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.

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.

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.

Site-specific features of active muscle stiffness and proximal aponeurosis strain in biceps femoris long head.

Limited information is available on site-specific features of muscle stiffness and aponeurosis strain of the biceps femoris long head (BFlh) during contractions. Therefore, understanding of the mechanics and etiology of hamstring strain injuries remains difficult. As a first step to gain further insight into them, the present study aimed to identify whether active muscle stiffness and proximal aponeurosis strain during contractions are varied along the long axis of the BFlh. The BFlh muscle shear wave speed (proxy for stiffness) was measured in the proximal, central, and distal sites during 20%, 50%, and 80% of maximal voluntary isometric contraction (MVC) of knee flexion exerted with the hip and knee joints flexed at 40° and 30°, respectively, using ultrasound shear wave elastography. Further, a segmental strain of the BFlh proximal aponeurosis was assessed in the proximal, central, and distal sites during isometric knee flexion, using B-mode ultrasonography. The shear wave speed was significantly higher in the distal site than the proximal and central sites at 20% MVC (p ≤ .002, with a large effect size), whereas no significant difference was found between the three sites at 50% and 80% MVC. The BFlh proximal aponeurosis strain showed no significant difference between the proximal, central, and distal sites at any contraction intensity. These findings indicate that site-specific differences in muscle stiffness and proximal aponeurosis strain are substantially small and that muscle stiffness and proximal aponeurosis strain of the BFlh at moderate-to-high contraction intensity is not exceptional in the site where a sprinting-type hamstring strain typically occurs.

Age-related differences in vastus lateralis fascicle behavior during fast accelerative leg-extension movements.

Leg-extensor rate of power development (RPD) decreases during aging. This study aimed to identify the underlying mechanism of the age-related decline in RPD during a fast acceleration in terms of in vivo vastus lateralis (VL) fascicle shortening behavior. Thirty-nine men aged between 25 and 69 years performed three maximal isokinetic leg-extensor tests with a fixed initial acceleration of 45° knee extension in 150 ms until 340°/s knee angular velocity. RPD, VL activity, and ultrasound images were recorded to assess (relative) fascicle shortening and mean shortening velocity for the phases of electromechanical delay, pretension, and acceleration. Our findings show that fascicle shortening and mean shortening velocity during a fast action increase with aging (0.002 per year, P = .035 and 0.005 s-1 per year, P = .097, respectively), mainly due to a higher amount of shortening in the phase of electromechanical delay. The ratio of VL fascicle length over upper leg length at rest showed a negative correlation (r = -.46, P = .004) with RPD/body mass, while pennation angle at rest showed a trend toward a positive correlation (r = .28, P = .089). To conclude, our findings indicate that the ability to reach high VL fascicle shortening velocities in vivo is not reduced in older men while performing preprogrammed fast accelerations. The greater amount of fascicle shortening in old age is probably the result of age-related differences in the tendinous properties of the muscle-tendon complex, forcing the fascicles to shorten more in order to transmit the muscle force to the segment.

Evolutionary anatomy of the plantar aponeurosis in primates, including humans.

The plantar aponeurosis in the human foot has been extensively studied and thoroughly described, in part, because of the incidence of plantar fasciitis in humans. It is commonly assumed that the human plantar aponeurosis is a unique adaptation to bipedalism that evolved in concert with the longitudinal arch. However, the comparative anatomy of the plantar aponeurosis is poorly known in most mammals, even among non-human primates, hindering efforts to understand its function. Here, we review previous anatomical descriptions of 40 primate species and use phylogenetic comparative methods to reconstruct the evolution of the plantar aponeurosis and its relationship to the plantaris muscle in primates. Ancestral state reconstructions suggest that the overall organization of the human plantar aponeurosis is shared with chimpanzees and that a similar anatomical configuration evolved independently in different primate clades as an adaptation to terrestrial locomotion. The presence of a plantar aponeurosis with clearly developed lateral and central bands in the African apes suggests that this structure is not prohibitive to suspensory locomotion and that these species possess versatile feet adapted for both terrestrial and arboreal locomotion. This plantar aponeurosis configuration would have been advantageous in enhancing foot stiffness for bipedal locomotion in the earliest hominins, prior to the evolution of a longitudinal arch. Hominins may have subsequently evolved thicker and stiffer plantar aponeuroses alongside the arch to enable a windlass mechanism and elastic energy storage for bipedal walking and running, although this idea requires further testing.

