Muscle complaint is a term used to describe any type of pain or discomfort that is felt in the muscles. Common muscle complaints include ACHE and KNOT, which are both four letter words. ACHE is a dull, continuous pain that is usually caused by overuse or strain. KNOT is a tight, hard lump of muscle tissue that can be caused by stress, tension, or injury. Both of these conditions can cause significant discomfort and can be treated with rest, massage, and physical therapy.