Age-related differences in Achilles tendon properties and triceps surae muscle architecture in vivo

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.

Muscle and tendon stiffness of lower extremity in older adults with fall history: Stiffness effect on physical performance and fall risk

Changes in muscle and tendon stiffness may lead to falls in older adults by affecting joint stability and muscle function. This study aims to investigate the changes in stiffness in lower extremity muscles and tendons in the older adults with a fall history. A cross-sectional research design was followed. 25 older adults with a fall history and 26 older adults without fall history were recruited study. Stiffness of the lower extremity muscles and tendons was measured using a MyotonPRO device. Balance and functional ability of the participants were evaluated. The stiffness of all the selected muscles and tendon was lower in the older adults with a history of fall compared to controls (p<0.05). The obtained results suggest the decrease in the stiffness of the lower extremity muscles and tendon may negatively affect muscle function and joint stability/ control, and it may increase the predisposition to falling in older adults.

Experiment-guided tuning of muscle-tendon parameters to estimate muscle fiber lengths and passive forces

The workflow to simulate motion with recorded data usually starts with selecting a generic musculoskeletal model and scaling it to represent subject-specific characteristics. Simulating muscle dynamics with muscle-tendon parameters computed from existing scaling methods in literature, however, yields some inconsistencies compared to measurable outcomes. For instance, simulating fiber lengths and muscle excitations during walking with linearly scaled parameters does not resemble established patterns in the literature. This study presents a tool that leverages reported in vivo experimental observations to tune muscle-tendon parameters and evaluates their influence in estimating muscle excitations and metabolic costs during walking. From a scaled generic musculoskeletal model, we tuned optimal fiber length, tendon slack length, and tendon stiffness to match reported fiber lengths from ultrasound imaging and muscle passive force-length relationships to match reported in vivo joint moment-angle relationships. With tuned parameters, muscle contracted more isometrically, and soleus's operating range was better estimated than with linearly scaled parameters. Also, with tuned parameters, on/off timing of nearly all muscles' excitations in the model agreed with reported electromyographic signals, and metabolic rate trajectories varied significantly throughout the gait cycle compared to linearly scaled parameters. Our tool, freely available online, can customize muscle-tendon parameters easily and be adapted to incorporate more experimental data.

Open and Percutaneous Approaches Have Similar Biomechanical Results for Primary Midsubstance Achilles Tendon Repair: A Meta-analysis

Purpose

To evaluate the biomechanical properties of open versus percutaneous Achilles tendon repair.

Methods

A systematic review of original research articles was performed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. To qualify for study inclusion, articles were required to be published in English, use a laboratory design using either human or animal tissue, and directly compare the biomechanical properties of open Achilles repair using a Krackow or Kessler technique with percutaneous repair using either a locking or nonlocking suture construct. The biomechanical outcomes evaluated were displacement (millimeters) and load to failure (Newtons).

Results

Twelve studies met inclusion criteria, including 234 specimens (open: 97, percutaneous locking: 73; percutaneous nonlocking: 64) that underwent primary midsubstance Achilles tendon repair. Pooled analysis demonstrated no statistically significant difference in displacement (P = .240) or load to failure (P = .912) between the open and percutaneous techniques. Among the percutaneous approaches, there was no difference in displacement (P = .109) between the locking and nonlocking tendon repair systems.

Conclusions

The results of this study suggest that both open and percutaneous techniques are biomechanically viable approaches for primary midsubstance Achilles tendon repair.

Clinical Relevance

In clinical studies, similar rerupture rates have been observed after open or percutaneous Achilles tendon repair. It may be beneficial for surgeons to understand whether biomechanical differences exist between these repair techniques.

EMG-Constrained and Ultrasound-Informed Muscle-Tendon Parameter Estimation in Post-Stroke Hemiparesis

Secondary morphological and mechanical property changes in the muscle-tendon unit at the ankle joint are often observed in post-stroke individuals. These changes may alter the force generation capacity and affect daily activities such as locomotion. This work aimed to estimate subject-specific muscle-tendon parameters in individuals after stroke by solving the muscle redundancy problem using direct collocation optimal control methods based on experimental electromyography (EMG) signals and measured muscle fiber length. Subject-specific muscle-tendon parameters of the gastrocnemius, soleus, and tibialis anterior were estimated in seven post-stroke individuals and seven healthy controls. We found that the maximum isometric force, tendon stiffness and optimal fiber length in the post-stroke group were considerably lower than in the control group. We also computed the root mean square error between estimated and experimental values of muscle excitation and fiber length. The musculoskeletal model with estimated subject-specific muscle tendon parameters (from the muscle redundancy solver), yielded better muscle excitation and fiber length estimations than did scaled generic parameters. Our findings also showed that the muscle redundancy solver can estimate muscle-tendon parameters that produce force behavior in better accordance with the experimentally-measured value. These muscle-tendon parameters in the post-stroke individuals were physiologically meaningful and may shed light on treatment and/or rehabilitation planning.

Aging Does Not Alter Ankle, Muscle, and Tendon Stiffness at Low Loads Relevant to Stance

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, there has been limited investigation into age-dependent changes in muscle stiffness. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads relevant to standing and the stance phase of walking, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p > 0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p = 0.008), muscle (37%; p = 0.02), and tendon stiffness (22%; p = 0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p = 0.004). Together, these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort.

BiLSTM-Based Joint Torque Prediction From Mechanomyogram During Isometric Contractions: A Proof of Concept Study

Quantifying muscle strength is an important measure in clinical settings; however, there is a lack of practical tools that can be deployed for routine assessment. The purpose of this study is to propose a deep learning model for ankle plantar flexion torque prediction from time-series mechanomyogram (MMG) signals recorded during isometric contractions (i.e., a similar form to manual muscle testing procedure in clinical practice) and to evaluate its performance. Four different deep learning models in terms of model architecture (based on a stacked bidirectional long short-term memory and dense layers) were designed with different combinations of the number of units (from 32 to 512) and dropout ratio (from 0.0 to 0.8), and then evaluated for prediction performance by conducting the leave-one-subject-out cross-validation method from the 10-subject dataset. As a result, the models explained more variance in the untrained test dataset as the error metrics (e.g., root-mean-square error) decreased and as the slope of the relationship between the measured and predicted joint torques became closer to 1.0. Although the slope estimates appear to be sensitive to an individual dataset, >70% of the variance in nine out of 10 datasets was explained by the optimal model. These results demonstrated the feasibility of the proposed model as a potential tool to quantify average joint torque during a sustained isometric contraction.

Structure and function of Achilles and patellar tendons following moderate slow resistance training in young and old men

The aim of this study was to investigate the effect of aging and resistance training with a moderate load on the size and mechanical properties of the patellar (PT) and Achilles tendon (AT) and their associated aponeuroses; medial gastrocnemius (MG) and vastus lateralis (VL). Young (Y55; 24.8 ± 3.8 yrs, n = 11) and old men (O55; 70.0 ± 4.6 yrs, n = 13) were assigned to undergo a training program (12 weeks; 3 times/week) of moderate slow resistance training [55% of one repetition maximum (RM)] of the triceps surae and quadriceps muscles. Tendon dimensions were assessed using 1.5 T magnetic resonance imaging before and after 12 weeks. AT and PT cross sectional area (CSA) were determined every 10% of tendon length. Mechanical properties of the free AT, MG aponeurosis, PT, and VL aponeurosis were assessed using ultrasonography (deformation) and tendon force measurements. CSA of the AT but not PT was greater in O55 compared with Y55. At baseline, mechanical properties were generally lower in O55 than Y55 for AT, MG aponeurosis and VL aponeurosis (Young's modulus) but not for PT. CSA of the AT and PT increased equally in both groups following training. Further, for a given force, stiffness and Young's modulus also increased equally for VL aponeurosis and AT, for boths groups. The present study highlights that except for the PT, older men have lower tendon (AT, MG aponeurosis, and VL aponeurosis) mechanical properties than young men and 12-weeks of moderate slow resistance training appears sufficient to improve tendon size and mechanical adaptations in both young and older men. New and Noteworthy: These novel findings suggest that short-term moderate slow resistance training induces equal improvements in tendon size and mechanics regardless of age.

