Your joints move by the power of your muscles. The muscles attach to your joints through tissues called tendons. The tendons transfer the force from your muscle across the joint and cause it to move. Sometimes this tendon becomes inflamed because it was injured (such as a direct blow to the thigh) or overused (such as biking up hills). When the tendon is inflamed, doctors add the ending "itis" and thus, we call this tendonitis. In some cases, the tendon actually tears in half and this is called a tendon rupture. If you can not straighten your knee because of pain over your tendons, seek medical treatment immediately. Tendonitis pain is usually described as an ache that can be sharp with exiting from a chair or with squatting. The pain is usually improved with rest.