Measurement of muscle and tendon stiffness in man

Effects of different contact angles during forefoot running on the stresses of the foot bones: a finite element simulation study

Introduction: The purpose of this study was to compare the changes in foot at different sole-ground contact angles during forefoot running. This study tried to help forefoot runners better control and improve their technical movements by comparing different sole-ground contact angles. Methods: A male participant of Chinese ethnicity was enlisted for the present study, with a recorded age of 25 years, a height of 183 cm, and a body weight of 80 kg. This study focused on forefoot strike patterns through FE analysis. Results: It can be seen that the peak von Mises stress of M1-5 (Metatarsal) of a (Contact angle: 9.54) is greater than that of b (Contact angle: 7.58) and c (Contact angle: 5.62) in the three cases. On the contrary, the peak von Mises stress of MC (Medial Cuneiform), IC (Intermediate Cuneiform), LC (Lateral Cuneiform), C (Cuboid), N (Navicular), T (Tarsal) in three different cases is opposite, and the peak von Mises stress of c is greater than that of a and b. The peak von Mises stress of b is between a and c. Conclusion: This study found that a reduced sole-ground contact angle may reduce metatarsal stress fractures. Further, a small sole-ground contact angle may not increase ankle joint injury risk during forefoot running. Hence, given the specialized nature of the running shoes designed for forefoot runners, it is plausible that this study may offer novel insights to guide their athletic pursuits.

The effects of plyometric jump training on lower-limb stiffness in healthy individuals: A meta-analytical comparison

PURPOSE

This study aimed to examine the effects of plyometric jump training (PJT) on lower-limb stiffness.

METHODS

Systematic searches were conducted in PubMed, Web of Science, and Scopus. Study participants included healthy males and females who undertook a PJT programme isolated from any other training type.

RESULTS

There was a small effect size (ES) of PJT on lower-limb stiffness (ES = 0.33, 95% confidence interval (95%CI): 0.07-0.60, z = 2.47, p = 0.01). Untrained individuals exhibited a larger ES (ES = 0.46, 95%CI: 0.08-0.84, p = 0.02) than trained individuals (ES = 0.15, 95%CI: ‒0.23 to 0.53, p = 0.45). Interventions lasting a greater number of weeks (>7 weeks) had a larger ES (ES = 0.47, 95%CI: 0.06-0.88, p = 0.03) than those lasting fewer weeks (ES = 0.22, 95%CI: ‒0.12 to 0.55, p = 0.20). Programmes with ≤2 sessions per week exhibited a larger ES (ES = 0.39, 95%CI: 0.01-0.77, p = 0.04) than programmes that incorporated >2 sessions per week (ES = 0.20, 95%CI: -0.10 to 0.50, p = 0.18). Programmes with <250 jumps per week (ES = 0.50, 95%CI: 0.02-0.97, p = 0.04) showed a larger effect than programmes with 250-500 jumps per week (ES = 0.36, 95%CI: 0.00-0.72, p = 0.05). Programmes with >500 jumps per week had negative effects (ES = -0.22, 95%CI: -1.10 to 0.67, p = 0.63). Programmes with >7.5 jumps per set showed larger effect sizes (ES = 0.55, 95%CI: 0.02-1.08, p = 0.04) than those with <7.5 jumps per set (ES = 0.32, 95%CI: 0.01-0.62, p = 0.04).

CONCLUSION

PJT enhances lower-body stiffness, which can be optimised with lower volumes (<250 jumps per week) over a relatively long period of time (>7 weeks).

A Biorealistic Computational Model Unfolds Human-Like Compliant Properties for Control of Hand Prosthesis

Objective: Human neuromuscular reflex control provides a biological model for a compliant hand prosthesis. Here we present a computational approach to understanding the emerging human-like compliance, force and position control, and stiffness adaptation in a prosthetic hand with a replica of human neuromuscular reflex. Methods: A virtual twin of prosthetic hand was constructed in the MuJoCo environment with a tendon-driven anthropomorphic hand structure. Biorealistic mathematic models of muscle, spindle, spiking-neurons and monosynaptic reflex were implemented in neuromorphic chips to drive the virtual hand for real-time control. Results: Simulation showed that the virtual hand acquired human-like ability to control fingertip position, force and stiffness for grasp, as well as the capacity to interact with soft objects by adaptively adjusting hand stiffness. Conclusion: The biorealistic neuromorphic reflex model restores human-like neuromuscular properties for hand prosthesis to interact with soft objects.

Inverse Saxophone—A Device to Study the Role of Individual Finger Perturbations on Grasp Stability

The efficient coordination of fingertip forces to maintain static equilibrium while grasping an object continues to intrigue scientists. While many studies have explored this coordination, most of these studies assumed that interactions of hands primarily occur with rigid inanimate objects. Instead, our daily interactions with living and nonliving entities involve many dynamic, compliant, or fragile bodies. This paper investigates the fingertip force coordination on a manipulandum that changes its shape while grasping it. We designed a five-finger perturbation system with linear actuators at positions corresponding to each finger that would protrude outward from the center of the handle or retract toward the center of the handle as programmed. The behavior of the perturbed fingers and the other fingers while grasping this device was studied. Based on previous experiments on expanding and contracting handles, we hypothesized that each finger would exhibit a comparable response to similar horizontal perturbations. However, the response of the little finger was significantly different from the other fingers. We speculate that the central nervous system demonstrates preferential recruitment of some fingers over others while performing a task.

