[Numerical simulation in musculoskeletal biomechanics : Application perspectives and possibilities]. / Numerische Simulation in der muskuloskelettalen Biomechanik : Anwendungsperspektiven und Möglichkeiten.

BACKGROUND: Computational research methods, such as finite element analysis (FEA) and musculoskeletal multi-body simulation (MBS), are important in musculoskeletal biomechanics because they enable a better understanding of the mechanics of the musculoskeletal system, as well as the development and evaluation of orthopaedic implants. These methods are used to analyze clinically relevant issues in various anatomical regions, such as the hip, knee, shoulder joints and spine. Preoperative simulation can improve surgical planning in orthopaedics and predict individual results. EXAMPLES FROM PRACTICE: In this article, the methods of FE analysis and MBS are explained using two practical examples, and the activities of the "Numerical Simulation" cluster of the "Musculoskeletal Biomechanics Research Network (MSB-NET)" are presented in more detail. An outlook classifies numerical simulation in the age of artificial intelligence and draws attention to the relevance of simulation in the (re)approval of implants.

Bridging the sim2real gap. Investigating deviations between experimental motion measurements and musculoskeletal simulation results-a systematic review.

Musculoskeletal simulations can be used to estimate biomechanical variables like muscle forces and joint torques from non-invasive experimental data using inverse and forward methods. Inverse kinematics followed by inverse dynamics (ID) uses body motion and external force measurements to compute joint movements and the corresponding joint loads, respectively. ID leads to residual forces and torques (residuals) that are not physically realistic, because of measurement noise and modeling assumptions. Forward dynamic simulations (FD) are found by tracking experimental data. They do not generate residuals but will move away from experimental data to achieve this. Therefore, there is a gap between reality (the experimental measurements) and simulations in both approaches, the sim2real gap. To answer (patho-) physiological research questions, simulation results have to be accurate and reliable; the sim2real gap needs to be handled. Therefore, we reviewed methods to handle the sim2real gap in such musculoskeletal simulations. The review identifies, classifies and analyses existing methods that bridge the sim2real gap, including their strengths and limitations. Using a systematic approach, we conducted an electronic search in the databases Scopus, PubMed and Web of Science. We selected and included 85 relevant papers that were sorted into eight different solution clusters based on three aspects: how the sim2real gap is handled, the mathematical method used, and the parameters/variables of the simulations which were adjusted. Each cluster has a distinctive way of handling the sim2real gap with accompanying strengths and limitations. Ultimately, the method choice largely depends on various factors: available model, input parameters/variables, investigated movement and of course the underlying research aim. Researchers should be aware that the sim2real gap remains for both ID and FD approaches. However, we conclude that multimodal approaches tracking kinematic and dynamic measurements may be one possible solution to handle the sim2real gap as methods tracking multimodal measurements (some combination of sensor position/orientation or EMG measurements), consistently lead to better tracking performances. Initial analyses show that motion analysis performance can be enhanced by using multimodal measurements as different sensor technologies can compensate each other's weaknesses.

A sensorimotor enhanced neuromusculoskeletal model for simulating postural control of upright standing.

The human's upright standing is a complex control process that is not yet fully understood. Postural control models can provide insights into the body's internal control processes of balance behavior. Using physiologically plausible models can also help explaining pathophysiological motion behavior. In this paper, we introduce a neuromusculoskeletal postural control model using sensor feedback consisting of somatosensory, vestibular and visual information. The sagittal plane model was restricted to effectively six degrees of freedom and consisted of nine muscles per leg. Physiologically plausible neural delays were considered for balance control. We applied forward dynamic simulations and a single shooting approach to generate healthy reactive balance behavior during quiet and perturbed upright standing. Control parameters were optimized to minimize muscle effort. We showed that our model is capable of fulfilling the applied tasks successfully. We observed joint angles and ranges of motion in physiologically plausible ranges and comparable to experimental data. This model represents the starting point for subsequent simulations of pathophysiological postural control behavior.

Contributing Components of Metabolic Energy Models to Metabolic Cost Estimations in Gait.

