Each fascicle is bound by an endotendineum, which is a delicate loose connective tissue containing thin collagen fibrils[4][5] and elastic fibers.

After secretion from the cell, cleaved by procollagen N- and C-proteases, the tropocollagen molecules spontaneously assemble into insoluble fibrils.

[16] The dermatan sulfate side chains of decorin aggregate in solution, and this behavior can assist with the assembly of the collagen fibrils.

[19] Blood vessels may be visualized within the endotendon running parallel to collagen fibres, with occasional branching transverse anastomoses.

Conversely, in sports requiring athletes to excel in actions such as running or jumping, it is beneficial to have longer than average Achilles tendon and a shorter calf muscle.

[20] Tendon length is determined by genetic predisposition, and has not been shown to either increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances and a lack of recovery and stretching.

[21] In addition tendons allow muscles to be at an optimal distance from the site where they actively engage in movement, passing through regions where space is premium, like the carpal tunnel.

This connection allows tendons to passively modulate forces during locomotion, providing additional stability with no active work.

During the last portion of the stride, as the foot plantar-flexes (pointing the toes down), the stored elastic energy is released.

The collagen fibrils are parallel to each other and closely packed, but show a wave-like appearance due to planar undulations, or crimps, on a scale of several micrometers.

[23] In tendons, the collagen fibres have some flexibility due to the absence of hydroxyproline and proline residues at specific locations in the amino acid sequence, which allows the formation of other conformations such as bends or internal loops in the triple helix and results in the development of crimps.

In addition, because the tendon is a multi-stranded structure made up of many partially independent fibrils and fascicles, it does not behave as a single rod, and this property also contributes to its flexibility.

More recently, tests carried out in vivo (through MRI) and ex vivo (through mechanical testing of various cadaveric tendon tissue) have shown that healthy tendons are highly anisotropic and exhibit a negative Poisson's ratio (auxetic) in some planes when stretched up to 2% along their length, i.e. within their normal range of motion.

[34] Several studies have demonstrated that tendons respond to changes in mechanical loading with growth and remodeling processes, much like bones.

[37] Tendinopathies can be caused by a number of factors relating to the tendon extracellular matrix (ECM), and their classification has been difficult because their symptoms and histopathology often are similar.

The extrinsic factors are often related to sports and include excessive forces or loading, poor training techniques, and environmental conditions.

[41] Tendons are capable of healing and recovering from injuries in a process that is controlled by the tenocytes and their surrounding extracellular matrix.

The three main stages of tendon healing are inflammation, repair or proliferation, and remodeling, which can be further divided into consolidation and maturation.

In this stage, the tenocytes are involved in the synthesis of large amounts of collagen and proteoglycans at the site of injury, and the levels of GAG and water are high.

[40] The final maturation stage occurs after ten weeks, and during this time there is an increase in crosslinking of the collagen fibrils, which causes the tissue to become stiffer.

[42] Matrix metalloproteinases (MMPs) have a very important role in the degradation and remodeling of the ECM during the healing process after a tendon injury.

Certain MMPs including MMP-1, MMP-2, MMP-8, MMP-13, and MMP-14 have collagenase activity, meaning that, unlike many other enzymes, they are capable of degrading collagen I fibrils.

In animal models, extensive studies have been conducted to investigate the effects of mechanical strain in the form of activity level on tendon injury and healing.

Changes in the actin cytoskeleton can activate integrins, which mediate "outside-in" and "inside-out" signaling between the cell and the matrix.

It is also recommended in survival guides as a material from which strong cordage can be made for items like traps or living structures.

Inuit and other circumpolar people utilized sinew as the only cordage for all domestic purposes due to the lack of other suitable fiber sources in their ecological habitats.

The elastic properties of particular sinews were also used in composite recurved bows favoured by the steppe nomads of Eurasia, and Native Americans.