Age-related changes in mechanical properties of the Achilles tendon

Predictive ability of Achilles tendon elastography for frailty in older adults

PURPOSE

The Achilles tendon (AT) is the largest and strongest tendon in the human body, and its elasticity is known to be affected by the aging process. However, the relation between AT stiffness and frailty in older individuals remains uncertain. This study aims to explore the potential of Achilles tendon shear wave elastography (AT-SWE) as a tool for assessing physical frailty in older adults.

METHODS

A total of 148 patients aged 65 years and over were included in this cross-sectional study. Patients with heart failure, AT injury, stroke history, active malignancy, and claudication were excluded. All patients underwent a comprehensive geriatric assessment. Physical frailty assessment was performed with the fried frailty phenotype. Achilles tendon elastography was measured by ultrasound.

RESULTS

The mean age of the participants was 73.8 years and 62.2% were female. 30.4% of the participants were defined as frail. Achilles tendon shear wave elastography measurements were statistically lower in the frail group (p < 0.05). In the multivariate regression analysis, AT-SWE demonstrated a statistically significant association with frailty independent of confounding factors (OR 0.982, 95% CI 0.965-0.999, p value = 0.038). In the ROC curve analysis, the area under the curve for AT-SWE was 0.647 (95% CI, 0.564-0.724, p < 0.01) and the optimum cut-off point was 124.1 kilopascals.

CONCLUSION

These findings highlight the value of AT-SWE as a non-invasive and objective tool for predicting frailty in older adults.

Youth Athletes With Sever's Disease Exhibit Altered Achilles Tendon Ultrasound Characteristics: A Retrospective Case-Control Study

OBJECTIVES

Calcaneal apophysitis (Sever's disease) is an overuse condition caused by repetitive traction stress to the calcaneal apophysis. Whether Achilles tendon morphology is altered in this young patient population remains unknown. Therefore, we aimed to identify differences in Achilles tendon morphology between youth athletes diagnosed with calcaneal apophysitis and healthy controls.

METHODS

This retrospective chart review included 46 patients (n = 23 Sever's disease, 15F/8M, 12.4 ± 2.3 years old) and (n = 23 healthy controls, 13F/10M, 15.9 ± 1.5 years old) who sought care in a Children's Hospital Sports Medicine/Orthopedics Department between 2012 and 2022. We measured ultrasound-derived degree of tendon thickening, Achilles tendon thickness (cm), and cross-sectional area (CSA [cm2]). Separate multivariate analyses of covariance (MANCOVAs) were used to compare degree of thickening, mass-normalized Achilles tendon thickness, and CSA between participant groups, covarying for age. Cohen's d effect sizes were used to assess the magnitude of mean differences and standard error (MDSE) between groups.

RESULTS

Young athletes with Sever's disease had a significantly greater degree of tendon thickening with a large effect compared with healthy controls (MDSE: 0.07 [0.01] mm P < .001, d = 1.39). Achilles tendon thickness and CSA did not statistically differ between groups; however, the magnitude of between-group differences for these measures (MDSE: 0.18 [0.05] cm, MDSE: 0.27 [0.07] cm2, respectively) were moderate.

CONCLUSIONS

Our findings demonstrate previously unrecognized differences in Achilles tendon morphology between young athletes with clinically diagnosed Sever's disease and healthy controls. Our study supports incorporating diagnostic ultrasound as part of a comprehensive examination to ensure appropriate diagnosis and clinical management for adolescents with heel pain.

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.

Distal hamstrings tendons mechanical properties at rest and contraction using free-hand 3-D ultrasonography

Tendon properties impact human locomotion, influencing sports performance, and injury prevention. Hamstrings play a crucial role in sprinting, particularly the biceps femoris long head (BFlh), which is prone to frequent injuries. It remains uncertain if BFlh exhibits distinct mechanical properties compared to other hamstring muscles. This study utilized free-hand three-dimensional ultrasound to assess morphological and mechanical properties of distal hamstrings tendons in 15 men. Scans were taken in prone position, with hip and knee extended, at rest and during 20%, 40%, 60%, and 80% of maximal voluntary isometric contraction of the knee flexors. Tendon length, volume, cross-sectional area (CSA), and anteroposterior (AP) and mediolateral (ML) widths were quantified at three locations. Longitudinal and transverse deformations, stiffness, strain, and stress were estimated. The ST had the greatest tendon strain and the lowest stiffness as well as the highest CSA and AP and ML width strain compared to other tendons. Biceps femoris short head (BFsh) exhibited the least strain, AP and ML deformation. Further, BFlh displayed the highest stiffness and stress, and BFsh had the lowest stress. Additionally, deformation varied by region, with the proximal site showing generally the lowest CSA strain. Distal tendon mechanical properties differed among the hamstring muscles during isometric knee flexions. In contrast to other bi-articular hamstrings, the BFlh high stiffness and stress may result in greater energy absorption by its muscle fascicles, rather than the distal tendon, during late swing in sprinting. This could partly account for the increased incidence of hamstring injuries in this muscle.

The effect of plyometric training and moderating variables on stretch-shortening cycle function and physical qualities in female post peak height velocity volleyball players

Purpose: Although several studies investigated the effect of plyometric training on physical performance, there is a lack of clarity regarding the effectiveness of plyometric training or its moderator variables in youth female volleyball players. The primary aim of this study was to explore the effect of horizontal plyometric training on explosive stretch-shortening cycle hops and jumps in the vertical and horizontal directions in female post peak height velocity (PHV) volleyball players. The secondary aim was to assess the influence of participant and training related moderators on horizontal plyometric training in post-PHV volleyball players. Methods: A total of 23 post-PHV volleyball players participated in this 8-week intervention with horizontal plyometric exercises, twice a week. Pre-testing and post-testing included bilateral and unilateral vertical sub-maximal hopping, horizontal jumping and hopping, and a drop jump test. The effectiveness of the intervention was assessed using a paired t-test. The influence of internal moderators such as age, maturity and body mass and external moderators such as training volume were assessed using regression and correlation analysis. Results: An 8-week plyometric training improved sub-maximal hopping at 2.5 Hz left by 4.4%, bilateral sub-maximal hopping at 2.0 Hz by 9.5% and bilateral sub-maximal hopping at 2.2 Hz by 6.8% in post-PHV female volleyball players. Horizontal jumping and hopping, reactive strength index and other sub-maximal hopping conditions did not improve significantly. Body mass had a large moderating effect on vertical unilateral sub-maximal hopping at 2.5 Hz right (p = 0.010, η 2 = 0.314), vertical unilateral hopping at 3.0 Hz right (p = 0.035, η 2 = 0.170), and vertical unilateral hopping at 3.0 Hz left (p = 0.043, η 2 = 0.203). Training volume together with generalized joint hypermobility moderated right leg triple broad hop performance, whereas maturity and age did not moderate any variables. Conclusion: This study determined that 8 weeks of horizontal plyometric training can improve unilateral absolute leg stiffness in post-PHV female volleyball players, and this training effect can be moderated by body mass. Furthermore, the training effect on triple hopping performance on the right leg can be moderated by combined training volume with generalized joint hypermobility.

A Longitudinal Study of the Physical Characteristics, Muscle-Tendon Structure Properties, and Skeletal Age in Preadolescent Boys

OBJECTIVES

The purpose of this study was to examine longitudinal growth changes in physical characteristics, muscle-tendon structure properties, and skeletal age in preadolescent boys and to compare the relationship between the changes in physical characteristics and muscle-tendon properties and the changes in chronological and skeletal ages.

METHODS

Fourteen prepubescent boys (10.9 ± 1.1 years old at the onset of the study) participated in this study over two years (yearly). Maximal muscle strength and maximal strain of tendon structure during ramp isometric contraction and muscle and tendon thickness for knee extensors and plantar flexors were measured. In addition, skeletal age was assessed using Tanner-Whitehouse three method.

RESULTS

Changes in height, thigh length, and lower leg length were highly correlated with changes in skeletal age but not chronological age. However, changes in the morphological and mechanical properties of muscle and tendon structure were not significantly associated with changes in chronological and skeletal ages.

CONCLUSION

The present preliminary results suggest that longitudinal growth changes in the long-axis of the body are highly correlated with skeletal age change, whereas those in the muscle-tendon structure properties were not.

Biomechanics in the onset and severity of spondyloarthritis: a force to be reckoned with

Increasing evidence suggests that there is a pivotal role for physical force (mechanotransduction) in the initiation and/or the perpetuation of spondyloarthritis; the review contained herein examines that evidence. Furthermore, we know that damage and inflammation can limit spinal mobility, but is there a cycle created by altered spinal mobility leading to additional damage and inflammation?Over the past several years, mechanotransduction, the mechanism by which mechanical perturbation influences gene expression and cellular behaviour, has recently gained popularity because of emerging data from both animal models and human studies of the pathogenesis of ankylosing spondylitis (AS). In this review, we provide evidence towards an appreciation of the unsolved paradigm of how biomechanical forces may play a role in the initiation and propagation of AS.

