There are three types of muscle in the pig:

• Involuntary or smooth muscle - Found in the digestive and genital systems and the blood vessel walls.
• Cardiac muscle - The heart consists largely of this muscle. It is involuntary.
• Voluntary or skeletal muscle - This is the main muscle mass forming the muscular-skeletal system. These muscles are attached to the surface membrane covering bones called the periosteum. Inflammation of this covering is called periostitis.

Disease of the muscles in the pig are common and the symptoms seen depend upon which muscle groups are involved. The failure of muscle development may also be due to nerve or bone diseases. The common clinical signs include swelling, pain, wasting or trembling of the muscles. In some diseases there is death of muscle cells. Porcine stress syndrome (PSS) is a common heritable condition (recessive gene) associated with the sudden onset of prolonged muscle spasms which causes failure of the normal metabolism and the development of acid conditions throughout the body. It frequently ends in death.

Terminology
Asymmetric hind quarter syndrome - One hind leg muscle mass appears less than the other. It can arise where poor quality iron injections are given or it may be a congenital condition. It may be part of the porcine stress syndrome (PSS).
Back muscle necrosis - Sudden acute lameness and swellings of the lumber muscle often associated with PSS.
Congenital muscle hypertrophy - A breeding defect with excessive muscle formations.
Dark firm dry muscle (DFDM) - Describes the appearance of abnormal muscle at slaughter. Considered part of the PSS condition.
Mulberry heart disease (MHD) - Heart muscle failure associated with unavailability of vitamin E and or selenium.
Muscle necrosis - Dead muscle tissue. This can arise due to loss of blood supply caused by bacterial thrombosis (bacteria clogging up the blood vessels), physical damage or toxic damage. Iron toxicity, vitamin E or selenium deficiency are further examples.
Myodegeneration - Loss of function of muscle due to muscle fibres degenerating. Common problems are associated with deficiencies of vitamin E and or selenium.
Myopathy - This term describes any muscle disease.
Myositis - Inflammation of muscle often caused by trauma or infection.
Pale soft exudative muscle (PSE) - Describes the appearance of abnormal muscle at slaughter. Part of the PSS condition.
Pietrain creeper syndrome - Progressive muscle weakness in pigs from 3-12 weeks old. Considered to have a hereditary basis.
Porcine stress syndrome (PSS) - A heritable condition involving defective muscle metabolism.