Objective: In cerebral palsy the ßexor spasticity in the arm causes a nonfunctional position of the elbow and the wrist. The peripheral contribution to differences between extensors and ßexors are not thoroughly understood. Materials/Methods: We studied muscle protein expression in 17 consecutive children with cerebral palsy (age 4–19 years, mean 8,8 years, tetraplegic CP n=3, diplegic CPn=4, hemiplegic CP n=10), who underwent hand surgery. Transverse serial cryosections were cut, and stained for myosin-ATPase, as well as with monoclonal antibodies. Morphometric analysis was performed using a computerized image analysis system. Results: Immunohistochemical stainings showed signiþcantly more developmental myosin (embryonic and fetal) in the ßexors compared to the extensors. Developmental myosins are normally not found in childrenñs muscles. In the ßexors, there were signs of ongoing transformation of type 2A þbers to type 2B þbers, indicated by a higher proportion of type 2B þbers and a larger size variability in the type 2AB and the type 2 B þbers compared to the extensors. The tetraplegic patients had, compared to the hemiplegic patients, signiþcant signs of transformation from type 1 towards type 2AB and type 2B þbers. Conclusions: These þndings indicate more aberrant regeneration and faster contracting myosins in ßexors compared to extensors, and more in tetraplegic cerebral palsy compared to hemiplegic cerebral palsy. This could affect the muscle adaptability after tendon transfer.