Final answer:
The Tibialis posterior muscle is not inserted into the calcaneal tendon, unlike the gastrocnemius, soleus, and plantaris. It serves to invert and plantarflex the foot and is part of the deep posterior leg compartment.
Step-by-step explanation:
The muscle of the posterior leg compartment that is not inserted into the calcaneal tendon is the Tibialis posterior. This muscle is part of the deep posterior compartment, which also includes the popliteus, flexor digitorum longus, and flexor hallucis longus. Other muscles in the superficial posterior compartment like the gastrocnemius, soleus, and plantaris do insert into the calcaneal tendon, also known as the Achilles tendon.
The Tibialis posterior muscle primarily serves to invert and plantarflex the foot, and it is not involved in the formation of the calcaneal tendon, thereby making option D ('Tibialis posterior') the correct answer to the original question, 'Which muscle of the posterior leg compartment is not inserted into the calcaneal tendon?'