Effect of acute resistance exercise and sex on human patellar tendon structural and regulatory mRNA expression

Assessing low-back loading during lifting using personalized electromyography-driven trunk models and NIOSH-based risk levels

Workplace injury risk due to physically demanding tasks (e.g., repeated lifting) is currently assessed using ergonomic guidelines. The Revised NIOSH Lifting Equation (RNLE) is a commonly used approach that assesses risk of low-back loading during different lifting tasks. Advances in musculoskeletal models have enabled the estimation of physiologically valid person-specific musculoskeletal models (pEMS) driven by surface electromyography and joint angle information. These models offer realistic estimates of objective parameters such as moments and compressive and shear loads at the lumbosacral joint. In this study, we applied both techniques (RNLE and pEMS) to assess risk and low-back loading in seven healthy participants performing lifting tasks at different risk levels. We found that the pEMS estimated objective parameters of low-back loading in line with the different risk levels proposed by RNLE. However, the low-back compressive and shear loads were higher than the limits proposed by the RNLE. Moreover, we show that the lumbosacral compressive loads can be a better parameter to demarcate risk levels. We recommend performing this assessment on a larger and diverse population for evaluation of personalized risk levels across lifting tasks in the industry. These approaches can be implemented with wearable sensorized garments to monitor personalized musculoskeletal health unobtrusively in the workplace providing us a better insight into possibility of individual risk.

Female Tendons are from Venus and Male Tendons are from Mars, But Does it Matter for Tendon Health?

Tendons play fundamental roles in the execution of human movement and therefore understanding tendon function, health and disease is important for everyday living and sports performance. The acute mechanical behavioural and physiological responses to short-term loading of tendons, as well as more chronic morphological and mechanical adaptations to longer term loading, differ between sexes. This has led some researchers to speculate that there may be a sex-specific injury risk in tendons. However, the link between anatomical, physiological and biomechanical sex-specific differences in tendons and their contributory role in the development of tendon disease injuries has not been critically evaluated. This review outlines the evidence surrounding the sex-specific physiological and biomechanical responses and adaptations to loading and discusses how this evidence compares to clinical evidence on tendon injuries and rehabilitation in the Achilles and patellar tendons in humans. Using the evidence available in both sports science and medicine, this may provide a more holistic understanding to improve our ability to enhance human tendon health and performance in both sexes.

Achilles tendinopathy research has a gender data gap: A systematic review and meta-analysis

PURPOSE

A persisting gender bias has been recently highlighted in orthopaedics and sports medicine. The aim of this study was to evaluate the volume of gender-specific data and gender-specific results in the treatment of a common tendon disease, Achilles tendinopathy.

METHODS

Pubmed, Cochrane, and Web of Science were searched to identify all clinical studies focusing on Achilles tendinopathy treatment. The Visual Analogue Scale (VAS) and Victorian Institute of Sport Assessment-Achilles (VISA-A) data of women and men of the studies that disaggregated results by gender were collected, and a meta-analysis was conducted. Treatment response within and in between gender categories was evaluated, focusing on overall gender-disaggregated data, as well as within each of the three treatment categories: conservative treatment, injective treatment and surgical treatment. A formal risk of bias analysis was conducted using Downs and Black's grading system.

RESULTS

Out of the 8796 papers screened, 178 were included after the screening. The number of female study participants grew from 20% up to 1990 to 48% in the years 2019-2022. Only 373 out of 3423 (11%) female patients and 685 of 4352 (16%) male patients were found in sex-disaggregated studies. A meta-analysis was conducted on the 14 papers that reported sex-disaggregated data for VAS and VISA-A. The meta-analysis revealed that there was no difference in the overall treatment response between women and men and that both genders showed an overall significant treatment benefit in terms of VAS and VISA-A values. However, significant differences were documented within the treatment categories. While no differences were found in surgical studies, in conservative treatment studies, men experienced lower posttreatment VAS values (p = 0.004). The largest difference was found in injective treatments, with men experiencing a larger change in VAS values (men = -3.0, women = -1.0, p = 0.016) and higher posttreatment VISA-A values (p = 0.032).

CONCLUSION

This systematic review and meta-analysis showed a lack of awareness of the importance of sex-specific data within Achilles tendinopathy treatment research. The proportion of female study subjects has grown over the years, but there is still a large data gap caused by the absence of sex-disaggregated data. The omission of sex-disaggregated data causes the loss of valuable knowledge on the true effectiveness of current Achilles tendinopathy treatment. The results of this study indicate that women profit less from available treatments, particularly injective approaches, which prompts further research for treatment adaptation by gender.

LEVEL OF EVIDENCE

Level IV.

The Effect of Specific Bioactive Collagen Peptides on Tendon Remodeling during 15 wk of Lower Body Resistance Training

PURPOSE

Collagen peptide supplementation has been reported to enhance synthesis rates or growth in a range of musculoskeletal tissues and could enhance tendinous tissue adaptations to resistance training (RT). This double-blind placebo-controlled study aimed to determine if tendinous tissue adaptations, size (patellar tendon cross-sectional area (CSA) and vastus lateralis (VL) aponeurosis area), and mechanical properties (patellar tendon), after 15 wk of RT, could be augmented with collagen peptide (CP) versus placebo (PLA) supplementation.

METHODS

Young healthy recreationally active men were randomized to consume either 15 g of CP ( n = 19) or PLA ( n = 20) once every day during a standardized program of lower-body RT (3 times a week). Measurements pre- and post-RT included patellar tendon CSA and VL aponeurosis area (via magnetic resonance imaging), and patellar tendon mechanical properties during isometric knee extension ramp contractions.

RESULTS

No between-group differences were detected for any of the tendinous tissue adaptations to RT (ANOVA group-time, 0.365 ≤ P ≤ 0.877). There were within-group increases in VL aponeurosis area (CP, +10.0%; PLA, +9.4%), patellar tendon stiffness (CP, +17.3%; PLA, +20.9%) and Young's modulus (CP, +17.8%; PLA, +20.6%) in both groups (paired t -tests (all), P ≤ 0.007). There were also within-group decreases in patellar tendon elongation (CP, -10.8%; PLA, -9.6%) and strain (CP, -10.6%; PLA, -8.9%) in both groups (paired t -tests (all), P ≤ 0.006). Although no within-group changes in patellar tendon CSA (mean or regional) occurred for CP or PLA, a modest overall time effect ( n = 39) was observed for mean (+1.4%) and proximal region (+2.4%) patellar tendon CSA (ANOVA, 0.017 ≤ P ≤ 0.048).

CONCLUSIONS

In conclusion, CP supplementation did not enhance RT-induced tendinous tissue remodeling (either size or mechanical properties) compared with PLA within a population of healthy young men.

Exercise builds the scaffold of life: muscle extracellular matrix biomarker responses to physical activity, inactivity, and aging

Skeletal muscle extracellular matrix (ECM) is critical for muscle force production and the regulation of important physiological processes during growth, regeneration, and remodelling. ECM remodelling is a tightly orchestrated process, sensitive to multi-directional tensile and compressive stresses and damaging stimuli, and its assessment can convey important information on rehabilitation effectiveness, injury, and disease. Despite its profound importance, ECM biomarkers are underused in studies examining the effects of exercise, disuse, or aging on muscle function, growth, and structure. This review examines patterns of short- and long-term changes in the synthesis and concentrations of ECM markers in biofluids and tissues, which may be useful for describing the time course of ECM remodelling following physical activity and disuse. Forces imposed on the ECM during physical activity critically affect cell signalling while disuse causes non-optimal adaptations, including connective tissue proliferation. The goal of this review is to inform researchers, and rehabilitation, medical, and exercise practitioners better about the role of ECM biomarkers in research and clinical environments to accelerate the development of targeted physical activity treatments, improve ECM status assessment, and enhance function in aging, injury, and disease.

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.