Effect of Fascicle Length Range on Force Generation of Model-Based Biomimetic Controller for Tendon-Driven Prosthetic Hand

Model-based biomimetic control with neuro-muscular reflex requires accurate representation of muscle fascicle length, which affects both force generation capability of muscle and dynamics of muscle spindle. However, physiological data are insufficient to guide the selection of range of fascicle length for task control. Here a reverse engineering approach was used to investigate the effects of different fascicle length range on controller's force control ability, so as to justify the selection of operating range of muscle length for a grasp force task. We compared 3 different ranges of fascicle length for their effects on force generation, i.e. R₁: 0.5 - 1.0 L_o, R₂: 0.5 - 1.3 Lo and R₃: 0.5 - 1.6 L_o. The rationale to test these range selections was based on both physiological realism and engineering considerations. The steady state force output and transient force responses were evaluated with a range of step inputs as controller input. Results show that the prosthetic finger can produce a linear steady state force response with all 3 ranges of fascicle length. Peak force was the largest with R3. Fascicle length range had no significant effect on the rise time in force generation tasks. Results suggest that a wider range of fascicle length may be more favorable for force capacity, since the contact point of force control may well fall near the optimal length (L_o) region.

Evaluation of Model-Based Biomimetic Control of Prosthetic Finger Force for Grasp

Restoring neuromuscular reflex properties in the control of a prosthetic hand may potentially approach human-level grasp functions in the prosthetic hand. Previous studies have confirmed the feasibility of real-time emulation of a monosynaptic spinal reflex loop for prosthetic control. This study continues to explore how well the biomimetic controller could enable the amputee to perform force-control tasks that required both strength and error-tolerance. The biomimetic controller was programmed on a neuromorphic chip for real-time emulation of reflex. The model-calculated force of finger flexor was used to drive a torque motor, which pulled a tendon that flexed prosthetic fingers. Force control ability was evaluated in a "press-without-break" task, which required participants to press a force transducer toward a target level, but never exceeding a breakage threshold. The same task was tested either with the index finger or the full hand; the performance of the biomimetic controller was compared to a proportional linear feedback (PLF) controller, and the contralateral normal hand. Data from finger pressing task in 5 amputees showed that the biomimetic controller and the PLF controller achieved 95.8% and 66.9% the performance of contralateral finger in success rate; 50.0% and 25.1% in stability of force control; 59.9% and 42.8% in information throughput; and 51.5% and 38.4% in completion time. The biomimetic controller outperformed the PLF controller in all performance indices. Similar trends were observed with full-hand grasp task. The biomimetic controller exhibited capacity and behavior closer to contralateral normal hand. Results suggest that incorporating neuromuscular reflex properties in the biomimetic controller may provide human-like capacity of force regulation, which may enhance motor performance of amputees operating a tendon-driven prosthetic hand.

Evaluation of functional muscle anatomy scalability in the canine hind limb

In contrast to other mammals, the large variation in dog sizes is not accompanied by any significant genetic re-organization. In order to study the relationship between body mass, limb length and the functional anatomical muscle parameters of the canine hind limb, a large dataset comprising of muscle masses, optimal muscle fibre lengths and physiological cross-sectional area's (PCSA) were acquired for twenty-five muscles in ten dogs of sizes varying between 20 kg and 52 kg. The potential of body mass and limb length for reliably scaling individual muscle masses, optimal muscle fibre lengths and PCSA's were examined. For the majority of the muscles of the canine hind limb, neither body mass nor limb length were reliable scaling parameter for either muscle masses, PCSA's and optimal fibre length. These results indicate the need of a breed-specific approach to musculoskeletal modelling in future canine musculoskeletal research.

Graft choice for isolated MPFL reconstruction: gracilis versus semitendinosus

INTRODUCTION

After the first patellar dislocation, most patients report damage of the medio-patellofemoral ligament (MPFL) and surgical reconstruction is required. The purpose of this study is to systematically review current evidence and to clarify the role of the gracilis and semitendinosus tendons as graft for isolated MPFL reconstruction.

MATERIALS AND METHODS

The present systematic review was conducted according to the PRISMA guidelines. The literature search was conducted in October 2019. All clinical trials using the semitendinosus and/or gracilis tendon grafts for isolated MPFL reconstruction in patients with patellofemoral instability were considered for inclusion. Only articles reporting a minimum of 12-month follow-up were considered. The PEDro score was used for the methodological quality assessment.

RESULTS

Data from 1491 procedures were collected. The mean follow-up was 36.12 months. There was comparability among the patient baseline. All the scores of interests (Kujala, Tegner, Lysholm) and range of motion scored better in the semitendinosus group. Moreover, in favour of the semitendinosus group, a statistically significant reduction of the revision surgeries and re-dislocations were evidenced. Apprehension test and persistent instability sensation found any statistical correlations.

CONCLUSION

Isolated MPFL reconstruction through semitendinosus tendon graft performed better than the gracilis in selected patients suffering from recurrent patellofemoral instability.

