Bio


Dr. Falisse is a postdoctoral fellow in Bioengineering working on computational approaches to study human movement disorders. He primarily uses optimization methods, biomechanical modeling, and data from various sources (wearables, videos, medical images) to get insights into movement abnormalities and design innovative treatments and rehabilitation protocols.

Dr. Falisse received his PhD from KU Leuven (Belgium) where he worked on modeling and simulating the locomotion of children with cerebral palsy. His research was supported by the Research Foundation Flanders (FWO) through a personal fellowship. Dr. Falisse received several awards for his PhD work, including the David Winter Young Investigator Award, the Andrzej J. Komor Young Investigator Award, the VPHi Thesis Award in In Silico Medicine, and the KU Leuven Research Council Award in Biomedical Sciences.

Institute Affiliations


Honors & Awards


  • Best Doctoral Thesis Award: Runner-up, The European Society of Biomechanics (2020)
  • Best VPHi Thesis in In Silico Medicine Award, The Virtual Physiological Human (VPH) Institute (2020)
  • Research Council Award, KU Leuven (2020)
  • Andrzej J. Komor Award, The International Society of Biomechanics: Technical Group on Computer Simulation (2019)
  • David Winter Award, The International Society of Biomechanics (2019)

Professional Education


  • Ph.D., KU Leuven, Biomedical Sciences (2019)
  • M.S., UC Louvain, Biomedical Engineering (2014)
  • B.S., UC Louvain, Engineering Science (2012)

Lab Affiliations


All Publications


  • Predictive simulations of running gait reveal a critical dynamic role for the tail in bipedal dinosaur locomotion. Science advances Bishop, P. J., Falisse, A., De Groote, F., Hutchinson, J. R. 2021; 7 (39): eabi7348

    Abstract

    [Figure: see text].

    View details for DOI 10.1126/sciadv.abi7348

    View details for PubMedID 34550734

  • Perspective on musculoskeletal modelling and predictive simulations of human movement to assess the neuromechanics of gait. Proceedings. Biological sciences De Groote, F., Falisse, A. 2021; 288 (1946): 20202432

    Abstract

    Locomotion results from complex interactions between the central nervous system and the musculoskeletal system with its many degrees of freedom and muscles. Gaining insight into how the properties of each subsystem shape human gait is challenging as experimental methods to manipulate and assess isolated subsystems are limited. Simulations that predict movement patterns based on a mathematical model of the neuro-musculoskeletal system without relying on experimental data can reveal principles of locomotion by elucidating cause-effect relationships. New computational approaches have enabled the use of such predictive simulations with complex neuro-musculoskeletal models. Here, we review recent advances in predictive simulations of human movement and how those simulations have been used to deepen our knowledge about the neuromechanics of gait. In addition, we give a perspective on challenges towards using predictive simulations to gain new fundamental insight into motor control of gait, and to help design personalized treatments in patients with neurological disorders and assistive devices that improve gait performance. Such applications will require more detailed neuro-musculoskeletal models and simulation approaches that take uncertainty into account, tools to efficiently personalize those models, and validation studies to demonstrate the ability of simulations to predict gait in novel circumstances.

    View details for DOI 10.1098/rspb.2020.2432

    View details for PubMedID 33653141

  • Physics-Based Simulations to Predict the Differential Effects of Motor Control and Musculoskeletal Deficits on Gait Dysfunction in Cerebral Palsy: A Retrospective Case Study FRONTIERS IN HUMAN NEUROSCIENCE Falisse, A., Pitto, L., Kainz, H., Hoang, H., Wesseling, M., Van Rossom, S., Papageorgiou, E., Bar-On, L., Hallemans, A., Desloovere, K., Molenaers, G., Van Campenhout, A., De Groote, F., Jonkers, I. 2020; 14: 40

    Abstract

    Physics-based simulations of walking have the theoretical potential to support clinical decision-making by predicting the functional outcome of treatments in terms of walking performance. Yet before using such simulations in clinical practice, their ability to identify the main treatment targets in specific patients needs to be demonstrated. In this study, we generated predictive simulations of walking with a medical imaging based neuro-musculoskeletal model of a child with cerebral palsy presenting crouch gait. We explored the influence of altered muscle-tendon properties, reduced neuromuscular control complexity, and spasticity on gait dysfunction in terms of joint kinematics, kinetics, muscle activity, and metabolic cost of transport. We modeled altered muscle-tendon properties by personalizing Hill-type muscle-tendon parameters based on data collected during functional movements, simpler neuromuscular control by reducing the number of independent muscle synergies, and spasticity through delayed muscle activity feedback from muscle force and force rate. Our simulations revealed that, in the presence of aberrant musculoskeletal geometries, altered muscle-tendon properties rather than reduced neuromuscular control complexity and spasticity were the primary cause of the crouch gait pattern observed for this child, which is in agreement with the clinical examination. These results suggest that muscle-tendon properties should be the primary target of interventions aiming to restore an upright gait pattern for this child. This suggestion is in line with the gait analysis following muscle-tendon property and bone deformity corrections. Future work should extend this single case analysis to more patients in order to validate the ability of our physics-based simulations to capture the gait patterns of individual patients pre- and post-treatment. Such validation would open the door for identifying targeted treatment strategies with the aim of designing optimized interventions for neuro-musculoskeletal disorders.

