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Murie-Fernández M, Bahamonde C, Graffigna D, Hontanilla B. Botulinum toxin type A infiltration in spasticity and cervical dystonia. Muscle morphology: an overlooked factor. Neurologia 2024; 39:523-529. [PMID: 37116692 DOI: 10.1016/j.nrleng.2021.09.012] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2021] [Accepted: 09/25/2021] [Indexed: 04/30/2023] Open
Abstract
INTRODUCTION Botulinum toxin type A is used to treat spasticity and dystonia. However, its relationship with muscle morphology has not been studied. The action mechanism of botulinum toxin is based on the inhibition of acetylcholine release. Therefore, larger doses of toxin would be needed to treat larger muscles. This study aims to establish whether there is a discrepancy between muscle morphology and the botulinum toxin doses administered. METHODS We dissected, and subsequently measured and weighed, muscles from the upper and lower limbs and the head of a fresh cadaver. We consulted the summary of product characteristics for botulinum toxin type A to establish the recommended doses for each muscle and calculated the number of units infiltrated per gramme of muscle. RESULTS Different muscles present considerable morphological variability, and the doses of botulinum toxin administered to each muscle are very similar. We observed great variability in the amount of botulinum toxin administered per gramme of muscle, ranging from 0.3 U/g in the biceps femoris to 14.6 U/g in the scalene muscles. The mean dose was 2.55 U/g. The doses administered for nearly all lower limb muscles were below this value. CONCLUSIONS There are significant differences in morphology between the muscles of the lower limbs, upper limbs, and head, but similar doses of botulinum toxin are administered to each muscle. These differences result in great variability in the number of units of botulinum toxin administered per gramme of muscle.
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Affiliation(s)
- M Murie-Fernández
- Unidad de Neurorrehabilitación, Hospital Ciudad de Telde, Telde, Las Palmas de Gran Canaria, Spain.
| | - C Bahamonde
- Unidad de Neurorrehabilitación, Hospital Ciudad de Telde, Telde, Las Palmas de Gran Canaria, Spain
| | - D Graffigna
- Unidad de Neurorrehabilitación, Hospital Ciudad de Telde, Telde, Las Palmas de Gran Canaria, Spain
| | - B Hontanilla
- Departamento de Cirugía Plástica, Clínica Universidad de Navarra, Pamplona, Spain
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White MS, Mancini LM, Stoneback L, Palmieri-Smith RM, Lepley LK. Chronic Adaptions in Quadriceps Fascicle Mechanics Are Related to Altered Knee Biomechanics After Anterior Cruciate Ligament Reconstruction. J Appl Biomech 2024; 40:346-355. [PMID: 39013455 DOI: 10.1123/jab.2023-0252] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2023] [Revised: 04/07/2024] [Accepted: 05/07/2024] [Indexed: 07/18/2024]
Abstract
Following anterior cruciate ligament reconstruction (ACLR), patients exhibit abnormal walking mechanics and quadriceps dysfunction. Quadriceps dysfunction has been largely attributed to muscle atrophy and weakness. While important, these factors do not capture intrinsic properties of muscle that govern its ability to generate force and withstand load. While fascicle abnormalities after ACLR have been documented in early stages of recovery (<12 mo), long-term effects of ACLR on fascicle mechanics remain unexplored. We evaluated quadriceps fascicle mechanics during walking 3 years post-ACLR and examined the relationship with knee mechanics. Participants included 24 individuals with ACLR and 24 Controls. Linear mixed models compared the ACLR, Contralateral, and Controls limbs for (1) quadriceps strength, (2) fascicle architecture and mechanics, and (3) knee mechanics. No difference in strength or overall fascicle length excursions was found between limbs. The ACLR limb exhibited longer fascicles at heel strike and peak knee extension moment (P < .001-.004), and smaller fascicle angles at heel strike, peak knee extension moment, and overall suppressed fascicle angle excursions (P < .001-.049) relative to the Contralateral and/or Control limb. This indicates an abnormality in fascicle architecture and mechanics following ACLR and suggests abnormalities in contractile function that cannot be explained by muscle weakness and may contribute to long-term gait irregularities.
