Advances in imaging for assessing the design and mechanics of skeletal muscle in vivo

Skeletal muscle shape influences joint torque exertion through the mechanical advantages

Skeletal muscle morphology is linked to its function. Extensive literature demonstrates that muscle volume is crucial for determining joint torque exertion, a primary function of muscle. However, whether muscle shape also influences torque exertion capacity remains unclear. This study illustrates that the three-dimensional shape of muscles independently determines joint torque exertion, irrespective of muscle volume, using a statistical shape model designed to quantify muscle shape features. The statistical shape model was developed from magnetic resonance images of the quadriceps femoris muscles in 33 healthy young adults (26 ± 5 yr; 18 males). We investigated the association between the shape components of each quadriceps femoris head and isometric knee extensor torque. The findings reveal that the mediolateral curvatures of the rectus femoris (R2 = 0.60) and the bulging in the distal region of the vastus medialis (R2 = 0.65) were associated with increased knee extensor torque despite muscle volumes. Moreover, the rectus femoris and vastus medialis shapes were correlated with the medial-directed line-of-action (r = -0.42 and ρ = -0.36). The vastus medialis shape was correlated with the moment arm of the patellar lateral spin (ρ = 0.45). Therefore, the three-dimensional muscle shape determines the joint torque exertion by forming the mechanical advantages that balance the force/torque output optimally. Our findings demonstrate that muscle shape is crucial in the mechanical output of skeletal muscle and provides a framework for enhancing the understanding of muscle morphology and its functionality.NEW & NOTEWORTHY Here, we developed a statistical shape model, a geometric model that can quantify muscle morphology, particularly the quadriceps femoris muscle, to determine the influence of three-dimensional muscle shape on its force-generating capacity in young adults. The results revealed that curvature of the rectus femoris and bulging of vastus medialis were determinants of isometric knee extension strength, coupled with their muscle volumes. This morphological functionality relies on the critical relationship between muscle shape and mechanical advantage.

Unveiling the link: exploring muscle oxygen saturation in fibromyalgia and its implications for symptomatology and therapeutic strategies

Fibromyalgia, characterized as a complex chronic pain syndrome, presents with symptoms of pervasive musculoskeletal pain, significant fatigue, and pronounced sensitivity at specific anatomical sites. Despite extensive research efforts, the origins of fibromyalgia remain enigmatic. This narrative review explores the intricate relationship between muscle oxygen saturation and fibromyalgia, positing that disruptions in the oxygenation processes within muscle tissues markedly influence the symptom profile of this disorder. Muscle oxygen saturation, crucial for muscle function, has been meticulously investigated in fibromyalgia patients through non-invasive techniques such as near-infrared spectroscopy and magnetic resonance imaging. The body of evidence consistently indicates substantial alterations in oxygen utilization within muscle fibers, manifesting as reduced efficiency in oxygen uptake during both rest and physical activity. These anomalies play a significant role in fibromyalgia's symptomatology, especially in terms of chronic pain and severe fatigue, potentially creating conditions that heighten pain sensitivity and accumulate metabolic byproducts. Hypothesized mechanisms for these findings encompass dysfunctions in microcirculation, mitochondrial irregularities, and autonomic nervous system disturbances, all meriting further research. Understanding the dynamics of muscle oxygen saturation in fibromyalgia is of paramount clinical importance, offering the potential for tailored therapeutic approaches to alleviate symptoms and improve the quality of life for sufferers. This investigation not only opens new avenues for innovative research but also fosters hope for more effective treatment strategies and improved outcomes for individuals with fibromyalgia.

Comparative study on muscle-tendon stiffness and balance impairment in postmenopausal women: a focus on osteosarcopenia and osteoporosis

BACKGROUND AND AIMS

This study set out to examine the stiffness of the gastrocnemius medialis (GM) and Achilles tendon across postmenopausal women with osteosarcopenia (OS), osteoporosis (OP), and normal bone mineral density. Furthermore, we explored the relationship between muscle-tendon stiffness and postural sway during a curve-tracking task in both sagittal (AP) and frontal (ML) planes.

METHODS

Seventy-three women volunteered to participate in this study. The participants were classified into OS (T-score ≤ - 2.5 and muscle mass below 5.5 kg/m2), OP (T-score ≤ - 2.5), and healthy (T-score >-1) groups. The shear wave elastography was used to determine GM and Achilles tendon stiffness during rest and activation. The postural sway was recorded using a force plate during the performance-based curve tracking (CT) task.

