Here's why:

* Tibialis anterior is located on the front (anterior) of the shin. It dorsiflexes the foot (lifts the toes up) and inverts the foot (turns the sole inward).

* Tibialis posterior is located on the back (posterior) of the shin. It plantarflexes the foot (points the toes down) and inverts the foot (turns the sole inward).

They are antagonistic muscles, meaning they have opposite actions. This allows for controlled movement of the foot and ankle.