Women Have Tendons… and Tendinopathy: Gender Bias is a "Gender Void" in Sports Medicine with a Lack of Women Data on Patellar Tendinopathy-A Systematic Review

INTRODUCTION

Patellar tendinopathy is one of the most common musculoskeletal problems associated with sport. While commonly perceived as a predominantly male problem, recent epidemiological studies revealed that it also affects a large number of sport-active women. The aim of this systematic review was to understand how the available treatments apply to women affected by patellar tendinopathy.

METHODS

We analysed the available literature with a systematic review on three databases (PubMed, Cochrane, Web of Science) on February 2021, retrieving a total of 136 studies published from 1983.

RESULTS

The overall scientific field offers an astonishingly low number of data on treatment results referring to only 78 women (2%) in the entire literature. Only 5% of the retrieved articles considered focusing only or mostly on men to be a limitation.

CONCLUSIONS

Women represent only a minority of patients studied for this topic. The few documented cases are further fragmented by being related to different treatments, thus basically offering no solid evidence for results and limitations of any therapeutic approach in women. This literature analysis showed a greater gender gap than what is recognized in science and general medicine; it showed a gender blindness in sports medicine when investigating a common problem like patellar tendinopathy.

The role of loading in murine models of rotator cuff disease

Rotator cuff disease pathogenesis is associated with intrinsic (e.g., age, joint laxity, muscle weakness) and extrinsic (e.g., mechanical load, fatigue) factors that lead to chronic degeneration of the cuff tissues. However, etiological studies are difficult to perform in patients due to the long duration of disease onset and progression. Therefore, the purpose of this study was to determine the effects of altered joint loading on the rotator cuff. Mice were subjected to one of three load-dependent rotator cuff tendinopathy models: underuse loading, achieved by injecting botulinum toxin-A into the supraspinatus muscle; overuse loading, achieved using downhill treadmill running; destabilization loading, achieved by surgical excision of the infraspinatus tendon. All models were compared to cage activity animals. Whole joint function was assessed longitudinally using gait analysis. Tissue-scale structure and function were determined using microCT, tensile testing, and histology. The molecular response of the supraspinatus tendon and enthesis was determined by measuring the expression of 84 wound healing-associated genes. Underuse and destabilization altered forepaw weight-bearing, decreased tendon-to-bone attachment strength, decreased mineral density of the humeral epiphysis, and reduced tendon strength. Transcriptional activity of the underuse group returned to baseline levels by 4 weeks, while destabilization had significant upregulation of inflammation, growth factors, and extracellular matrix remodeling genes. Surprisingly, overuse activity caused changes in walking patterns, increased tendon stiffness, and primarily suppressed expression of wound healing-related genes. In summary, the tendinopathy models demonstrated how divergent muscle loading can result in clinically relevant alterations in rotator cuff structure, function, and gene expression.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

Mechanical properties of human patellar tendon collagen fibrils. An exploratory study of aging and sex

Tendons are connective tissues that transmit mechanical forces from muscle to bone and consist mainly of nano-scale fibrils of type I collagen. Aging has been associated with reduced mechanical function of tendons at the whole-tendon level and also with increased glycation of tendon collagen fibrils. Yet, the mechanical effects of aging at the fibril level remain unknown. In vitro glycation has previously been reported to substantially increase fibril strength and stiffness in young rats, suggesting a potentially large effect of aging through the glycation mechanism. We therefore expected that aging would have a similar major impact on fibril mechanical properties. In addition, differences in fibril mechanical properties between men and women have never been studied. This study investigated human patellar tendon biopsies from young (26 ± 4 years) and elderly (66 ± 1 years), men and women by measuring the mechanical properties of individual collagen fibrils using a custom nano-mechanical device. There were no major mechanical differences with either age or sex, but there were modestly greater failure stress (22%) and tensile modulus at both low and high strain (16% and 26% respectively) in the elderly group. No significant differences in mechanical properties were observed between men and women. The slightly greater strength and stiffness in the elderly group are in contrasts to the age-related deficits observed for whole-tendons in vivo, although the study was not designed to investigate these minor differences.

The impact of collagen protein ingestion on musculoskeletal connective tissue remodeling: a narrative review

Collagen is the central structural component of extracellular connective tissue, which provides elastic qualities to tissues. For skeletal muscle, extracellular connective tissue transmits contractile force to the tendons and bones. Connective tissue proteins are in a constant state of remodeling and have been shown to express a high level of plasticity. Dietary-protein ingestion increases muscle protein synthesis rates. High-quality, rapidly digestible proteins are generally considered the preferred protein source to maximally stimulate myofibrillar (contractile) protein synthesis rates. In contrast, recent evidence demonstrates that protein ingestion does not increase muscle connective tissue protein synthesis. The absence of an increase in muscle connective tissue protein synthesis after protein ingestion may be explained by insufficient provision of glycine and/or proline. Dietary collagen contains large amounts of glycine and proline and, therefore, has been proposed to provide the precursors required to facilitate connective tissue protein synthesis. This literature review provides a comprehensive evaluation of the current knowledge on the proposed benefits of dietary collagen consumption to stimulate connective tissue remodeling to improve health and functional performance.

Human adipose and skeletal muscle tissue DNA, RNA, and protein content

The purpose of this project was to provide a profile of DNA, RNA, and protein content in adipose tissue, which is relatively understudied in humans, to gain more insight into the amount of tissue that may be required for various analyses. Skeletal muscle tissue was also investigated to provide a direct comparison into potential differences between these two highly metabolically active tissues. Basal adipose and skeletal muscle tissue samples were obtained from 10 (7 M, 3 W) recreationally active participants [25 ± 1 yr; 84 ± 3 kg, maximal oxygen consumption (V̇o_2max): 3.5 ± 0.2 L/min, body fat: 29 ± 2%]. DNA, RNA, and protein were extracted and subsequently analyzed for quantity and quality. DNA content of adipose and skeletal muscle tissue was 52 ± 14 and 189 ± 44 ng DNA·mg tissue^-1, respectively (P < 0.05). RNA content of adipose and skeletal muscle tissue was 46 ± 14 and 537 ± 72 ng RNA·mg tissue^-1, respectively (P < 0.05). Protein content of adipose and skeletal muscle tissue was 4 ± 1 and 177 ± 10 µg protein·mg tissue^-1, respectively (P < 0.05). In summary, human adipose had 28% of the DNA, 9% of the RNA, and 2% of the protein found in skeletal muscle per mg of tissue. This information should be useful across a wide range of human clinical investigation designs and various laboratory analyses.NEW & NOTEWORTHY This investigation studied DNA, RNA, and protein contents of adipose and skeletal muscle tissues from young active individuals. A series of optimization steps were investigated to aid in determining the optimal approach to extract high-yield and high-quality biomolecules. These findings contribute to the knowledge gap in adipose tissue requirements for molecular biology assays, which is of increasing importance due to the growing interest in adipose tissue research involving human exercise physiology research.

Tendon and multiomics: advantages, advances, and opportunities

Tendons heal by fibrosis, which hinders function and increases re-injury risk. Yet the biology that leads to degeneration and regeneration of tendons is not completely understood. Improved understanding of the metabolic nuances that cause diverse outcomes in tendinopathies is required to solve these problems. 'Omics methods are increasingly used to characterize phenotypes in tissues. Multiomics integrates 'omic datasets to identify coherent relationships and provide insight into differences in molecular and metabolic pathways between anatomic locations, and disease stages. This work reviews the current literature pertaining to multiomics in tendon and the potential of these platforms to improve tendon regeneration. We assessed the literature and identified areas where 'omics platforms contribute to the field: (1) Tendon biology where their hierarchical complexity and demographic factors are studied. (2) Tendon degeneration and healing, where comparisons across tendon pathologies are analyzed. (3) The in vitro engineered tendon phenotype, where we compare the engineered phenotype to relevant native tissues. (4) Finally, we review regenerative and therapeutic approaches. We identified gaps in current knowledge and opportunities for future study: (1) The need to increase the diversity of human subjects and cell sources. (2) Opportunities to improve understanding of tendon heterogeneity. (3) The need to use these improvements to inform new engineered and regenerative therapeutic approaches. (4) The need to increase understanding of the development of tendon pathology. Together, the expanding use of various 'omics platforms and data analysis resulting from these platforms could substantially contribute to major advances in the tendon tissue engineering and regenerative medicine field.

