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The objective of this work was to investigate myonuclear permanence and transcriptional regulation as mechanisms for cellular muscle memory after strength training in humans. Twelve untrained men and women performed 10 weeks of unilateral elbow-flexor strength training followed by 16 weeks of de-training. Thereafter, 10 weeks' re-training was conducted with both arms: the previously trained arm and the contralateral untrained control arm. Muscle biopsies were taken from the trained arm before and after both training periods and from the control arm before and after re-training. Muscle biopsies were analysed for fibre cross-sectional area (fCSA), myonuclei and global transcriptomics (RNA sequencing). During the first training period, myonuclei increased in type 1 (13 ± 17%) and type 2 (33 ± 23%) fibres together with a 30 ± 43% non-significant increase in mixed fibre fCSA (P = 0.069). Following de-training, fCSA decreased in both fibre types, whereas myonuclei were maintained, resulting in 33% higher myonuclear number in previously trained vs. control muscle in type 2 fibres. Furthermore, in the previously trained muscle, three differentially expressed genes (DEGs; EGR1, MYL5 and COL1A1) were observed. Following re-training, the previously trained muscle showed larger type 2 fCSA compared to the control (P = 0.035). However, delta change in type 2 fCSA was not different between muscles. Gene expression was more dramatically changed in the control arm (1338 DEGs) than in the previously trained arm (822 DEGs). The sustained higher number of myonuclei in the previously trained muscle confirms myonuclear accretion and permanence in humans. Nevertheless, because of the unclear effect on the subsequent hypertrophy with re-training, the physiological benefit remains to be determined. KEY POINTS: Muscle memory is a cellular mechanism that describes the capacity of skeletal muscle fibres to respond differently to training stimuli if the stimuli have been previously encountered. This study overcomes past methodological limitations related to the choice of muscles and analytical procedures. We show that myonuclear number is increased after strength training and maintained during de-training. Increased myonuclear number and differentially expressed genes related to muscle performance and development in the previously trained muscle did not translate into a clearly superior responses during re-training. Because of the unclear effect on the subsequent hypertrophy and muscle strength gain with re-training, the physiological benefit remains to be determined.

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Background: Despite the clear theoretical link between sarcomere arrangement and force production, the relationship between muscle architecture and function remain ambiguous in vivo. Methods: We used two frequently used ultrasound-based approaches to assess the relationships between vastus lateralis architecture parameters obtained in three common conditions of muscle lengths and contractile states, and the mechanical output of the muscle in twenty-one healthy subjects. The relationship between outcomes obtained in different conditions were also examined. Muscle architecture was analysed in panoramic ultrasound scans at rest with the knee fully extended and in regular scans at an angle close to maximum force (60°), at rest and under maximum contraction. Isokinetic and isometric strength tests were used to estimate muscle force production at various fascicle velocities. Results: Measurements of fascicle length, pennation angle and thickness obtained under different experimental conditions correlated moderately with each other (r = 0.40-.74). Fascicle length measured at 60° at rest correlated with force during high-velocity knee extension (r = 0.46 at 400° s-1) and joint work during isokinetic knee extension (r = 0.44 at 200° s-1 and r = 0.57 at 100° s-1). Muscle thickness was related to maximum force for all measurement methods (r = 0.44-0.73). However, we found no significant correlations between fascicle length or pennation angle and any measures of muscle force or work. Most correlations between architecture and force were stronger when architecture was measured at rest close to optimal length. Conclusion: These findings reflect methodological limitations of current approaches to measure fascicle length and pennation angle in vivo. They also highlight the limited value of static architecture measurements when reported in isolation or without direct experimental context.

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In this study, we tested the hypotheses that (i) rate of force development (RFD) is correlated to muscle architecture and dynamics and that (ii) force-length-velocity properties limit knee extensor RFD. Twenty-one healthy participants were tested using ultrasonography and dynamometry. Vastus lateralis optimal fascicle length, fascicle velocity, change in pennation angle, change in muscle length, architectural gear ratio, and force were measured during rapid fixed-end contractions at 60° knee angle to determine RFD. Isokinetic and isometric tests were used to estimate individual force-length-velocity properties, to evaluate force production relative to maximal potential. Correlation analyses were performed between force and muscle parameters for the first three 50 ms intervals. RFD was not related to optimal fascicle length for any measured time interval, but RFD was positively correlated to fascicle shortening velocity during all intervals (r = 0.49-0.69). Except for the first interval, RFD was also related to trigonometry-based changes in muscle length and pennation angle (r = 0.45-0.63) but not to architectural gear ratio. Participants reached their individual vastus lateralis force-length-velocity potential (i.e. their theoretical maximal force at a given length and shortening velocity) after 62 ± 24 ms. Our results confirm the theoretical importance of fascicle shortening velocity and force-length-velocity properties for rapid force production and suggest a role of fascicle rotation.