Simulations suggest walking with reduced propulsive force would not mitigate the energetic consequences of lower tendon stiffness

Aging elicits numerous effects that impact both musculoskeletal structure and walking function. Tendon stiffness (kT) and push-off propulsive force (FP) both impact the metabolic cost of walking and are diminished by age, yet their interaction has not been studied. We combined experimental and computational approaches to investigate whether age-related changes in function (adopting smaller FP) may be adopted to mitigate the metabolic consequences arising from changes in structure (reduced kT). We recruited 12 young adults and asked them to walk on a force-sensing treadmill while prompting them to change FP (±20% & ±40% of typical) using targeted biofeedback. In models driven by experimental data from each of those conditions, we altered the kT of personalized musculoskeletal models across a physiological range (2-8% strain) and simulated individual-muscle metabolic costs for each kT and FP combination. We found that kT and FP independently affect walking metabolic cost, increasing with higher kT or as participants deviated from their typical FP. Our results show no evidence for an interaction between kT and FP in younger adults walking at fixed speeds. We also reveal complex individual muscle responses to the kT and FP landscape. For example, although total metabolic cost increased by 5% on average with combined reductions in kT and FP, the triceps surae muscles experienced a 7% local cost reduction on average. Our simulations suggest that reducing FP during walking would not mitigate the metabolic consequences of lower kT. Wearable devices and rehabilitative strategies can focus on either kT or FP to reduce age-related increases in walking metabolic cost.

Does biologically categorised training alter the perceived exertion and neuromuscular movement profile of academy soccer players compared to traditional age-group categorisation?

The individual response to load is multifactorial and complicated by transient temporal changes in biological maturation. The period surrounding peak height velocity exposes potentially "fragile" individuals to systematic, age-related increases in training loads. Bio-banding allows practitioners to manage the biological diversity and align training to the individual development needs . This study explores the acute impact of maturation on neuromuscular performance and perceived intensity through comparing both chronological and bio-banded training sessions. 55 male soccer players (mean ± SD; age 13.8 ± 1.4 years) were recruited from an EPPP academy. Following a warm-up and standardised sub-maximal run (30-15IFT), players competed in five bouts of 5-min 6v6 small-sided games (SSGs) before repeating the standardised sub-maximal run. The sessions were repeated on three occasions with chronological SSGs and the same with bio-banded SSGs wearing foot-mounted inertial measurement units (PlayerMakerTM) with differential ratings of perceived exertion used to quantify internal loads. Mixed linear modelling indicated maturity-specific pre-post differences in neuromuscular response, stride length and cadence having contrasting responses pre- (reduced) and post-PHV (increased), and larger changes in post sessions stiffness for pre- (∼18.6 kN·m-1) and circa-PHV (∼12.1 kN·m-1) players. Secondly, there were small to large differences in neuromuscular response (RSI, stride length, stiffness, and contact time) and perceptions of intensity between conditions, with bio-banding generally reducing pre-post changes. Bio-banding may therefore offer a mechanism to prescribe maturity-specific training loads which may help to alleviate the impact of repeated exposure to high-intensity activity, thus reducing injury risk whilst promoting long-term player development.Highlights Utilising a sub-maximal running protocol (30-15IFT) with foot mounted accelerometers can detect maturity specific responses to football specific training activity, which aligns with subjective perceptions of intensity.Chronologically derived small-sided games elicit different acute responses between players of varying maturity status, which is somewhat negated when bio-banded small-sided games are used instead.Bio-banding training sessions may offer practitioners a practical way of managing maturity-specific trainings load to reduce injury risk and promote long-term players development.

In Vivo Strain Patterns in the Achilles Tendon During Dynamic Activities: A Comprehensive Survey of the Literature

Achilles' tendon (AT) injuries such as ruptures and tendinopathies have experienced a dramatic rise in the mid- to older-aged population. Given that the AT plays a key role at all stages of locomotion, unsuccessful rehabilitation after injury often leads to long-term, deleterious health consequences. Understanding healthy in vivo strains as well as the complex muscle-tendon unit interactions will improve access to the underlying aetiology of injuries and how their functionality can be effectively restored post-injury. The goals of this survey of the literature with a systematic search were to provide a benchmark of healthy AT strains measured in vivo during functional activities and identify the sources of variability observed in the results. Two databases were searched, and all articles that provided measured in vivo peak strains or the change in strain with respect to time were included. In total, 107 articles that reported subjects over the age of 18 years with no prior AT injury and measured while performing functional activities such as voluntary contractions, walking, running, jumping, or jump landing were included in this review. In general, unclear anatomical definitions of the sub-tendon and aponeurosis structures have led to considerable confusion in the literature. MRI, ultrasound, and motion capture were the predominant approaches, sometimes coupled with modelling. The measured peak strains increased from 4% to over 10% from contractions, to walking, running, and jumping, in that order. Importantly, measured AT strains were heavily dependent on measurement location, measurement method, measurement protocol, individual AT geometry, and mechanical properties, as well as instantaneous kinematics and kinetics of the studied activity. Through a comprehensive review of approaches and results, this survey of the literature therefore converges to a united terminology of the structures and their common underlying characteristics and presents the state-of-knowledge on their functional strain patterns.

Height is a predictor of hamstring tendon length and ACL graft characteristics in adolescents

BACKGROUND

Knowing the potential hamstring tendon length is relevant for planning ligament reconstructions in children and adolescents, as it is not uncommon to encounter small hamstring tendons intraoperatively. The aim of this study is to predict semitendinosus and gracilis tendon length based on anthropometric values in children and adolescents. The secondary aim is to analyse hamstring tendon autograft characteristics in a closed socket anterior cruciate ligament reconstructions and to evaluate the relationship with anthropometric variables. The hypothesis of this study was that height is predictor of hamstring tendon length and thereby graft characteristics.

METHODS

This observational study included two cohorts of adolescents undergoing ligament reconstructions between 2007-2014 and 2017-2020. Age, sex, height and weight were recorded preoperatively. Semitendinosus and gracilis tendon length and graft characteristics were measured intraoperatively. Regression analysis was performed on tendon length and anthropometric values. Subgroup analyses of the closed socket ACL reconstruction were performed and the relation between anthropometric values and graft characteristics were analysed.

RESULTS

The population consisted of 171 adolescents from 13 to 17 years of age, with a median age of 16 years [IQR 16-17]. The median semitendinosus tendon length was 29 cm [IQR 26-30] and gracilis tendon length was 27 cm [IQR 25-29]. Height was a significant predictor of semitendinosus and gracilis tendon length. Subgroup analysis of the closed socket ACL reconstructions showed that in 75% of the procedure, the semitendinosus tendon alone was sufficient to create a graft with a minimum diameter of 8.0 mm.

CONCLUSIONS

Height is a significant predictor of semitendinosus and gracilis tendon length in adolescents between 13 and 17 years of age and outcomes are similar to data in adults. In 75% of the closed socket ACL reconstructions, the semitendinosus tendon alone is sufficient to create an adequate graft with a minimum diameter of 8 mm. Additional use of the gracilis tendon is more often necessary in females and shorter patients.

LEVEL OF EVIDENCE: 3

Adolescent boys who participate in sports exhibit similar ramp torque control with young men despite differences in strength and tendon characteristics

PURPOSE

The goal of this paper was to determine if sports participation influences torque control differently for adolescent boys and young men during a slow ramp task.

METHODS

Twenty-one adolescent boys (11 athletes) and 31 young men (16 athletes) performed a slow ramp increase in plantar flexion torque from 0 to maximum. We quantified torque control as the coefficient of variation (CV) of torque during the ramp and quantified the Achilles tendon mechanical properties using ultrasonography.

RESULTS

Relative to adolescent boys, young men were taller, heavier, stronger, and had a longer and stiffer Achilles tendon. However, these characteristics were not different between athletes and non-athletes in adolescent boys. For the CV of torque, there was a significant interaction with sports participation, indicating that only adolescent boys who were non-athletes had greater variability than young men. The CV of torque of all participants was predicted from the maximum torque and torque oscillations from 1 to 2 Hz, whereas the CV of torque for adolescent boys was predicted only from torque oscillations from 1 to 2 Hz.

CONCLUSION

These findings suggested that adolescent boys who participate in sports exhibited lower torque variability during a slow ramp task, which was not explained by differences in Achilles tendon properties or strength.