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Affiliation(s)
- McKenzie S White
- School of Kinesiology, University of Michigan, Ann Arbor, MI, USA
| | - Lucia M Mancini
- School of Kinesiology, University of Michigan, Ann Arbor, MI, USA
| | - Luke Stoneback
- School of Kinesiology, University of Michigan, Ann Arbor, MI, USA
| | | | - Lindsey K Lepley
- School of Kinesiology, University of Michigan, Ann Arbor, MI, USA
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3
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Zhang L, Van Wouwe T, Yan S, Wang R. EMG-Constrained and Ultrasound-Informed Muscle-Tendon Parameter Estimation in Post-Stroke Hemiparesis. IEEE Trans Biomed Eng 2024; 71:1798-1809. [PMID: 38206783 DOI: 10.1109/tbme.2024.3352556] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/13/2024]
Abstract
Secondary morphological and mechanical property changes in the muscle-tendon unit at the ankle joint are often observed in post-stroke individuals. These changes may alter the force generation capacity and affect daily activities such as locomotion. This work aimed to estimate subject-specific muscle-tendon parameters in individuals after stroke by solving the muscle redundancy problem using direct collocation optimal control methods based on experimental electromyography (EMG) signals and measured muscle fiber length. Subject-specific muscle-tendon parameters of the gastrocnemius, soleus, and tibialis anterior were estimated in seven post-stroke individuals and seven healthy controls. We found that the maximum isometric force, tendon stiffness and optimal fiber length in the post-stroke group were considerably lower than in the control group. We also computed the root mean square error between estimated and experimental values of muscle excitation and fiber length. The musculoskeletal model with estimated subject-specific muscle tendon parameters (from the muscle redundancy solver), yielded better muscle excitation and fiber length estimations than did scaled generic parameters. Our findings also showed that the muscle redundancy solver can estimate muscle-tendon parameters that produce force behavior in better accordance with the experimentally-measured value. These muscle-tendon parameters in the post-stroke individuals were physiologically meaningful and may shed light on treatment and/or rehabilitation planning.
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Nava S, Conte G, Triulzi FM, Comi GP, Magri F, Velardo D, Cinnante CM. Diffusion tensor imaging reveals subclinical alterations in muscles of patients with Becker muscular dystrophy. Br J Radiol 2024; 97:947-953. [PMID: 38574384 DOI: 10.1093/bjr/tqae070] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2023] [Revised: 11/21/2023] [Accepted: 03/27/2024] [Indexed: 04/06/2024] Open
Abstract
OBJECTIVES Becker muscular dystrophy (BMD) is a relatively less investigated neuromuscular disease, partially overlapping the phenotype of Duchenne dystrophy (DMD). Physiopathological and anatomical patterns are still not comprehensively known, despite recent effort in the search of early biomarkers. Aim of this study was to selectively compare normal appearing muscles of BMD with healthy controls. METHODS Among a pool of 40 BMD patients and 20 healthy controls, Sartorius and gracilis muscles were selected on the basis of a blinded clinical quantitative/qualitative evaluation, if classified as normal (0 or 1 on Mercuri scale) and subsequently segmented on diffusion tensor MRI scans with a tractographic approach. Diffusion derived parameters were extracted. RESULTS Non-parametric testing revealed significant differences between normal and normal appearing BMD derived parameters in both muscles, the difference being more evident in sartorius. Bonferroni-corrected P-values (<.05) of Mann-Whitney test could discriminate between BMD and controls for standard deviation of all diffusion parameters (mean diffusivity, fractional anisotropy, axial and radial diffusivity) in both sartorius and gracilis, while in sartorius the significant difference was found also in the average values of the same parameters (with exception of RD). CONCLUSIONS This method could identify microstructural alterations in BMD normal appearing sartorius and gracilis. ADVANCES IN KNOWLEDGE Diffusion based MRI could be able to identify possible early or subclinical microstructural alterations in dystrophic patients with BMD.
