## 4 Discussion
It can be thought that the abnormal ratio of agonistic and antagonistic muscles of the KNEE to CP is a factor that can adversely affect not only gait ability but also exercise ability and is closely related to low aerobic capacity. Previous studies have reported that the muscles of CP children are smaller [18; 55] and weaker [53; 56] than normal healthy children. Additionally, the muscle strength is correlated with the gait patterns [55; 57] and the motor functions [58], and the muscle strength is more associated with the muscle function than the muscle convulsion [57]. In particular, [PERSON] et al. (2008) reported in CP's Gross Motor Function Classification System (GMFCS) that the areas of the greatest muscle strength between Level I, in which CP can gait free of anaid apparatus, and GMFCS Level III groups, where CP gaits with aid apparatus, are HIP Extension/Abduction and KNEE Extension muscles [11]. Additionally, in this study, 300\({}^{\circ}\)/s peak torque/BW of KNEE was found to have the greatest effect on VO2 peak of CP among lower extremities muscle strength (\(p<0.01\)), and it was found to be the predictive factor that could explain VO2 peak at the level of 66% in CPs. In addition, 45\({}^{\circ}\)/s peak torque/BW of HIP was found to have the greatest effect on HRpeak (\(p<0.05\)), and this factor was found to be the predictive factor that could explain the HRpeak of people with cerebral palsy at the level of 39%. Lack of flexibility and movement control of CP has been recognized as the main cause of the excessive energy use in the inefficient gait of CP. Previous studies investigated the energy requirement for CP adolescent gait, and as a result, 1.5 to 3 times of gait energy of normal adolescents was required depending on the degree and type of gait angle [59]. Additionally, the cost of gait was 3 times than that of normal adolescents [53]. To reduce the inefficient energy consumption during gait, CP adolescents require less convulsions, restoration of stability in posture changes, and removal of foot drag, all of which could be improved by strengthening the muscles of the torso and lower extremities, stressing the need for strengthening the muscle strength of lower extremities [58]. Additionally, the energy consumed during gait in CP children was investigated, and as a result, there was a close relationship between the KNEE extensor and the gait efficiency, suggesting that the KNEE extensor force is an important factor for the efficient use of energy [51]. On the other hand, although it was not a study related to CP, the two preceding studies investigated the kinetics of VO2 by applying a heavy weight training program for normal adults. As a result, it was reported that the running economy improved as the storage of elastic energy increased, and the running economy improved due to the effect of titin on the elastic properties of muscle fibers [59; 60]. As such, it can be thought that the lower limb muscle strength has a close relationship with the gait ability and also affects the aerobic capacity. As a result, it can be concluded that the unstable gait pattern of the lower extremities of CP requires a lot of energy among the aerobic capacity measurements in this study, which resulted in the decrease in aerobic capacity. In particular, for CP, the muscle endurance of lower limb of KNEE Joint and the muscle power of HIP Joint are very important factors that can both enable the efficient gait and predict the aerobic capacity.