I don't understand the physical motion. Simply raising the heel whilst sitting?
A bit of googling says plantar flexion is the same muscle movement needed for pushing the accelerator pedal while driving or ballet dancers standing on their toes.
My guess (not my field of expertise) is the muscle is activated strongly in isolation (the toe pushing down motion) and inducing a large range of motion. So it's not raising the heel so much as pushing the toe down.