Designing a serious game for walking rehabilitation requires compliance with the theory of motor learning. Motivation, repetition, variability and feedback are key elements in improving and relearning a new walking pattern. As a preamble to the development of an AR rehabilitation game, and in order to choose the most effective feedback to provide to the patient, this article presents a preliminary study on the impact of presentation modalities on walking speed. We investigate which visual concurrent feedback modalities allows to reach and maintain a target speed (maximum or intermediate). Our first results on children with motor disabilities (n=10) show that some modalities improved walking performance and helped patients to better control their walking speed. In particular, a combination of targets anchored in the real world with a time indication seems to be effective in maintaining maximum walking speed, while simple moving objects could be used to control speed.