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Affiliation(s)
- Simone Nava
- Neuroradiology Unit, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, via Francesco Sforza 35, 20122 Milan, Italy
| | - Giorgio Conte
- Neuroradiology Unit, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, via Francesco Sforza 35, 20122 Milan, Italy
| | - Fabio M Triulzi
- Neuroradiology Unit, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, via Francesco Sforza 35, 20122 Milan, Italy
| | - Giacomo P Comi
- Neuromuscular and Rare Diseases Unit, Department of Neuroscience, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, via Francesco Sforza 35, 20122 Milan, Italy
- Dino Ferrari Center, Department of Pathophysiology and Transplantation, University of Milan, via Francesco Sforza 35, 20122 MilanItaly
| | - Francesca Magri
- Neuromuscular and Rare Diseases Unit, Department of Neuroscience, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, via Francesco Sforza 35, 20122 Milan, Italy
- Dino Ferrari Center, Department of Pathophysiology and Transplantation, University of Milan, via Francesco Sforza 35, 20122 MilanItaly
| | - Daniele Velardo
- Neuromuscular and Rare Diseases Unit, Department of Neuroscience, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, via Francesco Sforza 35, 20122 Milan, Italy
- Dino Ferrari Center, Department of Pathophysiology and Transplantation, University of Milan, via Francesco Sforza 35, 20122 MilanItaly
| | - Claudia M Cinnante
- Radiology Department, Istituto Auxologico Italiano IRCCS, Piazzale Brescia 20, 20149 Milan, Italy
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Vetter S, Köhler HP, Hepp P, Steinke H, Schleifenbaum S, Theopold J, Kiem S, Witt M, Henkelmann J, Roth C. Diffusion tensor imaging: Influence of segmentation on fiber tracking in the supraspinatus muscle-An inter-operator reliability analysis. PLoS One 2023; 18:e0286280. [PMID: 37733809 PMCID: PMC10513221 DOI: 10.1371/journal.pone.0286280] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2023] [Accepted: 09/06/2023] [Indexed: 09/23/2023] Open
Abstract
The ability of muscle to generate force depends on its architecture and health condition. MR-based diffusion tensor imaging of muscle (mDTI) is an innovative approach for showing the fiber arrangement for the whole muscle volume. For accurate calculations of fiber metrics, muscle segmentation prior to tractography is regarded as necessary. Since segmentation is known to be operator dependent, it is important to understand how segmentation affects tractography. The aim of this study was to compare the results of deterministic fiber tracking based on muscle models generated by two independent operators. In addition, this study compares the results with a segmentation-free approach. Fifteen subjects underwent mDTI of the right shoulder. The results showed that mDTI can be successfully applied to complex joints such as the human shoulder. Furthermore, operator segmentation did not influence the results of fiber tracking and fascicle length (FL), fiber volume (FV), fractional anisotropy (FA), axial diffusivity (AD), radial diffusivity (RD), and mean diffusivity (MD) showed excellent intraclass correlation estimates (≥ 0.975). As an exploratory approach, the segmentation-free fiber tracking showed significant differences in terms of mean fascicle length. Based on these findings, we conclude that tractography is not sensitive to small deviations in muscle segmentation. Furthermore, it implies that mDTI and automatic segmentation approaches or even a segmentation-free analysis can be considered for evaluation of muscle architecture.
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Affiliation(s)
- Sebastian Vetter
- Sports Faculty Department of Biomechanics in Sports, Leipzig University, Leipzig, Germany
| | - Hans-Peter Köhler
- Sports Faculty Department of Biomechanics in Sports, Leipzig University, Leipzig, Germany
| | - Pierre Hepp
- Department of Orthopedics, Trauma and Plastic Surgery, Universitätsklinikum, Leipzig University, Leipzig, Germany
| | - Hanno Steinke
- Department of Anatomy, Universitätsklinikum, Leipzig University, Leipzig, Germany
| | - Stefan Schleifenbaum
- Department of Orthopedics, Trauma and Plastic Surgery, Universitätsklinikum, Leipzig University, Leipzig, Germany
| | - Jan Theopold
- Department of Orthopedics, Trauma and Plastic Surgery, Universitätsklinikum, Leipzig University, Leipzig, Germany
| | - Simon Kiem
- Institute of Sport and Motion Science, University of Stuttgart, Stuttgart, Germany
| | - Maren Witt
- Sports Faculty Department of Biomechanics in Sports, Leipzig University, Leipzig, Germany
| | - Jeanette Henkelmann
- Clinics of Diagnostic and Interventional Radiology, Leipzig University, Leipzig, Germany
| | - Christian Roth
- Clinics of Diagnostic and Interventional Radiology, Leipzig University, Leipzig, Germany
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Martín-Noguerol T, Barousse R, Wessell DE, Rossi I, Luna A. Clinical applications of skeletal muscle diffusion tensor imaging. Skeletal Radiol 2023; 52:1639-1649. [PMID: 37083977 DOI: 10.1007/s00256-023-04350-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/06/2023] [Revised: 04/16/2023] [Accepted: 04/17/2023] [Indexed: 04/22/2023]
Abstract
Diffusion tensor imaging (DTI) may allow the determination of new threshold values, based on water anisotropy, to differentiate between healthy muscle and various pathological processes. Additionally, it may quantify treatment monitoring or training effects. Most current studies have evaluated the potential of DTI of skeletal muscle to assess sports-related injuries or therapy, and training monitoring. Another critical area of application of this technique is the characterization and monitoring of primary and secondary myopathies. In this manuscript, we review the application of DTI in the evaluation of skeletal muscle in these and other novel clinical scenarios, with emphasis on the use of quantitative imaging-derived biomarkers. Finally, the main limitations of the introduction of DTI in the clinical setting and potential areas of future use are discussed.