    View details for DOI 10.3389/fnhum.2020.00040

    View details for Web of Science ID 000518657500001

    View details for PubMedID 32132911

    View details for PubMedCentralID PMC7040166

  • OpenSim Moco: Musculoskeletal optimal control. PLoS computational biology Dembia, C. L., Bianco, N. A., Falisse, A. n., Hicks, J. L., Delp, S. L. 2020; 16 (12): e1008493

    Abstract

    Musculoskeletal simulations are used in many different applications, ranging from the design of wearable robots that interact with humans to the analysis of patients with impaired movement. Here, we introduce OpenSim Moco, a software toolkit for optimizing the motion and control of musculoskeletal models built in the OpenSim modeling and simulation package. OpenSim Moco uses the direct collocation method, which is often faster and can handle more diverse problems than other methods for musculoskeletal simulation. Moco frees researchers from implementing direct collocation themselves-which typically requires extensive technical expertise-and allows them to focus on their scientific questions. The software can handle a wide range of problems that interest biomechanists, including motion tracking, motion prediction, parameter optimization, model fitting, electromyography-driven simulation, and device design. Moco is the first musculoskeletal direct collocation tool to handle kinematic constraints, which enable modeling of kinematic loops (e.g., cycling models) and complex anatomy (e.g., patellar motion). To show the abilities of Moco, we first solved for muscle activity that produced an observed walking motion while minimizing squared muscle excitations and knee joint loading. Next, we predicted how muscle weakness may cause deviations from a normal walking motion. Lastly, we predicted a squat-to-stand motion and optimized the stiffness of an assistive device placed at the knee. We designed Moco to be easy to use, customizable, and extensible, thereby accelerating the use of simulations to understand the movement of humans and other animals.

    View details for DOI 10.1371/journal.pcbi.1008493

    View details for PubMedID 33370252

  • Rapid predictive simulations with complex musculoskeletal models suggest that diverse healthy and pathological human gaits can emerge from similar control strategies JOURNAL OF THE ROYAL SOCIETY INTERFACE Falisse, A., Serrancoli, G., Dembia, C. L., Gillis, J., Jonkers, I., De Groote, F. 2019; 16 (157): 20190402

    Abstract

    Physics-based predictive simulations of human movement have the potential to support personalized medicine, but large computational costs and difficulties to model control strategies have limited their use. We have developed a computationally efficient optimal control framework to predict human gaits based on optimization of a performance criterion without relying on experimental data. The framework generates three-dimensional muscle-driven simulations in 36 min on average-more than 20 times faster than existing simulations-by using direct collocation, implicit differential equations and algorithmic differentiation. Using this framework, we identified a multi-objective performance criterion combining energy and effort considerations that produces physiologically realistic walking gaits. The same criterion also predicted the walk-to-run transition and clinical gait deficiencies caused by muscle weakness and prosthesis use, suggesting that diverse healthy and pathological gaits can emerge from the same control strategy. The ability to predict the mechanics and energetics of a broad range of gaits with complex three-dimensional musculoskeletal models will allow testing novel hypotheses about gait control and hasten the development of optimal treatments for neuro-musculoskeletal disorders.

    View details for DOI 10.1098/rsif.2019.0402

    View details for Web of Science ID 000484404700029

    View details for PubMedID 31431186

    View details for PubMedCentralID PMC6731507

  • Modeling toes contributes to realistic stance knee mechanics in three-dimensional predictive simulations of walking. PloS one Falisse, A., Afschrift, M., De Groote, F. 1800; 17 (1): e0256311

    Abstract

    Physics-based predictive simulations have been shown to capture many salient features of human walking. Yet they often fail to produce realistic stance knee and ankle mechanics. While the influence of the performance criterion on the predicted walking pattern has been previously studied, the influence of musculoskeletal mechanics has been less explored. Here, we investigated the influence of two mechanical assumptions on the predicted walking pattern: the complexity of the foot model and the stiffness of the Achilles tendon. We found, through three-dimensional muscle-driven predictive simulations of walking, that modeling the toes, and thus using two-segment instead of single-segment foot models, contributed to robustly eliciting physiological stance knee flexion angles, knee extension torques, and knee extensor activity. Modeling toes also slightly decreased the first vertical ground reaction force peak, increasing its agreement with experimental data, and improved stance ankle kinetics. It nevertheless slightly worsened predictions of ankle kinematics. Decreasing Achilles tendon stiffness improved the realism of ankle kinematics, but there remain large discrepancies with experimental data. Overall, this simulation study shows that not only the performance criterion but also mechanical assumptions affect predictive simulations of walking. Improving the realism of predictive simulations is required for their application in clinical contexts. Here, we suggest that using more complex foot models might contribute to such realism.

