLoS Comput Biol 2016; 12:e1004737. [PMID: 26867014 PMCID: PMC4750997 DOI: 10.1371/journal.pcbi.1004737] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2015] [Accepted: 01/05/2016] [Indexed: 11/18/2022] Open
Abstract
Much debate has arisen from research on muscle synergies with respect to both limb impedance control and energy consumption. Studies of limb impedance control in the context of reaching movements and postural tasks have produced divergent findings, and this study explores whether the use of synergies by the central nervous system (CNS) can resolve these findings and also provide insights on mechanisms of energy consumption. In this study, we phrase these debates at the conceptual level of interactions between neural degrees of freedom and tasks constraints. This allows us to examine the ability of experimentally-observed synergies—correlated muscle activations—to control both energy consumption and the stiffness component of limb endpoint impedance. In our nominal 6-muscle planar arm model, muscle synergies and the desired size, shape, and orientation of endpoint stiffness ellipses, are expressed as linear constraints that define the set of feasible muscle activation patterns. Quadratic programming allows us to predict whether and how energy consumption can be minimized throughout the workspace of the limb given those linear constraints. We show that the presence of synergies drastically decreases the ability of the CNS to vary the properties of the endpoint stiffness and can even preclude the ability to minimize energy. Furthermore, the capacity to minimize energy consumption—when available—can be greatly affected by arm posture. Our computational approach helps reconcile divergent findings and conclusions about task-specific regulation of endpoint stiffness and energy consumption in the context of synergies. But more generally, these results provide further evidence that the benefits and disadvantages of muscle synergies go hand-in-hand with the structure of feasible muscle activation patterns afforded by the mechanics of the limb and task constraints. These insights will help design experiments to elucidate the interplay between synergies and the mechanisms of learning, plasticity, versatility and pathology in neuromuscular systems. The manner in which the nervous system coordinates the multiple muscles in the body is complex. It has been studied for decades, but a more full understanding is needed to enable the development of effective evaluation and treatment methods in disorders that cause neuromuscular disability such as cerebral palsy and stroke. In addition, the computational control of robots has and will continue to improve as the brain’s methods of muscular control are progressively reverse-engineered. Here, we study the capacity of arm muscles to regulate the stiffness of the hand for tasks such as using tools, stabilizing hand-held objects, and using doors. Using a simplified but generalizable model, we show that there will be necessary trade-offs in the functional capabilities of the limb if the nervous system chooses to control muscles in functional groups. This adds to our understanding of the consequences of different strategies to control muscles for real-world tasks with multiple and often competing demands. It enables future research and clinical experiments on the learning and execution of the multiple tasks of varying difficulty encountered in real life. It also sheds light on the design of control strategies for robots to operate in human and unstructured environments.
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Affiliation(s)
- Joshua M. Inouye
- Department of Biomedical Engineering, University of Virginia, Charlottesville, Virginia, United States of America
- Department of Biomedical Engineering, University of Southern California, Los Angeles, California, United States of America
| | - Francisco J. Valero-Cuevas
- Department of Biomedical Engineering, University of Southern California, Los Angeles, California, United States of America
- Department of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, California, United States of America
- * E-mail:
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11
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Sohn MH, Ting LH. Suboptimal Muscle Synergy Activation Patterns Generalize their Motor Function across Postures. Front Comput Neurosci 2016; 10:7. [PMID: 26869914 PMCID: PMC4740401 DOI: 10.3389/fncom.2016.00007] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2015] [Accepted: 01/13/2016] [Indexed: 01/01/2023] Open
Abstract