Tendon Extracellular Matrix Assembly, Maintenance and Dysregulation Throughout Life

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

Lifting Activity Assessment Using Kinematic Features and Neural Networks

Work-related low-back disorders (WLBDs) can be caused by manual lifting tasks. Wearable devices used to monitor these tasks can be one possible way to assess the main risk factors for WLBDs. This study aims at analyzing the sensitivity of kinematic data to the risk level changes, and to define an instrument-based tool for risk classification by using kinematic data and artificial neural networks (ANNs). Twenty workers performed lifting tasks, designed by following the rules of the revised NIOSH lifting equation, with an increasing lifting index (LI). From the acquired kinematic data, we computed smoothness parameters together with kinetic, potential and mechanical energy. We used ANNs for mapping different set of features on LI levels to obtain an automatic risk estimation during these tasks. The results show that most of the calculated kinematic indexes are significantly affected by changes in LI and that all the lifting condition pairs can be correctly distinguished. Furthermore, using specific set of features, different topologies of ANNs can lead to a reliable classification of the biomechanical risk related to lifting tasks. In particular, the training sets and numbers of neurons in each hidden layer influence the ANNs performance, which is instead independent from the numbers of hidden layers. Reliable biomechanical risk estimation can be obtained by using training sets combining body and load kinematic features.

Factors associated with positive outcomes of platelet-rich plasma therapy in Achilles tendinopathy

BACKGROUND

The efficacy of platelet-rich plasma in the treatment for Achilles tendinopathy is debated. Therefore, it is important to know which factors, related to the subjects and/or the disease, are associated with positive or negative outcomes. Aim of this study was to evaluate in a large cohort of patients with Achilles mid-portion tendinopathy which variables were independently associated with a positive outcome after platelet-rich plasma treatment.

MATERIAL AND METHODS

Eighty-four subjects with Achilles tendinopathy were evaluated by means of VISA-A score and ultrasound and treated with a single platelet-rich plasma injection once a week for 3 weeks. Afterward, a rehabilitation program, based on eccentric training, was implemented. At 3 and 6 months, the relationship between the mean VISA-A score and the following putative predictors was evaluated: sex, age, physical activity, sport, smoking, metabolic risk factors, BMI, symptoms duration, tendon damage, neovessels, adherence to eccentric training. Finally, the percentage of clinically evident positive outcomes (defined as an increase in VISA-A score ≥ 20 points) related to each variable was computed.

RESULTS

At final follow-up, using the General Linear Model for Repeated Measures procedure, male sex (0.02), age ≤ 40 (0.05) and adequate eccentric training (0.02) were found to be independently associated with a significant increase in the mean VISA-A score. Moreover, the clinically evident positive outcomes, as previously defined, were significantly associated with male sex (0.01), age ≤ 40 (0.000), BMI ≤ 25 (0.001), symptoms duration ≤ 12 months (0.02) and good adherence to eccentric training (0.004).

CONCLUSION

Younger age, male sex and good adherence to eccentric training can be considered predictors of better results after platelet-rich plasma therapy in Achilles tendinopathy.

Whole body vibration elicits differential immune and metabolic responses in obese and normal weight individuals

Traditional aerobic exercise reduces the risk of developing chronic diseases by inducing immune, metabolic, and myokine responses. Following traditional exercise, both the magnitude and time-course of these beneficial responses are different between obese compared to normal weight individuals. Although obesity may affect the ability to engage in traditional exercise, whole body vibration (WBV) has emerged as a more tolerable form of exercise . The impact of WBV on immune, metabolic, and myokine responses as well as differences between normal weight and obese individuals, however, is unknown.

Purpose

To determine if WBV elicits differential magnitudes and time-courses of immune, metabolic, and myokine responses between obese and normal weight individuals.

Methods

21 participants [Obese (OB): n = 11, Age: 33 ± 4 y, percent body fat (%BF): 39.1 ± 2.4% & Normal weight (NW) n = 10, Age: 28 ± 8 y, %BF: 17.4 ± 2.1%] engaged in 10 cycles of WBV exercise [1 cycle = 1 min of vibration followed by 30 s of rest]. Blood samples were collected pre-WBV (PRE), immediately (POST), 3 h (3H), and 24 h (24H) post-WBV and analyzed for leukocytes, insulin, glucose, and myokines (IL-6, decorin, myostatin).

Results

The peak (3H) percent change in neutrophil counts (OB: 13.9 ± 17.4 vs. NW: 47.2 ± 6.2%Δ; p = 0.007) was different between groups. The percent change in neutrophil percentages was increased in NW (POST: -1.6 ± 2.0 vs. 3H: 13.0 ± 7.2%Δ, p = 0.019) but not OB (p > 0.05). HOMA β-cell function was increased at 24H (PRE: 83.4 ± 5.4 vs. 24H: 131.0 ± 14.1%; p = 0.013) in NW and was not altered in OB (p > 0.05). PRE IL-6 was greater in OB compared to NW (OB: 2.7 ± 0.6 vs. NW: 0.6 ± 0.1 pg/mL; p = 0.011); however, the percent change from PRE to peak (3H) was greater in NW (OB: 148.1 ± 47.9 vs. NW: 1277.9 ± 597.6 %Δ; p = 0.035). Creatine kinase, decorin, and myostatin were not significantly altered in either group (p > 0.05).

Conclusion

Taken together, these data suggest that acute whole body vibration elicits favorable immune, metabolic, and myokine responses and that these responses differ between obese and normal weight individuals.

Multiscale computational model of Achilles tendon wound healing: Untangling the effects of repair and loading

Mechanical stimulation of the healing tendon is thought to regulate scar anisotropy and strength and is relatively easy to modulate through physical therapy. However, in vivo studies of various loading protocols in animal models have produced mixed results. To integrate and better understand the available data, we developed a multiscale model of rat Achilles tendon healing that incorporates the effect of changes in the mechanical environment on fibroblast behavior, collagen deposition, and scar formation. We modified an OpenSim model of the rat right hindlimb to estimate physiologic strains in the lateral/medial gastrocnemius and soleus musculo-tendon units during loading and unloading conditions. We used the tendon strains as inputs to a thermodynamic model of stress fiber dynamics that predicts fibroblast alignment, and to determine local collagen synthesis rates according to a response curve derived from in vitro studies. We then used an agent-based model (ABM) of scar formation to integrate these cell-level responses and predict tissue-level collagen alignment and content. We compared our model predictions to experimental data from ten different studies. We found that a single set of cellular response curves can explain features of observed tendon healing across a wide array of reported experiments in rats–including the paradoxical finding that repairing transected tendon reverses the effect of loading on alignment–without fitting model parameters to any data from those experiments. The key to these successful predictions was simulating the specific loading and surgical protocols to predict tissue-level strains, which then guided cellular behaviors according to response curves based on in vitro experiments. Our model results provide a potential explanation for the highly variable responses to mechanical loading reported in the tendon healing literature and may be useful in guiding the design of future experiments and interventions.

Tendons and ligaments transmit force between muscles and bones throughout the body and are comprised of highly aligned collagen fibers that help bear high loads. The Achilles tendon is exposed to exceptionally high loads and is prone to rupture. When damaged Achilles tendons heal, they typically have reduced strength and stiffness, and while most believe that appropriate physical therapy can help improve these mechanical properties, both clinical and animal studies of mechanical loading following injury have produced highly variable and somewhat disappointing results. To help better understand the effects of mechanical loading on tendon healing and potentially guide future therapies, we developed a computational model of rat Achilles tendon healing and showed that we could predict the main effects of different mechanical loading and surgical repair conditions reported across a wide range of published studies. Our model offers potential explanations for some surprising findings of prior studies and for the high variability observed in those studies and may prove useful in designing future therapies or experiments to test new therapies.