    View details for DOI 10.1371/journal.pone.0256311

    View details for PubMedID 35077455

  • A Dynamic Optimization Approach for Solving Spine Kinematics While Calibrating Subject-Specific Mechanical Properties (Apr, 10.1007/s10439-021-02774-3, 2021) ANNALS OF BIOMEDICAL ENGINEERING Wang, W., Wang, D., Falisse, A., Severijns, P., Overbergh, T., Moke, L., Scheys, L., De Groote, F., Jonkers, I. 2021; 49 (7): 1784

    View details for DOI 10.1007/s10439-021-02791-2

    View details for Web of Science ID 000651367100001

    View details for PubMedID 34014422

  • Computational modelling of muscle fibre operating ranges in the hindlimb of a small ground bird (Eudromia elegans), with implications for modelling locomotion in extinct species. PLoS computational biology Bishop, P. J., Michel, K. B., Falisse, A., Cuff, A. R., Allen, V. R., De Groote, F., Hutchinson, J. R. 2021; 17 (4): e1008843

    Abstract

    The arrangement and physiology of muscle fibres can strongly influence musculoskeletal function and whole-organismal performance. However, experimental investigation of muscle function during in vivo activity is typically limited to relatively few muscles in a given system. Computational models and simulations of the musculoskeletal system can partly overcome these limitations, by exploring the dynamics of muscles, tendons and other tissues in a robust and quantitative fashion. Here, a high-fidelity, 26-degree-of-freedom musculoskeletal model was developed of the hindlimb of a small ground bird, the elegant-crested tinamou (Eudromia elegans, ~550 g), including all the major muscles of the limb (36 actuators per leg). The model was integrated with biplanar fluoroscopy (XROMM) and forceplate data for walking and running, where dynamic optimization was used to estimate muscle excitations and fibre length changes throughout both gaits. Following this, a series of static simulations over the total range of physiological limb postures were performed, to circumscribe the bounds of possible variation in fibre length. During gait, fibre lengths for all muscles remained between 0.5 to 1.21 times optimal fibre length, but operated mostly on the ascending limb and plateau of the active force-length curve, a result that parallels previous experimental findings for birds, humans and other species. However, the ranges of fibre length varied considerably among individual muscles, especially when considered across the total possible range of joint excursion. Net length change of muscle-tendon units was mostly less than optimal fibre length, sometimes markedly so, suggesting that approaches that use muscle-tendon length change to estimate optimal fibre length in extinct species are likely underestimating this important parameter for many muscles. The results of this study clarify and broaden understanding of muscle function in extant animals, and can help refine approaches used to study extinct species.

    View details for DOI 10.1371/journal.pcbi.1008843

    View details for PubMedID 33793558

  • Predictive Simulations of Musculoskeletal Function and Jumping Performance in a Generalized Bird. Integrative organismal biology (Oxford, England) Bishop, P. J., Falisse, A., De Groote, F., Hutchinson, J. R. 2021; 3 (1): obab006

