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.