Objective: Muscle volume is one of the major morphological parameters determining force and joint torque production capacity. An accurate, automatic muscle segmentation method, using imaging modalities such as Magnetic Resonance Image (MRI), is critical for precise volume measurements thus enhancing the utility of MRI-based quantitative muscle evaluation. Methods: We developed EdgeUNETR++, an encoder-decoder model based on a transformer architecture that incorporates an edge enhancement module. Our proposed model was applied to on an MRI dataset of major lower leg muscles from both able-bodied and post-stroke participants to extract the muscle volume information. To further assess its potential for use in biomechanical analysis, we evaluated the correlation between the segmented muscle volumes and optimal ankle joint torque. Results: EdgeUNETR + + achieved segmentation performance comparable to the baseline model, with modest improvements in certain muscles and in boundary-focused metrics. Across all segmentation scenarios, muscle volumes derived from both models were close to the ground truth and exhibited a similar positive trend in their relationship with optimal ankle torque across the four major muscles (i.e., soleus, lateral gastrocnemius, medial gastrocnemius, and tibialis anterior). Conclusion: In conclusion, EdgeUNETR + + may serve as an automated and practical tool for lower leg muscle segmentation and volume estimation from MRI, supporting investigations of muscle morphology and related biomechanical analyses.

Skeletal muscle contains a highly hierarchical structure, leading to anisotropic mechanical properties, with varying morphological responses to mechanical loadings from different directions. However, this feature is rarely studied in clinical studies, mainly due to the challenges in quantifying muscle anisotropic mechanical properties in vivo. The aim of the current study was to quantify the anisotropic mechanical properties of skeletal muscle using an integrated approach combining multi-frequency magnetic resonance elastography (MRE) and diffusion tensor imaging (DTI). Muscle fascicle orientation was determined through DTI tractography. Direct inversion of the curl-based wave equation was used to quantify three complex-valued moduli (μ⊥∗, μ‖∗, and E‖∗) assuming muscle as an incompressible transversely isotropic material. This approach was evaluated on one ex vivo muscle sample by comparing MRE-derived moduli to rheometry measurements, and further assessed in vivo in the ankle plantarflexors of nine able-bodied subjects. Consistency in the anisotropic ratio was observed between rheometry and MRE measurements in the ex vivo muscle sample, though discrepancies were noted in absolute shear moduli values. In vivo, the anisotropy of skeletal muscle was observed by the relationship of μ⊥∗≠1/3E‖∗ and μ‖∗≠1/3E‖∗ at different MRE driving frequencies with higher parallel shear modulus (μ‖∗) than the perpendicular shear modulus (μ⊥∗). This study demonstrated a promising approach for quantifying the muscle anisotropic mechanical properties in vivo, which can be useful in various clinical applications.

This chapter introduces advanced engineering approaches for in vivo biomechanical evaluation of the neuromusculoskeletal system, offering a comprehensive analysis of the neuromechanical functions of skeletal muscles in human movement. It presents various ultrasound- and magnetic resonance imaging-based techniques for precise skeletal muscle quantification, including architecture, intrinsic properties, and fat content. The electrophysiological properties of muscles are explored via high-density electromyography (HDEMG), which provides a noninvasive method for motor unit behavior observation. The clinical relevance of these methods is highlighted through their application across a wide range of clinical conditions. Finally, the chapter outlooks the potential advancements these methods could bring to biomechanical evaluation in tele-neurorehabilitation, highlighting the transformative role of wearable devices, AI-driven algorithms, and sensor innovations in improving patient care and rehabilitation outcomes. This overview underscores the transformative role of biomechanical assessment of the neuromusculoskeletal system in advancing clinical diagnostics, interventions, and research in motor disorders.

Skeletal muscle is crucial for enabling movement, maintaining posture, and stabilising joints. These functions are largely related to the morphology and mechanical properties of skeletal muscle. To quantify these properties in vivo, medical imaging techniques have been widely used, with ultrasonography and magnetic resonance imaging (MRI) being the most commonly used imaging modalities. This thesis presents four studies using different imaging techniques to quantify the morphology and mechanical properties of skeletal muscle. 

In the first study, we used three-dimensional freehand ultrasound (3DFUS) and MRI to quantify 3D skeletal muscle morphological parameters, including muscle volume, fascicle length, and pennation angle. We demonstrated that 3DFUS provided reliable and repeatable measurements, with strong agreement with MRI-based measurements. Given its lower cost and better accessibility, we suggest that 3DFUS could serve as a viable alternative to MRI for quantifying skeletal muscle morphology. 

