Musculoskeletal Simulation Research Articles

Just Don’t Fall: An AI Agent’s Learning Journey Towards Posture Stabilisation

Learning to maintain postural balance while standing requires a significant, fine coordination effort between the neuromuscular system and the sensory system. It is one of the key contributing factors towards fall prevention, especially in the older population. Using artificial intelligence (AI), we can similarly teach an agent to maintain a standing posture, and thus teach the agent not to fall. In this paper, we investigate the learning progress of an AI agent and how it maintains a stable standing posture through reinforcement learning. We used the Deep Deterministic Policy Gradient method (DDPG) and the OpenSim musculoskeletal simulation environment based on OpenAI Gym. During training, the AI agent learnt three policies. First, it learnt to maintain the Centre-of-Gravity and Zero-Moment-Point in front of the body. Then, it learnt to shift the load of the entire body on one leg while using the other leg for fine tuning the balancing action. Finally, it started to learn the coordination between the two pre-trained policies. This study shows the potentials of using deep reinforcement learning in human movement studies. The learnt AI behaviour also exhibited attempts to achieve an unplanned goal because it correlated with the set goal (e.g., walking in order to prevent falling). The failed attempts to maintain a standing posture is an interesting by-product which can enrich the fall detection and prevention research efforts.

Torus obstacle method as a wrapping approach of the deltoid muscle group for humeral abduction in musculoskeletal simulation

Musculoskeletal models of the shoulder complex are valuable research aids to investigate tears of the supraspinatus and the resulting mechanical impact during abduction of the humerus. One of the major contributors to this motion is the deltoid muscle group and for this, an accurate modeling of the lines of action is indispensable. The aim of this work was to utilize a torus obstacle wrapping approach for the deltoids of an existing shoulder model and assess the feasibility of the approach during humeral abduction. The shoulder model from the AnyBody™ modeling system was used as a platform. The size of the tori is based on a magnetic resonance imaging (MRI) approach and several kinematic couplings are implemented to determine the trajectories of the tori during abduction. To assess the model behavior, the moment arms of the virtual muscle elements and the resultant glenohumeral joint reaction force (GHJF) were compared with reference data from the literature during abduction of the humerus in the range 20°–120°. The root mean square error for the anterior, lateral and posterior part between the simulated muscle elements and reference data from the literature was 3.9, 1.7 and 5.8 mm, respectively. The largest deviation occurred on the outer elements of the muscle groups, with 12.6, 10.4 and 20.5 mm, respectively. During abduction, there is no overlapping of the muscle elements and these are in continuous contact with the torus obstacles, thus enabling a continuous force transmission. This results in a rising trend of the resultant GHJF.The torus obstacle approach as a wrapping method for the deltoid muscles provides a guided muscle pathing by simultaneously approximating the curvature of the deltoid muscle. The results from the comparison of the simulated moment arms and the resultant GHJF are in accordance with those in the literature in the range 20°–120° of abduction.Although this study shows the strength of the torus obstacle as a wrapping approach, the method of fitting the tori according to MRI data was not suitable. A cadaver study is recommended to better validate and mathematically describe the torus approach.

A three-experiment examination of iliotibial band strain characteristics during different conditions using musculoskeletal simulation

