The withdrawal reflex from stepping on a sharp object involves activating hip flexors to lift the foot and contralateral circuits to extend the opposite leg for balance.

“So, in the case of stepping on a sharp object like a piece of glass, or a nail, or a tack, you would essentially activate the hip flexor to lift up your foot as quickly as possible. In doing so, that same neural circuit would activate a contralateral, meaning opposite side of the body circuit, to ensure that the leg, the foot that's not stepping on the sharp object would do exactly the opposite and would extend to make sure that you don't fall over.”