Relationship Between the Changes of Tendon Elastic Moduli With Ultrasound Shear Wave Elastography and Mechanical Compression Test

OBJECTIVE

The purpose of this study was to investigate the consistency of the changes in the elastic modulus measured with ultrasound shear wave elastography (SWE) with changes measured through mechanical testing using tendons that were artificially altered by chemical modifications.

METHODS

Thirty-six canine flexor digitorum profundus tendons were used for this experiment. To mimic tendon mechanical property changes induced by tendinopathy conditions, tendons were treated with collagenase to soften the tissue by collagen digestion or with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) to stiffen the tissues through chemical crosslinking. Tendons were randomly assigned to one of three groups: immersion in phosphate-buffered saline (PBS) as a control group (n = 12), collagenase treatment (n = 12) or EDC treatment (n = 12). Immediately following SWE measurement of each tendon, mechanical compression testing was performed as a gold standard to validate the SWE measurement. Both tests were conducted before and after treatment.

RESULTS

The compressive modulus and SWE shear modulus significantly decreased after collagenase treatment. Conversely, both moduli significantly increased after EDC treatment. There was no significant difference in either modulus before or after PBS treatment. As a result of a regression analysis with the percentage change of the compressive modulus as the dependent variable and SWE shear modulus as the independent variable, the best-fit regression was found to be an exponential function and the coefficient of determination was 0.687.

CONCLUSION

The changes in the compressive moduli and SWE shear moduli in tendons induced by chemical treatments were correlated by approximately 70%.

Gender differences on the age-related distal-to-proximal shift in joint kinetics during running

The increased running participation in women and men over 40 years has contributed to scientific interest on the age-related and gender differences in running performance and biomechanics over the last decade. Gender differences in running biomechanics have been studied extensively in young runners, with inconsistent results. Understanding how gender influences the age-related differences in running mechanics could help develop population-specific training interventions or footwear to address any potential different mechanical demands. The purpose of this study was to assess gender and age effects on lower limb joint mechanics while running. Middle-aged men (57 ± 5 years) and women (57 ± 8 years) and young men (28 ± 6 years) and women (30 ± 6 years) completed five overground running trials at a set speed of 2.7 m/s while lower limb kinematics and ground reaction forces were collected. Lower limb joint kinetics were computed, normalized to body mass and compared between age and gender groups using two-factor analyses of variance. Women reported slower average running paces than men and middle-aged runners reported slower running paces than young runners. We confirmed that young runners run with more ankle, but less hip positive work and peak positive power compared to middle-aged runners (i.e., age-related distal-to-proximal shift in joint kinetics). We also present a novel finding that women run with more ankle, but less hip peak positive power compared to men suggesting an ankle dominant strategy in women at a preferred and comfortable running pace. However, the age-related distal-to-proximal shift in joint kinetics was not different between genders.

Attenuation of muscle spindle firing with artificially increased series compliance during stretch of relaxed muscle

Muscle spindles relay vital mechanosensory information for movement and posture, but muscle spindle feedback is coupled to skeletal motion by a compliant tendon. Little is known about the effects of tendon compliance on muscle spindle feedback during movement, and the complex firing of muscle spindles makes these effects difficult to predict. Our goal was to investigate changes in muscle spindle firing using added series elastic elements (SEEs) to mimic a more compliant tendon, and to characterize the accompanying changes in firing with respect to muscle-tendon unit (MTU) and muscle fascicle displacements (recorded via sonomicrometry). Sinusoidal, ramp-and-hold and triangular stretches were analysed to examine potential changes in muscle spindle instantaneous firing rates (IFRs) in locomotor- and perturbation-like stretches as well as serial history dependence. Added SEEs effectively reduced overall MTU stiffness and generally reduced muscle spindle firing rates, but the effect differed across stretch types. During sinusoidal stretches, peak and mean firing rates were not reduced and IFR was best-correlated with fascicle velocity. During ramp stretches, SEEs reduced the initial burst, dynamic and static responses of the spindle. Notably, IFR was negatively related to fascicle displacement during the hold phase. During triangular stretches, SEEs reduced the mean IFR during the first and second stretches, affecting the serial history dependence of mean IFR. Overall, these results demonstrate that tendon compliance may attenuate muscle spindle feedback during movement, but these changes cannot be fully explained by reduced muscle fascicle length or velocity, or MTU force.

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

The Influence of Footwear Longitudinal Bending Stiffness on Running Economy and Biomechanics in Older Runners

Purpose: This study assessed the effects of footwear longitudinal bending stiffness on running economy and biomechanics of rearfoot striking older runners. Methods: Nine runners over 60 years of age completed two running bouts at their preferred running pace in each of three footwear conditions: low (4.4 ± 1.8 N·m-1), moderate (5.7 ± 1.7 N·m-1), and high (6.4 ± 1.6 N·m-1) bending stiffness. Testing order was randomized and a mirror protocol was used (i.e., A,B,C,C,B,A). Expired gases, lower limb kinematics, and ground reaction forces were collected simultaneously and lower limb joint kinetics, running economy (i.e., VO2), leg stiffness, and spatio-temporal variables were calculated. Results: Running economy was not different among stiffness conditions (p = 0.60, p = 0.53 [mass adjusted]). Greater footwear stiffness reduced step length (p = 0.046) and increased peak vertical ground reaction force (p = 0.019) but did not change peak ankle plantarflexor torque (p = 0.65), peak positive ankle power (p = 0.48), ankle positive work (p = 0.86), propulsive force (p = 0.081), and leg stiffness (p = 0.46). Moderate footwear stiffness yielded greater peak negative knee power compared to low (p = 0.04) and high (p = 0.03) stiffness. Conclusions: These novel findings demonstrate that increasing footwear longitudinal bending stiffness using flat carbon fiber inserts does not improve running economy and generally does not alter lower limb joint mechanics of rearfoot strike runners over 60 years. Future studies should investigate how other footwear characteristics (e.g., midsole material, plate location, and sole curvature) influence economy and biomechanics in this population.

At matched loads, aging does not alter ankle, muscle, or tendon stiffness

Older adults have difficulty maintaining balance when faced with postural disturbances, a task that is influenced by the stiffness of the triceps surae and Achilles tendon. Age-related changes in Achilles tendon stiffness have been reported at matched levels of effort, but measures typically have not been made at matched loads, which is important due to age-dependent changes in strength. Moreover, age-dependent changes in muscle stiffness have yet to be tested. Here, we investigate how age alters muscle and tendon stiffness and their influence on ankle stiffness. We hypothesized that age-related changes in muscle and tendon contribute to reduced ankle stiffness in older adults and evaluated this hypothesis when either load or effort were matched. We used B-mode ultrasound with joint-level perturbations to quantify ankle, muscle, and tendon stiffness across a range of loads and efforts in seventeen healthy younger and older adults. At matched loads, there was no significant difference in ankle, muscle, or tendon stiffness between groups (all p>0.13). However, at matched effort, older adults exhibited a significant decrease in ankle (27%; p=0.008), muscle (37%; p=0.02), and tendon stiffness (22%; p=0.03) at 30% of maximum effort. This is consistent with our finding that older adults were 36% weaker than younger adults in plantarflexion (p=0.004). Together these results indicate that, at the loads tested in this study, there are no age-dependent changes in the mechanical properties of muscle or tendon, only differences in strength that result in altered ankle, muscle, and tendon stiffness at matched levels of effort.

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.