Digital Design and Automated Fabrication of Bespoke Collagen Microfiber Scaffolds

A great variety of natural and synthetic polymer materials have been utilized in soft tissue engineering as extracellular matrix (ECM) materials. Natural polymers, such as collagen and fibrin hydrogels, have experienced especially broad adoption due to the high density of cell adhesion sites compared to their synthetic counterparts, ready availability, and ease of use. However, these and other hydrogels lack the structural and mechanical anisotropy that define the ECM in many tissues, such as skeletal and cardiac muscle, tendon, and cartilage. Herein, we present a facile, low-cost, and automated method of preparing collagen microfibers, organizing these fibers into precisely controlled mesh designs, and embedding these meshes in a bulk hydrogel, creating a composite biomaterial suitable for a wide variety of tissue engineering and regenerative medicine applications. With the assistance of custom software tools described herein, mesh patterns are designed by a digital graphical user interface and translated into protocols that are executed by a custom mesh collection and organization device. We demonstrate a high degree of precision and reproducibility in both fiber and mesh fabrication, evaluate single fiber mechanical properties, and provide evidence of collagen self-assembly in the microfibers under standard cell culture conditions. This work offers a powerful, flexible platform for the study of tissue engineering and cell material interactions, as well as the development of therapeutic biomaterials in the form of custom collagen microfiber patterns that will be accessible to all through the methods and techniques described here. Impact Statement Collagen microfiber meshes have immediate and broad applications in tissue engineering research and show high potential for later use in clinical therapeutics due to their compositional similarities to native extracellular matrix and tunable structural and mechanical characteristics. Physical and biological characterizations of these meshes demonstrate physiologically relevant mechanical properties, native-like collagen structure, and cytocompatibility. The methods presented herein not only describe a process through which custom collagen microfiber meshes can be fabricated but also provide the reader with detailed device plans and software tools to produce their own bespoke meshes through a precise, consistent, and automated process.

The influence of model parameters on model validation

The study examined the sensitivity of two musculoskeletal models to the parameters describing each model. Two different models were examined: a phenomenological model of human jumping with parameters based on live subject data, and the second a model of the First Dorsal Interosseous with parameters based on cadaveric measurements. Both models were sensitive to the model parameters, with the use of mean group data not producing model outputs reflective of either the performance of any group member or the mean group performance. These results highlight the value of subject specific model parameters, and the problems associated with model validation.

Musculotendon excursion potential, tendon slack and muscle fibre length: the interaction of the canine gastrocnemius muscle and tendon

Although the form-function relation of muscles and tendons has been studied extensively, little in vivo data exist on the musculotendon properties of the gastrocnemius complex in dogs. Using a combination of ultrasound and 3D motion tracking, musculotendon parameters were obtained in vivo from the lateral gastrocnemius muscle and the gastrocnemius tendon in nine healthy Labrador Retrievers. These parameters include musculotendon length and excursion potential, tendon slack length, muscle belly length, muscle fibre length, pennation angle and architectural index. This study also examined the variation of muscle and tendon length contributions to musculotendon length, as well as the relation between musculotendon excursion potential and muscle fibre length or tendon length. To facilitate comparison between dog breeds, the femur length as a potential scaling parameter was examined. In the Labrador gastrocnemius musculotendon complex, the tendon contributes 41% (± 9%) of musculotendon length. In longer musculotendon complexes, the contribution of the muscle belly increases while the tendon contribution decreases. Longer muscle belly and musculotendon complexes were, however, associated with shorter muscle fibres. No significant relations were found between musculotendon excursion potential and muscle fibre length or tendon slack length, and femur length did not prove to be a reliable scale factor for the length-related musculotendon parameters examined in this study. Longer musculotendon complexes exhibit relatively longer muscle bellies, which are in turn associated with shorter muscle fibre lengths. This trade-off between gastrocnemius muscle belly length and muscle fibre length might have the advantage that muscle volume stays constant regardless of the length of the limbs.

A springs actuated finger exoskeleton: From mechanical design to spring variables evaluation

In the context of post-stroke patients, suffering of hemiparesis of the hand, robot-aided neuro-motor rehabilitation allows for intensive rehabilitation treatments and quantitative evaluation of patients' progresses. This work presents the design and evaluation of a spring actuated finger exoskeleton. In particular, the spring variables and the interaction forces between the assembly and the hand were investigated, in order to assess the effectiveness of the proposed exoskeleton.

Modeling and simulating the neuromuscular mechanisms regulating ankle and knee joint stiffness during human locomotion

This work presents an electrophysiologically and dynamically consistent musculoskeletal model to predict stiffness in the human ankle and knee joints as derived from the joints constituent biological tissues (i.e., the spanning musculotendon units). The modeling method we propose uses electromyography (EMG) recordings from 13 muscle groups to drive forward dynamic simulations of the human leg in five healthy subjects during overground walking and running. The EMG-driven musculoskeletal model estimates musculotendon and resulting joint stiffness that is consistent with experimental EMG data as well as with the experimental joint moments. This provides a framework that allows for the first time observing 1) the elastic interplay between the knee and ankle joints, 2) the individual muscle contribution to joint stiffness, and 3) the underlying co-contraction strategies. It provides a theoretical description of how stiffness modulates as a function of muscle activation, fiber contraction, and interacting tendon dynamics. Furthermore, it describes how this differs from currently available stiffness definitions, including quasi-stiffness and short-range stiffness. This work offers a theoretical and computational basis for describing and investigating the neuromuscular mechanisms underlying human locomotion.

Mechanical and neural changes in plantar-flexor muscles after spinal cord injury in humans

STUDY DESIGN

Cross-sectional study.

OBJECTIVES

To determine the effect of injury duration on plantar-flexor elastic properties in individuals with chronic spinal cord injury (SCI) and spasticity.

SETTING

National Rehabilitation Center for Persons with Disabilities, Japan.