Iliotibial band syndrome (ITBS) is a common chronic pathology mediated via excessive Iliotibial band (ITB) strain. The purpose using a three-experiment approach is to provide insight into the differences in strain between different athletic movements, the incidence of ITBS in females, the efficacy of different prophylactic modalities for ITBS and also the kinematic parameters associated with ITB strain. Experiment 1 examined male and female athletes performing run, 45° cut and one-legged hop movements, experiment 2 observed males and females, whilst running in five different orthotic conditions and experiment 3 examined males and females riding a cycle ergometer at 70, 80 and 90 RPM whilst in prophylactic knee brace and no-brace conditions. In each experiment, kinematics were obtained using a motion capture system and ITB strain was measured using a musculoskeletal simulation approach. In experiment 1, ITB strain was greater in the run (male = 3.87% and female = 4.37%; P < 0.001) and cut (male = 3.12% and female = 4.06%; P < 0.001) movements compared to hop (male = 0.87% and female = 1.54%). Experiment 2 showed that females exhibited increased ITB strain (male = 6.34% and female = 8.91%; P < 0.05) and ITB strain velocity (male = 57.17%/s and female = 77.41%/s; P < 0.05) and also in females that ITB strain velocity was greater (P ≤ 0.01) in lateral (80.22%/s) and no-orthotic (83.01%/s) conditions compared to medial (72.58%/s) and off the shelf orthoses (74.52%/s). The regression analyses across movements showed that ITB strain was predicted by sagittal and coronal plane mechanics at the hip (R2 = 0.15–0.30; P < 0.05) and sagittal, coronal and transverse plane kinematics at the knee joint (R2 = 0.15–0.22; P < 0.05). Further insight is provided into differences in ITB strain across functional athletic movements, the increased incidence of ITBS in females and the parameters linked most strongly with ITB strain during different movements is provided; whilst also highlighting the prophylactic efficacy of medial and off the shelf orthoses in female runners.

Surface-based modeling of muscles: Functional simulation of the shoulder

Musculoskeletal simulations are an essential tool for studying functional implications of pathologies and of potential surgical outcomes, e.g., for the complex shoulder anatomy. Most shoulder models rely on line-segment approximation of muscles with potential limitations. Comprehensive shoulder models based on continuum-mechanics are scarce due to their complexity in both modeling and computation.In this paper, we present a surface-based modeling approach for muscles, which simplifies the modeling process and is efficient for computation.We propose to use surface geometries for modeling muscles, and devise an automatic approach to generate such models, given the locations of the origin and insertion of tendons. The surfaces are expressed as higher-order tensor B-splines, which ensure smoothness of the geometrical representation. They are simulated as membrane elements within a finite element simulation. This is demonstrated on a comprehensive model of the upper limb, where muscle activations needed to perform desired motions are obtained by using inverse dynamics.In synthetic examples, we demonstrate our proposed surface elements both to be easy to customize (e.g., with spatially varying material properties) and to be substantially (up to 12 times) faster in simulation compared to their volumetric counterpart. With our presented automatic approach of muscle wrapping around bones, the humeral head is exemplified to be wrapped physiologically consistently with surface elements. Our functional simulation is shown to successfully replicate a tracked shoulder motion during activities of daily living.We demonstrate surface-based models to be a numerically stable and computationally efficient compromise between line-segment and volumetric models, enabling anatomical correctness, subject-specific customization, and fast simulations, for a comprehensive simulation of musculoskeletal motion.

Patellofemoral Mechanics: a Review of Pathomechanics and Research Approaches.

The patellofemoral joint is a complicated articulation of the patella and femur that is prone to pathologies. The purpose of this review is to report on the current methods of investigating patellofemoral mechanics, factors that affect joint function, and future directions in patellofemoral joint research with emerging technologies and techniques. While previous hypotheses have suggested that the patella is only a moment arm extender, recent literature has suggested that the patella influences the control of knee moments and forces acting on the tibia as well as contributes to various aspects of patellar function with minimal neural input. With advancements in simulating a six-degrees-of-freedom patellofemoral joint, we have gained a better understanding of patella motion and have shown that geometry and muscle activations impact patella mechanics. Research into influences on patella mechanics from other joints such as the hip and foot has become more prevalent. In this review, we report current in vivo, in vitro, and in silico approaches to studying the patellofemoral joint. Kinematic and anatomical factors that affect patellofemoral joint function such as patella alta and tilt or bone morphology and ligaments are discussed. Moving forward, we suggest that advanced in vivo dynamic imaging methods coupled to musculoskeletal simulation will provide further understanding of patellofemoral pathomechanics and allow engineers and clinicians to design interventions to mitigate or prevent patellofemoral pathologies.

Contact force path in total hip arthroplasty: effect of cup medialisation in a whole-body simulation.