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Purpose: Whole body vibration (WBV) triggers anabolic responses in various tissues, including tendons, without requiring high force production. In this waitlist-controlled equivalence trial, we tested its clinical effectiveness as an alternative treatment for patellar tendinopathy against conventional heavy slow resistance training (HSR). Methods: Thirty-nine patients were randomized to either 3 months of WBV training (n = 13), HSR training (n = 11), or a waitlist control (WLC) group (n = 15). In a partly cross-over design, 14 patients of the WLC group were redistributed to one of the two intervention groups (5 in WBV, 9 in HSR). Pre- and post-intervention testing included pain assessments (VAS), functional limitations (VISA-P), knee extension strength and tendon morphological, mechanical and material properties. Follow-up measurements (VAS, VISA-P) were performed in the WBV and HSR groups 6 months after the intervention. Results: Comparisons with the WLC group revealed significant improvements in VISA-P and VAS scores after HSR (41%, p = 003; 54%, p = 0.005) and WBV (22%, p = 0.022; 56%, p = 0.031) training. These improvements continued until follow-up (HSR: 43%, 56%; WBV: 24%, 37%). Pre-post improvements in VAS scores were equivalent between WBV and HSR groups but inconclusive for the VISA-P score and all pre-test to follow up comparisons. The mid-tendon cross-sectional area was significantly reduced after WBV (-5.7%, p = 0.004) and HSR (-3.0%, p = 0.004) training compared to WLC although the equivalence test between interventions was inconclusive. Conclusion: Whole body vibration improved symptoms typically associated with patellar tendinopathy. This type of intervention is as effective as HSR against maximum pain, although equivalence could not be confirmed for other variables. The beneficial responses to WBV and HSR treatments persisted for 6 months after the end of the intervention. Clinical Trial Registration: https://www.drks.de/drks_web/setLocale_EN.do, identifier DRKS00011338.

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INTRODUCTION: This study examined the effects of 24 wk of daily static stretching of the plantarflexors (unilateral 4 × 60-s stretching, whereas the contralateral leg served as a control; n = 26) on joint range of motion (ROM), muscle-tendon unit morphological and mechanical properties, neural activation, and contractile function. METHODS: Torque-angle/velocity was obtained in passive and active conditions using isokinetic dynamometry, whereas muscle-tendon morphology and mechanical properties were examined using ultrasonography. RESULTS: After the intervention, ROM increased (stretching, +11° ± 7°; control, 4° ± 8°), and passive torque (stretching, -10 ± 11 N·m; control, -7 ± 10 N·m) and normalized EMG amplitude (stretching, -3% ± 6%; control, -3% ± 4%) at a standardized dorsiflexion angle decreased. Increases were seen in passive tendon elongation at a standardized force (stretching, +1.3 ± 1.6 mm; control, +1.4 ± 2.1 mm) and in maximal passive muscle and tendon elongation. Angle of peak torque shifted toward dorsiflexion. No changes were seen in tendon stiffness, resting tendon length, or gastrocnemius medialis fascicle length. Conformable changes in ROM, passive dorsiflexion variables, tendon elongation, and angle of peak torque were observed in the nonstretched leg. CONCLUSIONS: The present findings indicate that habitual stretching increases ROM and decreases passive torque, altering muscle-tendon behavior with the potential to modify contractile function.