    Abstract

    Jumping is a common, but demanding, behavior that many animals employ during everyday activity. In contrast to jump-specialists such as anurans and some primates, jumping biomechanics and the factors that influence performance remains little studied for generalized species that lack marked adaptations for jumping. Computational biomechanical modeling approaches offer a way of addressing this in a rigorous, mechanistic fashion. Here, optimal control theory and musculoskeletal modeling are integrated to generate predictive simulations of maximal height jumping in a small ground-dwelling bird, a tinamou. A three-dimensional musculoskeletal model with 36 actuators per leg is used, and direct collocation is employed to formulate a rapidly solvable optimal control problem involving both liftoff and landing phases. The resulting simulation raises the whole-body center of mass to over double its standing height, and key aspects of the simulated behavior qualitatively replicate empirical observations for other jumping birds. However, quantitative performance is lower, with reduced ground forces, jump heights, and muscle-tendon power. A pronounced countermovement maneuver is used during launch. The use of a countermovement is demonstrated to be critical to the achievement of greater jump heights, and this phenomenon may only need to exploit physical principles alone to be successful; amplification of muscle performance may not necessarily be a proximate reason for the use of this maneuver. Increasing muscle strength or contractile velocity above nominal values greatly improves jump performance, and interestingly has the greatest effect on more distal limb extensor muscles (i.e., those of the ankle), suggesting that the distal limb may be a critical link for jumping behavior. These results warrant a re-evaluation of previous inferences of jumping ability in some extinct species with foreshortened distal limb segments, such as dromaeosaurid dinosaurs. Simulations predictives de la fonction musculo-squelettique et des performances de saut chez un oiseau generalise Sauter est un comportement commun, mais exigeant, que de nombreux animaux utilisent au cours de leurs activites quotidiennes. Contrairement aux specialistes du saut tels que les anoures et certains primates, la biomecanique du saut et les facteurs qui influencent la performance restent peu etudies pour les especes generalisees qui n'ont pas d'adaptations marquees pour le saut. Les approches de modelisation biomecanique computationnelle offrent un moyen d'aborder cette question de maniere rigoureuse et mecaniste. Ici, la theorie du controle optimal et la modelisation musculo-squelettique sont integrees pour generer des simulations predictives du saut en hauteur maximal chez un petit oiseau terrestre, le tinamou. Un modele musculo-squelettique tridimensionnel avec 36 actionneurs par patte est utilise, et une methode numerique nommee "direct collocation" est employee pour formuler un probleme de controle optimal rapidement resoluble impliquant les phases de decollage et d'atterrissage. La simulation qui en resulte eleve le centre de masse du corps entier a plus du double de sa hauteur debout, et les aspects cles du comportement simule reproduisent qualitativement les observations empiriques d'autres oiseaux sauteurs. Cependant, les performances quantitatives sont moindres, avec une reduction des forces au sol, des hauteurs de saut et de la puissance musculo-tendineuse. Une manoeuvre de contre-mouvement prononcee est utilisee pendant le lancement. Il a ete demontre que l'utilisation d'un contre-mouvement est essentielle a l'obtention de hauteurs de saut plus importantes, et il se peut que ce phenomene doive exploiter uniquement des principes physiques pour reussir; l'amplification de la performance musculaire n'est pas necessairement une raison immediate de l'utilisation de cette manoeuvre. L'augmentation de la force musculaire ou de la vitesse de contraction au-dessus des valeurs nominales ameliore grandement la performance de saut et, fait interessant, a le plus grand effet sur les muscles extenseurs des membres plus distaux (c'est-a-dire ceux de la cheville), ce qui suggere que le membre distal peut etre un lien critique pour le comportement de saut. Ces resultats justifient une reevaluation des deductions precedentes de la capacite de sauter chez certaines especes eteintes avec des segments de membres distaux raccourcis, comme les dinosaures dromeosaurides. Voorspellende simulaties van musculoskeletale functie en springprestaties bij een gegeneraliseerde vogel Springen is een veel voorkomend, maar veeleisend, gedrag dat veel dieren toepassen tijdens hun dagelijkse bezigheden. In tegenstelling tot de springspecialisten zoals de anura en sommige primaten, is de biomechanica van het springen en de factoren die de prestaties beinvloeden nog weinig bestudeerd voor algemene soorten die geen uitgesproken adaptaties voor het springen hebben. Computationele biomechanische modelbenaderingen bieden een manier om dit op een rigoureuze, mechanistische manier aan te pakken. Hier worden optimale controle theorie en musculoskeletale modellering geintegreerd om voorspellende simulaties te genereren van maximale hoogtesprong bij een kleine grondbewonende vogel, een tinamou. Een driedimensionaal musculoskeletaal model met 36 actuatoren per poot wordt gebruikt, en directe collocatie wordt toegepast om een snel oplosbaar optimaal controleprobleem te formuleren dat zowel de opstijg-als de landingsfase omvat. De resulterende simulatie verhoogt het lichaamszwaartepunt tot meer dan het dubbele van de stahoogte, en belangrijke aspecten van het gesimuleerde gedrag komen kwalitatief overeen met empirische waarnemingen voor andere springende vogels. De kwantitatieve prestaties zijn echter minder, met verminderde grondkrachten, spronghoogtes en spierpeeskracht. Tijdens de lancering wordt een uitgesproken tegenbewegingsmanoeuvre gebruikt. Aangetoond is dat het gebruik van een tegenbeweging van cruciaal belang is voor het bereiken van grotere spronghoogten, en dit fenomeen hoeft alleen op fysische principes te berusten om succesvol te zijn; versterking van de spierprestaties hoeft niet noodzakelijk een proximate reden te zijn voor het gebruik van deze manoeuvre. Het verhogen van de spierkracht of van de contractiesnelheid boven de nominale waarden verbetert de sprongprestatie aanzienlijk, en heeft interessant genoeg het grootste effect op de meer distale extensoren van de ledematen (d.w.z. die van de enkel), wat suggereert dat de distale ledematen een kritieke schakel kunnen zijn voor het springgedrag. Deze resultaten rechtvaardigen een herevaluatie van eerdere conclusies over springvermogen bij sommige uitgestorven soorten met voorgekorte distale ledematen, zoals dromaeosauride dinosauriers. Pradiktive Simulationen der muskuloskelettalen Funktion und Sprungleistung bei einem generalisierten Vogel Springen ist ein ubliches jedoch anstrengendes Verhalten, das viele Tiere bei ihren taglichen Aktivitaten einsetzen. Im Gegensatz zu Springspezialisten, wie Froschen und einigen Primaten, sind bei allgemeinen Arten, welche keine ausgepragten Anpassung fur Sprungverhalten aufweisen, die Biomechanik beim Springen und die Faktoren, welche die Leistungsfahigkeit beeinflussen, noch wenig untersucht. Computergestutzte biomechanische Modellierungsverfahren bieten hier eine Moglichkeit, dies in einer grundlichen, mechanistischen Weise anzugehen. In dieser Arbeit werden die optimale Steuerungstheorie und Muskel-Skelett-Modellierung zusammen eingesetzt, um die maximale Sprunghohe eines kleinen bodenlebenden Vogels, eines Perlsteisshuhns, zu simulieren und zu prognostizieren. Es wird ein dreidimensionales Muskel-Skelett-Modell mit 36 Aktuatoren pro Bein verwendet, und durch direkte Kollokation wird ein schnell losbares optimales Steuerungsproblem formuliert, das sowohl die Abstoss- als auch die Landephase umfasst. Die daraus folgende Simulation bringt den Ganzkorperschwerpunkt auf mehr als das Doppelte seiner Standhohe und entscheidende Aspekte des simulierten Verhaltens entsprechen qualitativ empirischen Beobachtungen fur andere springende Vogel. Allerdings ist die quantitative Leistungsfahigkeit geringer, mit reduzierten Bodenkraften, Sprunghohen und Muskel-Sehnen-Kraften. Beim Abstossen wird ein ausgepragtes Gegenbewegungsmanover durchgefuhrt. Die Durchfuhrung einer Gegenbewegung ist nachweislich entscheidend fur das Erreichen grosserer Sprunghohen, wobei dieses Phanomen moglicherweise nur physikalische Prinzipien auszuschopfen braucht, um erfolgreich zu sein. Die Verstarkung der Muskelleistung ist daher moglicherweise nicht zwingend ein unmittelbarer Grund fur die Verwendung dieses Manovers. Eine Erhohung der Muskelkraft oder der Kontraktionsgeschwindigkeit uber die Nominalwerte hinaus fuhrt zu einer erheblichen Zunahme der Sprungleistung und hat interessanterweise den grossten Effekt bei den weiter distal gelegenen Streckmuskeln der Beine (d.h. bei denjenigen des Sprunggelenks), was darauf hindeutet, dass die distale Gliedmasse ein entscheidendes Element fur das Sprungverhalten sein konnte. Diese Ergebnisse geben Anlass zur Uberprufung fruherer Schlussfolgerungen hinsichtlich der Sprungfahigkeit einiger ausgestorbener Arten mit verkurzten distalen Gliedmassen, wie beispielsweise bei dromaeosauriden Dinosauriern.