OBJECTIVE: As metabolic cost is a primary factor influencing humans' gait, we want to deepen our understanding of metabolic energy expenditure models. Therefore, this paper identifies the parameters and input variables, such as muscle or joint states, that contribute to accurate metabolic cost estimations. METHODS: We explored the parameters of four metabolic energy expenditure models in a Monte Carlo sensitivity analysis. Then, we analysed the model parameters by their calculated sensitivity indices, physiological context, and the resulting metabolic rates during the gait cycle. The parameter combination with the highest accuracy in the Monte Carlo simulations represented a quasi-optimized model. In the second step, we investigated the importance of input parameters and variables by analysing the accuracy of neural networks trained with different input features. RESULTS: Power-related parameters were most influential in the sensitivity analysis and the neural network-based feature selection. We observed that the quasi-optimized models produced negative metabolic rates, contradicting muscle physiology. Neural network-based models showed promising abilities but have been unable to match the accuracy of traditional metabolic energy expenditure models. CONCLUSION: We showed that power-related metabolic energy expenditure model parameters and inputs are most influential during gait. Furthermore, our results suggest that neural network-based metabolic energy expenditure models are viable. However, bigger datasets are required to achieve better accuracy. SIGNIFICANCE: As there is a need for more accurate metabolic energy expenditure models, we explored which musculoskeletal parameters are essential when developing a model to estimate metabolic energy.

The Determination of Assistance-as-Needed Support by an Ankle-Foot Orthosis for Patients with Foot Drop.

Patients who suffer from foot drop have impaired gait pattern functions and a higher risk of stumbling and falling. Therefore, they are usually treated with an assistive device, a so-called ankle-foot orthosis. The support of the orthosis should be in accordance with the motor requirements of the patient and should only be provided when needed, which is referred to as assistance-as-needed. Thus, in this publication, an approach is presented to determine the assistance-as-needed support using musculoskeletal human models. Based on motion capture recordings of multiple subjects performing gaits at different speeds, a parameter study varying the optimal force of a reserve actuator representing the ankle-foot orthosis added in the musculoskeletal simulation is conducted. The results show the dependency of the simulation results on the selected optimal force of the reserve actuator but with a possible identification of the assistance-as-needed support required from the ankle-foot orthosis. The required increase in support due to the increasing severity of foot drop is especially demonstrated with the approach. With this approach, information for the required support of individual subjects can be gathered, which can further be used to derive the design of an ankle-foot orthosis that optimally assists the subjects.

Methods for integrating postural control into biomechanical human simulations: a systematic review.

Understanding of the human body's internal processes to maintain balance is fundamental to simulate postural control behaviour. The body uses multiple sensory systems' information to obtain a reliable estimate about the current body state. This information is used to control the reactive behaviour to maintain balance. To predict a certain motion behaviour with knowledge of the muscle forces, forward dynamic simulations of biomechanical human models can be utilized. We aim to use predictive postural control simulations to give therapy recommendations to patients suffering from postural disorders in the future. It is important to know which types of modelling approaches already exist to apply such predictive forward dynamic simulations. Current literature provides different models that aim to simulate human postural control. We conducted a systematic literature research to identify the different approaches of postural control models. The different approaches are discussed regarding their applied biomechanical models, sensory representation, sensory integration, and control methods in standing and gait simulations. We searched on Scopus, Web of Science and PubMed using a search string, scanned 1253 records, and found 102 studies to be eligible for inclusion. The included studies use different ways for sensory representation and integration, although underlying neural processes still remain unclear. We found that for postural control optimal control methods like linear quadratic regulators and model predictive control methods are used less, when models' level of details is increasing, and nonlinearities become more important. Considering musculoskeletal models, reflex-based and PD controllers are mainly applied and show promising results, as they aim to create human-like motion behaviour considering physiological processes.

Method for Using IMU-Based Experimental Motion Data in BVH Format for Musculoskeletal Simulations via OpenSim.