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We tested whether explosive resistance training with partial range of motion (ROM) would be as effective as full ROM training using a noninferiority trial design. Fifteen subjects with strength training experience took part in an explosive-concentric only-leg press training program, three times per week for 10 weeks. One leg was randomly assigned to exercise with partial ROM (ie, 9º) and the other leg to full ROM. Before and after training, we assessed leg press performance, isokinetic concentric and isometric knee extension torque, and vastus lateralis muscle architecture. Overall, both training modalities increased maximal strength and rate of force development. Training with partial ROM yielded noninferior results compared to full ROM for leg press peak power (+69 ± 47% vs. +61 ± 64%), isokinetic strength (4-6 ± 6%-12% vs. 1-6 ± 6%-10% at 30, 60, and 180˚s-1 ), and explosive torque after 100 (47 ± 24 vs. 35 ± 22) and 150 ms (57 ± 22% vs. 42 ± 25%). The comparison was inconclusive for other functional parameters (ie, isokinetic peak torque (300˚s-1 ), joint angle at isokinetic peak torque, explosive torque after 50 ms, and electrically evoked torque) and for muscle fascicle length and thickness, although noninferiority was established for pennation angle. However, partial ROM was not found statistically inferior to full ROM for any measured variable. Under the present conditions, the effects of explosive heavy resistance training were independent of joint ROM. Instead, these data suggest that the distinct timing of muscle work in explosive contractions confers more influence to the starting joint angle than ROM on adaptations to this type of training.

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INTRODUCTION: The effect of chronic patellar tendinopathy on tissue function and integrity is currently unclear and underinvestigated. The aim of this cohort comparison was to examine morphological, material, and mechanical properties of the patellar tendon and to extend earlier findings by measuring the ability to store and return elastic energy in symptomatic tendons. METHODS: Seventeen patients with chronic (>3 months, VISA-P < 80), inferior pole patellar tendinopathy (24 ± 4 years; male = 12, female = 5) were carefully matched to controls (25 ± 3 years) for training status, pattern, and history of loading of the patellar tendon. Individual knee extension force, patellar tendon stiffness, stress, strain, Young's modulus, hysteresis, and energy storage capacity, were obtained with combined dynamometry, ultrasonography, magnetic resonance imaging, and electromyography. RESULTS: Anthropometric parameters did not differ between groups. VISA-P scores ranged from 28 to 78 points, and symptoms had lasted from 10 to 120 months before testing. Tendon proximal cross-sectional area was 61% larger in the patellar tendinopathy group than in the control group. There were no differences between groups in maximal voluntary isometric knee extension torque (p = 0.216; d < -0.31) nor in tensile tendon force produced during isometric ramp contractions (p = 0.185; d < -0.34). Similarly, tendon strain (p = 0.634; d < 0.12), hysteresis (p = 0.461; d < 0.18), and strain energy storage (p = 0.656; d < 0.36) did not differ between groups. However, patellar tendon stiffness (-19%; p = 0.007; d < -0.74), stress (-27%; p< 0.002; d < -0.90) and Young's modulus (-32%; p = 0.001; d < -0.94) were significantly lower in tendinopathic patients compared to healthy controls. DISCUSSION: In this study, we observed lower stiffness in affected tendons. However, despite the substantial structural and histological changes occurring with tendinopathy, the tendon capacity to store and dissipate energy did not differ significantly.

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In vivo measurements of muscle architecture (i.e. the spatial arrangement of muscle fascicles) are routinely included in research and clinical settings to monitor muscle structure, function and plasticity. However, in most cases such measurements are performed manually, and more reliable and time-efficient automated methods are either lacking completely, or are inaccessible to those without expertise in image analysis. In this work, we propose an ImageJ script to automate the entire analysis process of muscle architecture in ultrasound images: Simple Muscle Architecture Analysis (SMA). Images are filtered in the spatial and frequency domains with built-in commands and external plugins to highlight aponeuroses and fascicles. Fascicle dominant orientation is then computed in regions of interest using the OrientationJ plugin. Bland-Altman plots of analyses performed manually or with SMA indicate that the automated analysis does not induce any systematic bias and that both methods agree equally through the range of measurements. Our test results illustrate the suitability of SMA to analyse images from superficial muscles acquired with a broad range of ultrasound settings.

