Tendons form the strong but flexible linkage between the more fleshy muscles and rigid bones. Composed primarily of collagen, the tissue is woven with fibrils, and capable of bending with minimal elongation and no contraction. The overall rope-like structure is then encased in a sheath.
There's a greater risk of injury in a tendon that crosses two or more joints. Tendons that pass over a convex surface--think of the rotator cuff--are also more injury-prone. And tendons exposed to the high strain of repetitive eccentric contractions, as well as tendons involved in locomotion, are also more at risk. The Achilles tendon possesses three out of four of these traits (it doesn't pass over a convex surface).
Not all tendon pain is tendinitis. Many tendon injuries are in fact tendinosis, and an understanding of the distinction can help you to identify one injury or the other and to adopt strategies for properly healing an injury such as Achilles tendinosis.
How Tendons Are Injured
By and large, tendons may be injured in one of three ways: Acute trauma, which is a sudden event that causes serious damage to a previously normal tendon; chronic overuse injury, which amounts to cumulative damage from repetitive, micro-tear-causing stress that compromises the ability of a tendon to withstand tension; and sudden trauma brought on by repetitive overuse, as in an Achilles tendon rupture, which most often shows evidence of lesser injury prior to the rupture.
The most relevant among these for our purposes is tendon injury caused by chronic overuse, which arrives with warning signs that don't accompany acute trauma and serves as a precursor to the third category in any case. Chronic overuse injury is often the result of insufficient rest between workouts.
When the damage to the tendon is microscopic, akin to the fraying of a rope, the injury will often heal itself. Fibroblast cells already within the tendon possess all the materials necessary to perform repairs and direct the crucial next phase of healing: remodeling. This is the cross-linking of collagen fibers to ensure that the tendon will again withstand the tension imposed on it by repetitive stress. It's this phase that provides the crucial mechanical and structural integrity so that you may later resume training.
Tendinitis
When micro-tears and the ensuing repair and remodeling occur, the result is tendinitis. The pain associated with this condition stems from microscopic damage, followed by self-healing.
During this time it's important to protect the tendon not only from sudden, intense forces, but from repetitive, low-demand motions as well. And that can be difficult; it's equally important to avoid complete immobilization. As long as the demands don't overcome the tensile strength of the tendon, increased demand results in increased tendon strength. See Treatment below for more details about the rehabilitation process.
Tendinosis
By contrast, tendinosis is the result of failed healing. This condition is familiar to any runner who has experienced an Achilles tendon injury that takes unbearably long to resolve. It's not unheard of for these injuries to never heal normally. In many cases, the cause of this is an insufficient inflammatory response during the early phases of healing.
While the later, remodeling phase occurs internally in the tendon without the aid of the immune system, initially it's important for outside help: Lymphocytes, neutrophils and macrophages rush to the scene to clear out debris and prepare the damaged tissue for bridgework over torn fibers.
When remodeling occurs without this preliminary step, debris remains in the scar tissue, which results in weakened structures. Under an electron microscope these chemical remnants and unconnected collagen strands reveal the lack of proper tendon structure.
This weakened tissue is obviously not desirable in and of itself, but its additional danger is that it places undue demands on the healthier areas of the tendon. The result is a kind of structural virus, with more and more of the tendon becoming compromised.