Effect of Aging on Tendon Biology, Biomechanics and Implications for Treatment Approaches

Tendon aging is associated with an increasing prevalence of tendon injuries and/or chronic tendon diseases, such as tendinopathy, which affects approximately 25% of the adult population. Aged tendons are often characterized by a reduction in the number and functionality of tendon stem/progenitor cells (TSPCs), fragmented or disorganized collagen bundles, and an increased deposition of glycosaminoglycans (GAGs), leading to pain, inflammation, and impaired mobility. Although the exact pathology is unknown, overuse and microtrauma from aging are thought to be major causative factors. Due to the hypovascular and hypocellular nature of the tendon microenvironment, healing of aged tendons and related injuries is difficult using current pain/inflammation and surgical management techniques. Therefore, there is a need for novel therapies, specifically cellular therapy such as cell rejuvenation, due to the decreased regenerative capacity during aging. To augment the therapeutic strategies for treating tendon-aging-associated diseases and injuries, a comprehensive understanding of tendon aging pathology is needed. This review summarizes age-related tendon changes, including cell behaviors, extracellular matrix (ECM) composition, biomechanical properties and healing capacity. Additionally, the impact of conventional treatments (diet, exercise, and surgery) is discussed, and recent advanced strategies (cell rejuvenation) are highlighted to address aged tendon healing. This review underscores the molecular and cellular linkages between aged tendon biomechanical properties and the healing response, and provides an overview of current and novel strategies for treating aged tendons. Understanding the underlying rationale for future basic and translational studies of tendon aging is crucial to the development of advanced therapeutics for tendon regeneration.

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.

Attenuation of muscle spindle firing with artificially increased series compliance during stretch of relaxed muscle

Muscle spindles relay vital mechanosensory information for movement and posture, but muscle spindle feedback is coupled to skeletal motion by a compliant tendon. Little is known about the effects of tendon compliance on muscle spindle feedback during movement, and the complex firing of muscle spindles make these effects difficult to predict. Our goal was to investigate changes in muscle spindle firing using added series elastic elements (SEEs) to mimic a more compliant tendon, and to characterize the accompanying changes in firing with respect to muscle-tendon unit (MTU) and muscle fascicle displacements (recorded via sonomicrometry). Sinusoidal, ramp-hold-release, and triangular stretches were analyzed to examine potential changes in muscle spindle instantaneous firing rates (IFRs) in locomotor- and perturbation-like stretches as well as history dependence. Added SEEs effectively reduced overall MTU stiffness and generally reduced muscle spindle firing rates, but the effect differed across stretch types. During sinusoidal stretches, peak firing rates were reduced and IFR was strongly correlated with fascicle velocity. During ramp stretches, SEEs reduced the dynamic and static responses of the spindle during lengthening but had no effect on initial bursts at the onset of stretch. Notably, IFR was negatively related to fascicle displacement during the hold phase. During triangular stretches, SEEs reduced the mean IFR during the first and second stretches, affecting the history dependence of mean IFR. Overall, these results demonstrate that tendon compliance may attenuate muscle spindle feedback during movement, but these changes cannot be fully explained by reduced muscle fascicle length and velocity.

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

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

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.

Effect of age on shear modulus, muscle thickness, echo intensity of the upper limb, lower limb, and trunk muscles in healthy women

PURPOSE

This study aimed to examine the effect of age on the mechanical properties, muscle size, and muscle quality in the upper and lower limb and trunk muscles.

METHODS

We evaluated the shear modulus (G), muscle thickness (MT), and echo intensity (EI) of the upper and lower limb and trunk muscles of 83 healthy women (21-83-year-old). The G values of some limb muscles were measured in relaxed and stretched positions.

RESULTS

Regarding the effect of age on G at the distinct positions, the G of the upper limb muscles were not significantly correlated with age in the relaxed and stretched positions. In contrast, the G of the iliacus showed a significant negative correlation in both positions. Additionally, the G of the rectus femoris had a significant negative correlation only in the relaxed position. Regarding differences among body parts, the G of the lower limb and oblique abdominal muscles showed a significant negative correlation, but no correlation in the upper limb, rectus abdominis, and back muscles. Moreover, MT showed a significant negative correlation with age in the lower limb, abdominal, and erector spinae muscles, but no correlation was detected in the upper limb and lumbar multifidus muscles. EI had a significant positive correlation in all the muscles.

CONCLUSION

The effect of age on G depended on body parts, and the G of the lower limb and oblique abdominal muscles negatively associated with age. Additionally, G in the relaxed position may be more susceptible to aging than G in the stretched position.

Effect of polyester-based artificial tendons on movement biomechanics: A preliminary in vivo study

Artificial tendons may be valuable clinical devices for replacing damaged or missing biological tendons. In this preliminary study, we quantified the effect of polyester-suture-based artificial tendons on movement biomechanics. New Zealand White rabbits underwent surgical replacement of either the Achilles (n = 2) or tibialis cranialis (TC, n = 2) biological tendons with artificial tendons. Once pre-surgery and weekly from 2 to 6 weeks post-surgery, we quantified hindlimb kinematics and ground contact pressures during the stance phase of hopping gait. Post-surgical movement biomechanics were either consistent or improved over time in both groups. However, the Achilles group had greater overall biomechanical and muscle deficits than the TC group. In the TC group, at 6 weeks post-surgery, foot angles were about 10° greater than those in healthy controls during the first 30 % of stance. At 6 weeks post-surgery, the Achilles group exhibited lesser (i.e., more dorsiflexed) ankle angles (minimum angle = 31.5 ± 9.4°) and vertical ground reaction forces (37.4 ± 2.6 %BW) during stance than those in healthy controls (65.0 ± 11.2° and 50.2 ± 8.3 %BW, respectively). Future studies are needed to quantify long-term biomechanical function with artificial tendons, the effect of artificial tendons on muscle function and structure, and the effect of formal rehabilitation.

Age-related changes in gait biomechanics and their impact on the metabolic cost of walking: Report from a National Institute on Aging workshop

Changes in old age that contribute to the complex issue of an increased metabolic cost of walking (mass-specific energy cost per unit distance traveled) in older adults appear to center at least in part on changes in gait biomechanics. However, age-related changes in energy metabolism, neuromuscular function and connective tissue properties also likely contribute to this problem, of which the consequences are poor mobility and increased risk of inactivity-related disease and disability. The U.S. National Institute on Aging convened a workshop in September 2021 with an interdisciplinary group of scientists to address the gaps in research related to the mechanisms and consequences of changes in mobility in old age. The goal of the workshop was to identify promising ways to move the field forward toward improving gait performance, decreasing energy cost, and enhancing mobility for older adults. This report summarizes the workshop and brings multidisciplinary insight into the known and potential causes and consequences of age-related changes in gait biomechanics. We highlight how gait mechanics and energy cost change with aging, the potential neuromuscular mechanisms and role of connective tissue in these changes, and cutting-edge interventions and technologies that may be used to measure and improve gait and mobility in older adults. Key gaps in the literature that warrant targeted research in the future are identified and discussed.

Understanding altered contractile properties in advanced age: insights from a systematic muscle modelling approach

Age-related alterations of skeletal muscle are numerous and present inconsistently, and the effect of their interaction on contractile performance can be nonintuitive. Hill-type muscle models predict muscle force according to well-characterised contractile phenomena. Coupled with simple, yet reasonably realistic activation dynamics, such models consist of parameters that are meaningfully linked to fundamental aspects of muscle excitation and contraction. We aimed to illustrate the utility of a muscle model for elucidating relevant mechanisms and predicting changes in output by simulating the individual and combined effects on isometric force of several known ageing-related adaptations. Simulating literature-informed reductions in free Ca2+ concentration and Ca2+ sensitivity generated predictions at odds qualitatively with the characteristic slowing of contraction speed. Conversely, incorporating slower Ca2+ removal or a fractional increase in type I fibre area emulated expected changes; the former was required to simulate slowing of the twitch measured experimentally. Slower Ca2+ removal more than compensated for force loss arising from a large reduction in Ca2+ sensitivity or moderate reduction in Ca2+ release, producing realistic age-related shifts in the force-frequency relationship. Consistent with empirical data, reductions in free Ca2+ concentration and Ca2+ sensitivity reduced maximum tetanic force only slightly, even when acting in concert, suggesting a modest contribution to lower specific force. Lower tendon stiffness and slower intrinsic shortening speed slowed and prolonged force development in a compliance-dependent manner without affecting force decay. This work demonstrates the advantages of muscle modelling for exploring sources of variation and identifying mechanisms underpinning the altered contractile properties of aged muscle.

