Two 9 years old twin sisters, one with spastic cerebral palsy (C) and the other healthy (H) without any impairments, performed the pendulum drop test with EMG acquisitions. The purpose of this paper is to present a way to model the knee's spastic angular displacement of C and H using mechanical differential equations. Then, we propose a controller to correct the knee's angular displacement of C. and make it converge towards the one of H. To this end, we use a PID controller to correct the angular position. Both models of the spasticity and the controller were computed using a genetic algorithm (GA). Angular trajectories were modeled with a determination coefficient (R 2) higher than 95% for both spastic and non-spastic cases, and R 2 > 87% for the corrected angular position of C.