This will be a tad hard to explain, so bear with me. Taking into account only the legs what would be an accurate definition of the position of the upper legs, lower legs and feet with respect to time?

Let's say we have a person running on the spot (Not moving anywhere), from the reference point (Say the spot they're running on, or perhaps their waist) how can we define the position and angle of each of the upper legs, lower legs and feet with respect to time (let's say 0 to 1 for a single cycle of running)

I'm sure there's plenty of documentation on the subject but I'm unable to get anything useful from google.

The purpose is computer animation in an environment that doesn't support key-framing