Tendon compliance and preload must be considered when determining the in vivo force-velocity relationship from the torque-angular velocity relation

In vivo, the force-velocity relation (F-v-r) is typically derived from the torque-angular velocity relation (T-ω-r), which is subject to two factors that may influence resulting measurements: tendon compliance and preload prior to contraction. The in vivo plantar flexors' T-ω-r was determined during preloaded maximum voluntary shortening contractions at 0-200°/s. Additionally, we used a two factor block simulation study design to independently analyze the effects of preload and tendon compliance on the resulting T-ω-r. Therefore, we replicated the in vivo experiment using a Hill-type muscle model of the gastrocnemius medialis. The simulation results matched a key pattern observed in our recorded in vivo experimental data: during preloaded contractions, torque output of the muscle was increased when compared with non-preloaded contractions from literature. This effect increased with increasing contraction velocity and can be explained by a rapidly recoiling tendon, allowing the contractile element to contract more slowly, thus developing higher forces compared with non-preloaded contractions. Our simulation results also indicate that a more compliant tendon results in increased ankle joint torques. The simulation and the experimental data clearly show that the deduction of the in vivo F-v-r from the T-ω-r is compromised due to the two factors preloading and tendon compliance.

Towards modern understanding of the Achilles tendon properties in human movement research

The Achilles tendon (AT) is the strongest tendon in humans, yet it often suffers from injury. The mechanical properties of the AT afford efficient movement, power amplification and power attenuation during locomotor tasks. The properties and the unique structure of the AT as a common tendon for three muscles have been studied frequently in humans using in vivo methods since 1990's. As a part of the celebration of 50 years history of the International Society of Biomechanics, this paper reviews the history of the AT research focusing on its mechanical properties in humans. The questions addressed are: What are the most important mechanical properties of the Achilles tendon, how are they studied, what is their significance to human movement, and how do they adapt? We foresee that the ongoing developments in experimental methods and modeling can provide ways to advance knowledge of the complex three-dimensional structure and properties of the Achilles tendon in vivo, and to enable monitoring of the loading and recovery for optimizing individual adaptations.

Longitudinal changing relationships between growth tempo and vertical stiffness in movement across maturation

PURPOSE

To determine whether average growth tempo influenced longitudinal relationships between maturity status and coordination capability using a 15-s hopping task (Aim 1). To investigate how differences in absolute growth tempo were associated with change in coordination capability within and across peak growth (Aim 2).

METHODS

Participants were N₁ = 110 (Aim 1) and N₂ = 71 (Aim 2) Australian male competitive swimmers, aged 10-15 years, exposed to repeated-measures tracking (2-years, and 12-months respectively) of maturity status, growth tempo and movement coordination capability. Coordination capability was estimated via vertical stiffness (K_V) in a hopping task, reflected by participant mean K_V and between-jump coefficient of variation (CV).

RESULTS

For Aim 1, log-linear mixed model trends identified maturity status and growth tempo were significantly associated with K_V mean and K_V CV. For a given maturity status, mean K_V was 9% lower in the 'High' average growth tempo group than the 'Low' average tempo group. For Aim 2, mixed repeated-measures analyses of variance identified how time points of increased growth tempo were significantly associated with 7-11% reductions in mean K_V, with similar mean K_V decrements irrespective of growth tempo group. Meanwhile, K_VCV only illustrated progressive longitudinal reductions.

CONCLUSIONS

Within maturational progression, short-term accelerations in growth tempo corresponded with short-term decreases in K_V mean, suggesting temporary disruptions to movement coordination capability. Measuring growth tempo and applying hopping tasks in specific movement contexts could help consistently identify disturbances in motor coordination.

Optimal Drop Height in Prepubertal Boys Is Revealed by the Performance in Squat Jump

Drop jump (DJ) performance gain with increasing drop height is well documented in adults, but there is still no clear evidence of such gain in children. This study aimed to examine the differences in DJ performance gain in male adults and prepubescent boys by comparing drop heights tailored to each individual's performance and expressed as a percentage of their squat jump (SJ) performance. Fifteen boys (9-11 y) and 15 men (19-27 y) executed DJs from drop heights that were set at 75%, 100%, 125%, and 150% of their best performance in SJ (DJ75, DJ100, DJ125, and DJ150, respectively). Vertical ground reaction force (vGRF), contact time and kinematics of the lower extremities were captured. The results showed that boys jumped significantly lower than adults in DJs, and both age groups presented jumping gain with increasing drop height, up to DJ125. Boys demonstrated longer total contact time, lower angular velocity and vGRF during the propulsive phase, as well as smaller knee flexion at touchdown and lower reactive strength index. vGRF in DJ75 and DJ100 was lower than in DJ125 and DJ150. The highest value for maximum knee flexion was also presented at DJ150. It is concluded that in prepubescent boys, the appropriate drop height for an effective DJ is linked to their performance in SJ and might be between 75% and 125% of their maximum SJ performance.

Relationship between Muscle-Tendon Stiffness and Drop Jump Performance in Young Male Basketball Players during Developmental Stages

BACKGROUND

The relationship between stiffness and drop jump performance in athletes in various stages of development has yet to be fully investigated. The first aim of this study was to investigate the association between the stiffness of the patellar and quadriceps tendon (PT, QT), gastrocnemius-Achilles tendon unit (GAT), and rectus femoris (RF) using drop jump (DJ) performance in young basketball players. The second aim was to investigate possible variations in the stiffness levels of those tissues in different developmental stages.

METHODS

The stiffness levels of the GAT, PT, QT, and RF were measured in both limbs in 73 male basketball players aged 12 to 18 years. The reactive strength index (RSI), contact time (CT) and jump height (JH) during 30 and 40 cm DJs were also measured.

RESULTS

Pearson correlation coefficients showed a significant association between DJ performance and PT, QT, GAT, and RF dynamic stiffness. Moreover, the youngest subjects were found to have lower stiffness values than the older ones.

CONCLUSIONS

Tissue stiffness can affect athletic performance by modifying the stretch-shortening cycle in young basketball players. Stiffness of muscles and tendons increases during the maturation process. Further investigations could shed light on the effect of training on the stiffness of muscles and tendons.

Personalisation of Plantarflexor Musculotendon Model Parameters in Children with Cerebral Palsy

Neuromusculoskeletal models can be used to evaluate aberrant muscle function in cerebral palsy (CP), for example by estimating muscle and joint contact forces during gait. However, to be accurate, models should include representative musculotendon parameters. We aimed to estimate personalised parameters that capture the mechanical behaviour of the plantarflexors in children with CP and typically developing (TD) children. Ankle angle (using motion capture), torque (using a load-cell), and medial gastrocnemius fascicle lengths (using ultrasound) were measured during slow passive ankle dorsiflexion rotation for thirteen children with spastic CP and thirteen TD children. Per subject, the measured rotation was input to a scaled OpenSim model to simulate the torque and fascicle length output. Musculotendon model parameters were personalised by the best match between simulated and experimental torque–angle and fascicle length-angle curves according to a least-squares fit. Personalised tendon slack lengths were significantly longer and optimal fibre lengths significantly shorter in CP than model defaults and than in TD. Personalised tendon compliance was substantially higher in both groups compared to the model default. The presented method to personalise musculotendon parameters will likely yield more accurate simulations of subject-specific muscle mechanics, to help us understand the effects of altered musculotendon properties in CP.

Mechanical, Material and Morphological Adaptations of Healthy Lower Limb Tendons to Mechanical Loading: A Systematic Review and Meta-Analysis

BACKGROUND

Exposure to increased mechanical loading during physical training can lead to increased tendon stiffness. However, the loading regimen that maximises tendon adaptation and the extent to which adaptation is driven by changes in tendon material properties or tendon geometry is not fully understood.

OBJECTIVE

To determine (1) the effect of mechanical loading on tendon stiffness, modulus and cross-sectional area (CSA); (2) whether adaptations in stiffness are driven primarily by changes in CSA or modulus; (3) the effect of training type and associated loading parameters (relative intensity; localised strain, load duration, load volume and contraction mode) on stiffness, modulus or CSA; and (4) whether the magnitude of adaptation in tendon properties differs between age groups.

METHODS

Five databases (PubMed, Scopus, CINAHL, SPORTDiscus, EMBASE) were searched for studies detailing load-induced adaptations in tendon morphological, material or mechanical properties. Standardised mean differences (SMDs) with 95% confidence intervals (CIs) were calculated and data were pooled using a random effects model to estimate variance. Meta regression was used to examine the moderating effects of changes in tendon CSA and modulus on tendon stiffness.

RESULTS

Sixty-one articles met the inclusion criteria. The total number of participants in the included studies was 763. The Achilles tendon (33 studies) and the patella tendon (24 studies) were the most commonly studied regions. Resistance training was the main type of intervention (49 studies). Mechanical loading produced moderate increases in stiffness (standardised mean difference (SMD) 0.74; 95% confidence interval (CI) 0.62-0.86), large increases in modulus (SMD 0.82; 95% CI 0.58-1.07), and small increases in CSA (SMD 0.22; 95% CI 0.12-0.33). Meta-regression revealed that the main moderator of increased stiffness was modulus. Resistance training interventions induced greater increases in modulus than other training types (SMD 0.90; 95% CI 0.65-1.15) and higher strain resistance training protocols induced greater increases in modulus (SMD 0.82; 95% CI 0.44-1.20; p = 0.009) and stiffness (SMD 1.04; 95% CI 0.65-1.43; p = 0.007) than low-strain protocols. The magnitude of stiffness and modulus differences were greater in adult participants.

