Muscular dystrophy (MD) is a slowly progressive, hereditary muscle atrophy.
A variant in the COL6A3 gene has been identified in the American Staffordshire Terrier breed that causes congenital muscular dystrophy. The first symptoms such as progressive gait disorders and joint contractures (restriction of joint movement) become noticeable at the age of 6 months.

Affected dogs show diffuse muscle atrophy and multifocal joint contractures with limited range of motion and marked thickening of the elbow and knee joints, together with joint hyperlaxity (hyperextensible joints) of the distal limbs. Other symptoms include general weakness, difficulty standing up and walking, and tetraparesis with a stiff, choppy gait with short steps in all limbs, but without obvious ataxia. Weak withdrawal reflexes were observed in all limbs.