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Affiliation(s)
| | | | | | | | - Antonio Luna
- MRI Unit, Radiology Department, HT Médica, Jaén, Spain
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Klawitter F, Walter U, Axer H, Ehler J. [Intensive care unit-acquired weakness-Diagnostic value of neuromuscular ultrasound]. DIE ANAESTHESIOLOGIE 2023; 72:543-554. [PMID: 37310449 DOI: 10.1007/s00101-023-01300-5] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Accepted: 04/25/2023] [Indexed: 06/14/2023]
Abstract
Intensive care unit-acquired weakness (ICUAW) is one of the most common neuromuscular complications in intensive care medicine. The clinical diagnosis and assessment of the severity using established diagnostic methods (e.g., clinical examination using the Medical Research Council Sum Score or electrophysiological examination) can be difficult or even impossible, especially in sedated, ventilated and delirious patients. Neuromuscular ultrasound (NMUS) has increasingly been investigated in ICUAW as an easy to use noninvasive and mostly patient compliance-independent diagnostic alternative. It has been shown that NMUS appears to be a promising tool to detect ICUAW, to assess the severity of muscular weakness and to monitor the clinical progression. Further studies are needed to standardize the methodology, to evaluate the training effort and to optimize outcome predication. The formulation of an interdisciplinary neurological and anesthesiological training curriculum is warranted to establish NMUS as a complementary diagnostic method of ICUAW in daily clinical practice.
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Affiliation(s)
- Felix Klawitter
- Klinik und Poliklinik für Anästhesiologie, Intensivmedizin und Schmerztherapie, Universitätsmedizin Rostock, Schillingallee 35, 18057, Rostock, Deutschland
| | - Uwe Walter
- Klinik und Poliklinik für Neurologie, Universitätsmedizin Rostock, Gehlsheimer Str. 20, 18147, Rostock, Deutschland
| | - Hubertus Axer
- Klinik für Neurologie, Universitätsklinikum Jena, Am Klinikum 1, 07747, Jena, Deutschland
| | - Johannes Ehler
- Klinik für Anästhesiologie und Intensivmedizin, Universitätsklinikum Jena, Am Klinikum 1, 07747, Jena, Deutschland.
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Wang Z, Destro A, Petersson S, Cenni F, Wang R. In Vivo 3D Muscle Architecture Quantification Based on 3D Freehand Ultrasound and Magnetic Resonance Imaging. J Biomech 2023; 152:111567. [PMID: 37023558 DOI: 10.1016/j.jbiomech.2023.111567] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/29/2022] [Revised: 03/12/2023] [Accepted: 03/23/2023] [Indexed: 03/30/2023]
Abstract
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°, 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.
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Takahashi K, Shiotani H, Evangelidis PE, Sado N, Kawakami Y. Three-dimensional architecture of human medial gastrocnemius fascicles in vivo: Regional variation and its dependence on muscle size. J Anat 2022; 241:1324-1335. [PMID: 36004517 PMCID: PMC9644967 DOI: 10.1111/joa.13750] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2022] [Revised: 07/25/2022] [Accepted: 08/09/2022] [Indexed: 11/29/2022] Open
Abstract
Fascicle architecture (length and pennation angle) can vary regionally within a muscle. The architectural variability in human muscles has been evaluated in vivo, but the interindividual variation and its determinants remain unclear. Considering that within‐muscle non‐uniform changes in pennation angle are associated with change in muscle size by chronic mechanical loading, we hypothesized that the regional variation in fascicle architecture is dependent on interindividual variation in muscle size. To test this hypothesis, we reconstructed fascicles three‐dimensionally along and across the whole medial gastrocnemius in the right lower leg of 15 healthy adults (10 males and 5 females, 23.7 ± 3.3 years, 165.8 ± 8.3 cm, 61.9 ± 11.4 kg, mean ± standard deviation) in neutral ankle joint position with the knee fully extended, using magnetic resonance diffusion tensor imaging and tractography. The 3D‐reconstructed fascicles arose from the deep aponeurosis with variable lengths and angles both in sagittal and coronal planes. The fascicle length was significantly longer in the middle (middle‐medial: 52.4 ± 6.1 mm, middle‐lateral: 52.0 ± 5.1 mm) compared to distal regions (distal‐medial: 41.0 ± 5.0 mm, distal‐lateral: 38.9 ± 3.6 mm, p < 0.001). The 2D pennation angle (angle relative to muscle surface) was significantly greater in distal than middle regions, and medial than lateral regions (middle‐medial: 26.6 ± 3.1°, middle‐lateral: 24.1 ± 2.3°, distal‐medial: 31.2 ± 3.6°, distal‐lateral: 29.2 ± 3.0°, p ≤ 0.017), while only a proximo‐distal difference was significant (p < 0.001) for 3D pennation angle (angle relative to line of action of muscle). These results clearly indicate fascicle's architectural variation in 3D. The magnitude of regional variation evaluated as standard deviation across regions differed considerably among individuals (4.0–10.7 mm for fascicle length, 0.9–5.0° for 2D pennation angle, and 3.0–8.8° for 3D pennation angle), which was positively correlated with the muscle volume normalized to body mass (r = 0.659–0.828, p ≤ 0.008). These findings indicate muscle‐size dependence of the variability of fascicle architecture.