Biomechanical simulation allows for in silico estimations of biomechanical parameters such as muscle, joint and ligament forces. Experimental kinematic measurements are a prerequisite for musculoskeletal simulations using the inverse kinematics approach. Marker-based optical motion capture systems are frequently used to collect this motion data. As an alternative, IMU-based motion capture systems can be used. These systems allow flexible motion collection without nearly any restriction regarding the environment. However, one limitation with these systems is that there is no universal way to transfer IMU data from arbitrary full-body IMU measurement systems into musculoskeletal simulation software such as OpenSim. Thus, the objective of this study was to enable the transfer of collected motion data, stored as a BVH file, to OpenSim 4.4 to visualize and analyse the motion using musculoskeletal models. By using the concept of virtual markers, the motion saved in the BVH file is transferred to a musculoskeletal model. An experimental study with three participants was conducted to verify our method's performance. Results show that the present method is capable of (1) transferring body dimensions saved in the BVH file to a generic musculoskeletal model and (2) correctly transferring the motion data saved in the BVH file to a musculoskeletal model in OpenSim 4.4.

Design and Additive Manufacturing of a Passive Ankle-Foot Orthosis Incorporating Material Characterization for Fiber-Reinforced PETG-CF15.

The individualization of patient-specific ankle joint orthoses is becoming increasingly important and can be ideally realized by means of additive manufacturing. However, currently, there are no functional additively manufactured fiber-reinforced products that are used in the field of orthopedic treatment. In this paper, an approach as to how additively manufactured orthopedic products can be designed and produced quickly and flexibly in the future is presented. This is demonstrated using the example of a solid ankle-foot orthosis. For this purpose, test results on PETG-CF15, which were determined in a previous work, were integrated into a material map for an FEA simulation. Therewith, the question can be answered as to whether production parameters that were determined at the test specimen level can also be adapted to real, usable components. Furthermore, gait recordings were used as loading conditions to obtain exact results for the final product. In order to perfectly adapt the design of the splint to the user, a 3D scan of a foot was performed to obtain a perfect design space for topology optimization. This resulted in a patient-specific and stiffness-optimized product. Subsequently, it was demonstrated that the orthosis could be manufactured using fused layer modelling. Finally, a comparison between the conventional design and the consideration of AM-specific properties was made. On this basis, it can be stated that the wearing comfort of the patient-specific design is very good, but the tightening of the splint still needs to be improved.

Subject-specific tribo-contact conditions in total knee replacements: a simulation framework across scales.

Fundamental knowledge about in vivo kinematics and contact conditions at the articulating interfaces of total knee replacements are essential for predicting and optimizing their behavior and durability. However, the prevailing motions and contact stresses in total knee replacements cannot be precisely determined using conventional in vivo measurement methods. In silico modeling, in turn, allows for a prediction of the loads, velocities, deformations, stress, and lubrication conditions across the scales during gait. Within the scope of this paper, we therefore combine musculoskeletal modeling with tribo-contact modeling. In the first step, we compute contact forces and sliding velocities by means of inverse dynamics approach and force-dependent kinematic solver based upon experimental gait data, revealing contact forces during healthy/physiological gait of young subjects. In a second step, the derived data are employed as input data for an elastohydrodynamic model based upon the finite element method full-system approach taking into account elastic deformation, the synovial fluid's hydrodynamics as well as mixed lubrication to predict and discuss the subject-specific pressure and lubrication conditions.

Exploring the importance of a usable and emotional product design from the user's perspective.

Usability and emotionality are important components of user experience. However, an equal consideration of both constructs in product design is not always possible due to sometimes competitive objectives. In order to foster a user-oriented design decision in such conflicting situations, this paper examines the general importance of both constructs and their dimensions from the user's perspective while taking into account socio-demographic variables. Examination was realised by conducting a product independent anonymous online survey (n = 130). The findings confirm that both constructs are important, yet in a direct comparison, usability is perceived as more important than emotionality. Taking selected dimensions of both constructs into account, an intuitive, easy and learnable usage, suitability for the user's task and freedom from impairment are particularly important in terms of usability. An aesthetic and pleasurable product design as well as originality is essential in terms of emotionality.Practitioner summary: This paper aims for supporting user-oriented design decisions in the context of conflicting objectives occurring in the consideration of usability and emotionality in product design. The conducted survey (n = 130) revealed usability as perceived more important than emotionality. Usability may thus be prioritised within conflicting design decisions.Abbreviations: DFG: German Research Foundation; e.g.: for example; GESIS: Leibniz Institute for the Social Sciences; m: metric; M: Mean; n: number of participants; n: nominal; o: ordinal; p level: level of statistical significance; RQ: Research question; r: correlation coefficient; V: Cramer's V; χ2: Chi-square.