CONCLUSIONS

Mechanical loading leads to positive adaptation in lower limb tendon stiffness, modulus and CSA. Studies to date indicate that the main mechanism of increased tendon stiffness due to physical training is increased tendon modulus, and that resistance training performed at high compared to low localised tendon strains is associated with the greatest positive tendon adaptation. PROSPERO registration no.: CRD42019141299.

Foundational Principles and Adaptation of the Healthy and Pathological Achilles Tendon in Response to Resistance Exercise: A Narrative Review and Clinical Implications

Therapeutic exercise is widely considered a first line fundamental treatment option for managing tendinopathies. As the Achilles tendon is critical for locomotion, chronic Achilles tendinopathy can have a substantial impact on an individual's ability to work and on their participation in physical activity or sport and overall quality of life. The recalcitrant nature of Achilles tendinopathy coupled with substantial variation in clinician-prescribed therapeutic exercises may contribute to suboptimal outcomes. Further, loading the Achilles tendon with sufficiently high loads to elicit positive tendon adaptation (and therefore promote symptom alleviation) is challenging, and few works have explored tissue loading optimization for individuals with tendinopathy. The mechanism of therapeutic benefit that exercise therapy exerts on Achilles tendinopathy is also a subject of ongoing debate. Resultingly, many factors that may contribute to an optimal therapeutic exercise protocol for Achilles tendinopathy are not well described. The aim of this narrative review is to explore the principles of tendon remodeling under resistance-based exercise in both healthy and pathologic tissues, and to review the biomechanical principles of Achilles tendon loading mechanics which may impact an optimized therapeutic exercise prescription for Achilles tendinopathy.

Characterizing Musculoskeletal Tissue Mechanics Based on Shear Wave Propagation: A Systematic Review of Current Methods and Reported Measurements

Developing methods for the non-invasive characterization of the mechanics of musculoskeletal tissues is an ongoing research focus in biomechanics. Often, these methods use the speed of shear wave propagation to characterize tissue mechanics (e.g., shear wave elastography and shear wave tensiometry). The primary purpose of this systematic review was to identify, compare, and contrast current methods for exciting and measuring shear wave propagation in musculoskeletal tissues. We conducted searches in the Web of Science, PubMed, and Scopus databases for studies published from January 1, 1900, to May 1, 2020. These searches targeted both shear wave excitation using acoustic pushes and mechanical taps, and shear wave speed measurement using ultrasound, magnetic resonance imaging, accelerometers, and laser Doppler vibrometers. Two reviewers independently screened and reviewed the articles, identifying 524 articles that met our search criteria. Regarding shear wave excitation, we found that acoustic pushes are useful for exciting shear waves through the thickness of the tissue of interest, and mechanical taps are useful for exciting shear waves in wearable applications. Regarding shear wave speed measurement, we found that ultrasound is used most broadly to measure shear waves due to its ability to study regional differences and target specific tissues of interest. The strengths of magnetic resonance imaging, accelerometers, and laser Doppler vibrometers make them advantageous to measure shear wave speeds for high-resolution shear wave imaging, wearable measurements, and non-contact ex vivo measurements, respectively. The advantages that each method offers for exciting and measuring shear waves indicate that a variety of systems can be assembled using currently available technologies to determine musculoskeletal tissue material behavior across a range of innovative applications.

An Identical Twin Study on Human Achilles Tendon Adaptation: Regular Recreational Exercise at Comparatively Low Intensities Can Increase Tendon Stiffness

Achilles tendon adaptation is a key aspect of exercise performance and injury risk prevention. However, much debate exists about the adaptation of the Achilles tendon in response to exercise activities. Most published research is currently limited to elite athletes and selected exercise activities. Also, existing studies on tendon adaptation do not control for genetic variation. Our explorative cross-sectional study investigated the effects of regular recreational exercise activities on Achilles tendon mechanical properties in 40 identical twin pairs. Using a handheld oscillation device to determine Achilles tendon mechanical properties, we found that the Achilles tendon appears to adapt to regular recreational exercise at comparatively low intensities by increasing its stiffness. Active twins showed a 28% greater Achilles tendon stiffness than their inactive twin (p < 0.05). Further, our research extends existing ideas on sport-specific adaptation by showing that tendon stiffness seemed to respond more to exercise activities that included an aerial phase such as running and jumping. Interestingly, the comparison of twin pairs revealed a high variation of Achilles tendon stiffness (305.4–889.8 N/m), and tendon adaptation was only revealed when we controlled for genetic variance. Those results offer new insights into the impact of genetic variation on individual Achilles tendon stiffness, which should be addressed more closely in future studies.

Triceps Surae Muscle-Tendon Properties as Determinants of the Metabolic Cost in Trained Long-Distance Runners

Purpose: This study aimed to determine whether triceps surae's muscle architecture and Achilles tendon parameters are related to running metabolic cost (C) in trained long-distance runners. Methods: Seventeen trained male recreational long-distance runners (mean age = 34 years) participated in this study. C was measured during submaximal steady-state running (5 min) at 12 and 16 km h-1 on a treadmill. Ultrasound was used to determine the gastrocnemius medialis (GM), gastrocnemius lateralis (GL), and soleus (SO) muscle architecture, including fascicle length (FL) and pennation angle (PA), and the Achilles tendon cross-sectional area (CSA), resting length and elongation as a function of plantar flexion torque during maximal voluntary plantar flexion. Achilles tendon mechanical (force, elongation, and stiffness) and material (stress, strain, and Young's modulus) properties were determined. Stepwise multiple linear regressions were used to determine the relationship between independent variables (tendon resting length, CSA, force, elongation, stiffness, stress, strain, Young's modulus, and FL and PA of triceps surae muscles) and C (J kg-1m-1) at 12 and 16 km h-1. Results: SO PA and Achilles tendon CSA were negatively associated with C (r 2 = 0.69; p < 0.001) at 12 km h-1, whereas SO PA was negatively and Achilles tendon stress was positively associated with C (r 2 = 0.63; p = 0.001) at 16 km h-1, respectively. Our results presented a small power, and the multiple linear regression's cause-effect relation was limited due to the low sample size. Conclusion: For a given muscle length, greater SO PA, probably related to short muscle fibers and to a large physiological cross-sectional area, may be beneficial to C. Larger Achilles tendon CSA may determine a better force distribution per tendon area, thereby reducing tendon stress and C at submaximal speeds (12 and 16 km h-1). Furthermore, Achilles tendon morphological and mechanical properties (CSA, stress, and Young's modulus) and triceps surae muscle architecture (GM PA, GM FL, SO PA, and SO FL) presented large correlations with C.

Simple models highlight differences in the walking biomechanics of young children and adults

Adults conserve metabolic energy during walking by minimizing the step-to-step transition work performed by the legs during double support and by utilizing spring-like mechanisms in their legs, but little is known as to whether children utilize these same mechanisms. To gain a better understanding, we studied how children (5-6 years) and adults modulate the mechanical and metabolic demands of walking at their preferred speed, across slow (75%), preferred (100%) and fast (125%) step frequencies. We quantified (1) the positive mass-specific work done by the trailing leg during step-to-step transitions and (2) the leg's spring-like behavior during single support. On average, children walked with a 36% greater net cost of transport (COT; J kg-1 m-1) than adults (P=0.03), yet both groups increased their net COT at varying step frequencies. After scaling for speed, children generated ∼2-fold less trailing limb positive scaled mechanical work during the step-to-step transition (P=0.02). Unlike adults, children did not modulate their trailing limb positive work to meet the demands of walking at 75% and 125% of their preferred step frequency. In single support, young children operated their stance limb with much greater compliance than adults ( versus 11.35; P=0.023). Our observations suggest that the mechanics of walking in children aged 5-6 years are fundamentally distinct from the mechanics of walking in adults and may help to explain a child's higher net COT. These insights have implications for the design of assistive devices for children and suggest that children cannot be simply treated as scaled-down versions of adults.