METHODS

A total of 16 chronic SCI patients (age, 33±9.3 years; injury localization, C6-T12; injury duration, 11-371 months) participated. Spasticity of the ankle plantar-flexors was assessed using the Modified Ashworth Scale (MAS). The calf circumference and muscle thickness of the medial gastrocnemius (MG), lateral gastrocnemius and soleus were assessed using tape measure and ultrasonography. In addition, the ankle was rotated from 10° plantar-flexion to 20° dorsiflexion at 5 deg s(-1) with a dynamometer, and the ankle angle and torque were recorded. After normalizing the data (the initial points of angle and torque were set to zero), we calculated the peak torque and energy. Furthermore, angle-torque data (before and after normalization) were fitted with a second- and fourth-order polynomial, and exponential (Sten-Knudsen) models, and stiffness indices (SISOP, SIFOP, SISK) and AngleSLACK (the angle at which plantar-flexor passive torque equals zero) were calculated. The stretch reflex gain and offset were determined from 0-10° dorsiflexion at 50, 90, 120 and 150 deg s(-1). After logarithmic transformation, Pearson's correlation coefficients were calculated.

RESULTS

MAS, calf circumference, MG thickness, peak torque and SIFOP significantly decreased with injury duration (r log-log=-0.63, -0.69, -0.63, -0.53 and -0.55, respectively, P<0.05). The peak torque and SIFOP maintained significant relationships even after excluding impacts from muscle morphology.

CONCLUSION

Plantar-flexor elasticity in chronic SCI patients decreased with increased injury duration.

Critical damping conditions for third order muscle models: implications for force control

Experimental results presented in the literature suggest that humans use a position control strategy to indirectly control force rather than direct force control. Modeling the muscle-tendon system as a third-order linear model, we provide an explanation of why an indirect force control strategy is preferred. We analyzed a third-order muscle system and verified that it is required for a faithful representation of muscle-tendon mechanics, especially when investigating critical damping conditions. We provided numerical examples using biomechanical properties of muscles and tendons reported in the literature. We demonstrated that at maximum isotonic contraction, for muscle and tendon stiffness within physiologically compatible ranges, a third-order muscle-tendon system can be under-damped. Over-damping occurs for values of the damping coefficient included within a finite interval defined by two separate critical limits (such interval is a semi-infinite region in second-order models). An increase in damping beyond the larger critical value would lead the system to mechanical instability. We proved the existence of a theoretical threshold for the ratio between tendon and muscle stiffness above which critical damping can never be achieved; thus resulting in an oscillatory free response of the system, independently of the value of the damping. Under such condition, combined with high muscle activation, oscillation of the system can be compensated only by active control.

Active muscle stiffness in the human medial gastrocnemius muscle in vivo

The aims of this study were to 1) directly assess active muscle stiffness according to actual length changes in muscle fibers (fascicles) during short range stretching; and 2) compare actual measured active muscle and tendon stiffness using ultrasonography with the stiffness of active (i.e., muscle) and passive (i.e., tendon) parts in series elastic component of plantar flexors using the alpha method. Twenty-four healthy men volunteered for this study. Active muscle stiffness in the medial gastrocnemius muscle was calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions [10, 30, 50, 70, and 90% maximal voluntary contractions (MVC)]. Using the variables measured during this fast stretch experiment, the stiffness of active (i.e., muscle) and passive (i.e., tendon) parts in plantar flexors was assessed using alpha method. Tendon stiffness was determined during isometric plantar flexion by ultrasonography. Active muscle stiffness increased with the exerted torque levels. At 30, 50, 70, and 90% MVC, there were no significant correlations between muscle stiffness using ultrasonography and stiffness of active part (i.e., muscle) by alpha method, although this relationship at 10% MVC was significant ( r = 0.552, P = 0.005). In addition, no correlation was noted in tendon stiffness between the two different methods ( r = 0.226, P = 0.209). The present study demonstrated that ultrasonography could quantified active muscle stiffness in vivo. Furthermore, active muscle stiffness and tendon stiffness using ultrasonography were not related to active (i.e., muscle) or passive (i.e., tendon) stiffness in series elastic component of plantar flexors by alpha method.

Short communication: pennation angle variability in human muscle

The pennated arrangement of muscle fibers has important implications for muscle function in vivo, but complex arrangement of muscle fascicles in whole muscle raises the question whether the arrangement of fascicles produce variations in pennation angle throughout muscle. The purpose of this study was to describe the variability in pennation angle observed throughout the first dorsal interosseous (FDI) muscle using magnetic resonance imaging (MRI). Two cadaveric muscles were scanned in a 14.1 tesla MRI unit. Muscles were divided into slices and pennation angle was measured in the same representative location throughout the muscle in each slice for the medial-lateral and anterior posterior-image planes. Data showed large nonuniform variation in pennation angles throughout the muscles. For example, for cadaver 2, pennation angle in 287 planes along the medial-lateral axis ranged from 3.2° to 22.6°, while for the anterior-posterior axis, in 237 planes it ranged from 3.1° to 24.5°. The nonnormal distribution of pennation angles along each axis suggests a more complex distribution of fascicles than is assumed when a single pennation angle is used to represent an entire muscle. This distribution indicates that a single pennation angle may not accurately describe the arrangement of muscle fascicles in whole muscle.

Measuring subject specific muscle model parameters of the first dorsal interosseous in vivo

Subject specific musculoskeletal models typically base some or all of their parameters on a source other than the subject being modeled. Evidence demonstrates that cadaveric measurements do not always scale appropriately to every subject, yet many musculoskeletal models still rely heavily on cadaveric based data. This study focused on the First Dorsal interosseous (FDI) given its unique function as the sole abductor of the second metacarpophalangeal joint. There were two purposes to this study: (1) to describe the procedures that can be used in vivo to determine the properties of a model of the FDI. (2). To determine the model parameters required to characterize the FDI for a group of four subjects. Parameters were determined using ultrasound imaging and a custom-built finger dynamometer. Some parameters were measured directly while other parameters had to be estimated using a least-squares criterion. For example, the parameters for the force-length properties were determined by fitting a model to experimentally determined data, with maximum isometric force values ranging from 86 to 102 N, and optimum lengths from 41 to 53 mm. It was shown that full characterization is possible for the FDI with parameters that are physiologically reasonable, but which showed variability between subjects. This model and approach for parameter identification will allow for more detailed analysis of the function of the FDI.

