From mechanical loading to collagen synthesis, structural changes and function in human tendon

Abstract

The adaptive response of connective tissue to loading requires increased synthesis and turnover of matrix proteins, with special emphasis on collagen. Collagen formation and degradation in the tendon increases with both acute and chronic loading, and data suggest that a gender difference exists, in that females respond less than males with regard to an increase in collagen formation after exercise. It is suggested that estrogen may contribute toward a diminished collagen synthesis response in females. Conversely, the stimulation of collagen synthesis by other growth factors can be shown in both animal and human models where insulin‐like growth factor 1 (IGF‐I) and transforming growth factor‐β‐1 (TGF‐β‐1) expression increases to accompany or precede an increase in procollagen expression and collagen synthesis. In humans, it can be demonstrated that an increase in the interstitial concentration of TGF‐β, PGE2, IGF‐I plus its binding proteins and interleukin‐6 takes place after exercise. The increase in IGF‐I expression in tendon includes the isoform that has so far been thought only to exist in skeletal muscle (mechano growth factor). The increase in IGF‐I and procollagen expression showed a similar response whether the tendon was stimulated by concentric, isometric or eccentric muscle contraction, suggesting that strain rather that stress/torque determines the collagen‐synthesis stimulating response seen with exercise. The adaptation time to chronic loading is longer in tendon tissue compared with contractile elements of skeletal muscle or the heart, and only with very prolonged loading are significant changes in gross dimensions of the tendon observed, suggesting that habitual loading is associated with a robust change in the size and mechanical properties of human tendons. An intimate interplay between mechanical signalling and biochemical changes in the matrix is needed in tendon, such that chemical changes can be converted into adaptations in the morphology, structure and material properties.

The central nervous system – An additional consideration in ‘rotator cuff tendinopathy’ and a potential basis for understanding response to loaded therapeutic exercise

“…The term 'mechanotherapy' has been coined to describe how a programme of structured exercise might stimulate human tissue and reverse tendon deconditioning (Reeves, 2006;Abate et al, 2009;Khan and Scott, 2011). It is proposed that a progressive exercise regime will stimulate a process of re-conditioning and improve the capacity of the rotator cuff to withstand greater load and stress (McKenzie and May, 2000;Reeves, 2006;Kjaer et al, 2009). This idea has been substantiated in the literature where tendon tissue has been shown to become stronger through increases in tensile strength and elastic stiffness in response to programmes of structured exercise (Abate et al, 2009).…”

Vitamin C–enriched gelatin supplementation before intermittent activity augments collagen synthesis

“…While poor nutrition, genetics, and disease can make connective tissue prone to failure, adequate nutrition together with exercise normally improves the function of the matrix (5). Acute exercise is known to increase collagen synthesis (6)(7)(8) as well as the expression of the primary enzyme involved in collagen crosslinking, lysyl oxidase (9).…”

Age-related changes in structure and extracellular matrix protein expression levels in rat tendons

“…Usually, larger/stronger tendons are attached to larger/stronger muscles, while smaller and weaker muscles have smaller/weaker tendons (Narici and Maganaris 2007). Regular physical exercise leads to stronger muscle and correspondingly stronger tendon (reviewed in Kjaer et al 2009), while age-related decrease in physical activity and changes in structural composition could be related to weakened tendons and increased risk factor for tendinopathy in the elderly (reviewed in Magnusson et al 2008;Hess 2010). Tendons are composed primarily of collagens that contribute to the ability of the tissue to withstand high tensile forces.…”