Sensitivity of the shear wave speed-stress relationship to soft tissue material properties and fiber alignment

The use of shear wave propagation to noninvasively measure material properties and loading in tendons and ligaments is a growing area of interest in biomechanics. Prior models and experiments suggest that shear wave speed primarily depends on the apparent shear modulus (i.e., shear modulus accounting for contributions from all constituents) at low loads, and then increases with axial stress when axially loaded. However, differences in the magnitudes of shear wave speeds between ligaments and tendons, which have different substructures, suggest that the tissue's composition and fiber alignment may also affect shear wave propagation. Accordingly, the objectives of this study were to (1) characterize changes in the apparent shear modulus induced by variations in constitutive properties and fiber alignment, and (2) determine the sensitivity of the shear wave speed-stress relationship to variations in constitutive properties and fiber alignment. To enable systematic variations of both constitutive properties and fiber alignment, we developed a finite element model that represented an isotropic ground matrix with an embedded fiber distribution. Using this model, we performed dynamic simulations of shear wave propagation at axial strains from 0% to 10%. We characterized the shear wave speed-stress relationship using a simple linear regression between shear wave speed squared and axial stress, which is based on an analytical relationship derived from a tensioned beam model. We found that predicted shear wave speeds were both in-range with shear wave speeds in previous in vivo and ex vivo studies, and strongly correlated with the axial stress (R² = 0.99). The slope of the squared shear wave speed-axial stress relationship was highly sensitive to changes in tissue density. Both the intercept of this relationship and the apparent shear modulus were sensitive to both the shear modulus of the ground matrix and the stiffness of the fibers' toe-region when the fibers were less well-aligned to the loading direction. We also determined that the tensioned beam model overpredicted the axial tissue stress with increasing load when the model had less well-aligned fibers. This indicates that the shear wave speed increases likely in response to a load-dependent increase in the apparent shear modulus. Our findings suggest that researchers may need to consider both the material and structural properties (i.e., fiber alignment) of tendon and ligament when measuring shear wave speeds in pathological tissues or tissues with less well-aligned fibers.

Plasticity of the gastrocnemius elastic system in response to decreased work and power demand during growth

Elastic energy storage and release can enhance performance that would otherwise be limited by the force-velocity constraints of muscle. Although functional influence of a biological spring depends on tuning between components of an elastic system (the muscle, spring-driven mass and lever system), we do not know whether elastic systems systematically adapt to functional demand. To test whether altering work and power generation during maturation alters the morphology of an elastic system, we prevented growing guinea fowl (Numida meleagris) from jumping. We compared the jump performance of our treatment group at maturity with that of controls and measured the morphology of the gastrocnemius elastic system. We found that restricted birds jumped with lower jump power and work, yet there were no significant between-group differences in the components of the elastic system. Further, subject-specific models revealed no difference in energy storage capacity between groups, though energy storage was most sensitive to variations in muscle properties (most significantly operating length and least dependent on tendon stiffness). We conclude that the gastrocnemius elastic system in the guinea fowl displays little to no plastic response to decreased demand during growth and hypothesize that neural plasticity may explain performance variation.

Stimuli for Adaptations in Muscle Length and the Length Range of Active Force Exertion-A Narrative Review

Treatment strategies and training regimens, which induce longitudinal muscle growth and increase the muscles’ length range of active force exertion, are important to improve muscle function and to reduce muscle strain injuries in clinical populations and in athletes with limited muscle extensibility. Animal studies have shown several specific loading strategies resulting in longitudinal muscle fiber growth by addition of sarcomeres in series. Currently, such strategies are also applied to humans in order to induce similar adaptations. However, there is no clear scientific evidence that specific strategies result in longitudinal growth of human muscles. Therefore, the question remains what triggers longitudinal muscle growth in humans. The aim of this review was to identify strategies that induce longitudinal human muscle growth. For this purpose, literature was reviewed and summarized with regard to the following topics: (1) Key determinants of typical muscle length and the length range of active force exertion; (2) Information on typical muscle growth and the effects of mechanical loading on growth and adaptation of muscle and tendinous tissues in healthy animals and humans; (3) The current knowledge and research gaps on the regulation of longitudinal muscle growth; and (4) Potential strategies to induce longitudinal muscle growth. The following potential strategies and important aspects that may positively affect longitudinal muscle growth were deduced: (1) Muscle length at which the loading is performed seems to be decisive, i.e., greater elongations after active or passive mechanical loading at long muscle length are expected; (2) Concentric, isometric and eccentric exercises may induce longitudinal muscle growth by stimulating different muscular adaptations (i.e., increases in fiber cross-sectional area and/or fiber length). Mechanical loading intensity also plays an important role. All three training strategies may increase tendon stiffness, but whether and how these changes may influence muscle growth remains to be elucidated. (3) The approach to combine stretching with activation seems promising (e.g., static stretching and electrical stimulation, loaded inter-set stretching) and warrants further research. Finally, our work shows the need for detailed investigation of the mechanisms of growth of pennate muscles, as those may longitudinally grow by both trophy and addition of sarcomeres in series.

Muscle architecture and passive lengthening properties of the gastrocnemius medialis and Achilles tendon in children who idiopathically toe-walk

Children who idiopathically toe-walk (ITW) habitually operate at greater plantarflexion angles and thus, at shorter muscle-tendon unit (MTU) lengths than typically developing (TD) children. Therefore, it is often assumed that habitual use of the gastrocnemius muscle in this way will cause remodelling of the muscle-tendon architecture compared to TD children. However, the gastrocnemius muscle architecture of children who ITW has never been measured. It is essential that we gain a better understanding of these muscle-tendon properties, to ensure that appropriate clinical interventions can be provided for these children. Five children who ITW (age 8 ± 2 years) and 14 TD children (age 10 ± 2 years) participated in this study. Ultrasound was combined with isokinetic dynamometry and surface electromyography, to measure muscle architecture at common positions and passive lengthening properties of the gastrocnemius muscle and tendon across full range of motion. Regardless of which common condition groups were compared under, both the absolute and normalised to MTU muscle belly and fascicle lengths were always longer, and the Achilles tendon length was always shorter in children who ITW than TD children (p < 0.05; large effect sizes). The passive lengthening properties of the muscle and tendon were not different between groups (p > 0.05); however, passive joint stiffness was greater in children who ITW at maximum dorsiflexion (p = 0.001) and at a joint moment common to all participants (p = 0.029). Consequently, the findings of this pilot study indicate a remodelling of the relative MTU that does not support the concept that children who ITW commonly experience muscle shortening. Therefore, greater consideration of the muscle and tendon properties are required when prescribing clinical interventions that aim to lengthen the MTU, and treatments may be better targeted at the Achilles tendon in children who ITW.

The patellar ligament: A comprehensive review

The patellar ligament (PL) is an epiphyseal ligament and is part of the extensor complex of the knee. The ligament has gained attention due to its clinical relevance to autograft and tendinopathy. A variety of anatomical variations of the PL such as aplasia, numerical variations, and vascularity are being reported recently by clinicians and anatomists. The aim of this literature was to review the available literature to provide a consensus regarding anatomic variations of the PL, neurovasculature surrounding the PL, histology of the PL, and various aspects of PL measurements with relevance to the surgical considerations and sex and age-related differences. A narrative review of the patellar ligament was performed by conducting a detailed literature search and review of relevant articles. A total of 90 articles on the patellar ligament were included and were categorized into studies based on anatomical variations, neurovasculature, morphometrics, microanatomy, sex and age-related difference, and ACL reconstruction. The anatomical variations and morphometrics of the PL were found to correlate with the frequency of strain injuries, tendinopathy, and efficacy of the PL autograft in anterior cruciate ligament reconstruction. The sex differences in PL measurements and the effect of estrogen on collagen synthesis explained a higher incidence of patellar tendinopathy in women. An awareness of its variations enables careful selection of surgical incisions, thereby avoiding complications related to nerve injury. Accurate knowledge of the PL microanatomy assists in understanding the mechanism of ligament degeneration, rupture, autograft harvesting, and ligamentization results.

Adolescent Running Biomechanics - Implications for Injury Prevention and Rehabilitation

Global participation in running continues to increase, especially amongst adolescents. Consequently, the number of running-related injuries (RRI) in adolescents is rising. Emerging evidence now suggests that overuse type injuries involving growing bone (e.g., bone stress injuries) and soft tissues (e.g., tendinopathies) predominate in adolescents that participate in running-related sports. Associations between running biomechanics and overuse injuries have been widely studied in adults, however, relatively little research has comparatively targeted running biomechanics in adolescents. Moreover, available literature on injury prevention and rehabilitation for adolescent runners is limited, and there is a tendency to generalize adult literature to adolescent populations despite pertinent considerations regarding growth-related changes unique to these athletes. This perspective article provides commentary and expert opinion surrounding the state of knowledge and future directions for research in adolescent running biomechanics, injury prevention and supplemental training.

Normative Achilles and patellar tendon shear wave speeds and loading patterns during walking in typically developing children

BACKGROUND

Motion analysis is commonly used to evaluate joint kinetics in children with cerebral palsy who exhibit gait disorders. However, one cannot readily infer muscle-tendon forces from joint kinetics. This study investigates the use of shear wave tensiometry to characterize Achilles and patellar tendon forces during gait.

RESEARCH QUESTION

How do Achilles and patellar tendon wave speed and loading modulate with walking speed in typically developing children?