Impact of aging on tendon homeostasis, tendinopathy development, and impaired healing

Aging is a complex and progressive process where the tissues of the body demonstrate a decreased ability to maintain homeostasis. During aging, there are substantial cellular and molecular changes, with a subsequent increase in susceptibility to pathological degeneration of normal tissue function. In tendon, aging results in well characterized alterations in extracellular matrix (ECM) structure and composition. In addition, the cellular environment of aged tendons is altered, including a marked decrease in cell density and metabolic activity, as well as an increase in cellular senescence. Collectively, these degenerative changes make aging a key risk factor for the development of tendinopathies and can increase the frequency of tendon injuries. However, inconsistencies in the extent of age-related degenerative impairments in tendons have been reported, likely due to differences in how "old" and "young" age-groups have been defined, differences between anatomically distinct tendons, and differences between animal models that have been utilized to study the impact of aging on tendon homeostasis. In this review, we address these issues by summarizing data by well-defined age categories (young adults, middle-aged, and aged) and from anatomically distinct tendon types. We then summarize in detail how aging affects tendon mechanics, structure, composition, and the cellular environment based on current data and underscore what is currently not known. Finally, we discuss gaps in the current understanding of tendon aging and propose key avenues for future research that can shed light on the specific mechanisms of tendon pathogenesis due to aging.

Tendon Aging

Tendons are mechanosensitive connective tissues responsible for the connection between muscles and bones by transmitting forces that allow the movement of the body, yet, with advancing age, tendons become more prone to degeneration followed by injuries. Tendon diseases are one of the main causes of incapacity worldwide, leading to changes in tendon composition, structure, and biomechanical properties, as well as a decline in regenerative potential. There is still a great lack of knowledge regarding tendon cellular and molecular biology, interplay between biochemistry and biomechanics, and the complex pathomechanisms involved in tendon diseases. Consequently, this reflects a huge need for basic and clinical research to better elucidate the nature of healthy tendon tissue and also tendon aging process and associated diseases. This chapter concisely describes the effects that the aging process has on tendons at the tissue, cellular, and molecular levels and briefly reviews potential biological predictors of tendon aging. Recent research findings that are herein reviewed and discussed might contribute to the development of precision tendon therapies targeting the elderly population.

Does Mobility of the Ankle Joint Depends on Length of the Free Part of the Achilles Tendon?

The aim of our study was to evaluate whether the length of the free part of the Achilles tendon affects the mobility of the ankle joint in active motion without a load, as well as in functional motion with a body-weight load. We examined 36 healthy people, aged 21 to 30 years, and divided them into two groups: 1 (n = 15)—participants with a normal range of dorsiflexion in the ankle joint (20° or more), and 2 (n = 21)—participants with a reduced range of dorsiflexion in the ankle joint (below 20°). The length of the free part of the Achilles tendon was measured using ultrasonography. Ankle joint range of dorsiflexion was assessed, and a weight-bearing lunge test (WBLT) was conducted. Group 1 performed the WBLT better and demonstrated significantly greater Achilles tendon length compared to Group 2. A moderate, significant correlation was observed between ankle joint range of dorsiflexion and Achilles tendon length (r = 0.53, p < 0.05); between WBLT and Achilles tendon length (r = 0.61, p < 0.05); as well as between ankle joint range of dorsiflexion and WBLT (r = 0.63, p < 0.05). Thus, we can suggest that both the length of the tendon (measured by USG) and the ankle range of motion under a body-weight load (measured by WBLT) are good indicators regarding the range of foot dorsiflexion, but only up to specific values (6 cm of tendon length and 11 cm of WBLT reach). Therefore, Achilles tendon length, e.g., after injury and during tendon healing, may be monitored using the method of ultrasound imaging presented in this study.

Shorter muscle fascicle operating lengths increase the metabolic cost of cyclic force production

During locomotion, force-producing limb muscles are predominantly responsible for an animal's whole body metabolic energy expenditure. Animals can change the length of their force-producing muscle fascicles by altering body posture (e.g., joint angles), the structural properties of their biological tissues over time (e.g., tendon stiffness), or the body's kinetics (e.g., body weight). Currently, it is uncertain whether relative muscle fascicle operating lengths have a measurable effect on the metabolic energy expended during cyclic locomotion-like contractions. To address this uncertainty, we quantified the metabolic energy expenditure of human participants, as they cyclically produced two distinct ankle moments at three ankle angles (90°, 105°, and 120°) on a fixed-position dynamometer using their soleus. Overall, increasing participant ankle angle from 90° to 120° (more plantar flexion) reduced minimum soleus fascicle length by 17% (both moment levels, P < 0.001) and increased metabolic energy expenditure by an average of 208% across both moment levels (both P < 0.001). For both moment levels, the increased metabolic energy expenditure was not related to greater fascicle positive mechanical work (higher moment level, P = 0.591), fascicle force rate (both P ≥ 0.235), or model-estimated active muscle volume (both P ≥ 0.122). Alternatively, metabolic energy expenditure correlated with average relative soleus fascicle length (r = -0.72, P = 0.002) and activation (r = 0.51, P < 0.001). Therefore, increasing active muscle fascicle operating lengths may reduce metabolic energy expended during locomotion.NEW & NOTEWORTHY During locomotion, active muscles undergo cyclic length-changing contractions. In this study, we isolated confounding variables and revealed that cyclically producing force at relatively shorter fascicle lengths increases metabolic energy expenditure. Therefore, muscle fascicle operating lengths likely have a measurable effect on the metabolic energy expenditure during locomotion.

Strain sonoelastography in asymptomatic individuals and individuals with knee osteoarthritis: an evaluation of quadriceps and patellar tendon

An advanced ultrasound imaging technique, sonoelastography (SE) is used to evaluate tissue elasticity. To determine SE potential to detect pathological-related changes, and characteristics related to tendon pathology we aimed to (1) compare quadriceps and patellar tendon findings in individuals with knee osteoarthritis (KOA) and asymptomatic older adults (AC), and (2) explore associations between SE, participant characteristics (age, BMI, and leg circumference) and KOA status. 84 participants (47; KOA and 37; asymptomatic older adults) underwent SE examination of quadriceps (distal) and patellar (distal, proximal) tendon in a supine position with the knee bent at 30°. Colour score (CS) and Elasticity Ratio (ER) analysis were performed by a blinded experienced operator using Esaote Mylab 70 XVG Ultrasound equipment. Significantly reduced elasticity in the distal quadriceps (median (IQR) 2(2), 3(1), p = 0.033 for KOA and AC, respectively) and proximal patellar (3(1), 3(0), p = 0.001) tendons and more elastic distal patellar (1.50 (0.55), 1.87 (0.72), p = 0.034) tendons were observed in the KOA group. Significant associations) were identified between SE and participant BMI (Rs = − 0.249–0.750, p < 0.05) and leg circumference (Rs = − 0.260–0.903, p < 0.05). Age, BMI and KOA status, were independent explanatory variables of SE CS findings at the distal quadriceps tendon patellar tendon, proximal patellar tendon and distal patellar tendon, explaining 66%, 81% and 64% of variance, respectively. Age, BMI and KOA status were independent explanatory variables of SE ER findings at the distal patellar tendon explaining 19% of variance. Potentially clinically relevant altered tendon stiffness were observed between individuals with KOA and asymptomatic controls. Key KOA risk factors and participant characteristics explained variance in tendon stiffness. Findings provide context for future studies to investigate the potential for targeted SE detected early clinical management based on associated participant characteristics.