Tendinous Tissue Adaptation to Explosive- vs. Sustained-Contraction Strength Training

The effect of different strength training regimes, and in particular training utilizing brief explosive contractions, on tendinous tissue properties is poorly understood. This study compared the efficacy of 12 weeks of knee extensor explosive-contraction (ECT; n = 14) vs. sustained-contraction (SCT; n = 15) strength training vs. a non-training control (n = 13) to induce changes in patellar tendon and knee extensor tendon-aponeurosis stiffness and size (patellar tendon, vastus-lateralis aponeurosis, quadriceps femoris muscle) in healthy young men. Training involved 40 isometric knee extension contractions (three times/week): gradually increasing to 75% of maximum voluntary torque (MVT) before holding for 3 s (SCT), or briefly contracting as fast as possible to ∼80% MVT (ECT). Changes in patellar tendon stiffness and Young's modulus, tendon-aponeurosis complex stiffness, as well as quadriceps femoris muscle volume, vastus-lateralis aponeurosis area and patellar tendon cross-sectional area were quantified with ultrasonography, dynamometry, and magnetic resonance imaging. ECT and SCT similarly increased patellar tendon stiffness (20% vs. 16%, both p < 0.05 vs. control) and Young's modulus (22% vs. 16%, both p < 0.05 vs. control). Tendon-aponeurosis complex high-force stiffness increased only after SCT (21%; p < 0.02), while ECT resulted in greater overall elongation of the tendon-aponeurosis complex. Quadriceps muscle volume only increased after sustained-contraction training (8%; p = 0.001), with unclear effects of strength training on aponeurosis area. The changes in patellar tendon cross-sectional area after strength training were not appreciably different to control. Our results suggest brief high force muscle contractions can induce increased free tendon stiffness, though SCT is needed to increase tendon-aponeurosis complex stiffness and muscle hypertrophy.

The impact of loading, unloading, ageing and injury on the human tendon

A tendon transfers force from the contracting muscle to the skeletal system to produce movement and is therefore a crucial component of the entire muscle-tendon complex and its function. However, tendon research has for some time focused on mechanical properties without any major appreciation of potential cellular and molecular changes. At the same time, methodological developments have permitted determination of the mechanical properties of human tendons in vivo, which was previously not possible. Here we review the current understanding of how tendons respond to loading, unloading, ageing and injury from cellular, molecular and mechanical points of view. A mechanistic understanding of tendon tissue adaptation will be vital for development of adequate guidelines in physical training and rehabilitation, as well as for optimal injury treatment.

Gender associated muscle-tendon adaptations to resistance training

Purpose

To compare the relative changes in muscle-tendon complex (MTC) properties following high load resistance training (RT) in young males and females, and determine any link with circulating TGFβ-1 and IGF-I levels.

Methods

Twenty-eight participants were assigned to a training group and subdivided by sex (T males [TM] aged 20±1 year, n = 8, T females [TF] aged 19±3 year, n = 8), whilst age-matched 6 males and 6 females were assigned to control groups (ConM/F). The training groups completed 8 weeks of resistance training (RT). MTC properties (Vastus Lateralis, VL) physiological cross-sectional area (pCSA), quadriceps torque, patella tendon stiffness [K], Young’s modulus, volume, cross-sectional area, and length, circulating levels of TGFβ-1 and IGF-I were assessed at baseline and post RT.

Results

Post RT, there was a significant increase in the mechanical and morphological properties of the MTC in both training groups, compared to ConM/F (p<0.001). However, there were no significant sex-specific changes in most MTC variables. There were however significant sex differences in changes in K, with females exhibiting greater changes than males at lower MVC (Maximal Voluntary Contraction) force levels (10% p = 0.030 & 20% MVC p = 0.032) and the opposite effect seen at higher force levels (90% p = 0.040 & 100% MVC p = 0.044). There were significant increases (p<0.05) in IGF-I in both TF and TM following training, with no change in TGFβ-1. There were no gender differences (p>0.05) in IGF-I or TGFβ-1. Interestingly, pooled population data showed that TGFβ-1 correlated with K at baseline, with no correlations identified between IGF-I and MTC properties.

Conclusions

Greater resting TGFβ-1 levels are associated with superior tendon mechanical properties. RT can impact opposite ends of the patella tendon force-elongation relationship in each sex. Thus, different loading patterns may be needed to maximize resistance training adaptations in each sex.

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

AIM

The potential for tendinous tissues to adapt to functional overload, especially after several years of exposure to heavy-resistance training, is largely unexplored. This study compared the morphological and mechanical characteristics of the patellar tendon and knee extensor tendon-aponeurosis complex between young men exposed to long-term (4 years; n = 16), short-term (12 weeks; n = 15) and no (untrained controls; n = 39) functional overload in the form of heavy-resistance training.

METHODS

Patellar tendon cross-sectional area, vastus lateralis aponeurosis area and quadriceps femoris volume, plus patellar tendon stiffness and Young's modulus, and tendon-aponeurosis complex stiffness, were quantified with MRI, dynamometry and ultrasonography.

RESULTS

As expected, long-term trained had greater muscle strength and volume (+58% and +56% vs untrained, both P < .001), as well as a greater aponeurosis area (+17% vs untrained, P < .01), but tendon cross-sectional area (mean and regional) was not different between groups. Only long-term trained had reduced patellar tendon elongation/strain over the whole force/stress range, whilst both short-term and long-term overload groups had similarly greater stiffness/Young's modulus at high force/stress (short-term +25/22%, and long-term +17/23% vs untrained; all P < .05). Tendon-aponeurosis complex stiffness was not different between groups (ANOVA, P = .149).

CONCLUSION

Despite large differences in muscle strength and size, years of resistance training did not induce tendon hypertrophy. Both short-term and long-term overload demonstrated similar increases in high-force mechanical and material stiffness, but reduced elongation/strain over the whole force/stress range occurred only after years of overload, indicating a force/strain specific time-course to these adaptations.

Sex differences in tendon structure and function

Tendons play a critical role in the transmission of forces between muscles and bones, and chronic tendon injuries and diseases are among the leading causes of musculoskeletal disability. Little is known about sex-based differences in tendon structure and function. Our objective was to evaluate the mechanical properties, biochemical composition, transcriptome, and cellular activity of plantarflexor tendons from 4 month old male and female C57BL/6 mice using in vitro biomechanics, mass spectrometry-based proteomics, genome-wide expression profiling, and cell culture techniques. While the Achilles tendons of male mice were approximately 6% larger than female mice (p < 0.05), the cell density of female mice was around 19% greater than males (p < 0.05). No significant differences in mechanical properties (p > 0.05) of plantaris tendons were observed. Mass spectrometry proteomics analysis revealed no significant difference between sexes in the abundance of major extracellular matrix (ECM) proteins such as collagen types I (p = 0.30) and III (p = 0.68), but female mice had approximately twofold elevations (p < 0.05) in less abundant ECM proteins such as fibronectin, periostin, and tenascin C. The transcriptome of male and female tendons differed by only 1%. In vitro, neither the sex of the serum that fibroblasts were cultured in, nor the sex of the ECM in which they were embedded, had profound effects on the expression of collagen and cell proliferation genes. Our results indicate that while male mice expectedly had larger tendons, male and female tendons have very similar mechanical properties and biochemical composition, with small increases in some ECM proteins and proteoglycans evident in female tendons. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2117-2126, 2017.

Female hormones: do they influence muscle and tendon protein metabolism?

