The structure indicated is the biceps femoris muscle of the thigh.
The biceps femoris muscle is one of three muscles in the posterior compartment of the thigh. The other muscles are the semimembranosus, and the semitendinosus.
The muscles in the posterior compartment are often referred to as the “hamstrings” muscles. Collectively, these muscles are responsible for extending the hip joint, and flexing the knee joint.
The biceps femoris lies laterally, and the semitendinosus and semimembranosus lie medially.
Just like the biceps muscle in the arm, the biceps brachii , which is Latin for “two headed muscle (biceps) of the arm (brachii)”, the biceps femoris has two heads – a short head and a long head. The long head originates from the ischial tuberosity together with the semitendinosus muscle, whereas the short head originates from the shaft of the femur on the linear aspera. The two heads converge to a form a tendon which inserts laterally on the fibular head. The prime function of the biceps femoris is in flexion of the knee.
Origin: Long head – ischial tuberosity. Short head – linea aspera
Insertion: Fibula head and lateral tibial condyle
Action: Flexion of knee, lateral rotation of tibia, extension of hip.
Innervation: long head – tibial nerve. Short head – common peroneal nerve.
You can learn more about the muscles of the posterior compartment of the thigh in this video.