Biceps Femoris is one of the three muscles which form the hamstring group forming the back of the thigh. The muscle is often described as having a long head (the attachment from the ischium) and a short head (attached to the femur).
Gastrocnemius is the largest and most superficial of the calf muscles. Together the Gastrocnemius, Soleus and Plantaris are known as Triceps Surae. The Gastrocnemius is the main propellant in walking and running.
Gracilis is another muscle which works in conjunction with the groin muscles, or adductors.
The Sartorius is a two joint muscle and so is weak when the knee is flexed and the hip is flexed at the same time. It works better during single movements.
Semimembranosus is the most medial of the three hamstring muscles. Chronically tight hamstrings are often a contributary factor to lower back pain and knee pain.
When running the hamstrings act eccentrically to slow down the knee extension motion. Hamstring strains are common in individuals with chronically tight hamstrings or who do not warm-up thoroughly.
Popliteus is a small muscle which is often described as the key of the knee joint. It unlocks the knee joint by rotating the femur at the beginning of knee flexion to allow full knee flexion to occur.