Due to increased longevity, women can expect to live more than one-third of their lives in a post-menopausal state, which is characterised by low circulating levels of oestrogen and progesterone. The aim of this review is to provide insights into current knowledge of the effect of female hormones (or lack of female hormones) on skeletal muscle protein turnover at rest and in response to exercise. This review is primarily based on data from human trials. Many elderly post-menopausal women experience physical disabilities and loss of independence related to sarcopenia, which reduces life quality and is associated with substantial financial costs. Resistance training and dietary optimisation can counteract or at least decelerate the degenerative ageing process, but lack of oestrogen in post-menopausal women may reduce their sensitivity to these anabolic stimuli and accelerate muscle loss. Tendons and ligaments are also affected by sex hormones, but the effect seems to differ between endogenous and exogenous female hormones. Furthermore, the effect seems to depend on the age, and as a result influence the biomechanical properties of the ligaments and tendons differentially. Based on the present knowledge oestrogen seems to play a significant role with regard to skeletal muscle protein turnover. Therefore, oestrogen/hormonal replacement therapy may counteract the degenerative changes in skeletal muscle. Nevertheless, there is a need for greater insight into the direct and indirect mechanistic effects of female hormones before any evidence-based recommendations regarding type, dose, duration and timing of hormone replacement therapy can be provided.

Rat supraspinatus tendon responds acutely and chronically to exercise

The objective of this study was to identify acute responses and chronic adaptations of supraspinatus tendon to noninjurious exercise. We hypothesized that chronic exercise (EX) increases tendon mechanical properties, and a single exercise bout increases matrix metalloproteinase (MMP) activity acutely. Rats were divided into acute or chronic EX or cage activity groups. Animals in acute EX groups were euthanized, 3, 12, 24, 48, or 72 h upon completion of a single bout of exercise (10 m/min, 1 h) on a flat treadmill. Animals in chronic EX groups walked on a flat treadmill for 3 days or 1, 2, or 8 wk. Tendon histology, MMP activity, and mechanics were measured. A single bout of exercise trended toward reducing tendon mechanical properties, but 2 or 8 wk of chronic EX increased tendon mechanics. Cell density was not affected. Cells became rounder with chronic EX. All tendons were highly organized. MMP activity decreased after a single bout of exercise and returned to baseline by 72 h. MMP activity decreased after 8 wk of chronic EX. Decreased MMP activity may indicate an anabolic instead of catabolic response in contrast to injury. Results suggest that mild, acute decreases in MMP activity and tendon mechanics following a single exercise bout lead to enhanced tendon mechanical adaptations with repeated exercise bouts. This study defines acute and chronic changes of MMP activity, mechanical properties, and histology of the rat supraspinatus tendon in response to beneficial exercise and proposes a mechanism by which acute responses translate to chronic adaptations.NEW & NOTEWORTHY The line between beneficial exercise and overuse has not been elucidated. This study defines the acute and chronic temporal response to exercise of supraspinatus tendon in an in vivo model. We found that decreased matrix metalloproteinase activity and tendon mechanics after a single bout of exercise are followed by beneficial chronic adaptations of the tendon with repeated bouts. How the acute responses to exercise lead to chronic adaptations may distinguish beneficial exercise from overuse.

The ACL injury response: A collagen-based analysis

BACKGROUND

Anterior cruciate ligament (ACL) injuries do not effectively heal. Tendon graft tissue after reconstruction shows rapid tissue turnover and 'ligamentization.' It is unknown whether native torn ACL tissue undergoes significant collagen turnover after injury or is arrested by the intraarticular environment. It is also unknown whether injury mechanism or chronicity affect torn ligament tissue turnover.

METHODS

Thirty-three mid-substance ACL biopsies were obtained during primary arthroscopic ACL reconstruction (n=31; nine contact injuries, 22 non-contact injuries, 22 males, 11 females; mean age 28.5 years; median injury to surgery time 12 weeks), or from cadavers as uninjured ACL (n=2). As a marker for collagen turnover, immature collagen cross-link content was determined by ninhydrin reagent assays. The immature cross-link content was assessed against injury mechanism, patient age, and injury to surgery time. Histochemical analysis was conducted on two uninjured ACL cadaveric controls, a four-week-old ACL tear, and a four-year-old ACL tear.

RESULTS

Contact and non-contact groups were not demographically different with respect to sex, patient age, injury to surgery time, and activity involvement prior to injury, which ranged from basketball to logging. Collagen crosslink content was very low across all samples, suggesting high tissue turnover between injury and surgery regardless of injury mechanism (non-contact: 1.68ng/mol, CI 0.48-2.89; contact: 1.50ng/mol, CI 0.14-2.86; p=0.842).

CONCLUSION

Collagen turnover occurs rapidly after ACL injury regardless of contact or non-contact mechanism. Robust tissue turnover starts within the first several weeks after injury and persists to some extent throughout the life of the torn ACL.

Sex-based difference in Achilles peritendinous levels of matrix metalloproteinases and growth factors after acute resistance exercise

Several recent investigations have demonstrated that the ability of various tendons to alter structural and functional properties in response to exercise are muted in women compared with men. We hypothesize that this disparity between men and women may be due to a reduced tendon production of key mediators of tendon extracellular matrix (ECM) remodeling in response to mechanical loading, e.g., exercise. Using microdialysis before and after an acute bout of resistance exercise, we evaluated Achilles peritendinous levels of insulin-like growth factor-1 (IGF-1) and interleukin-6 (IL-6), which have both been shown to increase tendon collagen synthesis. Additionally, the matrix remodeling enzymes matrix metalloproteinase-2 (MMP-2), MMP-9, and tissue inhibitor of metalloproteinase-1 (TIMP-1) were also evaluated. IGF-1 levels were elevated ( P < 0.05) to a similar extent in men and women after 3 h of exercise but remained elevated at 4 h in only women. IL-6 levels were ~4-fold greater after exercise in both men and women ( P < 0.05). MMP-2 levels increased to a similar extent (~2-3-fold) in men and women ( P < 0.05). In contrast, MMP-9 increased with exercise but only in men ( P < 0.05). Last, TIMP-1 levels also increased ( P < 0.05) with exercise in men and women but the increase was more prolonged in women. In conclusion, we observed modest sex differences in tendon release of MMP-9, TIMP-1, and IGF-1 after acute resistance exercise. If such differences persist throughout a chronic exercise training, they may contribute to the reduced ability of women to adapt to exercise compared with men.

NEW & NOTEWORTHY In this investigation we utilized microdialysis of the peritendinous Achilles to evaluate potential differences between men and women in tendon production of key regulators of extracellular matrix remodeling. We demonstrate that a modest sex-specific difference exists in peritendinous levels of several key extracellular matrix modulators after an acute bout of resistance exercise.

Effect of aging and exercise on the tendon

Here, we review the literature on how tendons respond and adapt to ageing and exercise. With respect to aging, there are considerable changes early in life, but this seems to be maturation rather than aging per se. In vitro data indicate that aging is associated with a decreased potential for cell proliferation and a reduction in the number of stem/progenitor-like cells. Further, there is persuasive evidence that turnover in the core of the tendon after maturity is very slow or absent. Tendon fibril diameter, collagen content, and whole tendon size appear to be largely unchanged with aging, while glycation-derived cross-links increase substantially. Mechanically, aging appears to be associated with a reduction in modulus and strength. With respect to exercise, tendon cells respond by producing growth factors, and there is some support for a loading-induced increase in tendon collagen synthesis in humans, which likely reflects synthesis at the very periphery of the tendon rather than the core. Average collagen fibril diameter is largely unaffected by exercise, while there can be some hypertrophy of the whole tendon. In addition, it seems that resistance training can yield increased stiffness and modulus of the tendon and may reduce the amount of glycation. Exercise thereby tends to counteract the effects of aging.