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Low-load blood flow restriction (LL-BFR) training has gained increasing interest in the scientific community by demonstrating that increases in muscle mass and strength are comparable to conventional high-load (HL) resistance training. Although adaptations on the muscular level are well documented, there is little evidence on how LL-BFR training affects human myotendinous properties. Therefore, the aim of the present study was to investigate morphological and mechanical Achilles tendon adaptations after 14 wk of strength training. Fifty-five male volunteers (27.9 ± 5.1 yr) were randomly allocated into the following three groups: LL-BFR [20-35% of one-repetition maximum (1RM)], HL (70-85% 1RM), or a nonexercising control (CON) group. The LL-BFR and HL groups completed a resistance training program for 14 wk, and tendon morphology, mechanical as well as material properties, and muscle cross-sectional area (CSA) and isometric strength were assessed before and after the intervention. Both HL (+40.7%) and LL-BFR (+36.1%) training induced significant increases in tendon stiffness (P < 0.05) as well as tendon CSA (HL: +4.6%, LL-BFR: +7.8%, P < 0.001). These changes were comparable between groups without significant changes in Young's modulus. Furthermore, gastrocnemius medialis muscle CSA and plantar flexor strength significantly increased in both training groups (P < 0.05), whereas the CON group did not show significant changes in any of the evaluated parameters. In conclusion, the adaptive change in Achilles tendon properties following low-load resistance training with partial vascular occlusion appears comparable to that evoked by high-load resistance training.NEW & NOTEWORTHY Low-load blood flow restriction (LL-BFR) training has been shown to induce beneficial adaptations at the muscular level. However, studies examining the effects on human tendon properties are rare. The findings provide first evidence that LL-BFR can increase Achilles tendon mechanical and morphological properties to a similar extent as conventional high-load resistance training. This is of particular importance for individuals who may not tolerate heavy training loads but still aim for improvements in myotendinous function.

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The interaction between the Achilles tendon and the triceps surae muscles seems to be modulated differently with various task configurations. Here we tested the hypothesis that the increased forces and ankle joint work during running under contrasting conditions (altered speed or load) would be met by different, time-dependent adjustments at the muscle-tendon level. Ultrasonography, electromyography, kinematics, and ground reaction force measurements were used to examine Achilles tendon, gastrocnemius, and soleus muscle mechanics in 16 runners in four different running conditions, consisting of a combination of two different speeds (preferred and +20% of preferred speed) and two loading conditions (unloaded and +20% of body mass). Positive ankle joint work increased similarly (+13%) with speed and load. Gastrocnemius and soleus muscle fascicle length and peak velocity were not altered by either condition, suggesting that contractile conditions are mostly preserved despite the constraints imposed in this experimental design. However, at higher running speed, tendon length changes were unaltered but mean muscle electromyographic activity increased in gastrocnemius (+10%, P < 0.01) and soleus (+14%, P < 0.01). Conversely, when loading was increased, mean muscle activity remained similar to unloaded conditions but the mean velocity of gastrocnemius fascicles was reduced and tendon recoil increased (+29%, P < 0.01). Collectively, these results suggest that the neuromuscular system meets increased mechanical demands by favoring economical force production when enough time is available. NEW & NOTEWORTHY We demonstrate that muscle-tendon mechanics are adjusted differently when running under constraints imposed by speed or load, despite comparable increases in work. The neuromuscular system likely modulates the way force is produced as a function of availability of time and potential energy.

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BACKGROUND: During the stance phase of running, the elasticity of the Achilles tendon enables the utilisation of elastic energy and allows beneficial contractile conditions for the triceps surae muscles. However, the effect of changes in tendon mechanical properties induced by chronic loading is still poorly understood. We tested the hypothesis that a training-induced increase in Achilles tendon stiffness would result in reduced tendon strain during the stance phase of running, which would reduce fascicle strains in the triceps surae muscles, particularly in the mono-articular soleus. METHODS: Eleven subjects were assigned to a training group performing isometric single-leg plantarflexion contractions three times per week for ten weeks, and another ten subjects formed a control group. Before and after the training period, Achilles tendon stiffness was estimated, and muscle-tendon mechanics were assessed during running at preferred speed using ultrasonography, kinematics and kinetics. RESULTS: Achilles tendon stiffness increased by 18% (P < 0.01) in the training group, but the associated reduction in strain seen during isometric contractions was not statistically significant. Tendon elongation during the stance phase of running was similar after training, but tendon recoil was reduced by 30% (P < 0.01), while estimated tendon force remained unchanged. Neither gastrocnemius medialis nor soleus fascicle shortening during stance was affected by training. DISCUSSION: These results show that a training-induced increase in Achilles tendon stiffness altered tendon behaviour during running. Despite training-induced changes in tendon mechanical properties and recoil behaviour, the data suggest that fascicle shortening patterns were preserved for the running speed that we examined. The asymmetrical changes in tendon strain patterns supports the notion that simple in-series models do not fully explain the mechanical output of the muscle-tendon unit during a complex task like running.

