Summary
The popliteal fossa is the lozenge-shaped hollow tucked behind the knee, framed above by the semitendinosus and semimembranosus medially and the biceps femoris laterally. Below, the twin bellies of the gastrocnemius medial head and gastrocnemius lateral head close the diamond.
Deep inside runs the neurovascular bundle that supplies the leg, but for illustrators the fossa reads as a soft triangular recess that sharpens when the knee flexes. Observe how the medial hamstrings create a subtle ridge while the biceps tendon tightens laterally; their contrast helps you carve the hollow without over-modeling.