Where is the gastrocnemius muscle inserted?

The gastrocnemius muscle is inserted into the calcaneus via the Achilles tendon, which is a strong tendon that connects the calf muscles to the heel bone. This connection is crucial as it plays a significant role in the mechanics of walking, running, and jumping. The gastrocnemius, being one of the major muscles of the calf, acts to plantarflex the foot at the ankle joint and flex the leg at the knee joint. Its insertion at the calcaneus allows for effective transfer of the muscle's force to facilitate these movements.

The other options refer to different anatomical landmarks. The medial and lateral condyles of the femur are points of origin for the gastrocnemius, not its insertion. The soleal line and medial border of the tibia are related to the insertion of the soleus muscle, which functions in conjunction with the gastrocnemius but is distinctly different in its origin and action. Lastly, the posterior aspect of the fibular head pertains to the fibularis muscles and does not relate to the gastrocnemius, reinforcing the accuracy of the correct answer.