Morphological and mechanical properties of the human triceps surae aponeuroses taken from elderly cadavers: Implications for muscle-tendon interactions

Quantifying Plantar Flexor Muscles Stiffness During Passive and Active Force Generation Using Shear Wave Elastography in Individuals With Chronic Stroke

OBJECTIVES

This study aims to investigate the mechanical properties of paretic and healthy plantar flexor muscles and assesses the spatial distribution of stiffness between the gastrocnemius medialis (GM) and lateralis (GL) during active force generation.

METHODS

Shear wave elastography measurements were conducted on a control group (CNT, n=14; age=59.9±10.6 years; BMI=24.5±2.5 kg/m2) and a stroke survivor group (SSG, n=14; age=63.2±9.6 years; BMI=23.2±2.8 kg/m2). Shear modulus within the GM and GL was obtained during passive ankle mobilization at various angles of dorsiflexion (P0 =0°; P1=10°; P2=20°; P3=-20° and P4=-30°) and during different levels (30%, 50%, 70%, 100%) of maximal voluntary contraction (MVC). Muscle activations of GM, GL, soleus and tibialis anterior were also evaluated.

RESULTS

The results revealed a significant increase in passive stiffness within the paretic plantar flexor muscles under high tension during passive mobilization (p<0.05). Yet, during submaximal and maximal MVC, the paretic plantar flexors exhibited decreased active stiffness levels (p<0.05). A notable discrepancy was found between the stiffness of the GM and GL, with the GM demonstrating greater stiffness from 0° of dorsiflexion in the SSG (p<0.05), and from 10° of dorsiflexion in the CNT (p<0.05). No significant difference in stiffness was observed between the GM and GL muscles during active condition.

CONCLUSION

Stroke affects the mechanical properties differently depending on the state of muscle activation. Notably, the distribution of stiffness among synergistic plantar flexor muscles varied in passive condition, while remaining consistent in active condition.

Assessing site-specificity of the biomechanical properties of hamstring aponeuroses using MyotonPRO: A cadaveric study

BACKGROUND

Hamstring muscles are the most frequently reported sites of muscle strain injuries, especially near the bi-articular muscles' myotendinous junction, where aponeurosis provides a connective tissue network linking muscle fibers to the tendon. This study aimed to investigate the reliability and site-specific differences of hamstring aponeuroses under different conditions (formalin and urea) using MyotonPRO.

METHODS

Eight hamstring muscle groups were dissected from four human cadavers (two males and two females) aged 83-93 years. Measurements of the mechanical properties of the aponeuroses from the superficial and deep regions of biceps femoris long head, semitendinosus, and semimembranosus (after formalin solution immersion) were done using MyotonPRO (intra-rater reliability was examined within a 24-h interval), following which the hamstring aponeuroses were measured using a similar procedure after urea solution immersion.

FINDINGS

Test-retest (intra-rater) results revealed that the MyotonPRO measurement of tone, stiffness, relaxation, and creep of cadaveric aponeuroses presented good to excellent reliability (ICC: 0.86 to 0.98). There were no significant differences in tone, stiffness, elasticity, relaxation, and creep among the six sites of hamstring aponeuroses under both formalin and urea conditions. Significant differences between formalin and urea conditions were found in the tone, stiffness, relaxation, and creep of hamstring aponeuroses (P < 0.05).

INTERPRETATION

These results suggested that the biomechanical properties of hamstring aponeuroses showed homogeneity between the sites using MyotonPRO. Urea solution could potentially neutralize the effect of formalin on the biomechanical properties of cadaveric muscle-aponeurosis-tendon units. The present findings might influence the design of subsequent cadaveric studies on hamstring muscle strains.

Biomechanical assessment of gastrocnemii and Achilles tendon using MyotonPRO: in vivo measurements, and preliminary in situ measurements using formalin-fixed tissues

PURPOSE

This study aims to evaluate the reliability and validity of using MyotonPRO to quantify the mechanical properties of the muscle-tendon unit through in vivo measurements and preliminary in situ measurements using formalin-fixed tissues.

MATERIALS AND METHODS

The mechanical properties of gastrocnemii and the Achilles tendon of 12 healthy adults (six males and six females, 34.9 ± 5.8 years) were examined for in vivo test twice within a day and once post-24 hours using MyotonPRO, while nine human cadavers (formalin-fixed, 3 males and 6 females, 89.9 ± 5.1 years) were assessed for preliminary in situ test with identical time schedule to evaluate the within-day and inter-day reliability and validity.