Cup medialisation down to the true acetabular floor in total hip arthroplasty with a compensatory femoral offset increase seems to be mechanically advantageous for the abductor muscles due to the relocation of the lever arms (body weight lever arm decreased, abductor lever arm increased). However, limited information is currently available about the effects of this reconstruction type at the head cup interface, compared to an anatomical reconstruction that maintains the natural lever arms. Through a whole-body simulation analysis, we compared medialised versus anatomical reconstruction in THA to analyse the effects on: (1) contact force magnitude at the head cup interface; (2) contact force path in the cup; and (3) abductor activity. Musculoskeletal simulations were performed to calculate the above-mentioned parameters using inverse dynamics analysis. The differences between the virtually implanted THAs were calculated to compare the medialised versus anatomical reconstruction. Cup medialisation with compensatory femoral offset increase led to: (1) a reduction in contact force magnitude at the head cup interface up to 6.6%; (2) a similar contact force path in the cup in terms of sliding distance and aspect ratio; and (3) a reduction in abductor activity up to 17.2% (gluteus medius). In our opinion, these potential biomechanical gains do not generally justify a fully medialised reconstruction, especially in younger patients that are more likely to undergo revision surgery in their lifetime. Cup medialisation should be performed until sufficient press fit and bony coverage of a properly sized and oriented cup can be achieved.

Foot strike pattern during running alters muscle-tendon dynamics of the gastrocnemius and the soleus

Running is thought to be an efficient gait due, in part, to the behavior of the plantar flexor muscles and elastic energy storage in the Achilles tendon. Although plantar flexor muscle mechanics and Achilles tendon energy storage have been explored during rearfoot striking, they have not been fully characterized during forefoot striking. This study examined how plantar flexor muscle-tendon mechanics during running differs between rearfoot and forefoot striking. We used musculoskeletal simulations, driven by joint angles and electromyography recorded from runners using both rearfoot and forefoot striking running patterns, to characterize plantar flexor muscle-tendon mechanics. The simulations revealed that foot strike pattern affected the soleus and gastrocnemius differently. For the soleus, forefoot striking decreased tendon energy storage and fiber work done while the muscle fibers were shortening compared to rearfoot striking. For the gastrocnemius, forefoot striking increased muscle activation and fiber work done while the muscle fibers were lengthening compared to rearfoot striking. These changes in gastrocnemius mechanics suggest that runners planning to convert to forefoot striking might benefit from a progressive eccentric gastrocnemius strengthening program to avoid injury.

Sim-To-Real Transfer Learning Approach for Tracking Multi-DOF Ankle Motions Using Soft Strain Sensors

A data-driven approach has recently been investigated for identifying human joint angles by means of soft strain sensors because of the corresponding modeling difficulty. However, this approach commonly incurs a high computational burden due to the voluminous amount of data required and the time-series-oriented network architecture. Moreover, the nature of soft sensors makes the problem worse due to the inherent nonlinearity and hysteresis of the material. In this study, we developed a novel wearable sensing brace design for measuring multiple degrees of freedom (DOF) ankle motions to minimize hysteresis and to improve the measurement repeatability and developed a computationally efficient calibration method based on sim-to-real transfer learning. By attaching the soft sensors to shin links rather than directly to the ankle joint, the effects of external disturbances during joint motions were minimized. To calibrate the sensors to body motions, transfer learning was used based on the results from musculoskeletal simulation(OpenSim) and sensor data. The average tracking error for ankle motions using the proposed method was found to be 12.0° for five healthy subjects, while the direct deep neural network approach showed an error of 17.9°. The proposed method could be used to calibrate the soft sensors with 1000 times faster training speed while maintaining comparable tracking accuracy with a smaller amount of data.

Correction to: The Capacity of Generic Musculoskeletal Simulations to Predict Knee Joint Loading Using the CAMS-Knee Datasets.

