Bilateral Gastrocnemius Tertius Muscles: Cadaveric Findings of a Rare Variant

The posterior compartment of the leg typically contains three muscles in the superficial flexor group: the gastrocnemius, plantaris, and soleus. The gastrocnemius has medial and lateral heads (MH and LH) that originate from the medial and lateral condyles of the femur, respectively. However, a third head (TH) of the gastrocnemius, is a rare accessory muscle bundle of the gastrocnemius muscle that covers the surface of the popliteal fossa. Bilateral THs of gastrocnemius were identified in a 67-year-old male during a routine educational cadaveric dissection. Both gastrocnemius TH muscles consisted of a superficial belly with distinct neurovasculature heads and originated from the lateral condyle of the femur and inserted into the Achilles tendon. To our knowledge, the co-existence of bilateral gastrocnemius TH muscles has only been reported once. The male donor was found to exhibit an anatomical anomaly and could be clinically underdiagnosed due to its clinically silent nature and the lack of reports. Insight into the potential implications of bilateral and unilateral gastrocnemius TH and identification during clinical evaluation offers a path for future research to better identify and manage cases of gastrocnemius TH and its effects.