Plantar flexor muscle-tendon unit length and stiffness do not influence neuromuscular fatigue in boys and men.

PURPOSE: The twofold purpose of this study was (1) to compare differences in development and etiology of neuromuscular fatigue at different plantar flexor (PF) muscle-tendon unit (MTU) lengths between boys and men, and (2) to examine the relationship between musculotendinous stiffness and peripheral fatigue. METHODS: Nineteen pre-pubertal boys and 23 men performed three intermittent fatigue protocols at different PF MTU lengths (short: S, neutral: N and long: L), consisting of repeating maximal voluntary isometric contractions (MVIC) until the torque reached 60% of the initial value of MVIC. The etiology of the neuromuscular fatigue and the gastrocnemius medialis aponeuroses and tendon stiffness (KGM) were investigated using non-invasive methods. RESULT: The number of repetitions did not differ between men and boys, regardless of the PF MTU length (S: 16.5 ± 5.4 and 17.6 ± 5.8; N: 15.8 ± 4.5 and 13.3 ± 3.6; L: 13.6 ± 4.8 and 12.6 ± 4.6, respectively). Boys displayed a lower decrement of potentiated twitch torque (Qtwpot; p < 0.001) and greater decrease of voluntary activation level than men (p < 0.001). Although boys showed lower KGM values than men at S, no significant correlation was found between KGM and Qtwpot. CONCLUSION: PF MTU length had no effect on differences in the development and etiology of neuromuscular fatigue between boys and men. Although both groups displayed similar development of fatigue, central mechanisms mainly accounted for fatigue in boys and peripheral mechanisms were mainly involved in men. Additionally, musculotendinous stiffness did not account for difference in peripheral fatigue between children and adults.

Foot Motion Character During Forward and Backward Walking With Shoes and Barefoot.

Backward walking (BW) has been extensively used in athletic training and orthopedic rehabilitation as it may have value for enhancing balance. This study identified the differences in foot intersegment kinematics (forward walking (FW) vs. time-reversed BW) and plantar pressure parameters of 16 healthy habitually shod individuals walking FW and BW using flexible shoes (SH) and under barefoot conditions (BF). BW was found to have shorter stride length (SL) and higher stride frequency (SF) under BF conditions compared with SH, which indicates a better BW gait stability under BF conditions. Decreased HX/FF dorsiflexion at HO in BW induces less plantar aponeurosis tension which may inhibit the windlass mechanism compared to FW walking. Increased forefoot relative to hindfoot (FF/HF) pronation and sequentially hindfoot relative to tibia (HF/TB) eversion combined with medially distributed plantar pressure and a higher plantar contact area in the medial side in BW-BF maybe beneficial in maintaining balance. These results indicate that BW training may be more reliable under BF conditions compared to the SH conditions based on greater sensory information feedback from the plantar area resulting in better biomechanical behavior.

An intact lacertus fibrosus improves strength after reinsertion of the distal biceps tendon.

PURPOSE: The importance of an intact lacertus fibrosus in distal biceps tendon injury is uncertain. This study aimed to assess long-term outcome following distal biceps tendon repair with focus on the significance of the lacertus fibrosus. METHODS: Thirty-six patients surgically treated for primary distal biceps tendon rupture were identified. Medical records were reviewed for patient demographics in addition to surgery-related data. All patients underwent a targeted clinical examination to assess elbow function and they completed a patient reported questionnaire. Radiographs were obtained at time of follow-up and evaluated for the presence of osteoarthritis (OA) and heterotopic ossification (HO). RESULTS: All patients were male. Median age at injury was 48 years (34-69) and median time of follow-up of was 71 months (23-165). All patients presented functional range of motion in the elbow. Median flexion strength was 76 Nm (45-135) (median 99% of uninjured side; range 66-128) with intact lacertus fibrosus and 70 Nm (43-124) (88%, 62-114) with torn lacertus fibrosus. Median supination strength was 6 Nm (3-11) (86%, range 36-144) with intact lacertus fibrosus and 8 Nm (3-17) (67%, 28-118) with torn lacertus fibrosus. No signs of OA were revealed, but three patients had major HO of which one patient had minor limitations in range of elbow motion. CONCLUSION: An intact lacertus fibrosus contributes to elbow strength and should be preserved in distal biceps tendon repair. LEVEL OF EVIDENCE: III.