Interaction between gastrocnemius medialis fascicle and Achilles tendon compliance: a new insight on the quick-release method

The insufficient temporal resolution of imaging devices has made the analysis of very fast movements, such as those required to measure active muscle-tendon unit stiffness, difficult. Thus the relative contributions of tendon, aponeurosis, and fascicle to muscle-tendon unit compliance remain to be determined. The present study analyzed the dynamic interactions of fascicle, tendon, and aponeurosis in human gastrocnemius medialis during the first milliseconds of an ankle quick-release movement, using high-frame-rate ultrasonography (2,000 frames/s). Nine subjects performed the tests in random order at six levels of maximal voluntary contraction (MVC) (30% to 80% of MVC). These tests were carried out with the ultrasound probe placed on the muscle belly and on the myotendinous junction. Tendon, muscle fascicle, and aponeurosis length changes were quantified in relation to shortening of the muscle-tendon unit during the first few milliseconds following the release. The tendon was the main contributor (around 72%) to the shortening of the muscle-tendon unit, whereas the muscle fascicle and aponeurosis contributions were 18% and 10%, respectively. Because these structures can be considered in series, the quantified contributions can be regarded as relative contributions to muscle-tendon compliance. These contributions were not modified with the level of MVC or the time range used for the analysis between 10 and 25 ms. The constant contribution of tendon, muscle fascicle, and aponeurosis to muscle-tendon unit compliance may help to simplify the mechanism of compliance regulation and to maintain the important role of tendons in enhancing work output and movement efficiency.

Haptic recognition of dystonia and spasticity in simulated multi-joint hypertonia

This paper investigates the capability of naïve individuals to recognize dystonic- or spastic- like conditions through physical manipulation of a virtual arm. Subjects physically interact with a two-joint, six-muscle hypertonic arm model, rendered on a two degrees-of-freedom robotic manipulandum. This paradigm aims to identify the limitation of manual manipulation during diagnosis of hypertonia. Our results indicate that there are difficulties to discriminate between the two conditions at low to medium level of severity. We found that the sample entropy of the executed motion and the force experienced during physical manipulation, tended to be higher during incorrectly identified trials than in those correctly assessed.

Voluntary EMG-to-force estimation with a multi-scale physiological muscle model

BACKGROUND

EMG-to-force estimation based on muscle models, for voluntary contraction has many applications in human motion analysis. The so-called Hill model is recognized as a standard model for this practical use. However, it is a phenomenological model whereby muscle activation, force-length and force-velocity properties are considered independently. Perreault reported Hill modeling errors were large for different firing frequencies, level of activation and speed of contraction. It may be due to the lack of coupling between activation and force-velocity properties. In this paper, we discuss EMG-force estimation with a multi-scale physiology based model, which has a link to underlying crossbridge dynamics. Differently from the Hill model, the proposed method provides dual dynamics of recruitment and calcium activation.

METHODS

The ankle torque was measured for the plantar flexion along with EMG measurements of the medial gastrocnemius (GAS) and soleus (SOL). In addition to Hill representation of the passive elements, three models of the contractile parts have been compared. Using common EMG signals during isometric contraction in four able-bodied subjects, torque was estimated by the linear Hill model, the nonlinear Hill model and the multi-scale physiological model that refers to Huxley theory. The comparison was made in normalized scale versus the case in maximum voluntary contraction.

RESULTS

The estimation results obtained with the multi-scale model showed the best performances both in fast-short and slow-long term contraction in randomized tests for all the four subjects. The RMS errors were improved with the nonlinear Hill model compared to linear Hill, however it showed limitations to account for the different speed of contractions. Average error was 16.9% with the linear Hill model, 9.3% with the modified Hill model. In contrast, the error in the multi-scale model was 6.1% while maintaining a uniform estimation performance in both fast and slow contractions schemes.

CONCLUSIONS

We introduced a novel approach that allows EMG-force estimation based on a multi-scale physiology model integrating Hill approach for the passive elements and microscopic cross-bridge representations for the contractile element. The experimental evaluation highlights estimation improvements especially a larger range of contraction conditions with integration of the neural activation frequency property and force-velocity relationship through cross-bridge dynamics consideration.

Therapist recognition of impaired muscle groups in simulated multi-joint hypertonia

It is common in today's clinical practice for a therapist to physically manipulate patients' limbs to assess hypertonic conditions (e.g. spasticity, rigidity, dystonia, among others). We present a study that evaluates the capabilities of expert therapists to correctly identify the location of a hypertonic impairment of an arm through standard manipulation. Therapists interacted with a hypertonic virtual arms rendered on a robotic device. Our results show that testing joints independently can cause misjudgment of the mechanical contributions of pluri-articular muscles to multi-joint impairment.