The article The Capacity of Generic Musculoskeletal Simulations to Predict Knee Joint Loading Using the CAMS-Knee Datasets, written by Zohreh Imani Nejad et al., was originally published electronically on the publisher's internet portal on January 30, 2020 without open access. With the author(s)' decision to opt for Open Choice the copyright of the article changed on February 18, 2020 to © The Author(s) 2020 and the article is forthwith distributed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

Sex differences in medial and lateral tibiofemoral compartment loading during different functional sports tasks

This study examined sex differences in tibiofemoral compartment loading during sport specific movements. Fifteen males and fifteen females completed run and cut motions using an approach velocity of 4.0 m.s−1. Kinematics were obtained using a motion capture system and kinetics using a force platform. Tibiofemoral compartment loading was quantified using a musculoskeletal simulation approach. Irrespective of sex or movement the medial tibiofemoral compartment experienced greater peak loading (Male: run medial = 71.32; lateral = 33.49 N/kg/cut medial = 69.30; lateral = 59.17 N/kg & Female: run medial = 73.85; lateral = 33.61 N/kg/cut medial = 69.41; lateral = 64.03 N/kg). In addition females were shown to exhibit greater peak medial force per mile (6381.78 N/kg s) than males (5139.24 N/kg s) during running. Finally, irrespective of sex, peak lateral force was larger in the cut movement (Male = 59.17 & Female = 64.03 N/kg) compared to the run (Male = 33.49 & Female = 33.61 N/kg). This investigation shows that the different facets of the knee are loaded differently during functional athletic movements but also that both sex and the movement being executed may also be important. Specifically, female runners appear to be at increased risk of medial knee degeneration linked to the aetiology of osteoarthritis. Cette étude a examiné les différences liées au sexe dans compartiment tibiofémorale, chargement durant les mouvements particuliers de sport. Quinze hommes et quinze femmes terminées exécutez et couper des requêtes en utilisant une vitesse d’approche de 4,0 m.s−1. Cinématique ont été obtenues à l’aide d’un système de capture de mouvement et de la cinétique à l’aide d’une plateforme de force. Chargement de compartiment de tibiofémorale a été quantifiée à l’aide d’une approche de simulation musculosquelettiques. Sans distinction de sexe ou de mouvement le compartiment médial tibiofémorale a connu une plus grande charge de crête (hommes : exécutez médial = 71,32 ; latéral = 33,49 N/kg/cut médial = 69,30 ; latéral = N/kg 59,17 & femmes : exécuter médial = 73,85 ; latéral = 33,61 N/kg/cut médial = 69,41 ; latéral = 64,03 N/kg). En outre, les femmes ont montré d’exposer une plus grande force médiale maximale par mille (6381,78 N/kg s) que les hommes (5139,24 N/kg s) au cours de la course. Enfin, indépendamment du sexe, la force latérale maximale était plus importante dans le mouvement de coupe (hommes = 59,17 & femmes = 64,03 N/kg) par rapport à la course (hommes = 33,49 & femmes = 33,61 N/kg). Cette étude montre que les différentes facettes du genou sont chargés différemment au cours de mouvements athlétiques fonctionnels mais aussi que les sexe et le mouvement en cours d’exécution peuvent aussi être importants. Plus précisément, les coureurs féminins semblent être à un risque accru de dégénérescence médial genou lié à l’étiologie de l’arthrose.

Muscle contributions to medial and lateral tibiofemoral compressive loads during sidestep cutting

The tibiofemoral compressive forces experienced during functional activities are believed to be important for maintaining tibiofemoral stability. Previous studies have shown that both knee-spanning and non-knee-spanning muscles contribute to tibiofemoral joint compressive forces during walking. However, healthy individuals typically engage in more vigorous activities (e.g. jumping and cutting) that provide greater challenges to tibiofemoral stability. Despite this, no previous studies have investigated how both knee-spanning and non-knee-spanning muscles contribute to tibiofemoral compressive loading during such tasks. The present study investigated how muscles contributed to the medial and lateral compartment tibiofemoral compressive forces during sidestep cutting. Three-dimensional marker trajectories, ground reaction forces and muscle electromyographic signals were collected from eight healthy males whilst they completed unanticipated sidestep cutting. OpenSim was used to perform musculoskeletal simulations to compute the contribution of each lower-limb muscle to compressive loading of each compartment of the knee. The greatest contributors to medial compartment loading were the vasti, gluteus maximus and medius, and the medial gastrocnemius. The greatest contributors to lateral compartment loading were the vasti, adductors, medial and lateral gastrocnemius, and the soleus. The soleus displayed the greatest potential for unloading the medial compartment, whereas the gluteus maximus and medius displayed the greatest potential for unloading the lateral compartment. These findings may help to inform interventions aiming to modulate compressive loading at the knee.