RESULTS

The stiffness of the GM and Achilles tendon was found to be significantly lower in the OS group compared to the OP and healthy groups (P < 0.05). In the CT task, the OS group exhibited a significant decrease in the mean absolute (P = 0.011) and RMS error (P = 0.022) in the ML direction compared to the OP group. Additionally, a positive correlation was found between the ML mean absolute error and both GM and Achilles's stiffness during rest and activation (P < 0.05).

DISCUSSION AND CONCLUSION

The OS group exhibited the lowest muscle-tendon stiffness. The GM and Achilles stiffness was positively correlated with poor performance-based balance, particularly in the ML direction. This may increase the risk of falls and subsequent hip fractures during simple daily weight- shifting activities in women with osteosarcopenia.

A methodological approach for collecting simultaneous measures of muscle, aponeurosis, and tendon behaviour during dynamic contractions

Skeletal muscles and the tendons that attach them to bone are structurally complex and deform non-uniformly during contraction. While these tissue deformations dictate force production during movement, our understanding of this behaviour is limited due to challenges in obtaining complete measures of the constituent structures. To address these challenges, we present an approach for simultaneously measuring muscle, fascicle, aponeurosis, and tendon behaviour using sonomicrometry. To evaluate this methodology, we conducted isometric and dynamic contractions in in situ rabbit medial gastrocnemius. We found comparable patterns of strain in the muscle belly, fascicle, aponeurosis, and tendon during the isometric trials to those published in the literature. For the dynamic contractions, we found that our measures using this method were consistent across all animals and aligned well with our theoretical understanding of muscle-tendon unit behaviour. Thus, this method provides a means to fully capture the complex behaviour of muscle-tendon units across contraction types.

ISB clinical biomechanics award winner 2023: Medial gastrocnemius muscle and Achilles tendon interplay during gait in cerebral palsy

BACKGROUND

The interplay between the medial gastrocnemius muscle and the Achilles tendon is crucial for efficient walking. In cerebral palsy, muscle and tendon remodelling alters the role of contractile and elastic components. The aim was to investigate the length changes of medial gastrocnemius belly and fascicles, and Achilles tendon to understand their interplay to gait propulsion in individuals with cerebral palsy.

METHODS

Twelve young individuals with cerebral palsy and 12 typically developed peers were assessed during multiple gait cycles using 3D gait analysis combined with a portable ultrasound device. By mapping ultrasound image locations into the shank reference frame, the medial gastrocnemius belly, fascicle, and Achilles tendon lengths were estimated throughout the gait cycle. Participants with cerebral palsy were classified into equinus and non-equinus groups based on their sagittal ankle kinematics.

FINDINGS

In typically developed participants, the Achilles tendon undertook most of the muscle-tendon unit lengthening during stance, whereas in individuals with cerebral palsy, this lengthening was shared between the medial gastrocnemius belly and Achilles tendon, which was more evident in the equinus group. The lengthening behaviour of the medial gastrocnemius fascicles resembled that of the Achilles tendon in cerebral palsy.

INTERPRETATION

The findings revealed similar length changes of the medial gastrocnemius fascicles and Achilles tendon, highlighting the enhanced role of the muscle in absorbing energy during stance in cerebral palsy. These results, together with the current knowledge of increased intramuscular stiffness, suggest the exploitation of intramuscular passive forces for such energy absorption.

Motoneuron-driven computational muscle modelling with motor unit resolution and subject-specific musculoskeletal anatomy

The computational simulation of human voluntary muscle contraction is possible with EMG-driven Hill-type models of whole muscles. Despite impactful applications in numerous fields, the neuromechanical information and the physiological accuracy such models provide remain limited because of multiscale simplifications that limit comprehensive description of muscle internal dynamics during contraction. We addressed this limitation by developing a novel motoneuron-driven neuromuscular model, that describes the force-generating dynamics of a population of individual motor units, each of which was described with a Hill-type actuator and controlled by a dedicated experimentally derived motoneuronal control. In forward simulation of human voluntary muscle contraction, the model transforms a vector of motoneuron spike trains decoded from high-density EMG signals into a vector of motor unit forces that sum into the predicted whole muscle force. The motoneuronal control provides comprehensive and separate descriptions of the dynamics of motor unit recruitment and discharge and decodes the subject's intention. The neuromuscular model is subject-specific, muscle-specific, includes an advanced and physiological description of motor unit activation dynamics, and is validated against an experimental muscle force. Accurate force predictions were obtained when the vector of experimental neural controls was representative of the discharge activity of the complete motor unit pool. This was achieved with large and dense grids of EMG electrodes during medium-force contractions or with computational methods that physiologically estimate the discharge activity of the motor units that were not identified experimentally. This neuromuscular model advances the state-of-the-art of neuromuscular modelling, bringing together the fields of motor control and musculoskeletal modelling, and finding applications in neuromuscular control and human-machine interfacing research.