Save. Tendinitis is an inflammation and irritation of a tendon, which is a band of fibrous tissue that connects muscle to bone. Tendons around the knee, foot, elbow and shoulder are most commonly affected. Tendinitis results in varying degrees of discomfort.ometimes tendinitis causes weakness and restricted movement of the muscle attached to the tendon. The area might feel warm and the skin might be red. The most common areas of tendinitis pain are the outer or inner side of the forearm near the elbow, just above the heel, just below the kneecap and in the shoulder.