If it occurs, reinnervation of the skeletal muscle leads to fibre type grouping which contrasts with the chequer-board mosaic seen in normal skeletal muscle biopsies. Fibre type grouping is said to have occured when groups of 6 or more fibres of the same type can be found in the skeletal muscle biopsy. This is a group of hereditary primary myopathies. They show a chronic and progressive clinical course. The commonest forms are Duchenne muscular dystrophy, Becker dystrophy, limb girdle dystrophy, and myotonic dytrophy.
Muscle biopsies show scattered necrotic fibres and hypertrophied fibres.