Incidental finding detected on MRI done for internal derangement of the knee. 

An accessory 3rd head of the gastrocnemius muscle is noted, coursing lateral to the popliteal vessels. It is isointense to muscles on all sequences. It is not causing any vascular compression. 

The gastrocnemius muscle normally has two heads - medial and lateral, medial being slightly larger than the lateral. The accessory or the 3rd head of gastrocnemius muscle is also called as gastrocnemius tertius and is considered as most common variant of the gastrocnemius muscle. It can cause impingement on popliteal vessels or the tibial nerve. 

Knowledge of accessory/anomalous muscles is important so as not to confuse them with a soft tissue neoplasm. Also, the accessory muscles in the body are notorious for causing vascular / nerve compressions/entrapments.