Children with cerebral palsy often have spasticity of both the extensors and flexors, but how and why a flexion contracture of the wrist will develop during growth is not thoroughly understood. In order to understand the muscle adaptations that occur during contracture formation, the relationship between intraoperative sarcomere length and the extent of contracture was measured in 23 children (average age, 14.3 +/- 2.9 years) undergoing tendon transfers involving the flexor carpi ulnaris (FCU) or extensor carpi radialis brevis (ECRB) muscles. For both ECRB and FCU, sarcomere lengths measured intraoperatively were longer compared to sarcomere lengths predicted from a regression relationship obtained from "control" patients with radial nerve injury (P < 0.001). The most interesting aspect of the long FCU sarcomere lengths measured was that there was a highly significant correlation between the degree of contracture formation and intraoperative sarcomere length (r2 = 0.5, P < 0.005). These data clearly show that greater contracture severity is associated with longer intraoperative FCU sarcomere length. No such correlation was observed for the ECRB. The data suggest that the clinical presentation of the contracture is related to degree of FCU, but not ECRB adaptation, to the central nervous system lesion.