Evidence of a tunable biological spring: elastic energy storage in aponeuroses varies with transverse strain in vivo.

Tendinous structures are generally thought of as biological springs that operate with a fixed stiffness, yet recent observations on the mechanical behaviour of aponeuroses (broad, sheet-like tendons) have challenged this general assumption. During in situ contractions, aponeuroses undergo changes in both length and width and changes in aponeuroses width can drive changes in longitudinal stiffness. Here, we explore if changes in aponeuroses width can modulate elastic energy (EE) storage in the longitudinal direction. We tested this idea in vivo by quantifying muscle and aponeuroses mechanical behaviour in the turkey lateral gastrocnemius during landing and jumping, activities that require rapid rates of energy dissipation and generation, respectively. We discovered that when aponeurosis width increased (as opposed to decreased), apparent longitudinal stiffness was 34% higher and the capacity of aponeuroses to store EE when stretched in the longitudinal direction was 15% lower. These data reveal that biaxial loading of aponeuroses allows for variation in tendon stiffness and energy storage for different locomotor behaviours.

Mechanical properties, physiological behavior, and function of aponeurosis and tendon.

During human movement, the muscle and tendinous structures interact as a mechanical system in which forces are generated and transmitted to the bone and energy is stored and released to optimize function and economy of movement and/or to reduce risk of injury. The present review addresses certain aspects of how the anatomical design and mechanical and material properties of the force-transmitting tissues contribute to the function of the muscle-tendon unit and thus overall human function. The force-bearing tissues are examined from a structural macroscopic point of view down to the nanoscale level of the collagen fibril. In recent years, the understanding of in vivo mechanical function of the force-bearing tissues has increased, and it has become clear that these tissues adapt to loading and unloading and furthermore that force transmission mechanics is more complex than previously thought. Future investigations of the force-transmitting tissues in three dimensions will enable a greater understanding of the complex functional interplay between muscle and tendon, with relevance for performance, injury mechanisms, and rehabilitation strategies.

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

The human triceps surae (two gastrocnemii and soleus) has aponeuroses in the proximal and distal aspects, the latter of which insert into the calcaneus by sharing the common Achilles tendon. These tendinous tissues are known to have elasticity and upon muscle contraction the aponeurosis is stretched both longitudinally (along the muscle's line of action) and transversely. Higher aponeurosis transverse deformability has been documented, but there is a paucity of information on the morphology and mechanical properties of human aponeurosis. This study aimed to identify morphological and mechanical characteristics of the human triceps surae aponeuroses. Twenty-five triceps surae muscle-tendon units were procured from 13 human donors (formalin fixed, 6 males, 7 females) aged 67-91 years. Specimens of aponeuroses were excised from the eight regions (posterior and anterior regions of the gastrocnemius medialis and lateralis, medial and lateral parts of soleus; proximal, middle, and distal sites each, 2-4 cm × 2-4 cm). Aponeurosis thickness was measured using a digital caliper. Uniaxial tensile tests were implemented to determine the mechanical properties of specimens loaded longitudinally (along the muscle's line of action) and transversely. The aponeurosis thickness showed significant differences between muscles and sites, while Young's modulus showed direction-dependent (longitudinal vs. transverse) differences within sites. Results show different morphology and mechanical properties of aponeuroses between synergist muscles. The reason for site-dependent differences in stiffness is due to a reduced aponeurosis thickness rather than a reduction in the material property. The anisotropic elastic feature (differences between longitudinal and transverse directions) of the aponeuroses was more pronounced than previous in vivo findings, suggesting inherent material design of the aponeurosis that matches three-dimensional contractile behavior of muscle fibers.