METHODS

Twelve typically developing children (9-16 years old) walked on an instrumented treadmill with shear wave tensiometers over their Achilles (n = 11) and patellar (n = 9) tendons. Wave speeds were recorded at five leg length-normalized walking speeds (very slow to very fast). Achilles and patellar tendon moment arms were measured with synchronized ultrasound and motion capture. The tendon wave speed-load relationship was calibrated at the typical walking speed and used to estimate tendon loading at other walking speeds.

RESULTS

Characteristic Achilles and patellar tendon wave speed trajectories exhibited two peaks over a gait cycle. Peak Achilles tendon force closely aligned with peak ankle plantarflexor moment during pushoff, though force exhibited less modulation with walking speed. A second peak in late swing Achilles loading, which was not evident from the ankle moment, increased significantly with walking speed (p < 0.001). The two peaks in patellar tendon loading occurred at 12 ± 1% and 68 ± 6% of the gait cycle, matching the timing of peak knee extension moment in early stance and early swing. Both patellar tendon load peaks increased significantly with walking speed (p < 0.05).

SIGNIFICANCE

This is the first study to use shear wave tensiometry to characterize Achilles and patellar tendon loading during gait in children. These data could serve as a normative comparison when using tensiometry to identify abnormal tendon loading patterns in individuals who exhibit equinus and/or crouch gait.

Neonatal Spinal Cord Transection Decreases Hindlimb Weight-Bearing and Affects Formation of Achilles and Tail Tendons

Mechanical loading may be required for proper tendon formation. However, it is not well understood how tendon formation is impacted by the development of weight-bearing locomotor activity in the neonate. This study assessed tendon mechanical properties, and concomitant changes in weight-bearing locomotion, in neonatal rats subjected to a low thoracic spinal cord transection or a sham surgery at postnatal day (P)1. On P10, spontaneous locomotion was evaluated in spinal cord transected and sham controls to determine impacts on weight-bearing hindlimb movement. The mechanical properties of P10 Achilles tendons (ATs), as representative energy-storing, weight-bearing tendons, and tail tendons (TTs), as representative positional, non-weight-bearing tendons were evaluated. Non- and partial weight-bearing hindlimb activity decreased in spinal cord transected rats compared to sham controls. No spinal cord transected rats showed full weight-bearing locomotion. ATs from spinal cord transected rats had increased elastic modulus, while cross-sectional area trended lower compared to sham rats. TTs from spinal cord transected rats had higher stiffness and cross-sectional area. Collagen structure of ATs and TTs did not appear impacted by surgery condition, and no significant differences were detected in the collagen crimp pattern. Our findings suggest that mechanical loading from weight-bearing locomotor activity during development regulates neonatal AT lateral expansion and maintains tendon compliance, and that TTs may be differentially regulated. The onset and gradual increase of weight-bearing movement in the neonate may provide the mechanical loading needed to direct functional postnatal tendon formation.

Juvenile Spondyloarthritis: What More Do We Know About HLA-B27, Enthesitis, and New Bone Formation?

Juvenile spondyloarthritis (JSpA) refers to a diverse spectrum of immune-mediated inflammatory arthritides whose onset occurs in late childhood and adolescence. Like its adult counterpart, JSpA is typified by a strong association with human leukocyte antigen-B27 (HLA-B27) and potential axial involvement, while lacking rheumatoid factor (RF) and distinguishing autoantibodies. A characteristic manifestation of JSpA is enthesitis (inflammation of insertion sites of tendons, ligaments, joint capsules or fascia to bone), which is commonly accompanied by bone resorption and new bone formation at affected sites. In this Review, advances in the role of HLA-B27, enthesitis and its associated osteoproliferation in JSpA pathophysiology and treatment options will be discussed. A deeper appreciation of how these elements contribute to the JSpA disease mechanism will better inform diagnosis, prognosis and therapy, which in turn translates to an improved quality of life for patients.

The Influence of Growth, Maturation and Resistance Training on Muscle-Tendon and Neuromuscular Adaptations: A Narrative Review

The purpose of this article is to provide an overview of the growth, maturation and resistance training-related changes in muscle-tendon and neuromuscular mechanisms in youth, and the subsequent effect on performance. Sprinting, jumping, kicking, and throwing are common movements in sport that have been shown to develop naturally with age, with improvements in performance being attributed to growth and maturity-related changes in neuromuscular mechanisms. These changes include moderate to very large increases in muscle physiological cross-sectional area (CSA), muscle volume and thickness, tendon CSA and stiffness, fascicle length, muscle activation, pre-activation, stretch reflex control accompanied by large reductions in electro-mechanical delay and co-contraction. Furthermore, a limited number of training studies examining neuromuscular changes following four to 20 weeks of resistance training have reported trivial to moderate differences in tendon stiffness, muscle CSA, muscle thickness, and motor unit activation accompanied by reductions in electromechanical delay (EMD) in pre-pubertal children. However, the interaction of maturity- and training-related neuromuscular adaptions remains unclear. An understanding of how different neuromuscular mechanisms adapt in response to growth, maturation and training is important in order to optimise training responsiveness in youth populations. Additionally, the impact that these muscle-tendon and neuromuscular changes have on force producing capabilities underpinning performance is unclear.

Neuromuscular Fatigue After Long-Duration Adventure Racing in Adolescent Athletes

PURPOSE

To characterize the acute effects of a long-duration adventure race on knee extensor (KE) fatigue and the knee functional ratio in adolescent athletes.

METHODS

Twenty trained male adolescents (aged 14-17 y) performed an adventure race of 68.5 km. Maximal voluntary isometric contraction (MVIC) KE and knee flexor torques were measured before and immediately after the race. Central and peripheral components of neuromuscular fatigue were quantified from the maximal voluntary activation level and the doublet peak torque (Tw100), respectively. The peak eccentric knee flexor torque to concentric KE torque ratio was also measured to determine functional ratio.

RESULTS

The race completion time was 05:38 (00:20) hours. Significant reductions in MVICKE (-14.7%, P < .001) and MVICKF (-17.0%, P < .01) were observed after the race. Voluntary activation level decreased by 8.3% (P < .001) while Tw100 remained unchanged. Peak eccentric knee flexor torque decreased 16.0% (P < .001) while peak concentric KE torque did not change. This resulted in a significant reduction in functional ratio (-12.0%, P < .01).

CONCLUSION

The adventure race induced a moderate fatigue, which was mainly explained by central factors without significant peripheral fatigue. However, particular attention should be paid to the knee muscular imbalance incurred by the race, which could increase the risk of ligament injury in adolescent athletes.

Explaining Variability in the Prevalence of Achilles Tendon Abnormalities: A Systematic Review With Meta-analysis of Imaging Studies in Asymptomatic Individuals

OBJECTIVE

To estimate the prevalence of, and factors associated with, Achilles tendon abnormalities observed on imaging in asymptomatic individuals.

DESIGN

Systematic review with stratified meta-analysis and meta-regression.

LITERATURE SEARCH

Embase, Scopus, MEDLINE, CINAHL, SPORTDiscus, and Web of Science were searched from 1980 to August 2020.

STUDY SELECTION CRITERIA

We included studies that reported the prevalence of Achilles tendon abnormalities, observed with any imaging modality, in an asymptomatic population. We excluded studies if participant mean age was younger than 12 years or if participants had current/previous lower-limb tendon injuries/symptoms or other systemic conditions.

DATA SYNTHESIS

Random-effects proportion meta-analysis was used to estimate prevalence. We used meta-regression for continuous variables (mean age and body mass index [BMI], sample size, proportion of female participants) and stratified categorical variables (imaging modality and participation in physical activity) to explain between-study heterogeneity.

RESULTS

We included 91 studies (10 156 limbs, 5841 participants). The prevalence of Achilles tendon abnormalities on imaging ranged from 0% to 80% per participant. Between-study heterogeneity was high (I²>90%, P<.001), precluding data pooling. Between-study heterogeneity was partly explained by participant mean BMI (slope, 2.8% per 1-unit increase in BMI; 95% confidence interval: 0.57%, 5.03%; P = .015) and participation in physical activity per limb, and mean age of 40 years old or older (P = .022) per participant.

CONCLUSION

There was substantial variability in the prevalence of Achilles tendon abnormalities on imaging in asymptomatic individuals. Higher prevalence of abnormalities was associated with older age (40 years old or older), higher BMI, and participation in physical activity. A large proportion of heterogeneity remains unaccounted for, likely due to variations in abnormality definitions and study design. J Orthop Sports Phys Ther 2021;51(5):232-252. Epub 28 Mar 2021. doi:10.2519/jospt.2021.9970.

The combined effects of obesity and ageing on skeletal muscle function and tendon properties in vivo in men

PURPOSE

We investigated the combined impact of ageing and obesity on Achilles tendon (AT) properties in vivo in men, utilizing three classification methods of obesity.