    View details for DOI 10.1093/iob/obab006

    View details for PubMedID 34377939

  • A Dynamic Optimization Approach for Solving Spine Kinematics While Calibrating Subject-Specific Mechanical Properties. Annals of biomedical engineering Wang, W., Wang, D., Falisse, A., Severijns, P., Overbergh, T., Moke, L., Scheys, L., De Groote, F., Jonkers, I. 2021

    Abstract

    This study aims to propose a new optimization framework for solving spine kinematics based on skin-mounted markers and estimate subject-specific mechanical properties of the intervertebral joints. The approach enforces dynamic consistency in the entire skeletal system over the entire time-trajectory while personalizing spinal stiffness. 3D reflective markers mounted on ten vertebrae during spine motions were measured in ten healthy volunteers. Biplanar X-rays were taken during neutral stance of the subjects wearing the markers. Calculated spine kinematics were compared to those calculated using inverse kinematics (IK) and IK with imposed generic kinematic constraints. Calculated spine kinematics compared well with standing X-rays, with average root mean square differences of the vertebral body center positions below 10.1 mm and below [Formula: see text] for joint orientation angles. For flexion/extension and lateral bending, the lumbar rotation distribution patterns, as well as the ranges of rotations matched in vivo literature data. The approach outperforms state-of-art IK and IK with constraints methods. Calculated ratios reflect reduced spinal stiffness in low-resistance zone and increased stiffness in high-resistance zone. The patterns of calibrated stiffness were consistent with previously reported experimentally determined patterns. This approach will further our insight into spinal mechanics by increasing the physiological representativeness of spinal motion simulations.

