| Summary: | Popliteal angle (PA) and the knee position at the end of the swing phase during walking (K<sub>ts</sub>) are considered criteria for treatment selection and outcome prediction of hamstring lengthening surgery in individuals with cerebral palsy (CP). However, the relationships among K<sub>ts</sub>, PA, and hypertonia are not clear. This study aimed to determine whether hamstrings hypertonia affects the values of PA and K<sub>ts</sub>, and how it may affect the hamstring lengthening decision for CP. One hundred and twenty-six subjects with CP (male = 72, female = 54; age = 11.1 ± 3.9 years) underwent gait analysis and examination of hamstrings hypertonia (Tardieu scale) and length (PA). We found that K<sub>ts</sub> increased with PA (K<sub>ts</sub> = 5.00 + 0.31 × PA (r = 0.39; <i>p</i> < 0.001)). Every 10° change of PA leads to 3° improvement of knee position in walking. K<sub>ts</sub> were larger (<i>p</i> < 0.001) in the limbs with (20.40 ± 11.27°) than without (15.60 ± 9.99°) knee flexors hypertonia (Tardieu slow); and were larger (<i>p</i> < 0.001) in the limbs with (20.39 ± 11.01°) than without (14.85 ± 9.89°) knee flexors hypertonia (Tardieu fast). PAs were larger (<i>p</i> < 0.05) in the limbs with (42.81 ± 12.66°) than without (38.96 ± 14.38°) hypertonia (Tardieu fast). K<sub>ts</sub> = 13.93° and PA = 30° were cutoff values of the presence of hypertonia with sensitivities of 75.0% and 89.1%, respectively. K<sub>ts</sub> increased with the PA in ambulatory CP. It is estimated that pathological increase of K<sub>ts</sub> occurs at PA ≥ 40°. The hypertonia of knee flexors affected K<sub>ts</sub> and PA. The presence of knee flexors hypertonia should be considered in the decision-making of hamstring lengthening for individuals with CP if K<sub>ts</sub> ≥ 13.93° and PA ≥ 30°.
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