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Muscle wrapping influences its moment arm and alters the dynamic response of the musculoskeletal system. Conventional musculoskeletal multibody models often simplify muscle wrapping as the shortest path problem, while these models cannot consider the three-dimensional muscular geometry. In comparison, the finite element description of skeletal muscles using three-dimensional solid element discretization is computationally expensive. This paper establishes a novel flexible multibody modeling method of muscle wrapping based on absolute nodal coordinate formulation (ANCF), which can simultaneously deal with its fusiform geometry, distributed inertia, and active contractions. The skeletal muscle is discretized by full-parameterized ANCF beam elements with variable cross section. The three-dimensional shape of the muscle belly can be expressed by an explicit function, and its time-dependent change can be captured via the surface-to-surface contact detection algorithm. The muscle stress contains the effect of Hill-type active–passive contractions, muscular fascia hyperelasticity, and volume constancy based on continuum mechanics description. As a validation, a forward dynamics simulation of the sagittal elbow rotation driven by surface electromyography (sEMG) data was performed and compared with literature data and experimental results. The established numerical method provides an efficient analysis tool to describe muscle-bone force transmissions, presenting an approach for understanding the hidden biomechanics during human locomotion and human–machine interactions. [ABSTRACT FROM AUTHOR]

Many patients suffering from oral and maxillofacial tumors cannot open their mouths wide after mandibular reconstruction surgery. Musculoskeletal multibody modeling could be a valuable tool for predicting patient-specific jaw opening functions in the preoperative stage. In this study, a flexible multibody dynamics modeling framework of the human mandibular musculoskeletal system is proposed for surgical planning. In the model, the mandibular muscle bundles are discretized by a flexible cable element combining a typical Hill-type musculotendon model with distributed mass. The mandibular kinematics, together with the electromyographic activities of masticatory muscles, were measured in a patient with a unilateral mandibular tumor. Using the obtained experimental data, a forward–inverse dynamics procedure was proposed to realize the decoupled calculation of synergistic head movement and temporomandibular joint (TMJ) dynamics. The surgical planning simulation was driven by the measured activation patterns of the masticatory muscles and the calculated patterns of the jaw opening and pterygoid muscles. The muscle biomechanical parameters in the postoperative model were changed according to the medical imaging measurement of five patients before and after surgical interventions. As validation of the proposed surgical planning method, the predicted maximum jaw gape and mandibular deviations were compared with postoperative measurements. Numerical results further revealed the bistable characteristic of the TMJs together with the alternations of mandibular movement functions caused by muscle-release surgery. The proposed multibody simulation framework provides a novel method for understanding patient-specific pathology of suffering from trismus and assisting in designing treatments and rehabilitation strategies. [ABSTRACT FROM AUTHOR]

Multibody musculoskeletal modeling of human gait has been proved helpful in investigating the pathology of musculoskeletal disorders. However, conventional inverse dynamics methods rely on external force sensors and cannot capture the nonlinear muscle behaviors. Meanwhile, the forward dynamics approach is computationally demanding and only suited for relatively simple tasks. This study proposed an integrated simulation methodology to fulfill the requirements of estimating foot-ground reaction force, tendon elasticity, and muscle recruitment optimization. A hybrid motion capture system, which combines the marker-based infrared device and markerless tracking through deep convolutional neural networks, was developed to track lower limb movements. The foot-ground reaction forces were determined by a contact model for soft materials, and its parameters were estimated using a two-step optimization method. The muscle recruitment problem was first resolved via a static optimization algorithm, and the obtained muscle activations were used as initial values for further simulation. A torque tracking procedure was then performed by minimizing the errors of joint torques calculated by musculotendon equilibrium equations and inverse dynamics. The proposed approach was validated against the electromyography measurements of a healthy subject during gait. The simulation framework provides a robust way of predicting joint torques, musculotendon forces, and muscle activations, which can be beneficial for understanding the biomechanics of normal and pathological gait. [ABSTRACT FROM AUTHOR]

Practical multibody systems usually consist of flexible bodies of complex shapes, but existing dynamic modeling methods work efficiently only for the systems with bodies of simple and regular shapes. This study proposes a novel computational method for simulating dynamics of flexible multibody systems with flexible bodies of complex shapes via an integration of the finite cell method (FCM) and the absolute nodal coordinate formulation. The classic mesh of FCM is not aligned to the body boundaries, leading to a large number of integration points in cut cells. This study utilizes the Boolean FCM with compressed sub-cell method to reduce the number of integration points and improve computation efficiency. Seven static and dynamic numerical examples are used to validate the proposed method. [ABSTRACT FROM AUTHOR]

