The structure indicated is the soleal line of the tibia.
The soleal line is an oblique line visible on the posterior surface of the tibia. The popliteus muscle inserts above the soleal line.
Three other muscles in the posterior compartment of the leg take their origin, or part of their origin from the soleal line:
- Flexor digitorum longus
- Tibialis posterior
Learn all about the anatomy of the tibia and the fibula.