We used a musculoskeletal model to investigate the possible biomechanical and neural bases of using consistent muscle synergy patterns to produce functional motor outputs across different biomechanical conditions, which we define as generalizability. Experimental studies in cats demonstrate that the same muscle synergies are used during reactive postural responses at widely varying configurations, producing similarly-oriented endpoint force vectors with respect to the limb axis. However, whether generalizability across postures arises due to similar biomechanical properties or to neural selection of a particular muscle activation pattern has not been explicitly tested. Here, we used a detailed cat hindlimb model to explore the set of feasible muscle activation patterns that produce experimental synergy force vectors at a target posture, and tested their generalizability by applying them to different test postures. We used three methods to select candidate muscle activation patterns: (1) randomly-selected feasible muscle activation patterns, (2) optimal muscle activation patterns minimizing muscle effort at a given posture, and (3) generalizable muscle activation patterns that explicitly minimized deviations from experimentally-identified synergy force vectors across all postures. Generalizability was measured by the deviation between the simulated force direction of the candidate muscle activation pattern and the experimental synergy force vectors at the test postures. Force angle deviations were the greatest for the randomly selected feasible muscle activation patterns (e.g., >100°), intermediate for effort-wise optimal muscle activation patterns (e.g., ~20°), and smallest for generalizable muscle activation patterns (e.g., <5°). Generalizable muscle activation patterns were suboptimal in terms of effort, often exceeding 50% of the maximum possible effort (cf. ~5% in minimum-effort muscle activation patterns). The feasible muscle activation ranges of individual muscles associated with producing a specific synergy force vector was reduced by ~45% when generalizability requirements were imposed. Muscles recruited in the generalizable muscle activation patterns had less sensitive torque-producing characteristics to changes in postures. We conclude that generalization of function across postures does not arise from limb biomechanics or a single optimality criterion. Muscle synergies may reflect acquired motor solutions globally tuned for generalizability across biomechanical contexts, facilitating rapid motor adaptation.
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Affiliation(s)
- M Hongchul Sohn
- George W. Woodruff School of Mechanical Engineering, Georgia Institute of TechnologyAtlanta, GA, USA; Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory UniversityAtlanta, GA, USA
| | - Lena H Ting
- George W. Woodruff School of Mechanical Engineering, Georgia Institute of TechnologyAtlanta, GA, USA; Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory UniversityAtlanta, GA, USA
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12
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13
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Valero-Cuevas FJ, Cohn BA, Yngvason HF, Lawrence EL. Exploring the high-dimensional structure of muscle redundancy via subject-specific and generic musculoskeletal models. J Biomech 2015; 48:2887-96. [PMID: 25980557 PMCID: PMC5540666 DOI: 10.1016/j.jbiomech.2015.04.026] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2015] [Accepted: 04/04/2015] [Indexed: 11/27/2022]
Abstract
Subject-specific and generic musculoskeletal models are the computational instantiation of hypotheses, and stochastic techniques help explore their validity. We present two such examples to explore the hypothesis of muscle redundancy. The first addresses the effect of anatomical variability on static force capabilities for three individual cat hindlimbs, each with seven kinematic degrees of freedom (DoFs) and 31 muscles. We present novel methods to characterize the structure of the 31-dimensional set of feasible muscle activations for static force production in every 3-D direction. We find that task requirements strongly define the set of feasible muscle activations and limb forces, with few differences comparing individual vs. species-average results. Moreover, muscle activity is not smoothly distributed across 3-D directions. The second example explores parameter uncertainty during a flying disc throwing motion by using a generic human arm with five DoFs and 17 muscles to predict muscle fiber velocities. We show that the measured joint kinematics fully constrain the eccentric and concentric fiber velocities of all muscles via their moment arms. Thus muscle activation for limb movements is likely not redundant: there is little, if any, latitude in synchronizing alpha-gamma motoneuron excitation-inhibition for muscles to adhere to the time-critical fiber velocities dictated by joint kinematics. Importantly, several muscles inevitably exhibit fiber velocities higher than thought tenable, even for conservative throwing speeds. These techniques and results, respectively, enable and compel us to continue to revise the classical notion of muscle redundancy for increasingly more realistic models and tasks.