    View details for DOI 10.1007/s10439-021-02774-3

    View details for PubMedID 33851322

  • In Silico-Enhanced Treatment and Rehabilitation Planning for Patients with Musculoskeletal Disorders: Can Musculoskeletal Modelling and Dynamic Simulations Really Impact Current Clinical Practice? APPLIED SCIENCES-BASEL Killen, B. A., Falisse, A., De Groote, F., Jonkers, I. 2020; 10 (20)
  • Algorithmic differentiation improves the computational efficiency of OpenSim-based trajectory optimization of human movement PLOS ONE Falisse, A., Serrancol, G., Dembia, C. L., Gillis, J., De Groote, F. 2019; 14 (10): e0217730

    Abstract

    Algorithmic differentiation (AD) is an alternative to finite differences (FD) for evaluating function derivatives. The primary aim of this study was to demonstrate the computational benefits of using AD instead of FD in OpenSim-based trajectory optimization of human movement. The secondary aim was to evaluate computational choices including different AD tools, different linear solvers, and the use of first- or second-order derivatives. First, we enabled the use of AD in OpenSim through a custom source code transformation tool and through the operator overloading tool ADOL-C. Second, we developed an interface between OpenSim and CasADi to solve trajectory optimization problems. Third, we evaluated computational choices through simulations of perturbed balance, two-dimensional predictive simulations of walking, and three-dimensional tracking simulations of walking. We performed all simulations using direct collocation and implicit differential equations. Using AD through our custom tool was between 1.8 ± 0.1 and 17.8 ± 4.9 times faster than using FD, and between 3.6 ± 0.3 and 12.3 ± 1.3 times faster than using AD through ADOL-C. The linear solver efficiency was problem-dependent and no solver was consistently more efficient. Using second-order derivatives was more efficient for balance simulations but less efficient for walking simulations. The walking simulations were physiologically realistic. These results highlight how the use of AD drastically decreases computational time of trajectory optimization problems as compared to more common FD. Overall, combining AD with direct collocation and implicit differential equations decreases the computational burden of trajectory optimization of human movement, which will facilitate their use for biomechanical applications requiring the use of detailed models of the musculoskeletal system.

    View details for DOI 10.1371/journal.pone.0217730

    View details for Web of Science ID 000532567300004

    View details for PubMedID 31622352

    View details for PubMedCentralID PMC6797126

  • Subject-Exoskeleton Contact Model Calibration Leads to Accurate Interaction Force Predictions IEEE TRANSACTIONS ON NEURAL SYSTEMS AND REHABILITATION ENGINEERING Serrancoli, G., Falisse, A., Dembia, C., Vantilt, J., Tanghe, K., Lefeber, D., Jonkers, I., De Schutter, J., De Groote, F. 2019; 27 (8): 1597–1605

    Abstract

    Knowledge of human-exoskeleton interaction forces is crucial to assess user comfort and effectiveness of the interaction. The subject-exoskeleton collaborative movement and its interaction forces can be predicted in silico using computational modeling techniques. We developed an optimal control framework that consisted of three phases. First, the foot-ground (Phase A) and the subject-exoskeleton (Phase B) contact models were calibrated using three experimental sit-to-stand trials. Then, the collaborative movement and the subject-exoskeleton interaction forces, of six different sit-to-stand trials were predicted (Phase C). The results show that the contact models were able to reproduce experimental kinematics of calibration trials (mean root mean square differences - RMSD - coordinates ≤ 1.1° and velocities ≤ 6.8°/s), ground reaction forces (mean RMSD≤ 22.9 N), as well as the interaction forces at the pelvis, thigh, and shank (mean RMSD ≤ 5.4 N). Phase C could predict the collaborative movements of prediction trials (mean RMSD coordinates ≤ 3.5° and velocities ≤ 15.0°/s), and their subject-exoskeleton interaction forces (mean RMSD ≤ 13.1° N). In conclusion, this optimal control framework could be used while designing exoskeletons to have in silico knowledge of new optimal movements and their interaction forces.

    View details for DOI 10.1109/TNSRE.2019.2924536

    View details for Web of Science ID 000480356700011

    View details for PubMedID 31247556

  • SimCP: A Simulation Platform to Predict Gait Performance Following Orthopedic Intervention in Children With Cerebral Palsy FRONTIERS IN NEUROROBOTICS Pitto, L., Kainz, H., Falisse, A., Wesseling, M., Van Rossom, S., Hoang, H., Papageorgiou, E., Hallemans, A., Desloovere, K., Molenaers, G., Van Campenhout, A., De Groote, F., Jonkers, I. 2019; 13: 54

