Achilles tendonitis is an extremely common injury, not just in running but in many sports. The severity of overuse injuries to the Achilles tendon can range from dull soreness that barely impacts your ability to run or play sports to chronic, debilitating pain that can last for months or years. Fortunately, due to its high incidence in athletes, Achilles tendonitis is a fairly well-studied injury.
The majority of Achilles tendonitis cases occur at the midpoint of the Achilles, a few inches above the heel. But a minority of cases—between 20 and 24%, according to a few studies1, 2—occur where the Achilles tendon flattens out and inserts at the ankle. This is called insertional Achilles tendonitis. While the majority of studies focus on midpoint Achilles tendonitis, and as such it is the better-understood variant, there is nevertheless a good deal that can be learned about insertional Achilles tendonitis as well. This article deals specifically with midpoint Achilles tendonitis; if you are interested in insertional Achilles tendonitis, a detailed follow-up article is in the works, or you can read this article on insertional Achilles tendonitis that accompanied the original Achilles tendonitis post. Unless otherwise qualified in this article, "Achilles tendonitis" will refer specifically to midpoint Achilles overuse injury.
While "tendonitis" is by far the most common term used to refer to overuse injury to the Achilles tendon, it is not a strictly accurate term. The suffix "-itis" implies that the root cause or main feature of some condition is inflammation—as is the case in appendicitis, gingavitis, and so on. But, as we'll see in the research papers reviewed below, inflammation of the Achilles tendon or its surrounding tissues is not a common finding in athletes with overuse injuries to the Achilles. Rather, their pain is caused by real, physical damage to and degradation of the small fibers that make up the Achilles tendon. Because of this, some doctors and researchers advocate renaming the injury "Achilles tendonosis" or "Achilles tendinopathy" to make it clear that degeneration of the tendon fibers is the root of the problem. Despite my support for this idea, I will use "tendonitis" in this article, as it is still the most common term for the injury.
In older literature, you might also see phrases like "Achilles tenosyndovitis" or "Achilles peritendinitis," terms which indicates that the source of the problem lies in the tissue that surrounds the Achilles tendon proper. Research on this is limited, so it is not clear to what extent (if any) injury to the sheath surrounding the Achilles plays a role in Achilles tendonitis.