In the second and third studies, we used two elastography techniques, magnetic resonance elastography (MRE) and ultrasound shear wave elastography (SWE), to quantify the mechanical properties of skeletal muscle. We incorporated diffusion tensor imaging to determine the fascicle orientation and integrated this information into the direct inversion of the wave equation in MRE. This approach allowed for the quantification of anisotropic mechanical properties under the assumption that skeletal muscle behaves as an incompressible transversely isotropic material. This approach was first validated through comparison with ex vivo rheometry measurements, demonstrating a good agreement between the two techniques, and then applied in vivo to the medial gastrocnemius (MG), demonstrating muscle anisotropy as well. We also compared this technique with a commercial ultrasound SWE system, which assumes skeletal muscle to be isotropic, by measuring both ex vivo muscle samples and the MG in vivo. By quantifying shear wave velocity using both elastography techniques, we observed a moderate to strong correlation between SWE and MRE in ex vivo muscle samples and a strong correlation in the MG in vivo. These findings suggested that the isotropy assumption in commercial ultrasound SWE systems does not substantially affect the quantification of muscle mechanical properties. 

In the fourth study, we used MRI to evaluate changes in calf muscle morphology and intramuscular fat content 12 months after the first botulinum neurotoxin type A (BoNT-A) injection in children with cerebral palsy (CP), who were naive to muscle tone reduction therapy. Our findings showed that the calf muscle growth was not impaired 12 months after BoNT-A injection, as indicated by increased absolute muscle volume and unchanged normalized muscle volume. However, the calf muscle growth was compromised by concurrent intramuscular fat infiltration, evidenced by increased intramuscular fat content. 

The ultrasonography and MRI techniques presented in this thesis provide the biomechanics field with different options for quantifying skeletal muscle morphology and mechanical properties. These techniques not only contribute to the medical imaging methodological development but also offer practical implications for clinical assessments and rehabilitation strategies.

Skelettmuskulaturens funktion är viktigför förmågan att producera kraft som i kombination med neurologiska komponenter gör det möjligt för människan att röra sig, bibehålla hållningen och stabilisera lederna. Förmågan att producera kraft (ofta mätt som styrka) kan relateras till muskulaturensstruktur och mekaniska egenskaper. För att kvanitfiera dessa egenskaper in vivo används ofta medicinska avbildningstekniker varav ultraljud och magnetisk resonanstomografi (MRI) är de två mest använda metoderna. I denna avhandling som omfattar fyra studier presenteras olika avbildningstekniker för kvantifiering av skelettmuskulatur. 

I den första studien använde vi två tekniker, tredimensionell frihands-ultraljudsteknink (3DFUS) och MRI för 3D-kvantifiering av muskulaturens morfologiska egenskaper som inkluderade muskelvolym, fascikellängd och pennationsvinkel. Vi kunde visa att 3DFUS gav valida och reliabla resultat som överensstämde väl med resultaten från MRI undersökningarna. Resultaten visar att 3DFUS kan användas som ett kostnadseffektivt alternativ till MRI för att mäta muskelmorfologi. 

I den andra och tredje studien använde vi två tekniker, magnetisk resonans-elastografi (MRE) och ultraljuds-elastografi (SWE), för att kvantifiera muskulaturens mekaniska egenskaper. Med diffusionstensoravbildning, identifierade vi fascikelns orientering och integrerade denna information i den direkta inversionen av vågekvationen i MRE. Detta tillvägagångssätt möjliggjorde kvantifiering av skelettmuskulaturens anisotropa mekaniska egenskaper antaget att skelettmuskulaturen beter sig som inkompressibelt tvärgående isotropiskt material. Metoden som först valideras med reometrimätningar ex vivo och visade god överensstämmelse mellan MRE och reometrimätningar. Därefter tillämpades mätningen i vivo på mediala gastrocnemius (MG) som också visade god anisotropi. Vi jämförde sedan denna teknik med ett kommersiellt ultraljuds SWE-system, där skelettmuskulaturen antogs vara isotrop. Genom att kvantifiera skjuvvågshastighet från dessa två tekniker visade vi en måttlig till stark korrelation mellan SWE och MRE i ex vivo muskelprover och en stark korrelation mellan dessa två tekniker i MG in vivo. Resultat tyder på att mätningar med det kommersiella ultraljuds SWE-systemet inte signifikant påverkade kvantifieringen av muskelmekaniska egenskaper även med det isotropiska antagandet. 

