This paper presents a method to compute smooth ankle trajectories for lower limb exoskeletons with powered ankle joints. The proposed approach defines ankle trajectories using four polynomial functions, each representing one of the four primary phases of gait. These polynomials are computed according to different safety constraints. During the single support phase, ground contact constraints are enforced. In the swing phase, an optimization problem is solved to achieve minimum jerk planning while respecting a set of equality and inequality constraints designed to minimize the risk of stumbling. The used approach focuses on making the ankle joint able to smoothly adapt in real-time to different walking styles defined by user-selected gait parameters such as step length and clearance. The primary aim is to improve the user experience by producing a secure and comfortable walking pattern. To validate the effectiveness of the proposed method, the new ankle trajectories were tested on a group of healthy volunteers using the TWIN lower limb exoskeleton.