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

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Abstract

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

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Tendinopathy

Neal Millar, Karin G. Silbernagel, Kristian Thorborg … (2021)

Tendinopathy describes a complex multifaceted pathology of the tendon, characterized by pain, decline in function and reduced exercise tolerance. The most common overuse tendinopathies involve the rotator cuff tendon, medial and lateral elbow epicondyles, patellar tendon, gluteal tendons and the Achilles tendon. The prominent histological and molecular features of tendinopathy include disorganization of collagen fibres, an increase in the microvasculature and sensory nerve innervation, dysregulated extracellular matrix homeostasis, increased immune cells and inflammatory mediators, and enhanced cellular apoptosis. Although diagnosis is mostly achieved based on clinical symptoms, in some cases, additional pain-provoking tests and imaging might be necessary. Management consists of different exercise and loading programmes, therapeutic modalities and surgical interventions; however, their effectiveness remains ambiguous. Future research should focus on elucidating the key functional pathways implicated in clinical disease and on improved rehabilitation protocols.

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What are the Main Running-Related Musculoskeletal Injuries?

Alexandre Lopes, Luiz Carlos Hespanhol Junior, Simon Yeung … (2012)

Background Musculoskeletal injuries occur frequently in runners and despite many studies about running injuries conducted over the past decades it is not clear in the literature what are the main running-related musculoskeletal injuries (RRMIs). Objective The aim of this study is to systematically review studies on the incidence and prevalence of the main specific RRMIs. Methods An electronic database search was conducted using EMBASE (1947 to October 2011), MEDLINE (1966 to October 2011), SPORTDiscus™ (1975 to October 2011), the Latin American and Caribbean Center on Health Sciences Information (LILACS) [1982 to October 2011] and the Scientific Electronic Library Online (SciELO) [1998 to October 2011] with no limits of date or language of publication. Articles that described the incidence or prevalence rates of RRMIs were considered eligible. Studies that reported only the type of injury, anatomical region or incomplete data that precluded interpretation of the incidence or prevalence rates of RRMIs were excluded. We extracted data regarding bibliometric characteristics, study design, description of the population of runners, RRMI definition, how the data of RRMIs were collected and the name of each RRMI with their rates of incidence or prevalence. Separate analysis for ultra-marathoners was performed. Among 2924 potentially eligible titles, eight studies (pooled n = 3500 runners) were considered eligible for the review. In general, the articles had moderate risk of bias and only one fulfilled less than half of the quality criteria established. Results A total of 28 RRMIs were found and the main general RRMIs were medial tibial stress syndrome (incidence ranging from 13.6% to 20.0%; prevalence of 9.5%), Achilles tendinopathy (incidence ranging from 9.1% to 10.9%; prevalence ranging from 6.2% to 9.5%) and plantar fasciitis (incidence ranging from 4.5% to 10.0%; prevalence ranging from 5.2% to 17.5%). The main ultra-marathon RRMIs were Achilles tendinopathy (prevalence ranging from 2.0% to 18.5%) and patellofemoral syndrome (prevalence ranging from 7.4% to 15.6%). Conclusion This systematic review provides evidence that medial tibia stress syndrome, Achilles tendinopathy and plantar fasciitis were the main general RRMIs, while Achilles tendinopathy and patellofemoral syndrome were the most common RRMIs for runners who participated in ultra-marathon races.

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Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading.

Michael Kjær (2004)

The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.