Postinjury biomechanics of Achilles tendon vary by sex and hormone status

Achilles tendon ruptures are common injuries. Sex differences are present in mechanical properties of uninjured Achilles tendon, but it remains unknown if these differences extend to tendon healing. We hypothesized that ovariectomized females (OVX) and males would exhibit inferior postinjury tendon properties compared with females. Male, female, and OVX Sprague-Dawley rats (n = 32/group) underwent acclimation and treadmill training before blunt transection of the Achilles tendon midsubstance. Injured hindlimbs were immobilized for 1 wk, followed by gradual return to activity and assessment of active and passive hindlimb function. Animals were euthanized at 3 or 6 wk postinjury to assess tendon structure, mechanics, and composition. Passive ankle stiffness and range of motion were superior in females at 3 wk; however, by 6 wk, passive and active function were similar in males and females but remained inferior in OVX. At 6 wk, female tendons had greater normalized secant modulus, viscoelastic behavior, and laxity compared with males. Normalized secant modulus, cross-sectional area and tendon glycosaminoglycan composition were inferior in OVX compared with females at 6 wk. Total fatigue cycles until tendon failure were similar among groups. Postinjury muscle fiber size was better preserved in females compared with males, and females had greater collagen III at the tendon injury site compared with males at 6 wk. Despite male and female Achilles tendons withstanding similar durations of fatigue loading, early passive hindlimb function and tendon mechanical properties, including secant modulus, suggest superior healing in females. Ovarian hormone loss was associated with inferior Achilles tendon healing.

Sex Hormones and Tendon

The risk of overuse and traumatic tendon and ligament injuries differ between women and men. Part of this gender difference in injury risk is probably explained by sex hormonal differences which are specifically distinct during the sexual maturation in the teenage years and during young adulthood. The effects of the separate sex hormones are not fully elucidated. However, in women, the presence of estrogen in contrast to very low estrogen levels may be beneficial during regular loading of the tissue or during recovering after an injury, as estrogen can enhance tendon collagen synthesis rate. Yet, in active young female athletes, physiological high concentration of estrogen may enhance the risk of injuries due to reduced fibrillar crosslinking and enhanced joint laxity. In men, testosterone can enhance tendon stiffness due to an enhanced tendon collagen turnover and collagen content, but testosterone has also been linked to a reduced responsiveness to relaxin. The present chapter will focus on sex difference in tendon injury risk, tendon morphology and tendon collagen turnover, but also on the specific effects of estrogen and androgens.

The response of matrix metalloproteinase-9 and -2 to exercise

BACKGROUND

Matrix metalloproteinases (MMPs) are a major group of enzymes that play essential roles in normal functioning of diverse tissues during growth, development, and aging. However, among the MMPs little is known regarding the role of exercise in MMP-9 and MMP-2 function in humans.

OBJECTIVE

The aim of this study was to provide a systematic comprehensive review of the literature examining the effect of different exercise interventions on MMP-9 and MMP-2 in human investigations.

DATA SOURCES

A comprehensive systematic database search was performed, including PubMed/MEDLINE, Scopus, ScienceDirect, and Web of Science.

STUDY SELECTION

Both the acute and chronic effects of exercise were included for evaluation in this systematic review. Inclusion criteria included the use of any type of planned, structured, and repetitive movement and its effects on the MMP-2 and MMP-9 response (obtained from plasma samples), participants (humans only) of any age with or without diseases, sedentary participants and those involved in light, moderate, and vigorous activity, randomized controlled trials (RCTs) and clinical trials (CTs), full text article citations with no restrictions in terms of language, and scored at least 5/11 on the Physiotherapy Evidence Database (PEDro) quality scale.

STUDY APPRAISAL AND SYNTHESIS METHODS

The PEDro scale was used to appraise study quality of RCTs and CTs. Two reviewers independently reviewed the full texts of all potentially relevant articles for eligibility and disagreements were discussed and resolved.

RESULTS

Seven studies met the previously determined quality indicators and were reviewed; three were RCTs and four were CTs. In general, the quality of the studies ranged from 5 to 9 out of a maximum of 11 on the PEDro quality criteria scale. Results revealed that chronic aerobic training induces a decrease in MMP-9 and MMP-2 levels, possibly indicating a cardioprotective effect, while resistance exercise training displayed conflicting results.

CONCLUSION

Alterations in MMP-9 and MMP-2 plasma concentrations may be valuable biomarkers to reflect the influence of exercise on the inflammatory state. Nevertheless, the limited evidence available regarding the effects of exercise on the MMP-9 and MMP-2 response in human participants suggests that further studies are needed to fully define the connection between the role of exercise on the MMP-9 and MMP-2 response.

Eccentric exercise: acute and chronic effects on healthy and diseased tendons

Eccentric exercise can influence tendon mechanical properties and matrix protein synthesis. mRNA for collagen and regulatory factors thereof are upregulated in animal tendons, independent of muscular contraction type, supporting the view that tendon, compared with skeletal muscle, is less sensitive to differences in type and/or amount of mechanical stimulus with regard to expression of collagen, regulatory factors for collagen, and cross-link regulators. In overused (tendinopathic) human tendon, eccentric exercise training has a beneficial effect, but the mechanism by which this is elicited is unknown, and slow concentric loading appears to have similar beneficial effects. It may be that tendinopathic regions, as long as they are subjected to a certain magnitude of load at a slow speed, independent of whether this is eccentric or concentric in nature, can reestablish their normal tendon fibril alignment and cell morphology.

Proteomic differences between male and female anterior cruciate ligament and patellar tendon

The risk of anterior cruciate ligament (ACL) injury and re-injury is greater for women than men. Among other factors, compositional differences may play a role in this differential risk. Patellar tendon (PT) autografts are commonly used during reconstruction. The aim of the study was to compare protein expression in male and female ACL and PT. We hypothesized that there would be differences in key structural components between PT and ACL, and that components of the proteome critical for response to mechanical loading and response to injury would demonstrate significant differences between male and female. Two-dimensional liquid chromatography-tandem mass spectrometry and a label-free quantitative approach was used to identify proteomic differences between male and female PT and ACL. ACL contained less type I and more type III collagen than PT. There were tissue-specific differences in expression of proteoglycans, and ACL was enriched in elastin, tenascin C and X, cartilage oligomeric matrix protein, thrombospondin 4 and periostin. Between male and female donors, alcohol dehydrogenase 1B and complement component 9 were enriched in female compared to male. Myocilin was the major protein enriched in males compared to females. Important compositional differences between PT and ACL were identified, and we identified differences in pathways related to extracellular matrix regulation, complement, apoptosis, metabolism of advanced glycation end-products and response to mechanical loading between males and females. Identification of proteomic differences between male and female PT and ACL has identified novel pathways which may lead to improved understanding of differential ACL injury and re-injury risk between males and females.

Muscle and tendon connective tissue adaptation to unloading, exercise and NSAID

The extracellular matrix network of skeletal muscle and tendon connective tissue is primarily composed of collagen and connects the muscle contractile protein to the bones in the human body. The mechanical properties of the connective tissue are important for the effectiveness of which the muscle force is transformed into movement. Periods of unloading and exercise affect the synthesis rate of connective tissue collagen protein, whereas only sparse information exits regarding collagen protein degradation. It is likely, though, that changes in both collagen protein synthesis and degradation are required for remodeling of the connective tissue internal structure that ultimately results in altered mechanical properties of the connective tissue. Both unloading and exercise lead to increased production of growth factors and inflammatory mediators that are involved in connective tissue remodeling. Despite the fact that non-steroidal anti-inflammatory drugs seem to inhibit the healing process of connective tissue and the stimulating effect of exercise on connective tissue protein synthesis, these drugs are often consumed in relation to connective tissue injury and soreness. However, the potential effect of non-steroidal anti-inflammatory drugs on connective tissue needs further investigation.

What is the impact of inflammation on the critical interplay between mechanical signaling and biochemical changes in tendon matrix?