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Affiliation(s)
- F J Valero-Cuevas
- Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA; Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA, USA.
| | - B A Cohn
- Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA
| | - H F Yngvason
- Department of Computer Science, ETH Zurich, Switzerland
| | - E L Lawrence
- Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA
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14
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Feasible muscle activation ranges based on inverse dynamics analyses of human walking. J Biomech 2015; 48:2990-7. [PMID: 26300401 DOI: 10.1016/j.jbiomech.2015.07.037] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2014] [Revised: 07/30/2015] [Accepted: 07/30/2015] [Indexed: 12/12/2022]
Abstract
Although it is possible to produce the same movement using an infinite number of different muscle activation patterns owing to musculoskeletal redundancy, the degree to which observed variations in muscle activity can deviate from optimal solutions computed from biomechanical models is not known. Here, we examined the range of biomechanically permitted activation levels in individual muscles during human walking using a detailed musculoskeletal model and experimentally-measured kinetics and kinematics. Feasible muscle activation ranges define the minimum and maximum possible level of each muscle's activation that satisfy inverse dynamics joint torques assuming that all other muscles can vary their activation as needed. During walking, 73% of the muscles had feasible muscle activation ranges that were greater than 95% of the total muscle activation range over more than 95% of the gait cycle, indicating that, individually, most muscles could be fully active or fully inactive while still satisfying inverse dynamics joint torques. Moreover, the shapes of the feasible muscle activation ranges did not resemble previously-reported muscle activation patterns nor optimal solutions, i.e. static optimization and computed muscle control, that are based on the same biomechanical constraints. Our results demonstrate that joint torque requirements from standard inverse dynamics calculations are insufficient to define the activation of individual muscles during walking in healthy individuals. Identifying feasible muscle activation ranges may be an effective way to evaluate the impact of additional biomechanical and/or neural constraints on possible versus actual muscle activity in both normal and impaired movements.
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15
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Rácz K, Valero-Cuevas FJ. Spatio-temporal analysis reveals active control of both task-relevant and task-irrelevant variables. Front Comput Neurosci 2013; 7:155. [PMID: 24312045 PMCID: PMC3826108 DOI: 10.3389/fncom.2013.00155] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2013] [Accepted: 10/17/2013] [Indexed: 11/24/2022] Open
Abstract
The Uncontrolled Manifold (UCM) hypothesis and Minimal Intervention principle propose that the observed differential variability across task relevant (i.e., task goals) vs. irrelevant (i.e., in the null space of those goals) variables is evidence of a separation of task variables for efficient neural control, ranked by their respective variabilities (sometimes referred to as hierarchy of control). Support for this comes from spatial domain analyses (i.e., structure of) of kinematic, kinetic, and EMG variability. While proponents admit the possibility of preferential as opposed to strictly uncontrolled variables, such distinctions have only begun to be quantified or considered in the temporal domain when inferring control action. Here we extend the study of task variability during tripod static grasp to the temporal domain by applying diffusion analysis. We show that both task-relevant and task-irrelevant parameters show corrective action at some time scales; and conversely, that task-relevant parameters do not show corrective action at other time scales. That is, the spatial fluctuations of fingertip forces show, as expected, greater ranges of variability in task-irrelevant variables (>98% associated with changes in total grasp force; vs. only <2% in task-relevant changes associated with acceleration of the object). But at some time scales, however, temporal fluctuations of task-irrelevant variables exhibit negative correlations clearly indicative of corrective action (scaling exponents <0.5); and temporal fluctuations of task-relevant variables exhibit neutral and positive correlations clearly indicative of absence of corrective action (scaling exponents ≥0.5). In agreement with recent work in other behavioral contexts, these results propose we revise our understanding of variability vis-á-vis task relevance by considering both spatial and temporal features of all task variables when inferring control action and understanding how the CNS confronts task redundancy. Instead of a dichotomy of presence vs. absence of control, we should speak of a continuum of weaker to stronger—and potentially different—control strategies in specific spatiotemporal domains, indicated here by the magnitude of deviation from the 0.5 scaling exponent. Moreover, these results are counter examples to the UCM hypothesis and the Minimal Intervention principle, and the similar nature of control actions across time scales in both task-relevant and task-irrelevant spaces points to a level of modularity not previously recognized.
