Final answer:
The gastrocnemius and soleus muscles are responsible for eccentrically controlling ankle plantar flexion during gait, as they decelerate the foot's motion towards the ground.
Step-by-step explanation:
The muscles that eccentrically control ankle plantar flexion during gait are the ones responsible for decelerating the motion of the foot as it moves toward the ground after the heel strike and as the foot prepares to push off the ground. The primary muscles involved in this action are the gastrocnemius and the soleus, which are located in the superficial posterior compartment of the leg.
The gastrocnemius is the most superficial and visible muscle of the calf, while the soleus lies deep to the gastrocnemius. Both these muscles insert onto the calcaneal tendon (Achilles tendon) and act to control the speed at which plantar flexion occurs, preventing the foot from slapping down onto the ground too quickly.