Antagonist muscle architecture and aponeurosis/tendon strain of biceps femoris long head during maximal isometric efforts.

INTRODUCTION: Examination of the mechanical behavior of the hamstrings when acting as antagonists provides information about loading of this muscle group and its role for joint stability during forceful quadriceps contractions. The aim of this study was to quantify biceps femoris long head fascicle length (FL), angle of pennation (PA) and distal tendon/aponeurosis strain during maximum voluntary contraction efforts of the knee extensors using real-time ultrasound. METHODS: Fourteen participants performed passive joint movements and maximum voluntary knee extension and flexion efforts of the knee flexors at 0°, 45° and 90° of knee flexion. An ultrasound probe was used to visualize FL, PA and tendon/aponeurosis strain from the distal part of the muscle. RESULTS: Two-way analysis of variance designs indicated that: (a) antagonist BFlh tendon/aponeurosis strain increased significantly up to 2.77 ± 1.25% relative to rest (p < 0.05). The FL increased non-significantly (2.86 ± 6.81%) while the PA was unaltered during isometric MVC efforts of the knee extensors (p > 0.05) (b) FL, PA and tendon/aponeurosis strain of the BFlh when acting as antagonist were not significantly affected by knee joint angular position (p > 0.05). CONCLUSIONS: Antagonist hamstring function takes the form of a lengthened tendon/aponeurosis, no fascicle shortening and submaximal neural activation. Future research could examine whether exercise interventions that aim to alter tendon/aponeurosis mechanical properties of the hamstrings when acting as antagonists are beneficial for injury prevention and rehabilitation.

Effect of toe dorsiflexion on the regional distribution of plantar fascia shear wave velocity.

BACKGROUND: The plantar fascia is exposed to repetitive tensile stress induced by cyclic loads associated with daily activities, such as walking and running. Due to overuse or abnormal foot alignment, insertional and distal (i.e., mid-substance) regions within the plantar fascia may exhibit microtears, which leads to plantar fasciopathy. Ultrasound shear wave elastography is an imaging technique to measure shear wave velocity propagating through biological tissues, considered herein as an index of tensile stress. This study aimed to quantify the effect of toe dorsiflexion on the regional distribution of plantar fascia shear wave velocity. METHODS: Shear wave velocity of the plantar fascia was measured in the insertional and distal regions using ultrasound shear wave elastography in sixteen healthy participants (7 males and 9 females). The measurements were performed while the toes were maintained in neutral or dorsiflexed positions. FINDINGS: When considering the insertional region, there was no significant difference in shear wave velocity between neutral toe position [mean (SEM): 5.4 (0.6) m/s] and dorsiflexed toe position [5.5 (0.5) m/s] (P = 0.88; effect size = 0.05). When considering the distal region, there was a significant difference in shear wave velocity between the neutral position [7.8 (0.4) m/s] and dorsiflexed position [9.9 (0.3) m/s] (P = 0.002; effect size = 0.88). The difference in shear wave velocity between the insertional and distal regions showed a large effect size for either neutral (P = 0.010; effect size = 0.75) or dorsiflexed toe position (P = 0.003; effect size = 0.86). INTERPRETATION: In contrast to clinical beliefs, these findings suggest that toe dorsiflexion induces non-homogeneous changes in tensile stress within the plantar fascia.

Passive changes in muscle length.

This review, the first in a series of minireviews on the passive mechanical properties of skeletal muscles, seeks to summarize what is known about the muscle deformations that allow relaxed muscles to lengthen and shorten. Most obviously, when a muscle lengthens, muscle fascicles elongate, but this is not the only mechanism by which muscles change their length. In pennate muscles, elongation of muscle fascicles is accompanied by changes in pennation and changes in fascicle curvature, both of which may contribute to changes in muscle length. The contributions of these mechanisms to change in muscle length are usually small under passive conditions. In very pennate muscles with long aponeuroses, fascicle shear could contribute substantially to changes in muscle length. Tendons experience moderate axial strains even under passive loads, and, because tendons are often much longer than muscle fibers, even moderate tendon strains may contribute substantially to changes in muscle length. Data obtained with new imaging techniques suggest that muscle fascicle and aponeurosis strains are highly nonuniform, but this is yet to be confirmed. The development, validation, and interpretation of continuum muscle models informed by rigorous measurements of muscle architecture and material properties should provide further insights into the mechanisms that allow relaxed muscles to lengthen and shorten.