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Affiliation(s)
- Kornelius Rácz
- Department of Biomedical Engineering, and Neuroscience Graduate Program, University of Southern California Los Angeles, CA, USA
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16
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Keenan KG, Massey WV. Control of fingertip forces in young and older adults pressing against fixed low- and high-friction surfaces. PLoS One 2012; 7:e48193. [PMID: 23110210 PMCID: PMC3480490 DOI: 10.1371/journal.pone.0048193] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2012] [Accepted: 09/26/2012] [Indexed: 11/18/2022] Open
Abstract
Mobile computing devices (e.g., smartphones and tablets) that have low-friction surfaces require well-directed fingertip forces of sufficient and precise magnitudes for proper use. Although general impairments in manual dexterity are well-documented in older adults, it is unclear how these sensorimotor impairments influence the ability of older adults to dexterously manipulate fixed, low-friction surfaces in particular. 21 young and 18 older (65+ yrs) adults produced maximal voluntary contractions (MVCs) and steady submaximal forces (2.5 and 10% MVC) with the fingertip of the index finger. A Teflon covered custom-molded splint was placed on the fingertip. A three-axis force sensor was covered with either Teflon or sandpaper to create low- and high-friction surfaces, respectively. Maximal downward forces (Fz) were similar (p = .135) for young and older adults, and decreased by 15% (p<.001) while pressing on Teflon compared to sandpaper. Fluctuations in Fz during the submaximal force-matching tasks were 2.45× greater (p<.001) for older adults than in young adults, and reached a maximum when older adults pressed against the Teflon surface while receiving visual feedback. These age-associated changes in motor performance are explained, in part, by altered muscle activity from three hand muscles and out-of-plane forces. Quantifying the ability to produce steady fingertip forces against low-friction surfaces may be a better indicator of impairment and disability than the current practice of evaluating maximal forces with pinch meters. These age-associated impairments in dexterity while interacting with low-friction surfaces may limit the use of the current generation of computing interfaces by older adults.
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Affiliation(s)
- Kevin G. Keenan
- Department of Kinesiology, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States of America
- * E-mail:
| | - William V. Massey
- Department of Kinesiology, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States of America
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17
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Rácz K, Brown D, Valero-Cuevas FJ. An involuntary stereotypical grasp tendency pervades voluntary dynamic multifinger manipulation. J Neurophysiol 2012; 108:2896-911. [PMID: 22956798 DOI: 10.1152/jn.00297.2012] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We used a novel apparatus with three hinged finger pads to characterize collaborative multifinger interactions during dynamic manipulation requiring individuated control of fingertip motions and forces. Subjects placed the thumb, index, and middle fingertips on each hinged finger pad and held it-unsupported-with constant total grasp force while voluntarily oscillating the thumb's pad. This task combines the need to 1) hold the object against gravity while 2) dynamically reconfiguring the grasp. Fingertip force variability in this combined motion and force task exhibited strong synchrony among normal (i.e., grasp) forces. Mechanical analysis and simulation show that such synchronous variability is unnecessary and cannot be explained solely by signal-dependent noise. Surprisingly, such variability also pervaded control tasks requiring different individuated fingertip motions and forces, but not tasks without finger individuation such as static grasp. These results critically extend notions of finger force variability by exposing and quantifying a pervasive challenge to dynamic multifinger manipulation: the need for the neural controller to carefully and continuously overlay individuated finger actions over mechanically unnecessary synchronous interactions. This is compatible with-and may explain-the phenomenology of strong coupling of hand muscles when this delicate balance is not yet developed, as in early childhood, or when disrupted, as in brain injury. We conclude that the control of healthy multifinger dynamic manipulation has barely enough neuromechanical degrees of freedom to meet the multiple demands of ecological tasks and critically depends on the continuous inhibition of synchronous grasp tendencies, which we speculate may be of vestigial evolutionary origin.