Spinal sagittal alignment goals based on statistical modelling and musculoskeletal simulations

The definition of target alignment for spinal fusion surgery follows anatomical criteria and strongly relies on surgical experience. However, the optimal patient-specific alignment often remains unknown. Statistical models could provide information about physiological alignments, and musculoskeletal models are powerful tools to investigate biomechanics. We aimed to statistically predict alignments and hypothesized they would be biomechanically favorable.A statistical model was trained with 60 annotated radiographs to predict physiological sagittal alignment based on position of femoral heads and sacrum. Predicted alignments for 11 back pain patients were clinically evaluated in terms of balance and compared to Original alignments. The normative ranges for spinal balance parameters were obtained from Surgimap™. Musculoskeletal loads were furthermore simulated in upright standing and 30° forward flexion, using alignment-specific musculoskeletal models.For the majority of Predicted alignments (n = 9) at least two of three investigated balance parameters were within the normative range, as opposed to the minority of the Original alignments (n = 4). Predicted alignments resulted in significantly lowered overall muscle activity and compressive loads (all levels, both postures). Shear force magnitudes in upright standing decreased significantly at levels L1L2 (−68 N) and L2L3 (−69 N) and clearly yet not significantly at L3L4 (−39 N) and L4L5 (−152 N). Shear loads at level L5S1 remained the same. In flexed postures identical trends were observed.The statistical model was able to predict spinal alignments that led to both improved balance and reduced musculoskeletal loads. Further studies are needed to investigate clinical validity of such models.

Lower-limb muscle function is influenced by changing mechanical demands in cycling.

Although cycling is a seemingly simple, reciprocal task, muscles must adapt their function to satisfy changes in mechanical demands induced by higher crank torques and faster pedalling cadences. We examined whether muscle function was sensitive to these changes in mechanical demands across a wide range of pedalling conditions. We collected experimental data of cycling where crank torque and pedalling cadence were independently varied from 13 to 44 N m and 60 to 140 rpm. These data were used in conjunction with musculoskeletal simulations and a recently developed functional index-based approach to characterise the role of human lower-limb muscles. We found that in muscles that generate most of the mechanical power and work during cycling, greater crank torque induced shifts towards greater muscle activation, greater positive muscle-tendon unit (MTU) work and a more motor-like function, particularly in the limb extensors. Conversely, with faster pedalling cadence, the same muscles exhibited a phase advance in muscle activity prior to crank top dead centre, which led to greater negative MTU power and work and shifted the muscles to contract with more spring-like behaviour. Our results illustrate the capacity for muscles to adapt their function to satisfy the mechanical demands of the task, even during highly constrained reciprocal tasks such as cycling. Understanding how muscles shift their contractile performance under varied mechanical and environmental demands may inform decisions on how to optimise pedalling performance and to design targeted cycling rehabilitation therapies for muscle-specific injuries or deficits.

Contribution of hip and knee muscles to lateral knee stability during gait.

