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Purpose of the Study: In clinical practice, it is generally accepted that hamstring tightness results in incomplete knee extension when the hip is in flexion and in smaller conventional and modified popliteal angles. Similarly, a difference between the conventional popliteal angle and the modified popliteal angle (popliteal differential) would be associated with a permanent deficit in knee extension. The purpose of this study was to determine whether these two hypothesis correlate with clinical findings., Material and Methods: The series was composed of 35 walking cerebral palsy children, 16 girls and 19 boys, mean age 11+/-3.6 years with a pathological conventional popliteal angle. These children walked using the jump knee (n=24) or the crouch knee (n=11) pattern. Permanent hip flexion and the conventional and modified popliteal angles were noted. SPSS version 10.1.3 for Window was used to search for a correlation between the popliteal differential and the presence of permanent hip flexion using several values for the popliteal differential (5, 10, 15, 20, and 30 degrees ). Data were adjusted for age and gender., Results: The statistical analyses demonstrated a significant relationship between the presence of permanent hip flexion and a popliteal differential strictly less than 10 degrees and between the absence of permanent hip flexion and a popliteal angle greater or equal to 10 degrees . These statistically significant results, which demonstrated the opposite of what was expected, were independent of age and gender., Discussion: Our findings demonstrate that examination of the knee joint is indispensable but insufficient. The conventional popliteal angle is not a reliable indicator of hamstring tightness. The normal value of the modified popliteal angle has not been established so that it is impossible to determine what a pathological angle is. We do not know whether measurement of this angle is sufficient to establish indications for surgery. In the future, the development of muscle models coupled with gait analysis should enable more reliable prediction of outcome after surgery. At the present time, we recommend repeated physical examination using a standardized protocol, taking into consideration, several parameters including spasticity, selectivity and muscle force and to perform quantified gait analysis before scheduling hamstring lengthening surgery for walking cerebral palsy children.