Modelling the interaction between wearable assistive devices and digital human models-A systematic review.

Exoskeletons, orthoses, exosuits, assisting robots and such devices referred to as wearable assistive devices are devices designed to augment or protect the human body by applying and transmitting force. Due to the problems concerning cost- and time-consuming user tests, in addition to the possibility to test different configurations of a device, the avoidance of a prototype and many more advantages, digital human models become more and more popular for evaluating the effects of wearable assistive devices on humans. The key indicator for the efficiency of assistance is the interface between device and human, consisting mainly of the soft biological tissue. However, the soft biological tissue is mostly missing in digital human models due to their rigid body dynamics. Therefore, this systematic review aims to identify interaction modelling approaches between wearable assistive devices and digital human models and especially to study how the soft biological tissue is considered in the simulation. The review revealed four interaction modelling approaches, which differ in their accuracy to recreate the occurring interactions in reality. Furthermore, within these approaches there are some incorporating the appearing relative motion between device and human body due to the soft biological tissue in the simulation. The influence of the soft biological tissue on the force transmission due to energy absorption on the other side is not considered in any publication yet. Therefore, the development of an approach to integrate the viscoelastic behaviour of soft biological tissue in the digital human models could improve the design of the wearable assistive devices and thus increase its efficiency and efficacy.

Simplifying Computer Aided Ergonomics: A User-Product Interaction-Modeling Framework in CAD Based on a Taxonomy of Elementary Affordances.

OCCUPATIONAL APPLICATIONSThis contribution provides a framework for modeling user-product interactions (in CAD) for in-depth ergonomic analysis of product design, using digital human models. The framework aims to be applicable to a wide range of different products while being suitable for designers - especially those who do not have specialized ergonomic expertise or training in human behavior - by providing an intuitive, standardized, and time-efficient modeling procedure. The framework contains 31 elementary affordances, which describe mechanical dependencies between product geometries and human end effectors. These elementary affordances serve as a tool for interaction modeling. Additionally, the paper provides a taxonomy of elementary affordances, which can be used to formalize / abstract the nature of user-product interactions and to describe them as elementary affordances. Furthermore, an implementation of the interaction-modeling framework is presented in a CAD environment and provides an example of how the framework could be used in terms of a computer aided ergonomics tool.

TECHNICAL ABSTRACTBackground Digital human models (DHM) have not yet reached their full potential for proactive virtual assessment of ergonomics in engineering and industrial design. Modeling the interaction between user and product often is time demanding, cumbersome, unstandardized, or embedded insufficiently in the computer aided engineering environment. Existing interaction-modeling frameworks either address the simulation of occupational processes, are limited to a specific use cases, or offer insufficient usability.Purpose We present a framework for interaction modeling, its methodic background, as well as its implementation. The framework aims to provide ergonomic analyses of product designs, while being suitable for designers who do not have specific ergonomic knowledge or training in human behavior.Methods To resolve these partly contradictable demands, we utilize affordances, which serve as a tool for interaction modeling. We hypothesize, that many interaction concepts existing in human technology interaction can be reduced to a relatively small set of elementary affordances. We developed a taxonomy of elementary affordances to deduce elementary affordances from empirical interaction data.Results We present the resulting taxonomy, as well as the resulting 31 elementary affordances, which describe mechanical dependencies between product geometries and human end effectors. The identified elementary affordances are implemented as affordance features in a CAD environment (Siemens NX) and result in an interaction-modeling framework. A brief application example regarding the functionalities of the framework is presented.Conclusions The introduced framework demonstrates how the integration of interaction modeling into the computer aided engineering environment can be achieved in a comprehensible and straightforward way. The resulting simplicity and accessibility may constitute one key factor to help exploit the potential of DHM simulation as a computer aided ergonomics tool in engineering and industrial design.

Structured ergonomic guidance in early design phases by analysing the user-product interaction.

