You mean a sensor that will detect above normal resistance on the servo?? So that when your placing the foot downt hat you can detect when it hits the floor?
If so, you got a few different choices, you could That might work somewhat, but would probaly have flaws.
You couldput a bump sensor there and have it hooked to the bottom that when the bottom hits something, bump switch goes off.
THat could work.. it would be a near instant reaction time to the sensor, but your MCU would depend on how effective it is. Whether or not you hook it into an intterrupt would matter as well.