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Affiliation(s)
- Katsuki Takahashi
- Graduate School of Sport Sciences, Waseda University, Saitama, Japan.,Research Fellow of Japan Society for the Promotion of Science, Tokyo, Japan
| | - Hiroto Shiotani
- Faculty of Sport Sciences, Waseda University, Saitama, Japan.,Human Performance Laboratory, Comprehensive Research Organization of Waseda University, Tokyo, Japan
| | | | - Natsuki Sado
- Faculty of Health and Sport Sciences, University of Tsukuba, Ibaraki, Japan
| | - Yasuo Kawakami
- Faculty of Sport Sciences, Waseda University, Saitama, Japan.,Human Performance Laboratory, Comprehensive Research Organization of Waseda University, Tokyo, Japan
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Infiltración de toxina botulínica tipo A en espasticidad y distonía cervical. La morfología muscular, la gran olvidada. Neurologia 2022. [DOI: 10.1016/j.nrl.2021.09.016] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
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Passive Mechanical Properties of Human Medial Gastrocnemius and Soleus Musculotendinous Unit. BIOMED RESEARCH INTERNATIONAL 2021; 2021:8899699. [PMID: 33628828 PMCID: PMC7889354 DOI: 10.1155/2021/8899699] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/11/2020] [Revised: 12/15/2020] [Accepted: 01/21/2021] [Indexed: 11/17/2022]
Abstract
The in vivo characterization of the passive mechanical properties of the human triceps surae musculotendinous unit is important for gaining a deeper understanding of the interactive responses of the tendon and muscle tissues to loading during passive stretching. This study sought to quantify a comprehensive set of passive muscle-tendon properties such as slack length, stiffness, and the stress-strain relationship using a combination of ultrasound imaging and a three-dimensional motion capture system in healthy adults. By measuring tendon length, the cross-section areas of the Achilles tendon subcompartments (i.e., medial gastrocnemius and soleus aspects), and the ankle torque simultaneously, the mechanical properties of each individual compartment can be specifically identified. We found that the medial gastrocnemius (GM) and soleus (SOL) aspects of the Achilles tendon have similar mechanical properties in terms of slack angle (GM: -10.96° ± 3.48°; SOL: -8.50° ± 4.03°), moment arm at 0° of ankle angle (GM: 30.35 ± 6.42 mm; SOL: 31.39 ± 6.42 mm), and stiffness (GM: 23.18 ± 13.46 Nmm-1; SOL: 31.57 ± 13.26 Nmm-1). However, maximal tendon stress in the GM was significantly less than that in SOL (GM: 2.96 ± 1.50 MPa; SOL: 4.90 ± 1.88 MPa, p = 0.024), largely due to the higher passive force observed in the soleus compartment (GM: 99.89 ± 39.50 N; SOL: 174.59 ± 79.54 N, p = 0.020). Moreover, the tendon contributed to more than half of the total muscle-tendon unit lengthening during the passive stretch. This unequal passive stress between the medial gastrocnemius and the soleus tendon might contribute to the asymmetrical loading and deformation of the Achilles tendon during motion reported in the literature. Such information is relevant to understanding the Achilles tendon function and loading profile in pathological populations in the future.
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