Mechanical loading can influence tendon collagen homeostasis in animal models, while the dynamics of the human adult tendon core tissue are more debatable. Currently available data indicate that human tendon adaptation to loading may happen primarily in the outer tendon region. A role of inflammation in this peritendinous adaptation is supported by a rise in inflammatory mediators in the peritendinous area after physiological mechanical loading in humans. This plays a role in the exercise-induced rise in tendon blood flow and peritendinous collagen synthesis. Although inflammatory activity can activate proteolytic pathways in tendon, mechanical loading can protect against matrix degradation. Acute tendon injury displays an early inflammatory response that seems to be lowered when mechanical loading is applied during regeneration of tendon. Chronically overloaded tendons (tendinopathy) do neither at rest nor after acute exercise display any enhanced inflammatory activity, and thus the basis for using anti-inflammatory medication to treat tendon overuse seems limited.

A conceptual framework for computational models of Achilles tendon homeostasis

Computational modeling of tendon lags the development of computational models for other tissues. A major bottleneck in the development of realistic computational models for Achilles tendon is the absence of detailed conceptual and theoretical models as to how the tissue actually functions. Without the conceptual models to provide a theoretical framework to guide the development and integration of multiscale computational models, modeling of the Achilles tendon to date has tended to be piecemeal and focused on specific mechanical or biochemical issues. In this paper, we present a new conceptual model of Achilles tendon tissue homeostasis, and discuss this model in terms of existing computational models of tendon. This approach has the benefits of structuring the research on relevant computational modeling to date, while allowing us to identify new computational models requiring development. The critically important functional issue for tendon is that it is continually damaged during use and so has to be repaired. From this follows the centrally important issue of homeostasis of the load carrying collagen fibrils within the collagen fibers of the Achilles tendon. Collagen fibrils may be damaged mechanically-by loading, or damaged biochemically-by proteases. Upon reviewing existing computational models within this conceptual framework of the Achilles tendon structure and function, we demonstrate that a great deal of theoretical and experimental research remains to be done before there are reliably predictive multiscale computational model of Achilles tendon in health and disease.

Lack of tissue renewal in human adult Achilles tendon is revealed by nuclear bomb (14)C

Tendons are often injured and heal poorly. Whether this is caused by a slow tissue turnover is unknown, since existing data provide diverging estimates of tendon protein half-life that range from 2 mo to 200 yr. With the purpose of determining life-long turnover of human tendon tissue, we used the (14)C bomb-pulse method. This method takes advantage of the dramatic increase in atmospheric levels of (14)C, produced by nuclear bomb tests in 1955-1963, which is reflected in all living organisms. Levels of (14)C were measured in 28 forensic samples of Achilles tendon core and 4 skeletal muscle samples (donor birth years 1945-1983) with accelerator mass spectrometry (AMS) and compared to known atmospheric levels to estimate tissue turnover. We found that Achilles tendon tissue retained levels of (14)C corresponding to atmospheric levels several decades before tissue sampling, demonstrating a very limited tissue turnover. The tendon concentrations of (14)C approximately reflected the atmospheric levels present during the first 17 yr of life, indicating that the tendon core is formed during height growth and is essentially not renewed thereafter. In contrast, (14)C levels in muscle indicated continuous turnover. Our observation provides a fundamental premise for understanding tendon function and pathology, and likely explains the poor regenerative capacity of tendon tissue.

The time course of in vivo recovery of transverse strain in high-stress tendons following exercise

OBJECTIVE

To evaluate the time course of the recovery of transverse strain in the Achilles and patellar tendon following a bout of resistance exercise.

METHODS

Seventeen healthy adults underwent sonographic examination of the right patellar (n=9) and Achilles (n=8) tendons immediately prior to and following 90 repetitions of weight-bearing quadriceps and gastrocnemius-resistance exercise performed against an effective resistance of 175% and 250% body weight, respectively. Sagittal tendon thickness was determined 20 mm from the enthesis and transverse strain, as defined by the stretch ratio, was repeatedly monitored over a 24 h recovery period.

RESULTS

Resistance exercise resulted in an immediate decrease in Achilles (t7=10.6, p<0.01) and patellar (t8=8.9, p<0.01) tendon thickness, resulting in an average transverse stretch ratio of 0.86±0.04 and 0.82±0.05, which was not significantly different between tendons. The magnitude of the immediate transverse strain response, however, was reduced with advancing age (r=0.63, p<0.01). Recovery in transverse strain was prolonged compared with the duration of loading and exponential in nature. The average primary recovery time was not significantly different between the Achilles (6.5±3.2 h) and patellar (7.1±3.2 h) tendons. Body weight accounted for 62% and 64% of the variation in recovery time, respectively.

CONCLUSIONS

Despite structural and biochemical differences between the Achilles and patellar tendon, the mechanisms underlying transverse creep recovery in vivo appear similar and are highly time dependent. These novel findings have important implications concerning the time required for the mechanical recovery of high-stress tendons following an acute bout of exercise.

Effect of acute exercise on patella tendon protein synthesis and gene expression

Evidence suggests that habitual loading can result in patellar tendon hypertrophy, especially at the proximal and distal parts of the patellar tendon. The underlying protein kinetic changes and its regulation remains controversial and human data, investigating this topic, are limited. The present study investigated how acute exercise affects growth factor production and collagen fractional synthetic rate in patellar tendon tissue from patients undergoing an anterior cruciate ligament reconstruction operation. The operation was performed by use of the bone-patellar tendon-bone method under spinal anesthesia.

Twelve subjects were randomized to one of two groups: a control group or an exercise group (1-hr unilateral knee-extension 67% of W_max 24 hours before operation). Two hours before the anterior cruciate ligament operation a flooding-dose of L-[1-¹³C]proline was given. Tissue from the most proximal part of the patellar tendon was obtained during the operation. Tendon collagen fractional synthetic rate and mRNA concentrations of TGF-β-1, CTGF, and type I and III collagen were measured.

CTGF and type I collagen expression were higher in the exercise group compared to the control group (p < 0.05). Type III collagen expression (p = 0.11), TGF-β-1 expression (p = 0.34), and collagen fractional synthetic rate (p = 0.26) did not differ between groups.

Although the expression of CTGF and type I collagen were higher, the patellar tendon collagen fractional synthetic rate was not correspondingly higher after exercise. The elevated CTGF expression in the exercise group indicates that the TGF-beta pathway could be an important link between mechanical loading and stimulation of tendon tissue type I collagen expression.

The +1245g/t polymorphisms in the collagen type I alpha 1 (col1a1) gene in polish skiers with anterior cruciate ligament injury

Objectives

The aim of this study was to examine the association of +1245G/T polymorphisms in the COL1A1 gene with ACL ruptures in Polish male recreational skiers in a case-control study.

Methods

A total of 138 male recreational skiers with surgically diagnosed primary ACL ruptures, all of whom qualified for ligament reconstruction, were recruited for this study. The control group comprised 183 apparently healthy male skiers with a comparable level of exposure to ACL injury, none of whom had any self-reported history of ligament or tendon injury. DNA samples extracted from the oral epithelial cells were genotyped for the +1245G/T polymorphisms using real-time PCR method.

Results

Genotype distributions among cases and controls conformed to Hardy-Weinberg equilibrium (p = 0.2469 and p = 0.33, respectively). There was a significant difference in the genotype distribution between skiers and controls (p = 0.045, Fisher's exact test). There was no statistical difference in allele distribution: OR 1.43 (0.91-2.25), p = 0.101 (two-sided Fisher's exact test).

Conclusions

The risk of ACL ruptures was around 1.43 times lower in carriers of a minor allele G as compared to carriers of the allele T.

