Tendon injuries are the bane of anyone who regularly participates in exercise or sports. They seem to last forever or are chronic, and the current therapies to treat them don't seem too effective. While muscle injuries are more common than tendon injuries, minor muscle injuries, such as strains or sprains, often heal rapidly, while tendon injuries can go on for months or longer. The reason for the healing differences between the two types of tissue can be explained in two words: Blood supply. Muscles are enriched with a huge network of blood vessels that carry oxygen and nutrients to injured muscles, which greatly aids the healing process. Indeed, as noted in past issues of Applied Metabolics, muscle hypertrophy or growth itself involves a short period of injury and accompanying inflammation to trained muscle fibers. The body compensates for this injury by not only healing the injured fibers, a process greatly helped by the extensive blood supply in muscle, but also because in the process of muscle repair, a compensation effect occurs whereby the fibers level of thickness or circumference is increased by upgraded muscle protein synthesis processes. The net result is greater muscular size and strength.
Tendons differ from muscles in that they are largely connective tissue. The primary protein found in tendons is collagen, a tough, fibrous protein that provides strength to tendons. However, with the focus on strength in tendon structures because of collagen production, it was necessary to reduce the blood supply to connective tissue to low levels. As such, tendons, being largely connective tissue have a relatively poor blood supply in comparison to muscle. If tendons did have a larger blood supply, healing would progress far more rapidly than it typically does, but this increased blood supply would be at the expense of greater tensile strength of tendon structures. In short, a greater blood supply in tendons would be faster healing, but far more frequent injuries. But it's the relative lack of blood circulation in tendons that also produces the slow healing associated with tendon injuries.
The incidence of tendon injuries is considerably increased by certain risk factors. Foremost among these is age. Connective tissue is noted for its lack of hydration, and as you age, the hydration level of connective tissue drops gradually with the passing years. In practical terms, you can get away with doing things in your 20s that would prove disastrous in terms of connective tissue when you are 40 or over. For example, I did exercises in my 20s that put a level of stress on my tendons and connective tissue that if I attempted to do today would result in an immediate debilitating injury. One example of this was press-behind-neck to train the shoulder muscles. This exercise hyperextends the shoulder joint . . .