METHOD

Forty healthy, untrained men were categorised by age (young (18-49 years); older (50-80 years)), body mass index (BMI; normal weight (≥18.5-<25); overweight (≥25-<30); obese (≥30)), body fat% (normal adipose (<28%); high adiposity (≥28%)) and fat mass index (FMI; normal (3-6); excess fat (>6-9); high fat (>9). Assessment of body composition used dual-energy X-ray absorptiometry, gastrocnemius medialis (GM)/AT properties used dynamometry and ultrasonography and endocrine profiling used multiplex luminometry.

RESULTS

Older men had lower total range of motion (ROM; -11%; P = 0.020), GM AT force (-29%; P < 0.001), stiffness (-18%; P = 0.041), Young's modulus (-22%; P = 0.011) and AT stress (-28%; P < 0.001). All three methods of classifying obesity revealed obesity to be associated with lower total ROM (P = 0.014-0.039). AT cross sectional area (CSA) was larger with higher BMI (P = 0.030). However, after controlling for age, higher BMI only tended to be associated with greater tendon stiffness (P = 0.074). Interestingly, both AT CSA and stiffness were positively correlated with body mass (r = 0.644 and r = 0.520) and BMI (r = 0.541 and r = 0.493) in the young but not older adults. Finally, negative relationships were observed between AT CSA and pro-inflammatory cytokines TNF-α, IL-6 and IL-1β.

CONCLUSIONS

This is the first study to provide evidence of positive adaptations in tendon stiffness and size in vivo resulting from increased mass and BMI in young but not older men, irrespective of obesity classification.

The differences in viscoelastic properties of subtendons result from the anatomical tripartite structure of human Achilles tendon - ex vivo experimental study and modeling

The human Achilles tendon (AT) is a hierarchical structure macroscopically composed of three subtendons originating from the soleus (SOL) and gastrocnemius (GL, GM) muscles. According to recent reports, the divisible structure of the AT together with diverse material properties of its subtendons are suspected as a probable cause of non-homogeneous stress and strain distribution occurring in loaded AT. Despite numerous investigations on human AT, there is still relatively little knowledge regarding mechanical properties of subtendon-level hierarchy, which is crucial in fully understanding the multiscale relationship which governs tendon mechanics. In this paper we present the first ex vivo study conducted on SOL, GL, and GM subtendons of human AT. We investigate differences in viscoelastic properties of SOL, GM, and GL subtendons in terms of tensile modulus, mechanical hysteresis as well as stress relaxation observed at two different values of strain. Our results show that the most significant differences in mechanical properties exist between subtendon attached to the soleus muscle (SOL) and subtendons originating from the two heads of the gastrocnemius muscle (GM and GL). We used our experimental results to calibrate three different constitutive models: the hyperelastic Yeoh model with power-law flow, the microstructurally motivated Holzapfel-Gasser-Ogden model enhanced with strain-dependent Berström-Boyce flow and the phenomenological elasto-viscoplastic Arruda-Boyce-based model with strain-dependent Berström-Boyce flow supplemented with component representing matrix response. All calibrated models may be applied to commercial FEA software as a sufficient solution for rapid mechanical response modeling of human AT subtendons or for the purpose of future development of comprehensive patient-specific models of human lower limbs. STATEMENT OF SIGNIFICANCE: The divisible structure of the Achilles tendon together with diverse material properties of its subtendons are suspected as a probable cause of non-homogeneous stress and strain distribution occurring in loaded Achilles tendon. Despite numerous investigations on mechanical properties of Achilles tendon, there is still relatively little knowledge regarding mechanical properties of subtendon-level hierarchy, which is crucial in fully understanding the multiscale relationship which governs tendon mechanics. This study is the first reported ex vivo investigation conducted on SOL, GL, and GM human Achilles subtendons. We investigate differences in the viscoelastic properties of individual subtendons and demonstrate that the observed differences should be considered as muscle-dependent. Our experimental research is supported with a modeling study in which we calibrate three different constitutive models.

The Benefits of Strength Training on Musculoskeletal System Health: Practical Applications for Interdisciplinary Care

Global health organizations have provided recommendations regarding exercise for the general population. Strength training has been included in several position statements due to its multi-systemic benefits. In this narrative review, we examine the available literature, first explaining how specific mechanical loading is converted into positive cellular responses. Secondly, benefits related to specific musculoskeletal tissues are discussed, with practical applications and training programmes clearly outlined for both common musculoskeletal disorders and primary prevention strategies.

Motor Control and Achilles Tendon Adaptation in Adolescence: Effects of Sport Participation and Maturity

An important but unresolved research question in adolescent children is the following: “Does sport participation interact with maturation to change motor control and the mechanical and morphological properties of tendons?” Here, we address this important research question with a longitudinal study around the age of peak height velocity (PHV). Our purpose was to characterize the interactive effects of maturation and sports participation on motor control and the mechanical and morphological properties of the Achilles tendon (AT) in adolescent athletes and non-athletes. Twenty-two adolescent athletes (13.1 ± 1.1 years) and 19 adolescent non-athletes (12.8 ± 1.1 years) volunteered for this study. We quantified motor control as the coefficient of variation of torque during a ramp task. In addition, we quantified the AT morphological and mechanical properties using ultrasonography from 18 months before to 12 months after PHV. We found that motor control improved with maturation in both athletes and non-athletes. We found that athletes have a greater increase in body mass with maturation that relates to greater plantarflexion peak force and AT peak stress. Also, athletes have a thicker and longer AT, as assessed with resting cross-sectional area and length. Although the rate of increase in the morphological change with maturation was similar for athletes and non-athletes, the rate of increase in normalized AT stiffness was greater for athletes. This increased AT stiffness in athletes related to peak force and stress. In summary, maturation improves motor control in adolescent children. Further, we provide novel longitudinal evidence that sport participation interacts with maturation in adolescents to induce adaptive effects on the Achilles tendon morphology and mechanical properties. These findings have the potential to minimize the risk of injuries and maximize athletic development in talented adolescents.

The influence of animal species, gender and tissue on the structural, biophysical, biochemical and biological properties of collagen sponges

Although collagen type I is extensively used in biomedicine, no study to-date has assessed how the properties of the produced scaffolds are affected as a function of species, gender and tissue from which the collagen was extracted. Herein, we extracted and characterised collagen from porcine and bovine, male and female and skin and tendon tissues and we subsequently fabricated and assessed the structural, biophysical, biochemical and biological properties of collagen sponges. All collagen preparations were of similar purity and free-amine content (p > 0.05). In general, the porcine groups yielded more collagen; had higher (p < 0.05) denaturation temperature and resistance to enzymatic degradation; and lower (p < 0.05) swelling ratio and compression stress and modulus than the bovine groups of the same gender and tissue. All collagen preparations supported growth of human dermal fibroblasts and exhibited similar biological response to human THP-1 monocytes. These results further illustrate the need for standardisation of collagen preparations for the development of reproducible collagen-based devices. Assessment of the physicochemical and biological properties of collagen sponges as a function of animal species (bovine versus porcine), gender (male versus female) and tissue (skin versus tendon).

Nano-elastic modulus of tendon measured directly in living mice

The nano-biomechanical environment of the extracellular matrix is critical for cells to sense and respond to mechanical loading. However, to date, this important characteristic remains poorly understood in living tissue structures. This study reports the experimental measurement of the in vivo nano-elastic modulus of the tendon in a mouse tail model. The experiment was performed on the tail tendon of an 8-week-old C57BL/6 live mouse. Mechanical loading on tail tendons was regulated by changing both voltage and frequency of alternating current stimulation on the erector spinae. The nano-elastic modulus of the tail tendon was measured by atomic force microscope. The nano-elastic modulus showed significant variation (2.19-35.70 MPa) between different locations and up to 39% decrease under muscle contraction, suggesting a complicated biomechanical environment in which cells dwell. In addition, the nano-elastic modulus of the tail tendon measured in live mice was significantly lower than that measured in vitro, suggesting a disagreement of tissue mechanical properties in vivo and in vitro. This information is important for the designs of new extracellular biomaterial that can better mimic the biological environment, and improve clinical outcomes of musculoskeletal tissue degenerations and associated disorders.

Groin Injuries in Soccer: Investigating the Effect of Age on Adductor Muscle Forces

PURPOSE

The sudden rise in the injury incidence during adolescence is also evident in soccer-related injuries to the groin. Submaximal passing applies high stress on the adductor muscles and pubic symphysis and is therefore likely to be connected to the occurrence of groin injuries. Therefore, the purpose of the study was to compare hip joint kinematics and adductor muscle forces of different adolescent age groups during submaximal soccer passing.

METHODS

Sixty participants, in four groups, younger than 12, 15, 16. and 23 yr (U12, U15, U16, U23), were analyzed. A Footbonaut, equipped with a 3D motion capture system consisting of 16 cameras, was used to capture kinematic data of short passes. Inverse dynamic analysis was performed to calculate muscle forces of 10 passes of each subject.

