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Hamstring stretching is an integral component in the treatment for knee flexion contracture in cerebral palsy (CP). As hamstrings span across hip and knee joints, passive stretching of hamstrings applies force to the hip that is often dysplasia in CP. The purpose of this study is to measure the dynamic displacement of femoral head produced by passive stretching of hamstrings and to determine the factors associated with the phenomenon.Children with spastic CP were studied using computerized tomography (CT) of the pelvis. Two sets of CT studies were carried out, one with the knee flexed and the hip flexed (resting) and the other with the knee extended and the hip flexed to simulate manual hamstrings stretching. The distance from pelvic baseline to the posterior margin of femoral epiphysis was measured on the CT images and compared between resting and stretching for dynamic displacement. The dynamic displacement of the femoral head was expressed by a ratio to femoral epiphysis diameter.Twenty-seven CP children had CT studies at a mean age of 6.8 years (range: 4.5 to 9.6 y). Ten children were ambulators with or without devices and the other 17 children were nonambulators. On plain radiographs of the pelvis, the mean Reimer's migration percentage was 39% (range: 13% to 92%). On CT scan, dynamic displacement by stretching was 4.7% (range: −3.8% to 16.1%) of femoral epiphysis diameter (P<0.001 by paired t test). The displacement in the 33 hips with migration percentage of greater than 30% was significantly greater than the displacement in the other 21 hips with migration percentage of less than 30% (7.4% vs. 0.5%, P=0.001).Passive stretching of spastic hamstrings in the hip flexion position resulted in dynamic posterior displacement of the femoral head. Medical professionals should be aware of this fact, especially in spastic CP children with a higher migration percentage on radiograph.Diagnostic II.