Final answer:
The tibialis posterior (supinator) muscle is active during the midstance and terminal stance phases of gait.
Step-by-step explanation:
The tibialis posterior (supinator) muscle is active during midstance and terminal stance phases of gait, which are part of the stance phase. In the midstance phase, the muscle contracts eccentrically to control pronation of the foot, while in the terminal stance phase, it contracts concentrically to assist with supination.