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During rapid deceleration of the body, tendons buffer part of the elongation of the muscle-tendon unit (MTU), enabling safe energy dissipation via eccentric muscle contraction. Yet, the influence of changes in tendon stiffness within the physiological range upon these lengthening contractions is unknown. This study aimed to examine the effect of training-induced stiffening of the Achilles tendon on triceps surae muscle-tendon behavior during a landing task. Twenty-one male subjects were assigned to either a 10-week resistance-training program consisting of single-leg isometric plantarflexion (n = 11) or to a non-training control group (n = 10). Before and after the training period, plantarflexion force, peak Achilles tendon strain and stiffness were measured during isometric contractions, using a combination of dynamometry, ultrasound and kinematics data. Additionally, testing included a step-landing task, during which joint mechanics and lengths of gastrocnemius and soleus fascicles, Achilles tendon, and MTU were determined using synchronized ultrasound, kinematics and kinetics data collection. After training, plantarflexion strength and Achilles tendon stiffness increased (15 and 18%, respectively), and tendon strain during landing remained similar. Likewise, lengthening and negative work produced by the gastrocnemius MTU did not change detectably. However, in the training group, gastrocnemius fascicle length was offset (8%) to a longer length at touch down and, surprisingly, fascicle lengthening and velocity were reduced by 27 and 21%, respectively. These changes were not observed for soleus fascicles when accounting for variation in task execution between tests. These results indicate that a training-induced increase in tendon stiffness does not noticeably affect the buffering action of the tendon when the MTU is rapidly stretched. Reductions in gastrocnemius fascicle lengthening and lengthening velocity during landing occurred independently from tendon strain. Future studies are required to provide insight into the mechanisms underpinning these observations and their influence on energy dissipation.

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The different methods used to assess patellar tendon elongation in vivo may partly explain the large variation of mechanical properties reported in the literature. The present study investigated the effects of tracking landmark position and tibial point of resistive force application during leg extensions in a dynamometer. Nineteen adults performed isometric contractions with a proximal and distal dynamometer shank pad position. Knee joint moments were calculated employing an inverse dynamics approach. Tendon elongation was measured using the patellar apex and either the tibial tuberosity (T) or plateau (P) as tracking landmark. Using P for tracking introduced a bias towards greater values of tendon elongation at all force levels from 100 N to maximum tendon force (TFmax; p < 0.05). The differences between landmarks considering maximum tendon strain were greater at the proximal shank pad position (p < 0.05). Tendon stiffness was lower for P compared with T, but only in intervals up to 50% of TFmax (p < 0.05). The agreement between T and P for stiffness calculated between 50% and TFmax was acceptable with the distal, but poor with the proximal pad position. We demonstrated that using the tibia plateau and not the insertion as tracking landmark clearly affects the assessment of the force-elongation curve of the patellar tendon. However, using a distal point of resistive force application and calculating tendon stiffness between 50% and TFmax seems to yield an acceptable agreement between landmarks. These findings have important implications for the assessment of tendon properties in vivo and cross-study comparisons.

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The compliance of elastic elements allows muscles to dissipate energy safely during eccentric contractions. This buffering function is well documented in animal models but our understanding of its mechanism in humans is confined to non-specific tasks, requiring a subsequent acceleration of the body. The present study aimed to examine the behaviour of the human triceps surae muscle-tendon unit (MTU) during a pure energy dissipation task, under two loading conditions. Thirty-nine subjects performed a single-leg landing task, with and without added mass. Ultrasound measurements were combined with three-dimensional kinematics and kinetics to determine instantaneous length changes of MTUs, muscle fascicles, Achilles tendon and combined elastic elements. Gastrocnemius and soleus MTUs lengthened during landing. After a small concentric action, fascicles contracted eccentrically during most of the task, whereas plantar flexor muscles were activated. Combined elastic elements lengthened until peak ankle moment and recoiled thereafter, whereas no recoil was observed for the Achilles tendon. Adding mass resulted in greater negative work and MTU lengthening, which were accompanied by a greater stretch of tendon and elastic elements and a greater recruitment of the soleus muscle, without any further fascicle strain. Hence, the buffering action of elastic elements delimits the maximal strain and lengthening velocity of active muscle fascicles and is commensurate with loading constraints. In the present task, energy dissipation was modulated via greater MTU excursion and more forceful eccentric contractions. The distinct strain pattern of the Achilles tendon supports the notion that different elastic elements may not systematically fulfil the same function.

