Muscles are hard without exertion, the possible causes are muscle spasm, myofasciitis, muscle ossification and so on. 1. Muscle spasm: long-term non-exercise if the amount of sudden exercise is too large, may lead to muscle strain into the tension and the emergence of continuous muscle spasm, caused by the muscle is hard without exertion. 2. Myofasciitis: muscle is not hard, may be muscle myofasciitis, fascia inflammatory edema and thickening, and the muscle compensatory stiffness. 3. Muscle ossification: muscle ossification is common in patients with trauma. Due to trauma caused by limb or long bone fracture, in the process of fracture recovery, because the limbs can not be normal movement, calcium will be deposited in the muscle, so that the muscle, tendon gradually become hard, appearing muscle is hard without exertion. Muscles are hard without exertion may also have other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.