RESULTS

In vivo tests had very high within-day (ICC: 0.96-0.99) and inter-day reliability (ICC: 0.83-0.96), while in situ tests (formalin-fixed tissues) showed high within-day (ICC: 0.87-0.99) and inter-day reliability (ICC: 0.76-0.98) for the results of tone and stiffness. There was no significant difference in the stiffness of the free part of the Achilles tendon between in vivo and in situ conditions. The stiffness of the lateral gastrocnemius (r = 0.55, p = 0.018), proximal part of the Achilles tendon (r = 0.56, p = 0.015), and free part of the Achilles tendon (r = 0.47, p = 0.048) before removing the skin was significantly correlated with that after removing the skin condition.

CONCLUSIONS

The findings of the current study suggest that MyotonPRO is reliable and valid for evaluating tendon stiffness both in vivo and in situ (formalin-fixed tissues).

Aponeurosis structure-function properties: Evidence of heterogeneity and implications for muscle function

Aponeurosis is a sheath-like connective tissue that aids in force transmission from muscle to tendon and can be found throughout the musculoskeletal system. The key role of aponeurosis in muscle-tendon unit mechanics is clouded by a lack of understanding of aponeurosis structure-function properties. This work aimed to determine the heterogeneous material properties of porcine triceps brachii aponeurosis tissue with materials testing and evaluate heterogeneous aponeurosis microstructure with scanning electron microscopy. We found that aponeurosis may exhibit more microstructural collagen waviness in the insertion region (near the tendon) compared to the transition region (near the muscle midbelly) (1.20 versus 1.12, p = 0.055), which and a less stiff stress-strain response in the insertion versus transition regions (p < 0.05). We also showed that different assumptions of aponeurosis heterogeneity, specifically variations in elastic modulus with location can alter the stiffness (by more than 10x) and strain (by approximately 10% muscle fiber strain) of a finite element model of muscle and aponeurosis. Collectively, these results suggest that aponeurosis heterogeneity could be due to variations in tissue microstructure and that different approaches to modeling tissue heterogeneity alters the behavior of computational models of muscle-tendon units. STATEMENT OF SIGNIFICANCE: Aponeurosis is a connective tissue found in many muscle tendon units that aids in force transmission, yet little is known about the specific material properties of aponeurosis. This work aimed to determine how the properties of aponeurosis tissue varied with location. We found that aponeurosis exhibits more microstructural waviness near the tendon compared to near the muscle midbelly, which was associated with differences in tissue stiffness. We also showed that different variations in aponeurosis modulus (stiffness) can alter the stiffness and stretch of a computer model of muscle tissue. These results show that assuming uniform aponeurosis structure and modulus, which is common, may lead to inaccurate models of the musculoskeletal system.

Muscle-tendon unit design and tuning for power enhancement, power attenuation, and reduction of metabolic cost

The contractile elements in skeletal muscle fibers operate in series with elastic elements, tendons and potentially aponeuroses, in muscle-tendon units (MTUs). Elastic strain energy (ESE), arising from either work done by muscle fibers or the energy of the body, can be stored in these series elastic elements (SEEs). MTUs vary considerably in their design in terms of the relative lengths and stiffnesses of the muscle fibers and SEEs, and the force and work generating capacities of the muscle fibers. However, within an MTU it is thought that contractile and series elastic elements can be matched or tuned to maximize ESE storage. The use of ESE is thought to improve locomotor performance by enhancing contractile element power during activities such as jumping, attenuating contractile element power during activities such as landing, and reducing the metabolic cost of movement during steady-state activities such as walking and running. The effectiveness of MTUs in these potential roles is contingent on factors such as the source of mechanical energy, the control of the flow of energy, and characteristics of SEE recoil. Hence, we suggest that MTUs specialized for ESE storage may vary considerably in the structural, mechanical, and physiological properties of their components depending on their functional role and required versatility.