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Introduction: During running and jumping activities, elastic energy is utilized to enhance muscle mechanical output and efficiency. However, training-induced variations in tendon spring-like properties remain under-investigated. The present work extends earlier findings on sport-specific profiles of tendon stiffness and cross-sectional area to examine whether years of distinct loading patterns are reflected by tendons' ability to store and return energy. Methods:Ultrasound scans were performed to examine the morphological features of knee extensor and plantar flexor muscle-tendon units in elite ski jumpers, distance runners, water polo players, and sedentary controls. Tendon strain energy and hysteresis were measured with combined motion capture, ultrasonography, and dynamometry. Results: Apart from the fractional muscle-to-tendon cross-sectional area ratio being lower in the knee extensors of ski jumpers (-31%) and runners (-33%) than in water polo players, no difference in the considered muscle-tendon unit morphological features was observed between groups. Similarly, no significant difference in tendon energy storage or energy return was detected between groups. In contrast, hysteresis was lower in the patellar tendon of ski jumpers (-33%) and runners (-30%) compared to controls, with a similar trend for the Achilles tendon (significant interaction effect and large effect sizes η2 = 0.2). Normalized to body mass, the recovered strain energy of the patellar tendon was ~50% higher in ski jumpers than in water polo players and controls. For the Achilles tendon, recovered strain energy was ~40% higher in ski jumpers and runners than in controls. Discussion: Advantageous mechanical properties related to tendon spring-like function are observed in elite athletes whose sport require effective utilization of elastic energy. However, the mechanisms underpinning the better tendon capacity of some athletes to retain elastic energy could not be ascribed to intrinsic or morphological features of the lower limb muscle-tendon unit.

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The present study aimed to determine whether distinct sets of tendon properties are seen in athletes engaged in sports with contrasting requirements for tendon function and structural integrity. Patellar and Achilles tendon morphology and force-deformation relation were measured by combining ultrasonography, electromyography and dynamometry in elite ski jumpers, distance runners, water polo players and sedentary individuals. Tendon cross-sectional area normalized to body mass2/3 was smaller in water polo players than in other athletes (patellar and Achilles tendon; -28 to -24%) or controls (patellar tendon only; -9%). In contrast, the normalized cross-sectional area was larger in runners (patellar tendon only; +26%) and ski jumpers (patellar and Achilles tendon; +21% and +13%, respectively) than in controls. Tendon stiffness normalized to body mass2/3 only differed in ski jumpers, compared to controls (patellar and Achilles tendon; +11% and +27%, respectively) and to water polo players (Achilles tendon only; +23%). Tendon size appears as an adjusting variable to changes in loading volume and/or intensity, possibly to preserve ultimate strength or fatigue resistance. However, uncoupled morphological and mechanical properties indicate that functional requirements may also influence tendon adaptations.

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Low cellular activity and slow tissue turnover in human tendon may prolong resolution of tendinopathy. This may be stimulated by moderate localized traumas such as needle penetrations, but whether this results in a widespread cellular response in tendons is unknown. In an initial hypothesis-generating study, a trauma-induced tendon cell activity (increased total RNA and collagen I mRNA) was observed after repeated patellar tendon biopsies in young men. In a subsequent controlled study, 25 young men were treated with two 0.8-mm-diameter needle penetrations [n = 13, needle-group (NG)] or one 2.1-mm-diameter needle biopsy [n = 12, biopsy-group (BG)] in one patellar tendon. Four weeks later biopsies were taken from treated (5 mm lateral from trauma site) and contralateral tendons for analyses of RNA content (ribogreen assay), DNA content (PCR based), and gene expression for relevant target genes (Real-time RT-PCR) (NG, n = 11 and BG, n = 8). Intervention increased RNA content, and mRNA expression of collagen I and III and TGF-ß1 (P < 0.05), with biopsy treatment having greatest effect (tendency for RNA and collagen I). Results for DNA content were inconclusive, and no changes were detected in expression of insulin-like growth factor-I, connective tissue growth factor, scleraxis, decorin, fibromodulin, tenascin-C, tenomodulin, VEGFa, CD68, IL-6, MMP12, and MMP13. In conclusion, a moderate trauma to a healthy human tendon (e.g., biopsy sampling) results in a widespread upregulation of tendon cell activity and their matrix protein expression. The findings have implications for design of studies on human tendon and may provide perspectives in future treatment strategies in tendinopathy.

