Final answer:
The soleus is the muscle deep to the gastrocnemius and is a stronger plantarflexor of the foot.
Step-by-step explanation:
The muscle that is deep to the gastrocnemius and a stronger plantarflexor is the soleus. While the gastrocnemius is the most superficial and visible muscle of the calf, the soleus lies just beneath it. The soleus, along with the gastrocnemius, is part of the superficial posterior compartment of the leg and they both insert onto the calcaneal tendon (Achilles tendon). The soleus is considered a powerful muscle for plantarflexing the foot at the ankle, especially during activities such as walking, running, or jumping.
The plantaris muscle, although it runs obliquely between the gastrocnemius and the soleus, is a relatively small and vestigial muscle, and not as strong a plantarflexor as the soleus. Both the popliteus and tibialis anterior serve different functions; the popliteus is involved in flexing the leg at the knee, while the tibialis anterior dorsiflexes the foot.