The non-intuitive, in-vivo behavior of aponeuroses in a unipennate muscle

Experimental observations and theoretical models suggest that the loading of muscular aponeuroses is complex, causing strain patterns that are not reconcilable with the frequently assumed mechanical "in series" arrangement of aponeuroses with muscles and tendons. The purpose of this work was to measure the in-vivo longitudinal strains of the distal and proximal aponeuroses and force of the unipennate Medial Gastrocnemius (MG) muscle during locomotor activities. Sonomicrometry crystals and a force buckle transducer were implanted to measure aponeurosis strains and MG forces in the left hindlimb of four healthy female sheep while walking at different speeds and inclination angles on a motorized treadmill. The resulting aponeurosis strains versus the corresponding muscle forces resulted in a complex interaction that is not reconcilable with a mechanical "in series" arrangement of aponeuroses with either the free tendon or muscle, as has frequently been assumed when trying to determine the storage and release of mechanical energy in muscles or the stiffness and elastic modulus of in-vivo aponeurosis tissues. We conclude that the interaction of muscle tissue with aponeuroses in the sheep MG allows for elongation of the aponeuroses at low forces in the passive muscle, while elongation in the active muscle is greatly reduced possibly due to the complex shear forces and pressures produced when the muscle is activated. It is likely that the observed aponeurosis mechanics are similar in other unipennate skeletal muscles, but the current study was limited to a single muscle and therefore does not allow for such extrapolation at this time.

Soleus muscle and Achilles tendon compressive stiffness is related to knee and ankle positioning

Changes in fascicle length and tension of the soleus (SOL) muscle have been observed in humans using B-mode ultrasound to examine the knee from different angles. An alternative technique of assessing muscle and tendon stiffness is myometry, which is non-invasive, accessible, and easy to use. This study aimed to estimate the compressive stiffness of the distal SOL and Achilles tendon (AT) using myometry in various knee and ankle joint positions. Twenty-six healthy young males were recruited. The Myoton-PRO device was used to measure the compressive stiffness of the distal SOL and AT in the dominant leg. The knee was measured in two positions (90° of flexion and 0° of flexion) and the ankle joint in three positions (10° of dorsiflexion, neutral position, and 30° of plantar flexion) in random order. A three-way repeated-measures ANOVA test was performed. Significant interactions were found for structure × ankle position, structure × knee position, and structure × ankle position × knee position (p < 0.05). The AT and SOL showed significant increases in compressive stiffness with knee extension over knee flexion for all tested ankle positions (p < 0.05). Changes in stiffness relating to knee positioning were larger in the SOL than in the AT (p < 0.05). These results indicate that knee extension increases the compressive stiffness of the distal SOL and AT under various ankle joint positions, with a greater degree of change observed for the SOL. This study highlights the relevance of knee position in passive stiffness of the SOL and AT.

Posterior Leg Pain: Understanding Soleus Muscle Injuries

Soleus muscle injuries are frequently unrecognized, representing a common cause of sports inactivity. This is mainly because little is known about the anatomy of the soleus muscle and the clinical manifestations of injury. Unlike other muscles, the soleus muscle has a complex myoconnective structure with three intramuscular tendons, which makes the interpretation of muscle pathologic conditions challenging. Soleus muscle injuries can be acute or chronic and are usually considered to be a minor discomfort by both the patient and the sports medicine physician, leading to a relatively quick return to sporting activity with a high risk for reinjury. The authors review the soleus muscle anatomy and the importance of being familiar with the most frequent locations of injuries, which are fundamental aspects that every radiologist should understand to avoid underdiagnosis. The role of imaging, the clinical manifestations of injuries, and the differential diagnoses are key aspects to know when evaluating posterior leg pain. The online slide presentation from the RSNA Annual Meeting is available for this article. ^©RSNA, 2022.

A 3D model of the soleus reveals effects of aponeuroses morphology and material properties on complex muscle fascicle behavior

The soleus is an important plantarflexor muscle with complex fascicle and connective tissue arrangement. In this study we created an image-based finite element model representing the 3D structure of the soleus muscle and its aponeurosis connective tissue, including distinct fascicle architecture of the posterior and anterior compartments. The model was used to simulate passive and active soleus lengthening during ankle motion to predict tissue displacements and fascicle architecture changes. Both the model's initial architecture and changes incurred during passive lengthening were consistent with prior in vivo data from diffusion tensor imaging. Model predictions of active lengthening were consistent with axial plane muscle displacements that we measured in eight subjects' lower legs using cine DENSE (Displacement Encoding with Stimulated Echoes) MRI during eccentric dorsiflexion. Regional strains were variable and nonuniform in the model, but average fascicle strains were similar between the compartments for both passive (anterior: 0.18 ± 0.06, posterior: 0.19 ± 0.05) and active (anterior: 0.12 ± 0.05, posterior: 0.13 ± 0.06) lengthening and were two- to three-times greater than muscle belly strain (0.06). We used additional model simulations to investigate the effects of aponeurosis material properties on muscle deformation, by independently varying the longitudinal or transverse stiffness of the posterior or anterior aponeurosis. Results of model variations elucidate how properties of soleus aponeuroses contribute to fascicle architecture changes. Greater longitudinal stiffness of posterior compared to anterior aponeurosis promoted more uniform spatial distribution of muscle tissue deformation. Reduced transverse stiffness in both aponeuroses resulted in larger differences between passive and active soleus lengthening.