The role of intra-abdominal pressure (IAP) in spinal load reduction has remained controversial, partly because previous musculoskeletal models did not introduce the pressure generating mechanism. In this study, an integrated computational methodology is proposed to combine the IAP change with core muscle activations. An ideal gas relationship was introduced to calculate pressure distribution within the abdominal cavity. Additionally, based on flexible multibody dynamics, a muscle membrane element was developed by incorporating the muscular fiber deformation, inter-fiber stiffness, and volume constancy. This element was then utilized in discretizing the diaphragm and transversus abdominis, forming an IAP–muscle coupling system of the abdominal cavity. Based on this methodology, a forward dynamic simulation of spinal flexion was presented to examine the unloading effect of abdominal breathing. The results confirm that core muscle contraction during the abdominal breathing cycle can substantially reduce the forces of spinal compression together with trunk extensor muscles, and this effect is more pronounced when the IAP increase is produced by contraction of the transversus abdominis. This unloading effect still holds even with the co-activation of other abdominal muscles, providing a potential choice when designing trunk movements during weight-lifting tasks. [ABSTRACT FROM AUTHOR]

Skeletal muscles usually wrap over multiple anatomical features, and their mass moves along the curved muscle paths during human locomotion. However, existing musculoskeletal models simply lump the mass of muscles to the nearby body segments without considering the effect of mass flow, which has been shown to induce non-negligible errors. A mass-variable multibody formulation is proposed here to simultaneously characterize muscle wrapping and mass flow effects. To achieve this goal, a novel cable element of the muscle–tendon unit, which integrates the mass flow feature with a typical Hill-type constitutive relationship, was developed based on an arbitrary Lagrangian–Eulerian description. In addition, sliding joints were used to constrain the elements to move over the underlying bone geometries. After validating the proposed modeling method using two benchmark samples, it was applied to build a large-scale lower limb musculoskeletal model, where knee joint moments were calculated and compared with isokinetic dynamometry measurements of 12 healthy males. The results of the comparison confirm that muscular mass distribution play an important role in the force transmission of muscle wrapping, and the proposed mass-variable formulation provides a better way of predicting and understanding the dynamics of musculoskeletal systems. [ABSTRACT FROM AUTHOR]

Skeletal muscles are always embedded in sheets of connective tissues, which influences muscle biomechanics by shaping the fascicle geometry and encapsulating muscular mass flow. However, existing Hill-type muscle models typically take surrounding tissues into account as a nonlinear spring, without consideration of the muscle geometry and inertia. In this paper, a new muscle model is proposed to simultaneously account for soft tissue constraints on the muscle's shape together with mass flow during stretch. To accomplish this, a mass-variable cable element of the muscle–tendon unit, with parameterization of its geometrical influence on the force-producing capability, is newly formulated based on an arbitrary Lagrangian–Eulerian description. Also, sliding joints are presented to further constrain possible mass flow of the elements via epimuscular soft tissue connections between adjacent muscle bellies. Available experimental data from cat soleus and rat gastrocnemius medialis muscles validates the proposed method. For further verification, a planar model of the triceps surae is developed by integration of this modeling framework, and subject-specific simulations of the passive ankle dynamometry tests are performed and correlated with sonoelastographic evaluations of two male participants. The results confirm that the flow of the muscle mass can alternate its force-generating behaviors, and the established model provides an accurate prediction of muscle behavior under transverse loading. The proposed muscle element could be integrated with larger musculoskeletal models to better investigate biomechanical functions of muscles during locomotion, such as heel impact or vibration responses of the spine, when dynamic effects are substantial. [ABSTRACT FROM AUTHOR]

Fractional-order derivatives have been widely used to describe the spiking patterns of neurons, without considering their self-similar dendritic structures. In this study, a self-similar resistor–capacitor network is proposed to relate the spiny dendritic structure with fractional spiking properties. In order to achieve this goal, two types of networks comprising recursively staggered resistors and capacitors were developed to model the functional properties of smooth and spiny dendrites, respectively. Their overall electrotonic properties can be described by fractional order temporal operators derived by Heaviside operational calculus. According to this operator method, spiking patterns of spiny dendrites were controlled by the standard 0.5-order derivative, whereas an exponential modulation term was added in the governing fractional operator of the smooth dendrites. The application of these fractional operators in a leaky integrate-and-fire model reveals that the dendritic spine plays an important role in alternations of the spiking properties, including first-spike latency, firing rate adaptation, and afterhyperpolarization conductance. Further, the multilevel assembly of this network indicates that the fractional spiking behaviors of spiny neurons might originate from their hierarchical substructures, thereby highlighting possible functional consequences of alterations to dendritic self-similarity. [ABSTRACT FROM AUTHOR]

This study investigates the viscoelastic behaviour of soft bio-fibres in association with their fractal structures. A spring-dashpot fractal network with the self-similar topology, named the Π-type fractal ladder hyper-cell (FLHC), is abstracted from the micro/nano-structure of ligaments and tendons (LTs). Its constitutive operator is derived by the Heaviside operational calculus, which is of intrinsic fractional order. In terms of this operator, the long-term viscoelastic relaxation of bio-fibres arising from the fractal ladder topology is expounded. In addition, the fractional-order viscoelastic constitutive equation is obtained based on the FLHC of LTs, and its results are consistent with those of available human knee and spinal LT relaxation experiments. Results on the constitutive equation of FLHCs are formulated into two propositions. The multidisciplinary invariance and implications from the fractal ladder pattern of bio-fibres are also discussed. [ABSTRACT FROM AUTHOR]