Metabolic and molecular responses of human patellar tendon to concentric- and eccentric-type exercise in youth and older age

Exercise training can induce adaptive changes to tendon tissue both structurally and mechanically; however, the underlying compositional changes that contribute to these alterations remain uncertain in humans, particularly in the context of the ageing tendon. The aims of the present study were to determine the molecular changes with ageing in patellar tendons in humans, as well as the responses to exercise and exercise type (eccentric (ECC) and concentric (CON)) in young and old patellar tendon. Healthy younger males (age 23.5 ± 6.1 years; n = 27) and older males (age 68.5 ± 1.9 years; n = 27) undertook 8 weeks of CON or ECC training (3 times per week; at 60% of 1 repetition maximum (1RM)) or no training. Subjects consumed D₂O throughout the protocol and tendon biopsies were collected after 4 and 8 weeks for measurement of fractional synthetic rates (FSR) of tendon protein synthesis and gene expression. There were increases in tendon protein synthesis following 4 weeks of CON and ECC training (P < 0.01; main effect by ANOVA), with no differences observed between young and old males, or training type. At the transcriptional level however, ECC in young adults generally induced greater responses of collagen and extracellular matrix-related genes than CON, while older individuals had reduced gene expression responses to training. Different training types did not appear to induce differential tendon responses in terms of protein synthesis, and while tendons from older adults exhibited different transcriptional responses to younger individuals, protein turnover changes with training were similar for both age groups.

Tendon healing: a concise review on cellular and molecular mechanisms with a particular focus on the Achilles tendon

Tendon is a bradytrophic and hypovascular tissue, hence, healing remains a major challenge. The molecular key events involved in successful repair have to be unravelled to develop novel strategies that reduce the risk of unfavourable outcomes such as non-healing, adhesion formation, and scarring. This review will consider the diverse pathophysiological features of tendon-derived cells that lead to failed healing, including misrouted differentiation (e.g. de- or transdifferentiation) and premature cell senescence, as well as the loss of functional progenitors. Many of these features can be attributed to disturbed cell-extracellular matrix (ECM) or unbalanced soluble mediators involving not only resident tendon cells, but also the cross-talk with immigrating immune cell populations. Unrestrained post-traumatic inflammation could hinder successful healing. Pro-angiogenic mediators trigger hypervascularization and lead to persistence of an immature repair tissue, which does not provide sufficient mechano-competence. Tendon repair tissue needs to achieve an ECM composition, structure, strength, and stiffness that resembles the undamaged highly hierarchically ordered tendon ECM. Adequate mechano-sensation and -transduction by tendon cells orchestrate ECM synthesis, stabilization by cross-linking, and remodelling as a prerequisite for the adaptation to the increased mechanical challenges during healing. Lastly, this review will discuss, from the cell biological point of view, possible optimization strategies for augmenting Achilles tendon (AT) healing outcomes, including adapted mechanostimulation and novel approaches by restraining neoangiogenesis, modifying stem cell niche parameters, tissue engineering, the modulation of the inflammatory cells, and the application of stimulatory factors.Cite this article: Bone Joint Res 2022;11(8):561-574.

Morphological correlates of distal fibular morphology with locomotion in great apes, humans, and Australopithecus afarensis

OBJECTIVES

Recent studies highlighted the importance of the fibula to further our understanding of locomotor adaptations in fossil hominins. In this study, we present a three-dimensional geometric morphometric (3D-GM) investigation of the distal fibula in extant hominids and Australopithecus afarensis with the aim of pointing out morphological correlations to arboreal behavior.

METHODS

Three-dimensional surface meshes of the distal fibula were obtained using computer tomography for 40 extant hominid specimens and laser scanner for five A. afarensis specimens. Distal fibula morphology was quantified positioning 11 fixed landmarks, 40 curve semilandmarks, and 20 surface landmarks on each specimen. A generalized Procrustes analysis (GPA) was carried out on all landmark coordinates followed by Procrustes ANOVA. Principal component analysis (PCA) was performed on the GPA-aligned shape coordinates. Kruskal-Wallis tests and Mann-Whitney test were performed on scores along PCs.

RESULTS

Great apes are characterized by a shorter subcutaneous triangular surface (STS), more downward facing fibulotalar articular facets, more anteriorly facing lateral malleolus and wider/deeper malleolar fossa than humans. Within great apes, orangutans are characterized by more medially facing fibulotalar articular facets. Australopithecus afarensis shows a unique distal fibular morphology with several traits that are generally associated more to arboreality and less to bipedalism such as a short STS, a more anteriorly facing, laterally pointing malleolus and deeper and larger malleolar fossa.

CONCLUSIONS

The distal fibula morphology is indicative of locomotor patterns within extant hominids. The 3D-GM method presented here can be successfully used to further our understanding of arboreal adaptations in fossil hominins.

Muscle-tendon architecture in Kenyans and Japanese: Potential role of genetic endowment in the success of elite Kenyan endurance runners

AIM

The specificity of muscle-tendon and foot architecture of elite Kenyan middle- and long-distance runners has been found to contribute to their superior running performance. To investigate the respective influence of genetic endowment and training on these characteristics, we compared leg and foot segmental lengths as well as muscle-tendon architecture of Kenyans and Japanese males (i) from infancy to adulthood and (ii) non-athletes versus elite runners.

METHODS

The 676 participants were divided according to their nationality (Kenyans and Japanese), age (nine different age groups for non-athletes) and performance level in middle- and long-distance races (non-athlete, non-elite and elite adult runners). Shank and Achilles tendon (AT) lengths, medial gastrocnemius (MG) fascicle length, pennation angle and muscle thickness, AT moment arm (MA_AT ), and foot lever ratio were measured.

RESULTS

Above 8 years old, Kenyans had a longer shank and AT, shorter fascicle, greater pennation angle, thinner MG muscle as well as longer MA_AT , with lower foot lever ratio than age-matched Japanese. Among adults of different performance levels and independently of the performance level, Kenyans had longer shank, AT and MA_AT , thinner MG muscle thickness, and lower foot lever ratio than Japanese. The decrease in MG fascicle length and increase pennation angle observed for the adult Japanese with the increase in performance level resulted in a lack of difference between elite Kenyans and Japanese.

CONCLUSION

The specificity of muscle-tendon and foot architecture of elite Kenyan runners could result from genetic endowment and contribute to the dominance of Kenyans in middle- and long-distance races.

Role of Knee and Ankle Extensors' Muscle-Tendon Properties in Dynamic Balance Recovery from a Simulated Slip

Participants exposed to a simulated slip with forward loss of balance (FLB) develop large lower limb joint moments which may be a limiting factor for those whose muscle-tendon units’ (MTUs) properties are deteriorated. Whether the age-related decline in these properties limits participants’ capacity to recover their balance following a slip with FLB remains unclear. We combined isokinetic dynamometry, ultrasound and EMG to understand how knee extensor and ankle plantarflexor muscle strength and power, rate of moment development, electromechanical delay, and tendon stiffness affected the balance of young (25.3 ± 3.9 years) and older adults (62.8 ± 7.1 years) when recovering from a single slip with FLB triggered whilst walking on a split-belt instrumented treadmill. Except for the patellar tendon’s stiffness, knee extensor and ankle plantarflexor electromechanical delays, older adults’ MTUs properties were deteriorated compared to those of young participants (p < 0.05). We found no significant relationship between age or the MTUs properties of participants and balance recovery. These findings provide additional support that neither maximal nor explosive strength training are likely to be successful in preventing a fall for healthy older adults, and that other type of interventions, such as task-specific training that has already proved efficacious in reducing the risk of falling, should be developed.