The magnitude and character of resistance-training-induced increase in tendon stiffness at old age is gender specific

Human tendon mechanical properties are modified with loading. Moreover, there are indications that the training response in the tendon is gender specific. The aim of the current study was to examine whether in vivo patella tendon stiffness (K) differentially alters with training in older males compared with females. We also aimed to identify which endocrine pathway underlies the responses. Maximal knee extensor forces were also monitored to determine the training effect on muscle function. Fourteen healthy, habitually active older persons (seven males aged 74.0 ± 1.2 years (mean±SEM) and seven females aged 76.7 ± 1.2 years) were tested at baseline and after 12 weeks of weekly, progressive resistance training. With training, percentage increase in quadriceps maximum voluntary isometric force (MVC) was similar in males (2,469.6 ± 168.0 to 3,097.3 ± 261.9 N; +25.3 ± 6.1% (p < 0.01)) and females (1,728.8 ± 136.3 to 2,166.5 ± 135.8 N; +30.4 ± 15.1% (p < 0.05)), respectively. K increased more in males (338.0 ± 26.6 to 616.9 ± 58.7 N/mm; 79.8 ± 4.2% (p < 0.001)) compared to females (338.9 ± 31.0 to 373.2 ± 25.8 N/mm; +13.0 ± 3.7% (p < 0.001)). Interestingly, a pattern was found whereby below ~40% MVC, the females showed their greatest degree of K changes, whereas the males showed their greatest degree of K change above this relative force level. This gender contrast was also true at a standardised force level (1,200 N), with 5.8 ± 0.4% vs. 82.5 ± 1.8% increments in the females (i.e. value change from 380.3 ± 14.1 to 402.4 ± 13.3 N/mm) and the males (i.e. value change from 317.8 ± 13.8 to 580.2 ± 30.9 N/mm), respectively (p < 0.001). While circulating levels of both IGF-I and IL-6 did not alter with training, IGFBP-3 showed a significant training effect (19.1 ± 4.8%, p < 0.001) and only in the male sub-group (p = 0.038). We show here that with training, in vivo older females' tendon is less dramatically modulated than that of males'. We also show that the relative forces, at which the greatest adaptations are exhibited, differ by gender, with a suggestion of endocrine adaptations in males only. We thus propose that both training and rehabilitation regimens should consider gender-specific tendon responsiveness, at least in older persons.

Expression of extracellular matrix components and related growth factors in human tendon and muscle after acute exercise

Acute kicking exercise induces collagen synthesis in both tendon and muscle in humans, but it is not known if this relates to increased collagen transcription and if other matrix genes are regulated. Young men performed 1 h of one-leg kicking at 67% of max workload. Biopsies were taken from the patellar tendon and vastus lateralis muscle of each leg at 2 (n = 10), 6 (n = 11), or 26 h (n = 10) after exercise. Levels of messenger ribonucleic acid mRNA for collagens, noncollagenous matrix proteins, and growth factors were measured with real-time reverse transcription polymerase chain reaction. In tendon, gene expression was unchanged except for a decrease in insulin-like growth factor-IEa (IGF-IEa; P < 0.05). In muscle, collagen expression was not significantly altered, while levels of connective tissue growth factor (CTGF), IGF-IEa, transforming growth factor-β1, -2 (TGF-β), and the TGF-β receptor II mRNA were increased (P < 0.05). Matrix components tenascin-C, fibronectin, and decorin were also induced in loaded muscle (P < 0.05), while fibromodulin was unaffected. In conclusion, the relatively robust changes in matrix components and related growth factors in muscle indicate a stimulation of extracellular matrix even with moderate exercise. However, in tendon tissue, this exercise model does not appear to induce any anabolic response on the transcriptional level.

Cyclic loading of tendon fascicles using a novel fatigue loading system increases interleukin-6 expression by tenocytes

Repetitive strain or 'overuse' is thought to be a major factor contributing to the development of tendinopathy. The aims of our study were to develop a novel cyclic loading system, and use it to investigate the effect of defined loading conditions on the mechanical properties and gene expression of isolated tendon fascicles. Tendon fascicles were dissected from bovine-foot extensors and subjected to cyclic tensile strain (1 Hz) at 30% or 60% of the strain at failure, for 0 h (control), 15 min, 30 min, 1 h, or 5 h. Post loading, a quasi-static test to failure assessed damage. Gene expression at a selected loading regime (1 h at 30% failure strain) was analyzed 6 h post loading by quantitative real-time polymerase chain reaction. Compared with unloaded controls, loading at 30% failure strain took 5 h to lead to a significant decrease in failure stress, whereas loading to 60% led to a significant reduction after 15 min. Loading for 1 h at 30% failure strain did not create significant structural damage, but increased Collagen-1-alpha-chain-1 and interleukin-6 (IL6) expression, suggesting a role of IL6 in tendon adaptation to exercise. Correlating failure properties with fatigue damage provides a method by which changes in gene expression can be associated with different degrees of fatigue damage.

Influence of aerobic cycle exercise training on patellar tendon cross-sectional area in older women

Nine to 12 weeks of resistance exercise training in young individuals induces quadriceps muscle (∼6%) and region-specific patellar tendon (4-6%) hypertrophy. However, 12 weeks of resistance exercise training (∼1 h total exercise time) in older individuals (60-78 years) induces quadriceps muscle hypertrophy (9%) without impacting patellar tendon size. The current study examined if a different loading paradigm using cycle exercise would promote patellar tendon hypertrophy or alter the internal tendon properties, measured with magnetic resonance imaging signal intensity, in older individuals. Nine women (70 ± 2 years) completed 12 weeks of aerobic upright cycle exercise training (∼28 h total exercise time). Aerobic exercise training increased (P < 0.05) quadriceps muscle size (11 ± 2%) and VO2max (30 ± 9%). Mean patellar tendon cross-sectional area (CSA) (2 ± 1%) and signal intensity (-1 ± 2%) were unchanged (P > 0.05) over the 12 weeks of training. Region-specific CSA was unchanged (P > 0.05) at the proximal (-1 ± 3%) and mid regions (2 ± 2%) of the tendon but tended (P = 0.069) to increase at the distal region (5 ± 3%). Region-specific signal intensity differed along the tendon but was unchanged (P > 0.05) with training. Although more studies are needed, exercise-induced patellar tendon hypertrophy, compared with skeletal muscle, appears to be attenuated in older individuals, while the loading pattern associated with aerobic exercise seems to have more impact than resistance exercise in promoting patellar tendon hypertrophy.

The correlation between resting serum leptin and serum angiogenic indices at rest and after submaximal exercise

BACKGROUND

The effect of leptin as stimulant angiogenic factor has been studied. But the association of leptin levels and exercise-induced angiogenesis has not been studied. Accordingly, the researchers investigated whether there were any differences in circulating serum VEGF, MMP-2 and MMP-9 among high and low resting leptin individuals at rest or in response to submaximal exercise.

MATERIALS AND METHODS

For this purpose the researchers defined two groups with high and low resting leptin levels. Fifteen subjects with high resting leptin (23.57±9.14ng/ml and Vo(2) max=29.46±3.62ml/kg.min) and fifteen subjects with low resting leptin level (1.04±0.49ng/ml and Vo(2) max=37.99±4.63ml/kgmin) exercised for 1h (1h) at 70% of Vo(2) max. Antecubital vein blood was collected at rest, immediately and 2h post exercise. Serum VEGF, MMP-2 and MMP-9 was measured by ELISA method.

RESULTS

Results of the study showed that the resting serum levels of VEGF, MMP-2 and MMP-9 didn't have any correlation with basic levels of leptin. In low leptin group the levels of VEGF and MMP-2 in immediately post exercise decreased significantly, but in high leptin group, only VEGF decreased significantly. 2h post exercise; the VEGF level in the low resting leptin group was significantly lower than that of its basal level. Beside, MMP-2 in the high and low basic levels of leptin groups were significantly increased compared to that of immediately post exercise. But the amount of MMP-9 did not change significantly in response to exercise in two groups. There were not any differences in the changes of VEGF, MMP-2 and MMP-9 in response to exercise between two groups. Furthermore, resting leptin had a significant correlation with V0(2) max.

CONCLUSION

The obtained results showed that the serum VEGF, MMP-2 and MMP-9 did not have any correlation with basic levels of leptin. In addition, it was concluded that levels of different resting leptin is ineffective on serum levels of VEGF, MMP-2 and MMP-9 at rest and in response to exercise in normal healthy subjects.