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Affiliation(s)
- Kornelius Rácz
- Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA
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18
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Challenges and new approaches to proving the existence of muscle synergies of neural origin. PLoS Comput Biol 2012; 8:e1002434. [PMID: 22570602 PMCID: PMC3342930 DOI: 10.1371/journal.pcbi.1002434] [Citation(s) in RCA: 191] [Impact Index Per Article: 15.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2011] [Accepted: 02/02/2012] [Indexed: 11/19/2022] Open
Abstract
Muscle coordination studies repeatedly show low-dimensionality of muscle activations for a wide variety of motor tasks. The basis vectors of this low-dimensional subspace, termed muscle synergies, are hypothesized to reflect neurally-established functional muscle groupings that simplify body control. However, the muscle synergy hypothesis has been notoriously difficult to prove or falsify. We use cadaveric experiments and computational models to perform a crucial thought experiment and develop an alternative explanation of how muscle synergies could be observed without the nervous system having controlled muscles in groups. We first show that the biomechanics of the limb constrains musculotendon length changes to a low-dimensional subspace across all possible movement directions. We then show that a modest assumption—that each muscle is independently instructed to resist length change—leads to the result that electromyographic (EMG) synergies will arise without the need to conclude that they are a product of neural coupling among muscles. Finally, we show that there are dimensionality-reducing constraints in the isometric production of force in a variety of directions, but that these constraints are more easily controlled for, suggesting new experimental directions. These counter-examples to current thinking clearly show how experimenters could adequately control for the constraints described here when designing experiments to test for muscle synergies—but, to the best of our knowledge, this has not yet been done. How the brain and spinal cord control the body is a fundamental question of critical scientific and clinical importance. The preferred experimental approach to answer this question has been to infer the neural control strategy by analyzing recordings of muscle activity and limb mechanics collected while animals and people use their limbs. This has led to a popular, but not yet proven, hypothesis that the brain and spinal cord simplify the control of the numerous muscles by grouping them into few functional units called neural synergies. Our detailed experiments and simulations challenge the utility of this approach and the validity of its interpretation. We point out that mechanical constraints can also explain those experimental recordings. In particular, the anatomy of the limb combined with the type of tasks studied and analysis used, suffice to give the appearance of neural synergies. To be clear, we do not disprove the neural synergy hypothesis. Rather, in the tradition of scientific debate, by showing an alternative explanation to the available data we challenge the community and ourselves to design novel experiments and analyses to conclusively test that hypothesis by ruling out the confounds we point out.
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Keenan KG, Massey WV, Walters TJ, Collins JD. Sensitivity of EMG-EMG coherence to detect the common oscillatory drive to hand muscles in young and older adults. J Neurophysiol 2012; 107:2866-75. [PMID: 22378168 DOI: 10.1152/jn.01011.2011] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Multichannel surface electromyograms (EMGs) were used to examine the sensitivity of EMG-EMG coherence to infer changes in common oscillatory drive to hand muscles in young and older adults. Previous research has shown that measures of coherence calculated from different neurophysiological signals are influenced by the age of the subject, the visual feedback provided to the subject, and the task being performed. The change in the magnitude of EMG-EMG coherence across experimental conditions is often interpreted as a change in the oscillatory drive to motoneuron pools of a pair of muscles. However, signal processing (e.g., full-wave rectification) and electrode location are also reported to influence EMG-EMG coherence, which could decrease the sensitivity of EMG-EMG coherence to infer a change in common oscillatory drive to motoneurons. In this study, multichannel EMGs were used to compare EMG-EMG coherence in young (n = 11) and older (n = 10) adults during index finger abduction and pinch grip tasks performed at 2 and 3.5 N with a low and a high visual feedback gain. We found that, across all conditions, EMG-EMG coherence was influenced by electrode location (P < 0.001) but not by subject age, visual feedback gain, task, or signal processing. These results suggest that EMG-EMG coherence is most sensitive to electrode location. The results are discussed in terms of the potential issues related to inferring a common oscillatory drive to hand muscles with surface EMGs.
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Kutch JJ, Valero-Cuevas FJ. Muscle redundancy does not imply robustness to muscle dysfunction. J Biomech 2011; 44:1264-70. [PMID: 21420091 DOI: 10.1016/j.jbiomech.2011.02.014] [Citation(s) in RCA: 66] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2010] [Revised: 02/09/2011] [Accepted: 02/14/2011] [Indexed: 11/17/2022]
Abstract
It is well-known that muscle redundancy grants the CNS numerous options to perform a task. Does muscle redundancy, however, allow sufficient robustness to compensate for loss or dysfunction of even a single muscle? Are all muscles equally redundant? We combined experimental and computational approaches to establish the limits of motor robustness for static force production. In computer-controlled cadaveric index fingers, we find that only a small subset (<5%) of feasible forces is robust to loss of any one muscle. Importantly, the loss of certain muscles compromises force production significantly more than others. Further computational modeling of a multi-joint, multi-muscle leg demonstrates that this severe lack of robustness generalizes to whole limbs. These results provide a biomechanical basis to begin to explain why redundant motor systems can be vulnerable to even mild neuromuscular pathology.