Aging and matrix viscoelasticity affect multiscale tendon properties and tendon derived cell behavior

Aging is the largest risk factor for Achilles tendon associated disorders and rupture. Although Achilles tendon macroscale elastic properties are suggested to decline with aging, less is known about the effect of maturity and aging on multiscale viscoelastic properties and their effect on tendon cell behavior. Here, we show dose dependent changes in native multiscale tendon mechanical and structural properties and uncover several nanoindentation properties predicted by tensile mechanics and echogenicity. Alginate hydrogel systems designed to mimic juvenile tendon microscale mechanics revealed that stiffness and viscoelasticity affected Achilles tendon cell aspect ratio and proliferation during aging. This knowledge provides further evidence for the negative impact of maturity and aging on tendon and begins to elucidate how viscoelasticity can control tendon derived cell morphology and expansion. STATEMENT OF SIGNIFICANCE: Aging is the largest risk factor for Achilles tendon associated disorders and rupture. Although Achilles tendon macroscale elastic properties are suggested to decline with aging, less is known about the effect of maturity and aging on multiscale viscoelastic properties and their effect on tendon cell behavior. Here, we show dose dependent changes in native multiscale tendon mechanical and structural properties and uncover several nanoindentation properties predicted by tensile mechanics and echogenicity. Alginate hydrogel systems designed to mimic juvenile tendon microscale mechanics revealed that stiffness and viscoelasticity affected Achilles tendon cell spreading and proliferation during aging. This knowledge provides further evidence for the negative impact of maturity and aging on tendon and begins to elucidate how viscoelasticity can control tendon derived cell morphology and expansion.

Age- and muscle-specific reliability of muscle architecture measurements assessed by two-dimensional panoramic ultrasound

Background

Age-related changes in muscle properties affect daily functioning, therefore a reliable assessment of such properties is required. We examined the effects of age on reliability, muscle quality and interrelation among muscle architecture (MA) parameters of the gastrocnemius medialis (GM), tibialis anterior (TA), and vastus lateralis (VL) muscles.

Methods

Three raters scored ultrasound (US) scans of 12 healthy younger and older adults, on fascicle length (FL), pennation angle (PA) and muscle thickness (MT). Intra- and inter-rater reliability of MA measures in rest and contraction was assessed by intraclass correlation coefficients (ICC) and standard error of measurements (SEM, SEM%). The relationship between MA parameters was examined using Pearson correlation coefficients. Muscle quality (MQ) was examined using mean pixel intensity.

Results

Reliability was moderate to excellent for TA in both groups (ICCs: 0.64–0.99, SEM% = 1.6–14.8%), and for VL in the younger group (ICCs: 0.67–0.98, SEM% = 2.0–18.3%). VL reliability was poor to excellent in older adults (ICCs: 0.22–0.99, SEM% = 2.7–36.0%). For GM, ICCs were good to excellent (ICCs: 0.76–0.99) in both groups, but GM SEM% were higher in older adults (SEM%Younger = 1.5–10.7%, SEM%Older = 1.6–28.1%). Muscle quality was on average 19.0% lower in older vs. younger adults. In both groups, moderate to strong correlations were found for VL FL and MT (r ≥ 0.54), and TA PA and MT (r ≥ 0.72), while TA FL correlated with MT (r ≥ 0.67) in younger adults only.

Conclusions

In conclusion, age- and muscle-specificities were present in the relationships between MT and PA, and MT and FL at rest. Furthermore, the reliability of MA parameters assessed with 2D panoramic US is acceptable. However, the level of reliability varies with age, muscle and MA measure. In older adults notably, the lowest reliability was observed in the VL muscle. Among the MA parameters, MT appears to be the simplest and most easily reproducible parameter in all muscles and age groups.

Influence of Spinal Movements Associated with Physical Evaluation on Muscle Mechanical Properties of the Lumbar Paraspinal in Subjects with Acute Low Back Pain

This research aimed to identify changes in muscle mechanical properties (MMPs) when a standardized sequence of movements is performed and to determine the influence of acute low back pain (LBP) and age on the MMPs. Socio-demographic, clinical variables and MMPs were collected in 33 patients with LBP and 33 healthy controls. A 2 × 2 × 2 (group × age × time) analysis of variance (ANOVA) mixed model was used to determine the effect of the study factors on the different MMPs. There were no significant triple interactions. After the movements, tone and stiffness increased 0.37 Hz and 22.75 N/m, respectively, in subjects <35 years, independent of their clinical status. Relaxation showed differences by age in healthy subjects and creep in LBP subjects. Furthermore, elasticity was higher in <35 years (p < 0.001) without the influence of any other factor. In conclusion, sequenced movements can modify tone and stiffness as a function of age, while age-associated changes in viscoelastic characteristics depends on pain but not on movements. The MMPs should be assessed, not only at the beginning of the physical examination at rest, but also along the patient’s follow-up, depending on their pain and age, in a clinical setting.

Regional variation in lateral and medial gastrocnemius muscle fibre lengths obtained from diffusion tensor imaging

Assessment of regional muscle architecture is primarily done through the study of animals, human cadavers, or using b-mode ultrasound imaging. However, there remain several limitations to how well such measurements represent in vivo human whole muscle architecture. In this study, we developed an approach using diffusion tensor imaging and magnetic resonance imaging to quantify muscle fibre lengths in different muscle regions along a muscle's length and width. We first tested the between-day reliability of regional measurements of fibre lengths in the medial (MG) and lateral gastrocnemius (LG) and found good reliability for these measurements (intraclass correlation coefficient [ICC] = 0.79 and ICC = 0.84, respectively). We then applied this approach to a group of 32 participants including males (n = 18), females (n = 14), young (24 ± 4 years) and older (70 ± 2 years) adults. We assessed the differences in regional muscle fibre lengths between different muscle regions and between individuals. Additionally, we compared regional muscle fibre lengths between sexes, age groups, and muscles. We found substantial variability in fibre lengths between different regions within the same muscle and between the MG and the LG across individuals. At the group level, we found no difference in mean muscle fibre length between males and females, nor between young and older adults, or between the MG and the LG. The high variability in muscle fibre lengths between different regions within the same muscle, possibly expands the functional versatility of the muscle for different task requirements. The high variability between individuals supports the use of subject-specific measurements of muscle fibre lengths when evaluating muscle function.

Achilles Tendon Ruptures in Middle-Aged Rats Heal Poorly Compared With Those in Young and Old Rats [Formula: see text]

BACKGROUND

Achilles tendon ruptures are painful and debilitating injuries and are most common in middle-aged patients. There is a lack of understanding of the underlying causes for increased rupture rates in middle-aged patients and how healing outcomes after a rupture might be affected by patient age. Therefore, the objective of this study was to define age-specific Achilles tendon healing by assessing ankle functional outcomes and Achilles tendon mechanical and histological properties after a rupture using a rat model.

HYPOTHESIS

Rats representing the middle-aged patient population would demonstrate reduced healing capability after an Achilles tendon rupture, as demonstrated by a slower return to baseline ankle functional properties and inferior biomechanical and histological tendon properties.

STUDY DESIGN

Controlled laboratory study.

METHODS

Fischer 344 rats were categorized by age to represent young, middle-aged, and old patients, and Achilles tendon ruptures were induced in the right hindlimb. Animals were allowed to heal and were euthanized at 3 or 6 weeks after the injury. In vivo functional assays and ultrasound imaging were performed throughout the healing period, and ex vivo tendon mechanical and histological properties were assessed after euthanasia.

RESULTS

Rats representing middle-aged patients displayed reduced healing potential compared with the other age groups, as they demonstrated decreased recovery of in vivo functional and ultrasound assessment parameters and inferior mechanical and histological properties after an Achilles tendon rupture.

CONCLUSION

These findings may help explain the increased rupture rate observed clinically in middle-aged patients by suggesting that there may be altered tendon responses to daily trauma.

CLINICAL RELEVANCE

The results provide novel data on age-specific healing outcomes after an Achilles tendon rupture, which underscores the importance of considering a patient's age during treatment and expectations for outcomes.

Reduced Achilles Tendon Stiffness Disrupts Calf Muscle Neuromechanics in Elderly Gait

Older adults walk slower and with a higher metabolic energy expenditure than younger adults. In this review, we explore the hypothesis that age-related declines in Achilles tendon stiffness increase the metabolic cost of walking due to less economical calf muscle contractions and increased proximal joint work. This viewpoint may motivate interventions to restore ankle muscle-tendon stiffness, improve walking mechanics, and reduce metabolic cost in older adults.