Organization of the fascia and aponeurosis in the lumbar paraspinal compartment.

PURPOSE: The thoracolumbar fascia (TLF) and the erector spinae aponeurosis (ESA) play significant roles in the biomechanics of the spine and could be a source of low back pain. Attachment, collagen fiber direction, size and biomechanical properties of the TLF have been well documented. However, questions remain about the attachment of the TLF and ESA in relation to adjoining tissues in the lumbosacral region. Moreover, quantitative data in relation to the ESA have rarely been examined. The aim of this study was to further investigate the anatomical features of the TLF and ESA and to determine the attachments and sliding areas of the paraspinal compartment through dissection. MATERIALS AND METHODS: In 10 fresh cadavers (6 females, 4 males, mean age: 77 ± 10 years), we determined (1) the gross anatomy of the ESA and the TLF (attachments and sliding areas) and (2) the structure of the ESA and the TLF (thickness, width, orientation of collagen fibers). The pennation angle between the axis of the ES muscle fibers and the axis of the collagen fibers of the ESA were also measured. RESULTS: The TLF is an irregular dense connective tissue with a mean thickness of 0.95 mm. The distance between the spinous processes line and the site where the neurovascular bundles pierced the TLF, depending on the vertebral level, ranged from 29 mm at L1 to 75 mm at L3. The ESA constituted a band of regular longitudinally oriented connective fibers (mean thickness: 1.85 mm). Muscles fibers of the ES were strongly diagonally attached to the ESA (mean pennation angle 8° for the iliocostalis and 14° for the longissimus). To a lesser extent, the superficial multifidi were attached to the ESA at the lumbar level close to the midline and at the sacral level. CONCLUSION: The ESA, at twice the thickness of the pTLF, was the thickest dense connective tissue of the paraspinal compartment. The ESA and the TLF circumscribed subcompartments and sliding areas between the TFL and the lumbar paraspinal muscles, between the ES and the multifidus, and between the longissimus and the iliocostalis.

Aponeurosis influences the relationship between muscle gearing and force.

Aponeuroses are connective tissues found on the surface of pennate muscles and are in close association with muscle fascicles. In addition to transmitting muscle forces to the external tendon, aponeurosis has been hypothesized to influence the direction of muscle shape change during a contraction. Muscle shape changes affect muscle contractile force and velocity because they influence the gear ratio with which muscle fascicles transmit force and velocity to the tendon. If aponeurosis modulates muscle shape changes, altering the aponeurosis' radial integrity with incisions should alter gearing. We tested the hypothesis that incising the aponeurosis would lead to decreased gearing across force conditions with an in situ preparation of the turkey lateral gastrocnemius muscle. We found that multiple full-length incisions in the aponeurosis altered the relationship between gearing and force relative to the intact aponeurosis condition. Specifically, after multiple aponeurosis incisions, gear ratio decreased by 19% in the high-force contractions compared with the intact condition. These results suggest that aponeuroses influence muscle shape change and can alter muscle contractile force and speed through their effect on muscle gearing. NEW & NOTEWORTHY Muscle gearing is determined by muscle shape change during a contraction and varies with the force of contraction. Variable gearing influences muscle force and speed, but how gearing is modulated is not well understood. Incising the aponeurosis before and after contractions demonstrates that aponeurosis plays a role in modulating gearing.

Aponeurosis behaviour during muscular contraction: A narrative review.