Developing a Quantifying Device for Soft Tissue Material Prop-Erties around Lumbar Spines

Knowing the material properties of the musculoskeletal soft tissue could be important to develop rehabilitation therapy and surgical procedures. However, there is a lack of devices and information on the viscoelastic properties of soft tissues around the lumbar spine. The goal of this study was to develop a portable quantifying device for providing strain and stress curves of muscles and ligaments around the lumbar spine at various stretching speeds. Each sample was conditioned and applied for 20 repeatable cyclic 5 mm stretch-and-relax trials in the direction and perpendicular direction of the fiber at 2, 3 and 5 mm/s. Our device successfully provided the stress and strain curve of the samples and our results showed that there were significant effects of speed on the young's modulus of the samples (p < 0.05). Compared to the expensive commercial device, our lower-cost device provided comparable stress and strain curves of the sample. Based on our device and findings, various sizes of samples can be measured and viscoelastic properties of the soft tissues can be obtained. Our portable device and approach can help to investigate young's modulus of musculoskeletal soft tissues conveniently, and can be a basis for developing a material testing device in a surgical room or various lab environments.

Inhomogeneous and Anisotropic Mechanical Properties of the Triceps Surae Aponeuroses in Older Adults: Relationships With Muscle Strength and Walking Performance

This study investigated (a) site- and direction-dependent variations of passive triceps surae aponeurosis stiffness and (b) the relationships between aponeurosis stiffness and muscle strength and walking performance in older individuals. Seventy-nine healthy older adults participated in this study. Shear wave velocities of the triceps surae aponeuroses at different sites and in two orthogonal directions were obtained in a prone position at rest using supersonic shear imaging. The maximal voluntary isometric contraction torque of the plantar flexors and normal (preferred) and fast (fastest possible) walking speeds (5-m distance) were also measured. The shear wave velocities of the adjoining aponeuroses were weakly associated with plantar flexion torque (r = .23-.34), normal (r = .26), and fast walking speed (r = .25). The results show clear spatial variations and anisotropy of the triceps surae aponeuroses stiffness in vivo, and the aponeurosis stiffness was associated with physical ability in older adults.

Objective Assessment of Regional Stiffness in Achilles Tendon in Different Ankle Joint Positions Using the MyotonPRO

BACKGROUND Achilles tendinopathy commonly occurs in specific regions of the tendon, and Achilles tendon stiffness can be related to local pathological changes in the tendon. The MyotonPRO is a new handheld device that conveniently assesses stiffness of muscles and tendons. This study aimed to 1) evaluate the intra- and inter-rater reliability of stiffness measurements of the Achilles tendon at different ankle positions, 2) investigate the modulation of stiffness at different ankle joint angles, and 3) examine the differences between 2 regions of Achilles tendon stiffness. MATERIAL AND METHODS Thirty healthy young adults (15 men and 15 women) participated in this study. The regional Achilles tendon stiffness at 0 cm (AT-0) and 6 cm (AT-6) above the tendon insertion were evaluated by the MyotonPRO in the neutral position and 10° dorsiflexion of the ankle joint. Measurements of stiffness were taken by 2 raters on the first day and 5 days later. The stiffness data were compared by repeated measures analysis of variance (ANOVA). RESULTS The intra- and inter-rater reliability of stiffness measurements at AT-0 and AT-6 for each ankle position were good (all intraclass correlation coefficients >0.84). A significant modulation of Achilles tendon stiffness was obtained at different ankle joint angles (P<0.05). Stiffness at AT-0 was higher than at AT-6 (P<0.05) in both positions. CONCLUSIONS These results suggest the MyotonPRO reliably assessed Achilles tendon stiffness and monitors its modulation, and tendon stiffness increased with ankle dorsiflexion. Stiffness was also nonuniform along the length of the tendon.