Neural and muscular alterations of the plantar flexors in middle-aged women

INTRODUCTION

Considering the large population of middle-aged adults, it is important to understand the age-related change in lower limb muscles and the possible mechanisms before old age (> 65 years old). The purpose of this study was to investigate age-related neural and muscular alterations of the plantar flexors in young and middle-aged women.

METHODS

Twenty two middle-aged (54.0 ± 5.8 yrs) and 17 young (21.8 ± 1.4 yrs) recreationally active women performed rapid maximal voluntary isometric contractions (MVIC) of the plantar flexors. Absolute and normalized rate of torque development (RTD) and electromyography (EMG) were examined. Electrical stimulation was used to examine voluntary activation and contractile properties of the muscle. Ultrasonography was used to examine medial and lateral gastrocnemius muscle thickness and pennation angle. A 6-minute walk and sit to stand task were also performed by all participants.

RESULTS

The middle-aged women had significantly lower MVIC torque (141 ± 49 vs. 109 ± 30 Nm, P = 0.031), absolute RTD (753.0 ± 313.6 vs. 423.0 ± 156.1 Nm/s, P = 0.001), and normalized peak RTD (554.0 ± 191.0 vs. 388.0 ± 91.9% MVIC/s, P = 0.001). Normalized early RTD_0-50 and late RTD_100-200, voluntary activation, and EMG were similar between groups. Resting twitch data showed that time to peak (124.0 ± 20.4 vs. 143.0 ± 16.7 ms, P = 0.002) and half relaxation time (73.1 ± 15.2 vs. 107.0 ± 28.2 ms, P < 0.001) was significantly faster for the young women. Thickness was greater in the lateral gastrocnemius (1.6 ± 0.2 vs. 1.4 ± 0.2 cm, P = 0.006) for the young women. Pennation angle of both muscles were greater for the young women (15.8 ± 3.9 vs. 13.1 ± 2.7 degrees, P < 0.05). Performance of the 6-minute walk was similar between groups, however, the young women performed more repetitions during the sit to stand task (25.6 ± 6.7 vs. 18.3 ± 4.7 reps, P < 0.001).

CONCLUSION

Compared to young women, middle-age women were shown to have lower MVIC torque, peak RTD, and functional performance. Muscle architecture and contractile properties are affected by aging.

Muscle architecture and shape changes in the gastrocnemii of active younger and older adults

When muscles contract and change length, they also bulge in thickness and/or width. These shape changes extend the functional range of skeletal muscle by allowing individual muscle fibres to shorten at different velocities than the whole muscle. Age-related differences in muscle architecture and tissue properties influence how older muscles change shape and architecture during contractions, yet this remains unexplored in active older adults. The aim of this study was to quantify and compare in vivo muscle architecture and shape changes in the medial (MG) and lateral (LG) gastrocnemii of active younger and older adults during isometric plantarflexion contractions. Fifteen younger (21 ± 2y) and 15 older (70 ± 3y) participants performed contractions at 20%, 40%, 60%, 80%, and 100% of maximum voluntary contraction (MVC). B-mode ultrasound was used to measure fascicle length, pennation angle and muscle thickness in MG and LG. We found no influence of age on changes in normalized fascicle length and thickness, or absolute change in pennation angle during contractions. With increasing contraction level, MG and LG fascicle shortening (P < 0.001) and rotation (P < 0.001) increased. However, the change in muscle thickness increased at higher contraction levels in LG, and not MG. Similarly, increased changes in pennation angle were associated with increased muscle thickness in LG, but not MG at 80% and 100% MVC. These results suggest that (1) gastrocnemii shape changes are similar in active older and younger adults at matched levels of effort, and (2) the relationship between pennation angle and muscle thickness can differ between synergistics (LG and MG) and across contraction levels.

Resting Tendon Cross-Sectional Area Underestimates Biceps Brachii Tendon Stress: Importance of Measuring During a Contraction

Force produced by the muscle during contraction is applied to the tendon and distributed through the cross-sectional area (CSA) of the tendon. This ratio of force to the tendon CSA is quantified as the tendon mechanical property of stress. Stress is traditionally calculated using the resting tendon CSA; however, this does not take into account the reductions in the CSA resulting from tendon elongation during the contraction. It is unknown if calculating the tendon stress using instantaneous CSA during a contraction significantly increases the values of in vivo distal biceps brachii (BB) tendon stress in humans compared to stress calculated with the resting CSA. Nine young (22 ± 1 years) and nine old (76 ± 4 years) males, and eight young females (21 ± 1 years) performed submaximal isometric elbow flexion tracking tasks at force levels ranging from 2.5 to 80% maximal voluntary contraction (MVC). The distal BB tendon CSA was recorded on ultrasound at rest and during the submaximal tracking tasks (instantaneous). Tendon stress was calculated as the ratio of tendon force during contraction to CSA using the resting and instantaneous measures of CSA, and statistically evaluated with multi-level modeling (MLM) and Johnson–Neyman regions of significance tests to determine the specific force levels above which the differences between calculation methods and groups became statistically significant. The tendon CSA was greatest at rest and decreased as the force level increased (p < 0.001), and was largest in young males (23.0 ± 2.90 mm²) followed by old males (20.87 ± 2.0 mm²) and young females (17.08 ± 1.54 mm²) (p < 0.001) at rest and across the submaximal force levels. Tendon stress was greater in the instantaneous compared with the resting CSA condition, and young males had the greatest difference in the values of tendon stress between the two conditions (20 ± 4%), followed by old males (19 ± 5%), and young females (17 ± 5%). The specific force at which the difference between the instantaneous and resting CSA stress values became statistically significant was 2.6, 6.6, and 10% MVC for old males, young females, and young males, respectively. The influence of using the instantaneous compared to resting CSA for tendon stress is sex-specific in young adults, and age-specific in the context of males. The instantaneous CSA should be used to provide a more accurate measure of in vivo tendon stress in humans.

Effects of Self-Myofascial Release Using a Foam Roller on the Stiffness of the Gastrocnemius-Achilles Tendon Complex and Ankle Dorsiflexion Range of Motion

Increased muscle stiffness can contribute to reduced range of motion (ROM) and impaired function. Reduced ankle dorsiflexion ROM has been associated with increased injury risk in the ankle. Self-myofascial release (SMR) has been widely used in clinical and sports settings, but the effects of SMR on gastrocnemius and Achilles tendon (AT) stiffness are unclear. Therefore, we investigated the effects of self-myofascial release using a foam roller (FR) on the stiffness of the gastrocnemius–AT complex and ankle dorsiflexion ROM. Fifty healthy, untrained, and non-sedentary participants (age=22.5±2.6years) were randomly divided into an intervention group (FR group) and a control group. The subjects in the intervention group received a single foam roller intervention (three sets of 1min), while the subjects in the control group performed a 5-min sedentary rest. Stiffness of the gastrocnemius–AT complex was evaluated using MyotonPRO and the ankle dorsiflexion ROM was assessed using the weight-bearing lunge test. For the foam roller and control groups, the between-group analysis revealed a statistically significant difference in gastrocnemius stiffness and ankle dorsiflexion ROM after intervention (p<0.05). Within-group analysis revealed a significant increase in ROM and a significant decrease in medial and lateral gastrocnemius (LG) stiffness for the foam roller group after the intervention (p<0.05). In addition, further analysis of the preintervention data revealed a significant negative correlation between ankle dorsiflexion ROM and AT stiffness (r=−0.378 and p=0.007). These results suggest that self-myofascial release using a foam roller on the calf is an effective method for decreasing the stiffness of the gastrocnemius and increasing ankle dorsiflexion ROM.

