Final answer:
The gastrocnemius muscle, with its medial and lateral heads, is the primary muscle strengthened through calf raises. This exercise also engages the soleus muscle and emphasizes the role of the Achilles tendon in plantarflexion movement.
Step-by-step explanation:
The muscle being strengthened through the exercise known as calf raises is the gastrocnemius muscle. This muscle has two parts, the medial and lateral heads, which both attach to the Achilles tendon at the back of the leg. During a calf raise, these two muscles contract, pulling on the Achilles tendon and lifting the heel off the ground, with a resulting simplified lever system action at the foot. This movement exercises and strengthens the gastrocnemius as well as the underlying soleus muscle, although the primary focus is on the gastrocnemius due to its more superficial and larger muscle mass.
Additionally, the question highlights the concept that muscles often come in pairs, such as the hamstrings and quadriceps, which work together to move the legs during activities like running. The gastrocnemius, together with the soleus muscle, helps in plantarflexion of the foot, allowing movements such as standing on one's toes which is essentially what happens when performing calf raises.