There is an abundance of evidence that suggests elastic tendons can enhance both animal and human muscle performance. However, in many terrestrial animals, including humans, a large proportion of the elastic tissue within the muscle-tendon unit is located within the muscle. This continuous elastic sheet, which provides muscle fibre attachment, is known as the aponeurosis. The aponeurosis has a much more complicated shape than the free tendon and it undergoes a more complicated loading regime during contraction, due to its relationship with the bulging muscle fibres, which remain isovolumetric during force production. Muscle contraction may dynamically modulate the stiffness of the aponeurosis at the same active versus passive force, by increasing the intramuscular pressure and transverse forces within the muscle, which may stretch the aponeurosis in width and subsequently reduce its longitudinal strain. Some evidence also suggests that the aponeurosis mechanical properties may be affected by muscle length, which appears to reduce the fascicle strains for a given muscle force at longer muscle lengths. This narrative review outlines the animal and human studies that have investigated aponeurosis behaviour during contraction and discusses how an elastic sheet with a variable stiffness under activation might be beneficial for muscle performance. While it is clear that our understanding of the role of aponeurosis is lacking, it is hoped that further work will attempt to determine how this tissue contributes to power amplification and elastic energy savings during locomotion and potentially uncover how aponeurosis behaviour contributes to injury risk.

Muscle-tendon length and force affect human tibialis anterior central aponeurosis stiffness in vivo.

The factors that drive variable aponeurosis behaviors in active versus passive muscle may alter the longitudinal stiffness of the aponeurosis during contraction, which may change the fascicle strains for a given muscle force. However, it remains unknown whether these factors can drive variable aponeurosis behaviors across different muscle-tendon unit (MTU) lengths and influence the subsequent fascicle strains during contraction. Here, we used ultrasound and elastography techniques to examine in vivo muscle fascicle behavior and central aponeurosis deformations of human tibialis anterior (TA) during force-matched voluntary isometric dorsiflexion contractions at three MTU lengths. We found that increases in TA MTU length increased both the length and apparent longitudinal stiffness of the central aponeurosis at low and moderate muscle forces (P < 0.01). We also found that increased aponeurosis stiffness was directly related to reduced magnitudes of TA muscle fascicle shortening for the same change in force (P < 0.01). The increase in slope and shift to longer overall lengths of the active aponeurosis force-length relationship as MTU length increased was likely due to a combination of parallel lengthening of aponeurosis and greater transverse aponeurosis strains. This study provides in vivo evidence that human aponeurosis stiffness is increased from low to moderate forces and that the fascicle strains for a given muscle force are MTU length dependent. Further testing is warranted to determine whether MTU length-dependent stiffness is a fundamental property of the aponeurosis in pennate muscles and evaluate whether this property can enhance muscle performance.

Tendinous tissue properties after short- and long-term functional overload: Differences between controls, 12 weeks and 4 years of resistance training.

AIM: The potential for tendinous tissues to adapt to functional overload, especially after several years of exposure to heavy-resistance training, is largely unexplored. This study compared the morphological and mechanical characteristics of the patellar tendon and knee extensor tendon-aponeurosis complex between young men exposed to long-term (4 years; n = 16), short-term (12 weeks; n = 15) and no (untrained controls; n = 39) functional overload in the form of heavy-resistance training. METHODS: Patellar tendon cross-sectional area, vastus lateralis aponeurosis area and quadriceps femoris volume, plus patellar tendon stiffness and Young's modulus, and tendon-aponeurosis complex stiffness, were quantified with MRI, dynamometry and ultrasonography. RESULTS: As expected, long-term trained had greater muscle strength and volume (+58% and +56% vs untrained, both P < .001), as well as a greater aponeurosis area (+17% vs untrained, P < .01), but tendon cross-sectional area (mean and regional) was not different between groups. Only long-term trained had reduced patellar tendon elongation/strain over the whole force/stress range, whilst both short-term and long-term overload groups had similarly greater stiffness/Young's modulus at high force/stress (short-term +25/22%, and long-term +17/23% vs untrained; all P < .05). Tendon-aponeurosis complex stiffness was not different between groups (ANOVA, P = .149). CONCLUSION: Despite large differences in muscle strength and size, years of resistance training did not induce tendon hypertrophy. Both short-term and long-term overload demonstrated similar increases in high-force mechanical and material stiffness, but reduced elongation/strain over the whole force/stress range occurred only after years of overload, indicating a force/strain specific time-course to these adaptations.