We've learned a lot about the Achilles tendon in the last 20 years. Gone are the days when the sight of a red, thickened Achilles tendon and stories of stiffness after rest and creaking sensations would get a therapist reaching for the anti-inflammatories and ice.

It's now universally agreed that overuse tendon injuries such as in the Achilles are not inflammatory in nature, and instead are a degenerative condition. The misleading term tendonitis — in the professional arena at least — has been replaced by tendinopathy or tendinosis.

While our knowledge has grown, the precise pathology remains unproven. Several theories exist to explain the exact mechanism of injury and degeneration. The one theory that appears to have gained the most backing is the tendinopathy continuum, proposed by Cook and Purdam in 2009.

The tendinopathy continuum

There are three stages within the tendinopathy continuum, through which the condition of the tendon deteriorates and scope for reversal becomes less likely.

1. Reactive tendinopathy

The first stage of the tendinopathy continuum is reactive tendinopathy. At this point, the tendon is reacting to an acute overload — maybe an increase in training or change in activity. The result is a thickened tendon, previously mistaken for an inflamed tendon.

This thickening is due to increased cellular activity and proliferation, and a rise in the synthesis of a certain type of proteoglycan that absorbs water, swells and pushes the collagen fibers apart. Pain felt in the tendon at this point is thought to be due to the release of nociceptive substances.

The key feature at this stage is that the tendon is still intact and there is minimal change in collagen integrity. This allows a good scope for reversal of the condition with no long-term effects.

2. Disrepair

The second stage of tendinopathy is the disrepair phase. If a reactive tendinopathy is allowed to continue, then a failed healing response occurs. As this progresses, neovessels will start to develop and production of type 3 collagen begins.

This is weaker and more irregular in pattern and shape than the type 1 collagen found in healthy tendons. The importance of the neovessels is controversial with some researchers correlating their presence with pain — while others disagree.

Due to the impact these changes have on the structure of the tendon, a limited capacity for full repair is reported.

3. Degenerative tendinopathy

The final stage of tendinopathy is characterized by large changes in the tendon matrix, far less healthy type 1 collagen and a further increase in the irregular type 3 collagen. The tendon is hypercellular with areas of reduced cellularity.

At this stage, the tendon is far more liable to rupture when placed under sudden, heavy load. A reduction in pain levels in a degenerative tendon is common, and so patients are less guarded. In fact, 97 percent of Achilles tendon tears show degenerative changes which were present prior to the rupture.

Clinical implications

Based on this theory, the treatment of a tendinopathy should vary depending on its stage of injury. For example, eccentric loading exercises such as the heel drop are often recommended for Achilles tendinopathies. While this may well help in the later, degenerative stages, in the reactive phase this will only aggravate the tendon by overloading it further.

So, in the initial reactive phase, it is recommended to address the problem by reducing the load on the tendon. This may be through resting or modifying sport- or work-related activities. Maintain some degree of load through the tendon with isometric (static) contractions, which have been shown effective in the reactive phase for pain reduction and strength maintenance.

A relatively short rest period (Cook says most reactive tendons settle within 5-10 days) can be followed by a gradual introduction of increased load to the tendon, using stretching, concentric and then eccentric exercises. Watch out for a delayed response to activity. Tendons often react up to 24 hours after loading, rather than at the time of exertion.

When it comes to treating a degenerative tendinopathy, combining load management and strengthening is the key. Knowing what aggravates the tendon in each case is important (e.g. hill running), and working within these limits is vital. Adapt training programs or daily activities to work within such restraints.

Eccentric loading at this stage is effective at easing tendon pain and improving function, although there are no hard and fast rules regarding how to approach this type of strengthening protocol. Don't work into pain, and allow a rest day between strengthening sessions to allow for delayed reactive responses.

Always bear in mind with a degenerative tendon, that it is possible to have both a reactive and degenerative tendon at the same time. This is due to the tendon not being fully degenerated, but instead having some areas of "normal" tissue alongside the degenerated ones.

It is this tissue that can react to overload in the same way a 100 percent healthy tendon would. In these cases, the tendon should be managed as a reactive tendon until the increase in pain — which indicates this scenario — has eased.

This theory is not proven, and there are certainly opposing theories that hold some merit, but this is by far the most accepted theory of overuse tendon injury pathology. We can use this model to treat those who present with Achilles tendon injuries in our clinics or sports teams.