Anisotropic and viscoelastic tensile mechanical properties of aponeurosis: Experimentation, modeling, and tissue microstructure

Aponeuroses are stiff sheath-like components of the muscle-tendon unit that play a vital role in force transmission and thus locomotion. There is clear importance of the aponeurosis in musculoskeletal function, but there have been relatively few studies of aponeurosis material properties to date. The goals of this work were to: 1) perform tensile stress-relaxation tests, 2) perform planar biaxial tests, 3) employ computational modeling to the data from 1 to 2, and 4) perform scanning electron microscopy to determine collagen fibril organization for aponeurosis tissue. Viscoelastic modeling and statistical analysis of stress-relaxation data showed that while relaxation rate differed statistically between strain levels (p = 0.044), functionally the relaxation behavior was nearly the same. Biaxial testing and associated modeling highlighted the nonlinear (toe region of ~2-3% strain) and anisotropic (longitudinal direction linear modulus ~50 MPa, transverse ~2.5 MPa) tensile mechanical behavior of aponeurosis tissue. Comparisons of various constitutive formulations showed that a transversely isotropic Ogden approach balanced strong fitting (goodness of fit 0.984) with a limited number of parameters (five), while damage modeling parameters were also provided. Scanning electron microscopy showed a composite structure of highly aligned, partially wavy collagen fibrils with more random collagen cables for aponeurosis microstructure. Future work to expand microstructural analysis and use these data to inform computational modeling would benefit this work and the field.

Site dependent elastic property of human iliotibial band and the effect of hip and knee joint angle configuration

The iliotibial band (ITB) is the lateral thickening of the fascia lata. The ITB has been extensively studied for its relevance to injury, but not much is known about its elastic properties. We aimed to investigate the site- and joint angle-dependence of ITB elasticity. We tested twelve healthy males (22-30 years; in vivo) and twelve male cadavers (69-93 years; cadaver). The Young's modulus of the ITB was measured in the longitudinal direction at five sites (over the proximal, middle, and distal bellies of the vastus lateralis (VL), superior border of the patella, and between femur and tibia) of the right limb, by ultrasound shear wave elastography (in vivo) and the tensile test (cadaver). Joint angle-dependence was also studied for nine different positions (knee angles at 0, 25, 90˚ x hip angles at 0, 40, 90˚) (in vivo). Over VL, the ITB was more compliant at the distal (17.6-190.1 kPa; in vivo, 219.4 ± 68.8 MPa; cadaver, mean ± SD) than other sites (24.2-221.4 kPa, 337.9-362.7 MPa). The ITB at the superior border of the patella and between femur and tibia was stiffer in vivo (31.8-271.8 and 50.9-208.8 kPa), while it was more compliant in cadavers (113.4 ± 63.7 and 130.4 ± 73.7 MPa), compared to other sites. The ITB became stiffer associated with increasing hip extension angle and knee flexion angle, and the hip remarkably affecting the values regardless of site (in vivo). Our findings have clinical significance with respect to the site- and joint angle-dependence of ITB-related overuse injury.

Muscle-tendon morphology and function following long-term exposure to repeated and strenuous mechanical loading

We mapped structural and functional characteristics of muscle-tendon units in a population exposed to very long-term routine overloading. Twenty-eight military academy cadets (age = 21.00 ± 1.1 years; height = 176.1 ± 4.8 cm; mass = 73.8 ± 7.0 kg) exposed for over 24 months to repetitive overloading were profiled via ultrasonography with a senior subgroup of them (n = 11; age = 21.4 ± 1.0 years; height = 176.5 ± 4.8 cm; mass = 71.4 ± 6.6 kg) also tested while walking and marching on a treadmill. A group of eleven ethnicity- and age-matched civilians (age = 21.6 ± 0.7 years; height = 176.8 ± 4.3 cm; mass = 74.6 ± 5.6 kg) was also profiled and tested. Cadets and civilians exhibited similar morphology (muscle and tendon thickness and cross-sectional area, pennation angle, fascicle length) in 26 out of 29 sites including the Achilles tendon. However, patellar tendon thickness along the entire tendon was greater (P < .05) by a mean of 16% for the senior cadets compared with civilians. Dynamically, cadets showed significantly smaller ranges of fascicle length change and lower shortening velocity in medial gastrocnemius during walking (44.0% and 47.6%, P < .05-.01) and marching (27.5% and 34.3%, P < .05-.01) than civilians. Furthermore, cadets showed lower normalized soleus electrical activity during walking (22.7%, P < .05) and marching (27.0%, P < .05). Therefore, 24-36 months of continuous overloading, primarily occurring under aerobic conditions, leads to more efficient neural and mechanical behavior in the triceps surae complex, without any major macroscopic alterations in key anatomical structures.