This paper was published in 2016 by Professor Jill Cook, a top guru in the field of tendon rehabilitation, and presented three models for the cause of tendonitis.

The first model is that the collagen itself that makes up the tendon is torn and broken, but recent studies have shown that collagen fibers tend to be twisted and/or stretched rather than tearing unless they are seriously injured. It is further suggested that understimulation, or lack of exercise, is a typical cause of this distortion or expansion of collagen fibers. Understimulation is the opposite of overload, but conversely, when the tendon weakens due to lack of exercise, even a little overload can cause strain on the tendon, so it can be said that sustained overload is the actual cause, not understimulation.

The second model is due to inflammation, but usually, when an inflammatory reaction occurs due to an injury, the inflammatory substance called 'cytokine' increases 100-1000 times, but in tendinitis, this level increases 1.5 times on average. Therefore, it cannot be attributed to inflammation and this is why many experts are reluctant to use the term 'tendinitis' and prefer to use the term 'tendinopathy' instead.

- 'Tendinitis' means that the tendon is inflamed.

- 'Tendinopathy' means that the tendon has a pathology

The final third model is that the cells that make up the outside of the tendon respond to external shocks or compressive loading by the bone and this can cause tendon problems. Cook believes that this third model most logically explains the phenomenon, but noted that it can occur in combination with the two aforementioned reasons. In a paper published in 2012, compressive loading by bones had a more harmful effect when it occurs with tensile load (Increased muscle tension).

In summary, when muscles get tight due to body imbalance, muscle function loss, overload, etc., the tendons connected to them get pulled away from its attachment. This can increase compression/friction between its bony attachment and the inner part of the affected tendon, which can result in tendon problems. The muscles often affected by tennis elbow symptoms are also wrapped around the protruding elbow bone. Therefore, if these muscles get tight, they fail to respond to the combination of increased tension and compression, hence the elbow pain.

References

Cook, J. L., & Purdam, C. R. (2009). Is tendon pathology a continuum? A pathology model to explain the clinical presentation of load-induced tendinopathy. British journal of sports medicine, 43(6), 409-416.

Cook, J. L & Purdam, C. (2012). Is compressive load a factor in the development of tendinopathy? British Journal of Sports Medicine, 46(3), 163-168.

Cook, J. L., Rio, E., Purdam, C. R., & Docking, S. I. (2016). Revisiting the continuum model of tendon pathology: what is its merit in clinical practice and research?. British journal of sports medicine, 50(19), 1187-1191.