Gathering information for an early, proactive integration of ergonomic user requirements is challenging due to the unstructured character of available knowledge. Knowledge acquisition and processing is therefore costly and time-consuming. This contribution presents and evaluates InProCo, an approach for structured ergonomic guidance that aims to improve accessibility and clarity of ergonomic requirements in early design phases by providing interaction-based ergonomic properties. InProCo reduces the complexity of gathering knowledge and provide a novel way to describe interactions. Within a three-stage evaluation process, a survey assessed standard interactions for completeness, the output given by the graphical user interface of the approach (GUI) was evaluated for correctness and the knowledge base was validated by comparing the approaches output with properties identified by participants within a one-day workshop. The results showed that there are enough predefined standard interactions, the output of the GUI is valid and the knowledge base contains high quality data. Practitioner Summary: InProCo is an approach that provides structured interaction-based ergonomic properties to improve accessibility and clarity of ergonomic requirements in early design phases. This contribution presents and evaluates InProCo building the prerequisite for further use in an industrial context.

Challenges in interaction modelling with digital human models - A systematic literature review of interaction modelling approaches.

Digital human models (DHM) allow for a proactive ergonomic assessment of products by applying different models describing the user-product interaction. In engineering design, DHM tools are currently not established as computer-aided ergonomics tools, since (among other reasons) the interaction models are either cumbersome to use, unstandardised, time-demanding or not trustworthy. To understand the challenges in interaction modelling, we conducted a systematic literature review with the aim of identification, classification and examination of existing interaction models. A schematic user-product interaction model for DHM is proposed, abstracting existing models and unifying the corresponding terminology. Additionally, nine general approaches to proactive interaction modelling were identified by classifying the reviewed interaction models. The approaches are discussed regarding their scope, limitations, strength and weaknesses. Ultimately, the literature review revealed that prevalent interaction models cannot be considered unconditionally suitable for engineering design since none of them offer a satisfactory combination of genuine proactivity and universal validity. Practitioner summary: This contribution presents a systematic literature review conducted to identify, classify and examine existing proactive interaction modelling approaches for digital human models in engineering design. Ultimately, the literature review revealed that prevalent interaction models cannot be considered unconditionally suitable for engineering design since none of them offer a satisfactory combination of genuine proactivity and universal validity. Abbreviations: DHM: digital human model; CAE: computer-aided engineering; RQ: research question.

Musculoskeletal modeling of user groups for virtual product and process development.

Although biomechanical digital human models find their way into virtual engineering processes, biomechanical considerations are currently still unrecognized to a large extent. One major obstacle lies in the fact that even though subject-specific modeling procedures are developed, virtual user groups or populations are still missing. The objective of this contribution is to create such groups of musculoskeletal models. Therefore, a modeling procedure based upon population data is described. First of all, two generic three-dimensional models, one with female and one with male average anthropometric dimensions, were obtained. These models constitute the starting point for the following model adjustment phases. Evenly distributed dimensionless values for gender, age, height, mass, range of motion and strength are sampled and translated into more expressive parameters for the mentioned modeling domains serving as input data for the creation of each individual model of the desired population or user group. The most sophisticated step of the adaption is the strength mapping aiming to create models matching arbitrary target joint torques. Finally, the models' maximal strength is assessed in a manual material handling task and compared to empirical strength data. The approach is shown using the example of the German population.

New Design Process for Anatomically Enhanced Osteosynthesis Plates.

This study evaluated the implementation and effectiveness of an iterative process aimed to quantify and enhance the anatomical fit of an osteosynthesis plate design for the fifth metacarpal bone regarding a defined shape-based acceptance criterion (SAC) while complying with basic clinical requirements and engineering limitations. The process was based on employing virtual tools (a database of individual three-dimensional bone models, statistical analysis of the bone geometry, and proprietary software tools) to evaluate conformity between plate designs and bone shape. The conformity was quantified by the mean distance between plate and bone (MBP). The enhancement was completed when the median MBP of the population was below the SAC threshold. This was fulfilled by the third plate design (two enhancement iterations). The intentionally abstract enhancement process may serve as a guideline for development of plate designs for other indications. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1508-1517, 2019.