Tendons are thick cords that join your muscles to your bones. When these tendons become irritated or inflamed, it is called tendinitis. This condition causes acute pain and tenderness, making it difficult to move the affected joint.Tendinitis can be a result of an injury, repetitive movement, aging, or disease, such as rheumatoid arthritis.f your condition is more severe, your doctor may also recommend: 1  corticosteroid injections: a single injection can reduce pain and inflammation, but repeated injections can cause the tendon to weaken and increase your chances of injury.