RESULTS

The U15 group showed reduced angular velocities. A rise in hip adductor muscle forces was evident from the youngest group up to the oldest groups. The largest increase (49%) was found between U12 and U15. Lower-limb mass was identified as the best predictor for the increasing adductor force.

CONCLUSIONS

The reduced angular velocities of the U15 and the increase in muscle forces between all age groups were attributed to the increasing segment masses and length. This increases the moments of inertia of the leg segments thereby demanding higher forces to accelerate the segments. Most likely, the stress put upon the adductors apophyses increases during adolescence, as tendons are known to adapt slower than muscles, increasing the risk for overuse injuries. Coaches could use lower-limb mass as an indicator for fast increases in the force demand to identify players who would benefit from a reduced training volume.

The influence of a multidisciplinary intervention program on Achilles tendon structure in children with overweight and obesity

The effects of childhood obesity on tendon structure and its relation to physical activity are barely known. We aimed to investigate the influence of a 6-month multidisciplinary childhood obesity management program (which included dietary intervention and an exercise program) on Achilles tendon structure in overweight/obese children. Twenty-five overweight/obese children (overweight group) who participated in a 6-month multidisciplinary childhood obesity program, and 27 normal-weight controls were examined for anthropometric measurements and for Achilles tendon structure (using ultrasound tissue characterization to capture a three-dimensional structure of four echo type fibers and the cross-sectional area) at baseline, 3, and 6 months. The BMI of the overweight group significantly decreased from the pre- to the post-intervention period, (p = .002, η² = .229), yet among the control group, the BMI significantly increased (p = .002, η² = .222). Interactions (pre-post × group) showed a significant decrease in the prevalence of echo type I and in echo type III fibers and a significant increase in the prevalence of echo type II fibers and in the cross-sectional area along the intervention. No significant changes in echo type fibers were found in the control group along the 6 months.Conclusion: At pre-intervention, the overweight group had significantly "worse" tendon structure, with a lower prevalence of echo type II fiber and a higher prevalence of echo type III fibers compared with the control group. Following the 6-month intervention, the tendon structure of the overweight group was "positively" changed, with reduced echo type III fibers and increased echo type II fibers. What is Known: • Physical activity and weight reduction programs are efficient for obese children. • Obese children tend to suffer orthopedic problems and pain during physical activity. What is New: • Pre-intervention, obese children had 'worse' tendon structure compared to controls. • Following 6-month program, tendon structure of obese children 'positively' changed.

Evaluation of patellar tendinopathy using the single leg decline squat test: Is pain location important?

STUDY DESIGN

A cross-sectional study of non-elite volleyball players aged 13-17years.

OBJECTIVES

To evaluate the presence and location of pain during the single leg decline squat (SLDS) and compare patellar tendon thickness, structure, neovascularisation and symptom severity between SLDS-derived groups.

METHODS

32 male and 25 female participants attending a 5-day volleyball training camp underwent clinical evaluation by SLDS, describing the location of pain during this test using a pain map. The patellar tendon was examined using ultrasound imaging, performed by an assessor blinded to other assessments. Differences between participants experiencing local patellar tendon pain (PTP), other knee pain (OKP) or no-pain during the SLDS were evaluated.

RESULTS

Fifteen (26.3%) participants experienced pain during the SLDS. Local PTP was recorded for 12.3% and OKP for 10.5% of right legs. The PTP group was distinguished from the other groups by larger thickness and cross-sectional area of the mid-patellar tendon (p < 0.001), more frequent neovascularisation (p = 0.005) and greater pain and disability (p < 0.036). No differences between OKP and no-pain groups was observed.

CONCLUSION

Adolescent non-elite volleyball players reported symptoms indicative of patellar tendinopathy. In this cohort, the SLDS test combined with a pain map was associated with imaging and questionnaire-based outcomes.

LEVEL OF EVIDENCE

Diagnosis, Level 2; Cross-sectional study.

Altering the Mechanical Load Environment During Growth Does Not Affect Adult Achilles Tendon Properties in an Avian Bipedal Model

Tendon mechanical properties respond to altered load in adults, but how load history during growth affects adult tendon properties remains unclear. To address this question, we adopted an avian model in which we altered the mechanical load environment across the growth span. Animals were divided at 2 weeks of age into three groups: (1) an exercise control group given the opportunity to perform high-acceleration movements (EXE, n = 8); (2) a sedentary group restricted from high-intensity exercise (RES, n = 8); and (3) a sedentary group also restricted from high-intensity exercise and in which the gastrocnemius muscles were partially paralyzed using repeated bouts of botulinum toxin-A injections (RES-BTX, n = 8). Video analysis of bird movement confirmed the restrictions eliminated high-intensity exercise and did not alter time spent walking and sitting between groups. At skeletal maturity (33-35 weeks) animals were sacrificed for analysis, consisting of high-field MRI and material load testing, of both the entire free Achilles tendon and the tendon at the bone-tendon junction. Free tendon stiffness, modulus, and hysteresis were unaffected by variation in load environment. Further, the bone-tendon junction cross-sectional area, stress, and strain were also unaffected by variations in load environment. These results suggest that: (a) a baseline level of low-intensity activity (standing and walking) may be sufficient to maintain tendon growth; and (b) if this lower threshold of tendon load is met, non-mechanical mediated tendon growth may override the load-induced mechanotransduction signal attributed to tendon remodeling in adults of the same species. These results are important for understanding of musculoskeletal function and tendon health in growing individuals.

Muscle and Tendon Morphology in Early-Adolescent Athletes and Untrained Peers

Adolescent athletes can feature significantly greater muscle strength and tendon stiffness compared to untrained peers. However, to date, it is widely unclear if radial muscle and tendon hypertrophy may contribute to loading-induced adaptation at this stage of maturation. The present study compares the morphology of the vastus lateralis (VL) and the patellar tendon between early-adolescent athletes and untrained peers. In 14 male elite athletes (A) and 10 untrained controls (UC; 12–14 years of age), the VL was reconstructed from full muscle segmentations of magnetic resonance imaging (MRI) sequences and ultrasound imaging was used to measure VL fascicle length and pennation angle. The physiological cross-sectional area (PCSA) of the VL was calculated by dividing muscle volume by fascicle length. The cross-sectional area (CSA) of the patellar tendon was measured over its length based on MRI segmentations as well. Considering body mass as covariate in the analysis, there were no significant differences between groups considering the VL anatomical cross-sectional area (ACSA) over its length or maximum ACSA (UC: 24.0 ± 8.3 cm², A: 28.1 ± 5.3 cm², p > 0.05), yet athletes had significantly greater VL volume (UC: 440 ± 147 cm³, A: 589 ± 121 cm³), PCSA (UC: 31 ± 9 cm², A: 46 ± 9 cm²), pennation angle (UC: 8.2 ± 1.4°, A: 10.1 ± 1.3°), and average patellar tendon CSA (UC: 1.01 ± 0.18 cm², A: 1.21 ± 0.18 cm²) compared to the untrained peers (p < 0.05). However, the ratio of average tendon CSA to VL PCSA was significantly lower in athletes (UC: 3.4 ± 0.1%, A: 2.7 ± 0.5%; p < 0.05). When inferring effects of athletic training based on the observed differences between groups, these results suggest that both muscle and tendon of the knee extensors respond to athletic training with radial growth. However, the effect seems to be stronger in the muscle compared to the tendon, with an increase of pennation angle contributing to the marked increase of muscle PCSA. A disproportionate response to athletic training might be associated with imbalances of muscle strength and tendon stiffness and could have implications for the disposition towards tendon overuse injury.

The effects of growth on structural properties of the Achilles and Patellar tendons: A cross-sectional study

The purpose of this study was to investigate the structural properties (length and cross-sectional area) of both the Patellar and Achilles tendons at around adolescent growth spurt. One hundred-twenty children and adolescents participated in this study. Based on estimated age at peak height velocity, the participants were separated into three groups (before takeoff of adolescent growth spurt group, from takeoff of adolescent growth spurt until peak height velocity group and after peak height velocity group). An ultrasonography technique was used to determine structural properties of the Patellar and Achilles tendons. Significant group difference was observed in tendon length for the Patellar and Achilles tendons among groups. However, there were no significant differences in the ratio of the Patellar tendon to upper leg length and the ratio of the Achilles tendon to lower leg length among groups. The cross-sectional area of all regions for the Patellar and Achilles tendons in adolescents with after takeoff adolescent growth spurt group was greater than those of before takeoff adolescent growth spurt group. These results indicate that the cross-sectional area of both the Patellar and Achilles tendons increase with takeoff of adolescent growth spurt and tendons lengthen without the changes in the ratio of tendon length to bone length. In addition, the increases in the cross-sectional area of both the Patellar and Achilles tendons occur in whole regions but not specific regions.