Experimental parameter identification of a multi-scale musculoskeletal model controlled by electrical stimulation: application to patients with spinal cord injury

We investigated the parameter identification of a multi-scale physiological model of skeletal muscle, based on Huxley's formulation. We focused particularly on the knee joint controlled by quadriceps muscles under electrical stimulation (ES) in subjects with a complete spinal cord injury. A noninvasive and in vivo identification protocol was thus applied through surface stimulation in nine subjects and through neural stimulation in one ES-implanted subject. The identification protocol included initial identification steps, which are adaptations of existing identification techniques to estimate most of the parameters of our model. Then we applied an original and safer identification protocol in dynamic conditions, which required resolution of a nonlinear programming (NLP) problem to identify the serial element stiffness of quadriceps. Each identification step and cross validation of the estimated model in dynamic condition were evaluated through a quadratic error criterion. The results highlighted good accuracy, the efficiency of the identification protocol and the ability of the estimated model to predict the subject-specific behavior of the musculoskeletal system. From the comparison of parameter values between subjects, we discussed and explored the inter-subject variability of parameters in order to select parameters that have to be identified in each patient.

Haptic perception of multi-joint hypertonia during simulated patient-therapist physical tele-interaction

A potential solution to provide individualized physical therapy in remote areas is tele-interaction via robotic devices. To maintain stability during tele-interaction, transmission delay-compensation algorithms bound the impedance between the patient and the therapist. This can compromise the haptic perception of the patient being assessed, which can in turn lead to a bad diagnosis or intervention. We investigated how the perception of the severity of hypertonia (a common condition after neurological disorders) varied by modifying the connection impedance on a physical simulator. We found that assessing hypetonia using a low impedance connection may result in an overestimation of mild impairments.

Third-Order Muscle Models: The Role of Oscillatory Behavior In Force Control

This paper presents the analysis of a third-order linear differential equation representing a muscle-tendon system, including the identification of critical damping conditions. We analytically verified that this model is required for a faithful representation of muscle-skeletal muscles and provided numerical examples using the biomechanical properties of muscles and tendon reported in the literature. We proved the existence of a theoretical threshold for the ratio between tendon and muscle stiffness above which critical damping can never be achieved, thus resulting in an oscillatory free response of the system, independently of the value of the damping. Oscillation of the limb can be compensated only by active control, which requires creating an internal model of the limb mechanics. We demonstrated that, when admissible, over-damping of the muscle-tendon system occurs for damping values included within a finite interval between two separate critical limits. The same interval is a semi-infinite region in second-order models. Moreover, an increase in damping beyond the second critical point rapidly brings the system to mechanical instability.

Control of position and movement is simplified by combined muscle spindle and Golgi tendon organ feedback

Whereas muscle spindles play a prominent role in current theories of human motor control, Golgi tendon organs (GTO) and their associated tendons are often neglected. This is surprising since there is ample evidence that both tendons and GTOs contribute importantly to neuromusculoskeletal dynamics. Using detailed musculoskeletal models, we provide evidence that simple feedback using muscle spindles alone results in very poor control of joint position and movement since muscle spindles cannot sense changes in tendon length that occur with changes in muscle force. We propose that a combination of spindle and GTO afferents can provide an estimate of muscle-tendon complex length, which can be effectively used for low-level feedback during both postural and movement tasks. The feasibility of the proposed scheme was tested using detailed musculoskeletal models of the human arm. Responses to transient and static perturbations were simulated using a 1-degree-of-freedom (DOF) model of the arm and showed that the combined feedback enabled the system to respond faster, reach steady state faster, and achieve smaller static position errors. Finally, we incorporated the proposed scheme in an optimally controlled 2-DOF model of the arm for fast point-to-point shoulder and elbow movements. Simulations showed that the proposed feedback could be easily incorporated in the optimal control framework without complicating the computation of the optimal control solution, yet greatly enhancing the system's response to perturbations. The theoretical analyses in this study might furthermore provide insight about the strong physiological couplings found between muscle spindle and GTO afferents in the human nervous system.

The arrangement of fascicles in whole muscle

The architecture of the muscle fascicles, here meaning their lengths and their arrangement relative to one another, has important implications for the force a muscle can produce. Therefore, quantifying this architectural arrangement and understanding the implications of the architecture are important for understanding muscle function in vivo. There were two purposes of this study: (1) to assess, via blunt dissection, the number and the length of all the fascicles comprising the First Dorsal Interosseous (FDI) muscle and (2) to visually identify, via magnetic resonance imaging (MRI), the arrangement of the fascicles comprising the FDI. Simple blunt dissection of all the fascicles comprising four FDI muscles and their subsequent measurement demonstrated that the fascicles comprising the whole muscle were not as long as the muscle belly from which they were extracted. Muscle fascicles are surrounded by connective tissue hence the paths of the fascicles in two whole FDI muscles were identified via MRI by tracking the connective tissue surrounding the fascicles. The fascicles had a spiral pattern along the length of each muscle, within both muscles many of the fascicles were arranged in series with other fascicles. These architectural features of the fascicles of the FDI have important implications for the force-length and force-velocity properties of the whole muscle.

DESIGN OF A THRESHOLD FES CONTROL SYSTEM FOR ARM MOVEMENT

The existing functional electrical stimulation (FES) techniques often required to solve the complex "inverse dynamic problem" to calculate the muscle torques for moving along a desired trajectory. According to the threshold control theory of voluntary motor control, a bio-mimetic threshold control strategy for the FES controller is designed and tested in the human arm movement. The arm is modeled as three segments connected by two hinges joints. The movement is driven by seven muscles and limited in the horizontal plane. All muscles are described by a modified Hill-type muscle model. Simulation results suggest that the threshold FES control system can realize point to point movement and can approximately follow the desired traces in presence of feedback delays up to 20 ms. The movement can also maintain stability under external perturbation or external load. The control system can be employed in clinical application because of the following advantages: (1) The control strategy includes some mature control techniques which had been realized in hardware. (2) Only sophisticated sensors of goniometer and the surface electrodes are needed to provide feedbacks and muscle stimulation. (3) The performance of the control system will not be critically influenced by the slight change of musculo-tendon parameters and feedback delays, and even the parameters of controller are fixed.

