A system building a list of the most fundamental categories of reality has to decide how finely to cut it, and Samkhya's decision to give locomotion its own dedicated tattva, distinct from grasping, distinct from speech, is a specific and revealing choice. Pada is not treated as a special case of pani, movement is not analysed as a kind of full-body grasping, nor is it folded into some more general category of "bodily action." It stands as one of the twenty-five, exactly as basic, in the system's own accounting, as space, mind or fire.
The gross seat pada is named for, the feet, is again distinguished from the underlying tattva: the capacity for locomotion is what Samkhya counts as fundamental, and the feet are simply where that capacity happens to be physically expressed in a typical human body, a distinction the system draws consistently across all five Karmendriyas and all five Jnanendriyas alike. Produced from the sattvic aspect of ahamkara, pada shares its origin with vak, pani, payu, upastha and manas, the full complement of organs that gives an embodied stream of experience its capacity to act on, rather than merely register, the world.
Locomotion's inclusion also underlines how comprehensive Samkhya intends its twenty-five-category scheme to be: a complete inventory of what exists has to account not only for consciousness and matter in the abstract but for the specific, ordinary capacities, moving, speaking, grasping, that make an embodied life what it is.