    Abstract

    Gait deficits in cerebral palsy (CP) are often treated with a single-event multi-level surgery (SEMLS). Selecting the treatment options (combination of bony and soft tissue corrections) for a specific patient is a complex endeavor and very often treatment outcome is not satisfying. A deterioration in 22.8% of the parameters describing gait performance has been reported and there is need for additional surgery in 11% of the patients. Computational simulations based on musculoskeletal models that allow clinicians to test the effects of different treatment options before surgery have the potential to drastically improve treatment outcome. However, to date, no such simulation and modeling method is available. Two important challenges are the development of methods to include patient-specific neuromechanical impairments into the models and to simulate the effect of different surgical procedures on post-operative gait performance. Therefore, we developed the SimCP framework that allows the evaluation of the effect of different simulated surgeries on gait performance of a specific patient and includes a graphical user interface (GUI) that enables performing virtual surgery on the models. We demonstrated the potential of our framework for two case studies. Models reflecting the patient-specific musculoskeletal geometry and muscle properties are generated based solely on data collected before the treatment. The patient's motor control is described based on muscle synergies derived from pre-operative EMG. The GUI is then used to modify the musculoskeletal properties according to the surgical plan. Since SEMLS does not affect motor control, the same motor control model is used to define gait performance pre- and post-operative. We use the capability gap (CG), i.e., the difference between the joint moments needed to perform healthy walking and the joint moments the personalized model can generate, to quantify gait performance. In both cases, the CG was smaller post- then pre-operative and this was in accordance with the measured change in gait kinematics after treatment.

    View details for DOI 10.3389/fnbot.2019.00054

    View details for Web of Science ID 000477745300002

    View details for PubMedID 31379550

    View details for PubMedCentralID PMC6650580

  • A spasticity model based on feedback from muscle force explains muscle activity during passive stretches and gait in children with cerebral palsy PLOS ONE Falisse, A., Bar-On, L., Desloovere, K., Jonkers, I., De Groote, F. 2018; 13 (12): e0208811

    Abstract

    Muscle spasticity is characterized by exaggerated stretch reflexes and affects about 85% of the children with cerebral palsy. However, the mechanisms underlying spasticity and its influence on gait are not well understood. Here, we first aimed to model the response of spastic hamstrings and gastrocnemii in children with cerebral palsy to fast passive stretches. Then, we evaluated how the model applied to gait. We developed three models based on exaggerated proprioceptive feedback. The first model relied on feedback from muscle fiber length and velocity (velocity-related model), the second model relied on feedback from muscle fiber length, velocity, and acceleration (acceleration-related model), and the third model relied on feedback from muscle force and its first time derivative (force-related model). The force-related model better reproduced measured hamstrings and gastrocnemii activity during fast passive stretches (coefficients of determination (R2): 0.73 ± 0.10 and 0.60 ± 0.13, respectively, and root mean square errors (RMSE): 0.034 ± 0.031 and 0.009 ± 0.007, respectively) than the velocity-related model (R2: 0.46 ± 0.15 and 0.07 ± 0.13, and RMSE: 0.053 ± 0.051 and 0.015 ± 0.009), and the acceleration-related model (R2: 0.47 ± 0.15 and 0.09 ± 0.14, and RMSE: 0.052 ± 0.050 and 0.015 ± 0.008). Additionally, the force-related model predicted hamstrings and gastrocnemii activity that better correlated with measured activity during gait (cross correlations: 0.82 ± 0.09 and 0.85 ± 0.06, respectively) than the activity predicted by the velocity-related model (cross correlations: 0.49 ± 0.17 and 0.71 ± 0.22) and the acceleration-related model (cross correlations: 0.51 ± 0.16 and 0.67 ± 0.20). Our results therefore suggest that force encoding in muscle spindles in combination with altered feedback gains and thresholds underlie activity of spastic muscles during passive stretches and gait. Our model of spasticity opens new perspectives for studying movement impairments due to spasticity through simulation.

    View details for DOI 10.1371/journal.pone.0208811

    View details for Web of Science ID 000452640900048

    View details for PubMedID 30532154

    View details for PubMedCentralID PMC6286045

  • OpenSim Versus Human Body Model: A Comparison Study for the Lower Limbs During Gait. Journal of applied biomechanics Falisse, A., Van Rossom, S., Gijsbers, J., Steenbrink, F., van Basten, B. J., Jonkers, I., van den Bogert, A. J., De Groote, F. 2018: 1-7

    Abstract

    Musculoskeletal modeling and simulations have become popular tools for analyzing human movements. However, end users are often not aware of underlying modeling and computational assumptions. This study investigates how these assumptions affect biomechanical gait analysis outcomes performed with Human Body Model and the OpenSim gait2392 model. The authors compared joint kinematics, kinetics, and muscle forces resulting from processing data from 7 healthy adults with both models. Although outcome variables had similar patterns, there were statistically significant differences in joint kinematics (maximal difference: 9.8° [1.5°] in sagittal plane hip rotation), kinetics (maximal difference: 0.36 [0.10] N·m/kg in sagittal plane hip moment), and muscle forces (maximal difference: 8.51 [1.80] N/kg for psoas). These differences might be explained by differences in hip and knee joint center locations up to 2.4 (0.5) and 1.9 (0.2) cm in the posteroanterior and inferosuperior directions, respectively, and by the offset in pelvic reference frames of about 10° around the mediolateral axis. The choice of model may not influence the conclusions in clinical settings, where the focus is on interpreting deviations from the reference data, but it will affect the conclusions of mechanical analyses in which the goal is to obtain accurate estimates of kinematics and loading.