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INTRODUCTION: In vivo measurements have been used in the past two decades to investigate the effects of increased loading on tendon properties, yet the current understanding of tendon macroscopic changes to training is rather fragmented, limited to reports of tendon stiffening, supported by changes in material properties and/or tendon hypertrophy. The main aim of this review was to analyze the existing literature to gain further insights into tendon adaptations by extracting patterns of dose-response and time-course. METHODS: PubMed/Medline, SPORTDiscus, and Google Scholar databases were searched for studies examining the effect of training on material, mechanical, and morphological properties via longitudinal or cross-sectional designs. RESULTS: Thirty-five of 6440 peer-reviewed articles met the inclusion criteria. The key findings were i) the confirmation of a nearly systematic adaptation of tendon tissue to training, ii) the important variability in the observed changes in tendon properties between and within studies, and iii) the absence of a consistent incremental pattern regarding the dose-response or the time-course relation of tendon adaptation within the first months of training. However, long-term (years) training was associated with a larger tendon cross-sectional area, without any evidence of differences in material properties. Our analysis also highlighted several gaps in the existing literature, which may be addressed in future research. CONCLUSIONS: In line with some cross-species observations about tendon design, tendon cross-sectional area allegedly constitutes the ultimate adjusting parameter to increased loading. We propose here a theoretical model placing tendon hypertrophy and adjustments in material properties as parts of the same adaptive continuum.

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Costameres are mechano-sensory sites of focal adhesion in the sarcolemma that provide a structural anchor for myofibrils. Their turnover is regulated by integrin-associated focal adhesion kinase (FAK). We hypothesized that changes in content of costamere components (beta 1 integrin, FAK, meta-vinculin, gamma-vinculin) with increased and reduced loading of human anti-gravity muscle would: (i) relate to changes in muscle size and molecular parameters of muscle size regulation [p70S6K, myosin heavy chain (MHC)1 and MHCIIA]; (ii) correspond to adjustments in activity and expression of FAK, and its negative regulator, FRNK; and (iii) reflect the temporal response to reduced and increased loading. Unloading induced a progressive decline in thickness of human vastus lateralis muscle after 8 and 34 days of bedrest (-4% and -14%, respectively; n = 9), contrasting the increase in muscle thickness after 10 and 27 days of resistance training (+5% and +13%; n = 6). Changes in muscle thickness were correlated with changes in cross-sectional area of type I muscle fibers (r = 0.66) and beta 1 integrin content (r = 0.76) at the mid-point of altered loading. Changes in meta-vinculin and FAK-pY397 content were correlated (r = 0.85) and differed, together with the changes of beta 1 integrin, MHCI, MHCII and p70S6K, between the mid- and end-point of resistance training. By contrast, costamere protein level changes did not differ between time points of bedrest. The findings emphasize the role of FAK-regulated costamere turnover in the load-dependent addition and removal of myofibrils, and argue for two phases of muscle remodeling with resistance training, which do not manifest at the macroscopic level.

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BACKGROUND: Although differences in mechanical properties between symptomatic and healthy tendons have been observed for the Achilles tendon, the impact of tendinopathy on patellar tendon mechanics is not fully documented. The aim of the present case-control study was to assess the mechanical properties of the tendon and jump performance in elite athletes with and without patellar tendinopathy. METHODS: We identified 17 male volleyball players with patellar tendinopathy and 18 healthy matched controls from a 5-year prospective cohort study on junior elite volleyball players. Outcome variables included three measures of maximal vertical jump performance and ultrasound-based assessments of patellar tendon cross-sectional area, stiffness and Young's modulus. RESULTS: The proximal cross-sectional area of the patellar tendon was significantly larger in the tendinopathic group (133 ± 11 vs 112 ± 9 mm(2), respectively; p < 0.001). Pathological tendons presented lower stiffness (2254 ± 280 vs 2826 ± 603 N/mm, respectively; p = 0.006) and Young's modulus (0.99 ± 0.16 vs 1.17 ± 0.25 GPa, respectively; p = 0.04) than healthy tendons. However, the difference between the countermovement jump height and the squat jump height (3.4 ± 2.2 vs 1.2 ± 1.5 cm, p = 0.005) was significantly higher in the tendinopathic group compared with the control group. CONCLUSIONS: Patellar tendinopathy is associated with a decrease in the mechanical and material properties of the tendon in elite athletes subjected to a high volume of jumping activity. However, compared with their healthy counterparts, tendinopathic volleyball players have a better ability to utilise the stretch-shortening cycle when jumping.

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