[Purpose] Lateral knee instability is frequently observed in patients with knee injury or risk factors associated with knee osteoarthritis. Physical exercises can strengthen muscles that stabilize the knee joint. The purpose of this study was to define the contribution of the knee and hip muscles to lateral knee stability by comparing the muscle forces, as assessed by musculoskeletal simulation using one or two degrees-of-freedom (1-DOF and 2-DOF) knee models. [Participants and Methods] We evaluated the normal gait of 15 healthy subjects. We conducted a three-dimensional gait analysis using a motion analysis system and a force plate. We considered a muscle as a lateral knee stabilizer when the calculated muscle force was greater with the 2-DOF model than with the 1-DOF model. [Results] During early and late stance, the muscle forces of the lateral knee and hip joint increased in the 2-DOF model as opposed to in the 1-DOF model. In contrast, the forces of the medial knee muscles decreased. Furthermore, hip muscle forces increased during the late stance. [Conclusion] Our results show that the lateral knee and hip muscles contribute to lateral knee stability. Thus, exercises to strengthen these muscles could improve lateral knee stability.

Muscle and joint force dependence of scaling and skill level of athletes in high-speed overhead task: Musculoskeletal simulation study

Abstract The badminton overhead forehand smash (OFS) is a high-speed overhead task in which various muscle-tendon units generate the forces of motion. It is impossible to examine all muscles and joints forces during the motion directly. The goals of the current study were: (1) to estimate the muscle and joint load during OFS, (2) to compare results between generic and scaled-generic models. 20 badminton athletes categorized to two amateur and professional groups. A motion analysis system was used to record the marker trajectories. The shoulder musculoskeletal model developed in OpenSim which includes 5 segments, 10° of freedom, 26 muscle-tendon units. The inverse kinematics was used to evaluate joint kinematics during OFS. The static optimization and joint reaction force tools were utilized to estimate the joint and muscles forces through the OFS, respectively. Results present the joint and muscles forces pattern through the OFS for amateur and professional players. Significant differences were detected between generic and scaled-generic musculoskeletal models in the majority of muscles and joints forces. In addition, significant differences were found between amateur and professional athletes during OFS. The muscle and joint forces results are useful for athletes, coaches, trainer, and sports medicine. Recognition of which muscles and how to play an effective role to produce the force on the correct direction is vital for coaches and trainers due to design effective training to enhance the performance of athletes. On the other hand, the pattern of joint loads may help to evaluate which joints and in which sequences are high-risk. This study is a first and comprehensive study that examine the overhead high-speed motion in two different skill levels.

Estimation of lower limb muscle activity during jumps by using a musculoskeletal simulation model

Estimation of lower limb muscle activity during jumps by using a musculoskeletal simulation model

In vitro virtual reality: an anatomically explicit musculoskeletal simulation powered by in vitro muscle using closed-loop tissue-software interaction.

Muscle force-length dynamics are governed by intrinsic contractile properties, motor stimulation and mechanical load. Although intrinsic properties are well characterised, physiologists lack in vitro instrumentation to account for combined effects of limb inertia, musculoskeletal architecture and contractile dynamics. We introduce in vitro virtual reality (in vitro-VR) which enables in vitro muscle tissue to drive a musculoskeletal jumping simulation. In hardware, muscle force from a frog plantaris was transmitted to a software model where joint torques, inertia and ground reaction forces were computed to advance the simulation at 1 kHz. To close the loop, simulated muscle strain was returned to update in vitro length. We manipulated (1) stimulation timing and (2) the virtual muscle's anatomical origin. This influenced interactions among muscular, inertial, gravitational and contact forces dictating limb kinematics and jump performance. We propose that in vitro-VR can be used to illustrate how neuromuscular control and musculoskeletal anatomy influence muscle dynamics and biomechanical performance.

Patterns of Limb and Epaxial Muscle Activity During Walking in the Fire Salamander, Salamandra salamandra.

