Most common muscle affected by fibrosis is

Fibrosis of sternocleisomastoid occurs in Less than 0.4 percnt of all newborns but it&; is the most common cause of congenital toicollis The sterno-mastoid muscle on one side of the neck is fibrosed and fails to elongate as the child grows, and thus results in a progressive deformity. The cause of fibrosis is not known, but it is possibly a result of ischaemic necrosis of the sterno-mastoid muscle at bih. Evidence in our of this theory is the presence of a lump in the sterno-mastoid muscle in the first few weeks of life, probably a swollen ischaemic muscle. Ref: Maheshwari and Mhaskar 9th ed pg 321.