I too am working on determining initial contact during forefoot running using just kinematic data. I have had little success with using the position of a distally placed marker. It appears to determine IC late compared to when I visually determine IC. I have not tried using ankle plantarflexion to determine IC but I have been using an ankle angle. I currently have been focusing on ankle eversion. It appears the ankle stays in a position of inversion until the foot strikes the ground. I am still working on which variable to use - angle, velocity, or acceleration, but currently feel eversion is an appropriate motion to use to assess IC. One other reason I moved towards using eversion vs. a marker position or perhaps even plantarflexion is that I was looking for an algorithm I could use regardless of whether the IC was forefoot or rearfoot. Eversion appears to be accurate with both conditions.

Good luck,