###### Abstract  
Popliteal angle (PA) and the knee position at the end of the swing phase during walking (K\({}_{\rm ts}\)) are considered criteria for treatment selection and outcome prediction of hamstring lengthening surgery in individuals with cerebral palsy (CP). However, the relationships among K\({}_{\rm ts}\), PA, and hypertonia are not clear. This study aimed to determine whether hamstrings hypertonia affects the values of PA and K\({}_{\rm ts}\), 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 \(\pm\) 3.9 years) underwent gait analysis and examination of hamstrings hypertonia (Tardieu scale) and length (PA). We found that K\({}_{\rm ts}\) increased with PA (K\({}_{\rm ts}\) = 5.00 \(\pm\) 0.31 \(\times\) PA (r = 0.39; \(p\) < 0.001)). Every 10\({}^{\circ}\) change of PA leads to 3\({}^{\circ}\) improvement of knee position in walking. K\({}_{\rm ts}\) were larger (\(p\) < 0.001) in the limbs with (20.40 \(\pm\) 11.27\({}^{\circ}\)) than without (15.60 \(\pm\) 9.99\({}^{\circ}\)) knee flexors hypertonia (Tardieu slow); and were larger (\(p\) < 0.001) in the limbs with (20.39 \(\pm\) 11.01\({}^{\circ}\)) than without (14.85 \(\pm\) 9.89\({}^{\circ}\)) knee flexors hypertonia (Tardieu fast). PAs were larger (\(p\) < 0.05) in the limbs with (42.81 \(\pm\) 12.66\({}^{\circ}\)) than without (38.96 \(\pm\) 14.38\({}^{\circ}\)) hypertonia (Tardieu fast). K\({}_{\rm ts}\) = 13.93\({}^{\circ}\) and PA = 30\({}^{\circ}\) were cutoff values of the presence of hypertonia with sensitivities of 75.0% and 89.1%, respectively. K\({}_{\rm ts}\) increased with the PA in ambulatory CP. It is estimated that pathological increase of K\({}_{\rm ts}\) occurs at PA \(\geq\) 40\({}^{\circ}\). The hypertonia of knee flexors affected K\({}_{\rm ts}\) and PA. The presence of knee flexors hypertonia should be considered in the decision-making of hamstring lengthening for individuals with CP if K\({}_{\rm ts}\)\(\geq\) 13.93\({}^{\circ}\) and PA \(\geq\) 30\({}^{\circ}\). c 2022 2022 2022 20222 20222 20 Ashworth scale (MAS) and the Tardieu scale (TAS), where TAS better complies with the concept of spasticity evoked by either slow or fast stretch [4]. The instrumented three-dimensional gait analysis can quantify gait abnormality in individuals with neuromuscular deficits. Limited knee flexion range of motion (ROM) during gait is commonly observed in individuals with CP. The value of knee flexion at the end of the swing phase during walking (Kts) is one of the criteria for treatment selection (Figure 2). Both muscle shortness and spastic hypertonia may affect the knee ROM during walking. The exaggerated stretch reflexes in response to muscle stretch could affect gait [5; 6; 7; 8; 9]. As a consequence, it is difficult to differentiate whether limitations in dynamic knee ROM during walking originate from hypertonia or shortness of knee flexors. Figure 1: Standard popliletal angle (PA) test. Figure 2: Knee flexion angle at the end of the swing phase during walking (Kts). The role of PA test as a criterion for hamstring lengthening surgery and as a predictor of knee flexion during walking is debatable because the conditions of testing on an examination bench are radically different than during walking [10]. Moreover, increasingly researchers emphasize the influence of hypertonia on PA test. The activation of ipsilateral and contralateral muscles of the legs has been reported [1; 11; 12]. The hamstring lengthening surgery is not always effective and the improvement of knee ROM during walking is not consistent across individuals [13; 14; 15]. Thus, muscle shortness may be only one of the contributing factors to gait pathology. Emerging data recognize improper links between neurological injury, neuromuscular deficits, and gait abnormalities in patients with CP. It seems that data obtained with gait analysis and clinical tests are not direct and obvious [16; 17; 18]. PA is a result of clinical evaluation performed on the examination table.