Decline in gait propulsion in older adults over age decades

BACKGROUND

Despite strong evidence that walking speed and forward propulsion decline with increasing age, their relationship is still poorly understood. While changes in the ankle and hip mechanics have been described, few studies have reported the effect of ageing on the whole leg's contribution to propulsion.

RESEARCH QUESTION

The aim of this study was to investigate age-related changes in the work performed by the leg on the center of mass (COM) push-off power during walking in adults aged 20-86 years. Specifically, we evaluated how deterioration in COM push-off power relates to changes in ankle and hip kinetics as well as age and walking speed.

METHODS

Motion, ground reaction forces and gastrocnemius muscle activity were recorded in 138 adults during overground walking at self-selected speed. Age-related differences in variables between decades were analyzed with an ANOVA, while the relation between COM push-off power and joint kinetic variables, as well as walking speed and biological age, was evaluated using correlations and multiple regression analysis.

RESULTS

From the age of 70 years and onwards, COM push-off power was significantly decreased. The decline in COM push-off power was mostly explained by a decline in average ankle push-off power (72 %), and to a lesser extent by peak hip extension moment (3 %). There was no re-distribution of ankle-to-hip push-off power. The decline in COM push-off power seemed more related to walking speed (explaining 54 % of the variance) than biological age (only 4 %).

SIGNIFICANCE

Findings indicate that age-related decline in COM push-off power in able-bodied adults starts from the age of 70 years, which is before changes have been found in kinematics, but still later than generally presumed. This decrease in push-off power was more related to walking speed than biological age, which emphasizes the need to better understand the reason for speed decline in older adults.

Immediate Effect of Whole Body Vibration on Knee Extensor Tendon Stiffness in Hemiparetic Stroke Patients

Background and Objectives: Whole body vibration is widely used to enhance muscle performance, but evidence of its effects on the tendon stiffness of the knee extensor tendon in stroke remains inconclusive. Our study was aimed to determine the difference in patellar and quadriceps tendon stiffness between hemiparetic and unaffected limbs in stroke patients and to investigate the immediate effect of whole body vibration on tendon stiffness. Materials and Methods: The patellar and quadriceps tendon stiffness of first-ever hemiplegic stroke patients was evaluated with elastography to compare the differences between hemiparetic and unaffected limbs. After one 20 min session of whole body vibration exercise in the standing position, tendon stiffness was again measured to evaluate the immediate effects of whole body vibration on tendon stiffness. Results: The results showed no significant differences in the tendon stiffness of the patellar and quadriceps tendons between hemiparetic and unaffected limbs. However, significant associations were found between the tendon stiffness of the patellar and quadriceps tendons and knee extensor spasticity on the hemiparetic side (ρ = 0.62; p = 0.044). There were no significant changes in tendon stiffness after a single session of whole body vibration. Conclusions: In conclusion, knee extensor tendon stiffness in hemiparetic limbs is positively correlated to the degree of knee extensor spasticity in stroke patients. However, a single session of whole body vibration does not alter tendon stiffness.

Habitual side-specific loading leads to structural, mechanical and compositional changes in the patellar tendon of young and senior life-long male athletes

Effects of life-long physical activity on tendon function have been investigated in cross-sectional studies, but these are at risk of "survivorship" bias. Here, we investigate if life-long side-specific loading is associated with greater cross-sectional area (CSA), mechanical properties, cell density (DNA content) and collagen cross-link composition of the male human patellar tendon (PT), in vivo. Nine seniors and six young male life-long elite badminton players and fencers were included. CSA of the PT obtained by 3-tesla MRI, and ultrasonography-based bilateral PT mechanics were assessed. Collagen fibril characteristics, enzymatic cross-links, non-enzymatic glycation (autofluorescence), collagen and DNA content were measured biochemically in PT biopsies. The elite athletes had a ≥15% side-to-side difference in maximal knee extensor strength, reflecting chronic unilateral sport-specific loading patterns. The PT CSA was greater on the lead extremity compared with the non-lead extremity (17 %, p=0.0001). Furthermore, greater tendon stiffness (18 %, p=0.0404) together with lower tendon stress (22 %, p=0.0005) and tendon strain (18 %, p=0.0433) were observed on the lead extremity. No effects were demonstrated from side-to-side for glycation, enzymatic cross-link, collagen, and DNA content (50%, p=0.1160). Moreover, tendon fibril density was 87±28 fibrils/μm² on the lead extremity and 68±26 fibrils/μm² on the non-lead extremity (28%, p=0.0544). Tendon fibril diameter was 86±14 nm on the lead extremity and 94±14 nm on the non-lead extremity (-9%, p=0.1076). These novel data suggest that life-long side-specific loading in males yields greater patellar tendon size and stiffness possibly with concomitant greater fibril density but without changes of collagen cross-link composition.

Muscle and tendon adaptations to moderate load eccentric vs. concentric resistance exercise in young and older males

Resistance exercise training (RET) is well-known to counteract negative age-related changes in both muscle and tendon tissue. Traditional RET consists of both concentric (CON) and eccentric (ECC) contractions; nevertheless, isolated ECC contractions are metabolically less demanding and, thus, may be more suitable for older populations. However, whether submaximal (60% 1RM) CON or ECC contractions differ in their effectiveness is relatively unknown. Further, whether the time course of muscle and tendon adaptations differs to the above is also unknown. Therefore, this study aimed to establish the time course of muscle and tendon adaptations to submaximal CON and ECC RET. Twenty healthy young (24.5 ± 5.1 years) and 17 older males (68.1 ± 2.4 years) were randomly allocated to either isolated CON or ECC RET which took place 3/week for 8 weeks. Tendon biomechanical properties, muscle architecture and maximal voluntary contraction were assessed every 2 weeks and quadriceps muscle volume every 4 weeks. Positive changes in tendon Young's modulus were observed after 4 weeks in all groups after which adaptations in young males plateaued but continued to increase in older males, suggesting a dampened rate of adaptation with age. However, both CON and ECC resulted in similar overall changes in tendon Young's modulus, in all groups. Muscle hypertrophy and strength increases were similar between CON and ECC in all groups. However, pennation angle increases were greater in CON, and fascicle length changes were greater in ECC. Notably, muscle and tendon adaptations appeared to occur in synergy, presumably to maintain the efficacy of the muscle-tendon unit.

Recovery from Achilles Tendon Repair: A Combination of Postsurgery Outcomes and Insufficient Remodeling of Muscle and Tendon

INTRODUCTION

Achilles tendon rupture (ATR) patients have persistent functional deficits in the triceps surae muscle-tendon unit (MTU). The complex remodeling of the MTU accompanying these deficits remains poorly understood. The purpose of the present study was to associate in vivo and in silico data to investigate the relations between changes in MTU properties and strength deficits in ATR patients.

METHODS

Eleven male subjects who had undergone surgical repair of complete unilateral ATR were examined 4.6 ± 2.0 (mean ± SD) yr after rupture. Gastrocnemius medialis (GM) tendon stiffness, morphology, and muscle architecture were determined using ultrasonography. The force-length relation of the plantar flexor muscles was assessed at five ankle joint angles. In addition, simulations (OpenSim) of the GM MTU force-length properties were performed with various iterations of MTU properties found between the unaffected and the affected side.

RESULTS

The affected side of the patients displayed a longer, larger, and stiffer GM tendon (13% ± 10%, 105% ± 28%, and 54% ± 24%, respectively) compared with the unaffected side. The GM muscle fascicles of the affected side were shorter (32% ± 12%) and with greater pennation angles (31% ± 26%). A mean deficit in plantarflexion moment of 31% ± 10% was measured. Simulations indicate that pairing an intact muscle with a longer tendon shifts the optimal angular range of peak force outside physiological angular ranges, whereas the shorter muscle fascicles and tendon stiffening seen in the affected side decrease this shift, albeit incompletely.

CONCLUSIONS

These results suggest that the substantial changes in MTU properties found in ATR patients may partly result from compensatory remodeling, although this process appears insufficient to fully restore muscle function.