Final answer:
The hamstring group of muscles crosses the hip and knee posteriorly and is responsible for flexing the knee and extending the thigh.
Step-by-step explanation:
The muscle group that crosses the hip and knee posteriorly is the hamstring group. This group consists of three long muscles located on the back of the thigh: the biceps femoris, semitendinosus, and semimembranosus. These muscles are responsible for flexing the knee and extending the thigh.