Gait recognition is not sufficient for your purpose. Consider people on crutches, people in wheelchairs, people on unicycles, bicycles, or tricycles. Some of the robots being built in Japan apparently have quite good bipedal motion and look quite realistic, but those are not human.
Recognizing leg length accurately in order to calculate proportions in order to distinguish humans from other primates (e.g., gorillas) is difficult when one considers skirts, dresses, and long winter coats. The proportions in humans vary considerably -- consider for example dwarfism. Even without disorders such as that, if you take a male human and a female human of the same height, the female will statistically have longer legs. And I know that my proportions are at least two standard deviations from the mean.
For the purpose of this project, are you restricting "human" to Homo sapiens sapiens, or are you including Homo neanderthalensis, Homo habilis, and so on?