Preliminary design of a simple passive toe exercise apparatus with a flexible metal hydride actuator for pressure ulcer prevention

In an aging society, social demands for home-based rehabilitation and assistive technologies by healthcare and welfare services are globally increasing. The progress of quality-of-life technologies and rehabilitation science is a very important and urgent issue for elderly and disabled individuals as well as for their caregivers. Thus, there is a substantial need to develop simple bedside apparatuses for both continuous exercise of joints and for power assistance for standing to prevent and manage disuse syndromes (e.g., pressure ulcers, joint contractures and muscular atrophy). Unfortunately, there are currently no commercially-available actuators compatible with the human requirements of flexibility, quietness, lightness and a high power-to-weight ratio. To fulfill the above demands, we have developed a novel actuation device using a metal hydride (MH) alloy and a laminate film, called the flexible MH actuator, as a human-friendly force generator for healthcare and welfare services. In this paper, we show the basic structure and characteristics of the flexible MH actuator used to create a passive exercise system for preventing disuse syndromes. To evaluate the efficiency of passive exercise for bedsore prevention, subcutaneous blood flow during passive exercise at common pressure-ulcer sites is measured by a laser blood flow meter. The force and range-of-motion angle required for a passive exercise apparatus is also examined with the help of a professional physical therapist. Based on these findings, a prototype of a passive exercise apparatus is fabricated using the flexible MH actuator technology, and its operation characteristics are preliminarily verified using a thermoelectric control system.

Architectural properties of the first dorsal interosseous muscle

Muscle architecture is considered to reflect the function of muscle in vivo, and is important for example to clinicians in designing tendon-transfer and tendon-lengthening surgeries. The purpose of this study was to quantify the architectural properties of the FDI muscle. It is hypothesized that there will be consistency, that is low variability, in the architectural parameters used to describe the first dorsal interosseous muscle because of its clear functional role in index finger motion. The important architectural parameters identified were those required to characterize a muscle adequately by modeling. Specifically the mass, cross-sectional area, and length of the tendon and muscle were measured in cadavers along with the muscle fiber optimum length and pennation angle, and the moment arm of the first dorsal interosseous at the metacarpophalangeal joint. These parameters provide a characterization of the architecture of the first dorsal interosseous, and were used to indicate the inherent variability between samples. The results demonstrated a large amount of variability for all architectural parameters measured; leading to a rejection of the hypothesis. Ratios designed to describe the functioning of the muscles in vivo, for example the ratio of tendon to fiber optimum lengths, also demonstrated a large variability. The results suggest that function cannot be deduced from form for the first dorsal interosseous, and that subject-specific architectural parameters may be necessary for the formulation of accurate musculoskeletal models or making clinical decisions.

Prehension stability: experiments with expanding and contracting handle

We studied adjustments in digit forces and moments during holding a vertically oriented handle under slow, externally imposed changes in the width of the grasp. Subjects (n = 8) grasped a customized motorized handle with five digits and held it statically in the air. The handle width either increased (expanded) or decreased (contracted) at a rate of 1.0, 1.5, or 2.0 mm/s, while the subjects were asked to ignore the handle width changes, and their attention was distracted. External torques of 0.0, 0.25, and 0.5 Nm were applied to the handle in two directions. Forces and moments at the digit tips were measured with six-component sensors. The analysis was performed at the virtual finger (VF) and individual finger (IF) levels (VF is an imagined finger that produces the same wrench, i.e., the force and moment, as several fingers combined). In all the tasks, the normal VF and thumb forces increased with the handle expansion and decreased with the handle contraction. Similar behavior was seen for the thumb tangential force. In contrast, the VF tangential force decreased with the handle expansion and increased with the handle contraction. The changes in the tangential forces assisted the perturbations in the tasks requiring exertion of the supination moments and annulled the perturbation in the pronation effort tasks. In the former tasks, the equilibrium was maintained by the changes of the moments of normal forces, whereas in the latter tasks, the equilibrium was maintained by the changes of the moments of the tangential forces. Analysis at the IF level has shown that the resultant force and moment exerted on the object could arise from dissimilar adjustments of individual fingers to the same handle width change. The complex adjustments of digit forces to handle width change may be viewed as coming from two sources. First, there are local spring-like adjustments of individual digit forces and moments caused by both mechanical properties of the digits and the action of spinal reflexes. These stiffness-like reactions mainly assist in perturbing the rotational equilibrium of the object rather than in maintaining it. Second, there are tilt-preventing adjustments defined by the common task constraints that unite the digits into a task-specific synergy. The "virtual springs theory" developed in robotics literature is insufficient for describing the phenomena observed in human grasping.

Force-length characteristics of in vivo human skeletal muscle

In the present study, the in vivo force-length relations of the human soleus (SOL) and tibialis anterior (TA) muscles were estimated. Measurements were taken in six men at ankle angles from 30 degrees of dorsiflexion to 45 degrees of plantarflexion in steps of 15 degrees, and involved dynamometry, electrical stimulation, ultrasonography and magnetic resonance imaging (MRI). For each muscle and ankle angle studied the following three measurements were carried out: (1) dynamometry-based measurement of maximal voltage tetanic moment, (2) ultrasound-based measurement of pennation angle and fibre length and (3) MRI-based measurement of tendon moment arm length. Tendon forces were calculated dividing moments by moment arm lengths, and muscle forces were calculated dividing tendon forces by the cosine of pennation angles. In the transition from 30 degrees of dorsiflexion to 45 degrees of plantarflexion the SOL muscle fibre length decreased from 3.8 to 2.4 cm and its force decreased from 3330 to 290 N. Over the same range of ankle angles the TA muscle fibre length increased from 3.7 to 6 cm and its force increased from 157 to 644 N. Over the longest muscle fibre lengths reached the force of both muscles remained approximately constant. These results indicate that the intact human SOL and TA muscles operate in the ascending limb and plateau region of the force-length relationship. Similar conclusions were reached when calculating the theoretical operating range of the two muscle sarcomeres in the study.