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Affiliation(s)
- Jason J Kutch
- Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, USA
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21
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Keenan KG, Collins JD, Massey WV, Walters TJ, Gruszka HD. Coherence between surface electromyograms is influenced by electrode placement in hand muscles. J Neurosci Methods 2010; 195:10-4. [PMID: 21074555 DOI: 10.1016/j.jneumeth.2010.10.018] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2010] [Revised: 10/18/2010] [Accepted: 10/19/2010] [Indexed: 11/19/2022]
Abstract
We used multi-channel surface electromyograms (EMGs) to examine if electrode location influences coherence measures derived from pairs of EMGs recorded from two hand muscles during a pinch task. A linear probe of 16 electrodes was used to estimate the location of the innervation zone in first dorsal interosseous (FDI) and abductor pollicis brevis (APB). Four electrodes were then placed on the skin overlying each muscle and three bipolar electrode configurations were constructed with their center points directly over the innervation zone, and 15mm distal and proximal to the innervation zone. Ten subjects performed two force-matching tasks for 120s at 2N and 3.5N by pressing a force sensor held between the thumb and index finger. Coherence spectra were calculated from pairs of EMGs recorded from the two muscles. Maximal coherence from 1 to 15Hz and 16 to 32Hz was calculated at both force levels from the EMGs with electrodes centered over the innervation zones of FDI and APB. These values were compared to the maximal coherence from all other EMG comparisons across muscles recorded with electrodes that avoided the innervation zones. ANOVA revealed significant main effects only for electrode location, with a 58.1% increase (p=0.001) in maximal coherence for EMGs detected from pairs of electrodes that avoided the innervation zone (from 0.11±0.02 to 0.18±0.03; mean±95% confidence interval). These results indicate that electrode location relative to the innervation zone influences EMG-EMG coherence and should be carefully considered when placing EMG electrodes on hand muscles.
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Affiliation(s)
- Kevin G Keenan
- Neuromechanics Lab, Dept. of Human Movement Sciences,College of Health Sciences, University of Wisconsin-Milwaukee, 1600 E. Hartford Ave, Milwaukee, WI 53201, USA.
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Dideriksen JL, Farina D, Enoka RM. Influence of fatigue on the simulated relation between the amplitude of the surface electromyogram and muscle force. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2010; 368:2765-2781. [PMID: 20439272 DOI: 10.1098/rsta.2010.0094] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/29/2023]
Abstract
A linear relation between surface electromyogram (EMG) amplitude and muscle force is often assumed and used to estimate the contributions of selected muscles to various tasks. In the presence of muscle fatigue, however, changes in the properties of muscle fibre action potentials and motor unit twitch forces can alter the relation between surface EMG amplitude and force. A novel integrative model of motor neuron control and the generation of muscle fibre action potentials was used to simulate surface EMG signals and muscle force during three fatigue protocols. The change in the simulated relation between surface EMG amplitude and force depended on both the level of fatigue and the details of the fatiguing contraction. In general, surface EMG amplitude overestimated muscle force when fatigue was present. For example, surface EMG amplitudes corresponding to 60 per cent of the amplitude obtained at maximal force without fatigue corresponded to forces in the range 10-40% of the maximal force across three representative fatigue protocols. The results indicate that the surface EMG amplitude cannot be used to predict either the level of muscle activation or the magnitude of muscle force when the muscle exhibits any fatigue.
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Affiliation(s)
- Jakob L Dideriksen
- Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Aalborg University, , Fredrik Bajers Vej 7 D-3, DK-9220 Aalborg, Denmark
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