    View details for DOI 10.1123/jab.2017-0156

    View details for PubMedID 29809082

  • The influence of maximum isometric muscle force scaling on estimated muscle forces from musculoskeletal models of children with cerebral palsy. Gait & posture Kainz, H., Goudriaan, M., Falisse, A., Huenaerts, C., Desloovere, K., De Groote, F., Jonkers, I. 2018; 65: 213-220

    Abstract

    Musculoskeletal models do not include patient-specific muscle forces but rely on a scaled generic model, with muscle forces left unscaled in most cases. However, to use musculoskeletal simulations to inform clinical decision-making in children with cerebral palsy (CP), inclusion of subject-specific muscle forces is of utmost importance in order to represent each child's compensation mechanisms introduced through muscle weakness.The aims of this study were to (i) evaluate if maximum isometric muscle forces (MIMF) in musculoskeletal models of children with CP can be scaled based on strength measurements obtained with a hand-held-dynamometer (HHD), (ii) evaluate the impact of the HHD based scaling approach and previously published MIMF scaling methods on computed muscle forces during gait, and (iii) compare maximum muscle forces during gait between CP and typically developing (TD) children.Strength and motion capture data of six CP and motion capture data of six TD children were collected. The HHD measurements to obtain hip, knee and ankle muscle strength were simulated in OpenSim and used to modify MIMF of the 2392-OpenSim model. These muscle forces were compared to the MIMF scaled on the child's body mass and a scaling approach, which included the body mass and muscle-tendon lengths. OpenSim was used to calculate peak muscle forces during gait.Ankle muscle strength was insufficient to reproduce joint moments during walking when MIMF were scaled based on HHD. During gait, peak hip and knee extensor muscle forces were higher and peak ankle dorsi-flexor forces were lower in CP compared to TD participants.HHD measurements can be used to scale MIMF for the hip and knee muscle groups but underestimate the force capacity of the ankle muscle groups during walking. Muscle-tendon-length and mass based scaling methods affected muscle activations but had little influence on peak muscle forces during gait.

    View details for DOI 10.1016/j.gaitpost.2018.07.172

    View details for PubMedID 30558934

  • EMG-Driven Optimal Estimation of Subject-SPECIFIC Hill Model Muscle-Tendon Parameters of the Knee Joint Actuators IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING Falisse, A., Van Rossom, S., Jonkers, I., De Groote, F. 2017; 64 (9): 2253–62

    Abstract

    the purpose of this paper is to propose an optimal control problem formulation to estimate subject-specific Hill model muscle-tendon (MT-) parameters of the knee joint actuators by optimizing the fit between experimental and model-based knee moments. Additionally, this paper aims at determining which sets of functional motions contain the necessary information to identify the MT-parameters.the optimal control and parameter estimation problem underlying the MT-parameter estimation is solved for subject-specific MT-parameters via direct collocation using an electromyography-driven musculoskeletal model. The sets of motions containing sufficient information to identify the MT-parameters are determined by evaluating knee moments simulated based on subject-specific MT-parameters against experimental moments.the MT-parameter estimation problem was solved in about 30 CPU minutes. MT-parameters could be identified from only seven of the 62 investigated sets of motions, underlining the importance of the experimental protocol. Using subject-specific MT-parameters instead of more common linearly scaled MT-parameters improved the fit between inverse dynamics moments and simulated moments by about 30% in terms of the coefficient of determination (from [Formula: see text] to [Formula: see text]) and by about 26% in terms of the root mean square error (from [Formula: see text] to [Formula: see text] ). In particular, subject-specific MT-parameters of the knee flexors were very different from linearly scaled MT-parameters.we introduced a computationally efficient optimal control problem formulation and provided guidelines for designing an experimental protocol to estimate subject-specific MT-parameters improving the accuracy of motion simulations.the proposed formulation opens new perspectives for subject-specific musculoskeletal modeling, which might be beneficial for simulating and understanding pathological motions.

    View details for DOI 10.1109/TBME.2016.2630009

    View details for Web of Science ID 000408111500023

    View details for PubMedID 27875132