I den fjärde studien använde vi MRI för att utvärdera eventuella förändringar i vadmuskulaturen avseende morfologiska egenskaper och fettsammansättning 12 månader efter spasticitetsreducerande behandling med botulinumtoxin typ A (BoNT-A) injektioner hos barn med cerebral pares (CP), som inte fått muskeltonsreducerande behandling tidigare. Vi visade att vadmuskeltillväxten inte var försämrad 12 månader efter BoNT-A-injektion, vilket indikeras av ökad absolut muskelvolym och oförändrad normaliserad muskelvolym. Muskeltillväxten omfattades av samtidig fettinfiltration i form av ökning av intramuskulära fett. 

Ultraljuds- och MRI-teknikerna som presenteras i denna avhandling bidrar till ökad kunskap om metoder för kvantifiering av skelettmuskulaturens morfologi och dess mekaniska egenskaper att använda inom biomekaniken. Metoderna bidrar till ny kunskap och utveckling av medicinska avbildningstekniker som kan implementeras i kliniken och användas vid bedömning av muskelfunktion och för utvärdering av rehabiliteringsåtgärder. 

Accurate evaluation of electrophysiological and morphological characteristics of the skeletal muscles is critical to establish a comprehensive assessment of the human neuromusculoskeletal function in vivo. However, current technological challenges lie in unsynchronized and unparallel operation of separate acquisition systems such as surface electromyography (sEMG) and ultrasonography. Key problem is the lack of ultrasound transparency of current electrophysiological electrodes. In this work, ultrasound (US) transparent electrode based on cellulose nanofibrils (CNF) substrate are proposed to solve the issue. US transparency of the electrodes are evaluated using a standard US phantom. The effects of nanocellulose type and ion-bond introduction on electrode performance is investigated. Simultaneous US image and sEMG signal acquisition of biceps brachii during isometric muscle contraction are studied. Reliable correlation analysis of the US and sEMG signals is realized which is rarely reported in the previous literatures. Recyclability and biodegradability of the current electrode are evaluated. The reported technology opens up new pathways to provide coupled anatomical and electrical information of the skeletal muscles, enables reliable anatomical and electrical information correlation analysis and largely simplify the sensor integration for assessment of the human neuromusculoskeletal function.

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.

Muscle architecture parameters, such as the fascicle length, pennation angle, and volume, are important muscle morphology characteristics. Accurate in vivo quantification of these parameters allows to detect changes due to pathologies, interventions, and rehabilitation trainings, which ultimately impact on muscles' force-producing capacity. In this study, we compared three-dimensional (3D) muscle architecture parameters of the tibialis anterior and gastrocnemius medialis, which were quantified by 3D freehand ultrasound (3DfUS) and a magnetic resonance imaging (MRI) technique, diffusion tensor imaging (DTI), respectively. Sixteen able-bodied subjects were recruited where seven of them received both 3DfUS and MRI measurement, while the rest underwent 3DfUS measurements twice. Good to excellent intra-rater reliability and inter-session repeatability were found in 3DfUS measurements (intra-class correlation coefficient > 0.81). Overall, the two imaging modalities yielded consistent measurements of the fascicle length, pennation angle, and volume with mean differences smaller than 2.9 mm, 1.8 degrees, and 5.7 cm3, respectively. The only significant difference was found in the pennation angle of the tibialis anterior, although the discrepancy was small. Our study demonstrated, for the first time, that 3DfUS measurement had high reliability and repeatability for measurement of muscle architecture in vivo and could be regarded as an alternative to MRI for 3D evaluation of muscle morphology.

Skeletal muscle contains a highly hierarchical structure, leading to anisotropic mechanical properties, with varying morphological responses to mechanical loadings from different directions. However, this feature is rarely studied in clinical studies, mainly due to the challenges in quantifying muscle anisotropic mechanical properties in vivo. The aim of the current study was to quantify the anisotropic mechanical properties of skeletal muscle using an integrated approach combining multi-frequency magnetic resonance elastography (MRE) and diffusion tensor imaging (DTI). Muscle fascicle orientation was determined through DTI tractography. Direct inversion of the curl-based wave equation was used to quantify three complex-valued moduli ( , , and ) assuming muscle as an incompressible transversely isotropic material. This approach was evaluated on one ex vivo muscle sample by comparing MRE-derived moduli to rheometry measurements, and further assessed in vivo in the ankle plantarflexors of nine able-bodied subjects. Consistency in the anisotropic ratio was observed between rheometry and MRE measurements in the ex vivo muscle sample, though discrepancies were noted in absolute shear moduli values. In vivo, the anisotropy of skeletal muscle was observed by the relationship of and at different MRE driving frequencies with higher parallel shear modulus () than the perpendicular shear modulus (). This study demonstrated a promising approach for quantifying the muscle anisotropic mechanical properties in vivo, which can be useful in various clinical applications.