A student in our lab was performing this calculation recently (for push-ups, not gait, but the same principles apply) and we noticed the same thing concerning mass that Phil and Allan mentioned. The mass measured on the force plates vs. the mass needed for the calculated CoM velocity over a single repetition to be ~ periodic were often quite different, sometimes by up to 4 kg or so. There are several potential sources of this error but it surprised me that it was so big.