Load-elongation characteristics of in vivo human tendon and aponeurosis

In the present study, we measured the in vivo load-elongation characteristics of the human tibialis anterior tendon and its central aponeurosis. Measurements were taken in five men using dynamometry, muscle electrical stimulation and ultrasonography. Percutaneous tetanic stimulation of the muscle at successive voltages corresponding to 20, 40, 60, 80 and 100 % of maximum isometric dorsiflexion moment was applied. During electrical stimulation, we recorded the displacements of the tibialis anterior tendon origin and its aponeurosis proximal end using B-mode ultrasonography. Aponeurosis displacement was calculated by subtracting tendon displacement from the displacement of the aponeurosis proximal end. Tendon and aponeurosis displacements increased curvilinearly from 1.3 to 4 mm and from 3.7 to 12 mm, respectively, as a function of dorsiflexion load. Scaling of the displacements recorded to the resting lengths (measured over the skin) yielded strain values that increased curvilinearly with load, from 0.8 to 2.5% in the tendon and from 2.1 to 7% in the aponeurosis. Tendon strain was smaller by between 61 and 64% compared with aponeurosis strain at any given contraction level. These findings are in line with reports from in vitro isolated material testing and have important implications for muscle modelling.

Elasticity of tendon structures of the lower limbs in sprinters

The present study aims to investigate the elasticity of tendon structures of the lower limbs in sprinters and its relation with sprint performance. Subjects were 10 male sprinters and 14 controls whose anthropometric variables and isometric maximum strength were similar. The elongation (L) of the tendon and aponeurosis of vastus lateralis (VL) and medial gastrocnemius muscles (MG) during isometric knee extension and planter flexion, respectively, were determined using a real-time ultrasonic apparatus in vivo, while the subjects developed a gradually increasing torque from zero (relax) to maximal effort (MVC) within 5 s. While sprinters compared with controls showed significantly greater L above 500 N (about 50% of MVC) and higher dL/dF for VL at less than 20% of MVC during knee extension, there were no significant differences between the two groups in L and dL/dF for MG at every 10% of MVC during plantar flexion. Moreover, the average value of dL/dF above 50% of MVC, proposed as the compliance of tendon structures, did not significantly differ between sprinters and controls in either VL or MG. In a regression analysis within sprinters, the compliance of VL was negatively correlated to 100-m sprint time, r=-0.757 (P < 0.05), but that of MG was not, r=0.228 (P > 0.05). Thus the present results indicate that the elasticity of tendon structures of VL and MG at high force production levels, which might be assumed to associate with the storage and subsequent release of energy during exercises involving the stretch-shortening cycle, are similar in both sprinters and controls. For sprinters, however, the tendon structures of VL are more compliant than that for controls at low force production levels, and its elasticity at high force production levels may influence sprint performance.

In vivo human tendon mechanical properties

1. The aim of the present study was to measure the mechanical properties of human tibialis anterior (TA) tendon in vivo. 2. Measurements were taken in five males at the neutral ankle position and involved: (a) isometric dynamometry upon increasing the voltage of percutaneous electrical stimulation of the TA muscle, (b) real-time ultrasonography for measurements of the TA tendon origin displacement during contraction and tendon cross-sectional area, and (c) magnetic resonance imaging for estimation of the TA tendon length and moment arm. 3. From the measured joint moments and estimated moment arms, the values of tendon force were calculated and divided by cross-sectional area to obtain stress values. The displacements of the TA tendon origin from rest to all contraction intensities were normalized to tendon length to obtain strain values. From the data obtained, the tendon force-displacement and stress-strain relationships were determined and the tendon stiffness and Young's modulus were calculated. 4. Tendon force and stress increased curvilinearly as a function of displacement and strain, respectively. The tendon force and displacement at maximum isometric load were 530 N and 4.1 mm, and the corresponding stress and strain values were 25 MPa and 2.5 %, respectively. The tendon stiffness and Young's modulus at maximum isometric load were 161 N mm-1 and 1.2 GPa, respectively. These results are in agreement with previous reports on in vitro testing of isolated tendons and suggest that under physiological loading the TA tendon operates within the elastic 'toe' region.

Comparison of muscle and tendon stiffness, jumping ability, muscle strength and fatigue in the plantar flexors

An isokinetic dynamometer was used to measure plantar flexion muscle strength at 60 degrees/s and 200 degrees/s in 10 healthy young men (mean age 25 years). Muscle and tendon stiffnesses were determined on the dynamometer by the use of electrical stimulation and passive stretch (200 degrees/s). Differences in jumping heights between squat and counter-movement jumps were calculated from flight times. The number of heel-rises performed until exhaustion, standing on one leg, were counted. Stepwise regression analysis showed that differences in jumping height increased with lower muscle strength and with higher muscle and tendon stiffnesses, indicating that elastic components may be of more importance in persons with lower muscle strength. The number of heel-rises was negatively dependant on tendon stiffness, indicating that increased stiffness may enhance the development of fatigue.