Medial surface of tibia, just below the insertion of the sartorius and gracilis muscles. From a common tendon with the semitendinosus muscle and from the upper and medial portion of the ischial tuberosity. From the linea aspera and lateral portion of the lateral supracondylar line. These are all two joint muscles - hip and knee. Extension at the hip, flexion at the knee.
Minor functions are rotary in nature.
The semimembranosus and semitendinosus will externally rotate the tibia when the hip is extended and will internally rotate the tibia when the hip is flexed. Hamstring pulls occur often in many sports that involve sprinting. One of the primary reasons for a hamstring pull to occur is relative inflexibility in this area. This injury does not seem to me to be caused by weak hamstrings, but rather in hamstrings that were originally of adequate strength but the athlete had not worked on their flexibility. Frequently there is a muscle strength imbalance.