SynopsisSalamanders and newts (urodeles) are often used as a model system to elucidate the evolution of tetrapod locomotion. Studies range from detailed descriptions of musculoskeletal anatomy and segment kinematics, to bone loading mechanics and inferring central pattern generators. A further area of interest has been in vivo muscle activity patterns, measured through electromyography (EMG). However, most prior EMG work has primarily focused on muscles of the forelimb or hindlimb in specific species or the axial system in others. Here we present data on forelimb, hindlimb, and epaxial muscle activity patterns in one species, Salamandra salamandra, during steady state walking. The data are calibrated to limb stride cycle events (stance phase, swing phase), allowing direct comparisons to homologous muscle activation patterns recorded for other walking tetrapods (e.g., lizards, alligators, turtles, mammals). Results demonstrate that Salamandra has similar walking kinematics and muscle activity patterns to other urodele species, but that interspecies variation does exist. In the forelimb, both the m. dorsalis scapulae and m. latissimus dorsi are active for 80% of the forelimb swing phase, while the m. anconaeus humeralis lateralis is active at the swing–stance phase transition and continues through 86% of the stance phase. In the hindlimb, both the m. puboischiofemoralis internus and m. extensor iliotibialis anterior are active for 30% of the hindlimb swing phase, while the m. caudofemoralis is active 65% through the swing phase and remains active for most of the stance phase. With respect to the axial system, both the anterior and posterior m. dorsalis trunci display two activation bursts, a pattern consistent with stabilization and rotation of the pectoral and pelvic girdles. In support of previous assertions, comparison of Salamandra muscle activity timings to other walking tetrapods revealed broad-scale similarities, potentially indicating conservation of some aspects of neuromuscular function across tetrapods. Our data provide the foundation for building and testing dynamic simulations of fire salamander locomotor biomechanics to better understand musculoskeletal function. They could also be applied to future musculoskeletal simulations of extinct species to explore the evolution of tetrapod locomotion across deep-time.

Investigation of the effect of tonus on the change in postural control strategy using musculoskeletal simulation

BackgroundHumans use different postural control strategies depending on perturbations. The shift from an ankle strategy to a hip strategy occurs at different perturbation magnitudes for different individuals. Although such differences relate to the differences in body parameters such as muscle strength, the parameter changes that affect the strategy shift are unclear. The relationship between tonus and strategy is especially unclear, but humans control tonus, which contributes to body stability. The objective of this study was to investigate the influence of tonus on postural control strategy. Research questionIs there a trend toward the use of a hip strategy with a decrease in the magnitude of tonus? MethodsPredictive simulations were performed for changing parameters of muscle weakness and increased sensory noise, which are considered the causes of different strategies and decreased tonus. A musculoskeletal model with 70 muscles and 15 degrees of freedom of joints was controlled using a neural controller model, and the support surface was translated backward to introduce perturbations. The parameters of the musculoskeletal and neural controller models were changed for 48 conditions of different muscle strengths, sensory noise, and tonus. The control parameters were optimized for each condition. Simulations were performed with the optimized control parameters to calculate an evaluation index to show the difference in postural control strategies (peak hip angle), and the relationship between the index and parameters was analyzed using analysis of variance and multiple regression. ResultsThe main effects of muscle weakness and decreased tonus and their interaction were confirmed. The results recognized a positive response to the research question. SignificanceThe study emphasizes the importance of considering tonus while investigating the postural control strategy. Furthermore, it was suggested that when the magnitude of tonus was larger than a threshold, only the ankle strategy was used, regardless of muscle strength.

A Bioinspired Lightweight Wrist for High-DoF Robotic Prosthetic Arms

The prosthetic arms generally face several challenges: most critically, they are not lightweight to the users and do not provide full degrees of freedom (DoF) for the arms. In order to address this problem, we propose a novel bioinspired lightweight and high-DoF wrist for prosthetic arms. The proposed robotic wrist is lightweight, albeit more dexterous by simplifying the human wrist structure into a two-row ellipsoid joint and implementing novel ligament routing. The two-row ellipsoid structure effectively reduces the weight of the full-DoF wrist within sufficient ranges of motion required for its mobility. A novel routing-by-ligament tunnel allows for the employment of stiffer ligaments in a compact structure, thereby securing the full DoF of the wrist without compromising its load capacity. We conducted a musculoskeletal simulation to show that the bioinspired wrist helps to increase the energy efficiency of the entire upper limb. Our experimental results demonstrated that the proposed wrist greatly reduces the weight of the entire prosthetic arm while improving its range of motion, tensile stiffness, and the control performance.