Question: Does tendinitis cause weakness?

Answer: Yes.

Question: Does weakness cause tendinitis?

Answer: Not directly (but possibly indirectly).

Tendinitis causes weakness if the injured tendon is not treated and restored to normal function soon after the injury is sustained. When a tendon is injured, the body automatically - and usually unconsciously - reduces the use of that muscle-tendon unit, which causes gradual atrophy or weakening of both the muscle and tendon. For instance, a person who injures a shoulder tendon would likely use his or her uninjured arm when reaching for a box, opening a heavy door or carrying a bag, and would probably do so unconsciously. The less the injured shoulder is used, the more quickly it atrophies and becomes vulnerable to re-injury or additional injury. However, overusing the uninjured shoulder to protect the weakened side can cause injury to the overused tendon, as well. This pattern is frequent in those with bilateral injuries in the elbows; shoulders; wrists; knees; and feet.

Weakness does not directly cause tendonitis, as one is not exerted beyond his or her capability. A person who is extremely weak in a particular part of the body due to a lack of exercise; irregular exercise; debilitating disease; severe injury; or the "couch potato" mentality, might have weakened parts of the body that are particularly vulnerable to injury. If this person finds himself or herself in a physically compromising or dangerous position, the sudden use of a weak muscle-tendon unit will more likely cause injury.

The moral of the story is to encourage your clients to exercise regularly so they become (and stay) strong. Encourage them to tend to injuries that impair function as soon as possible, so that normal use may be restored.