Final answer:
The biceps femoris, a muscle in the hamstrings group, primarily performs knee flexion and hip extension. It acts as an agonist during these actions and as an antagonist when opposite movements occur, such as knee extension.
Step-by-step explanation:
The biceps femoris is part of the group of muscles known as the hamstrings, which are located on the posterior thigh. When it comes to its role in the knee and hip joint actions, the biceps femoris primarily performs two main functions. At the knee joint, this muscle acts as an agonist in knee flexion, bending the knee. At the hip, it serves as an agonist for hip extension, helping to move the thigh backward.
In scenarios requiring the opposite movement, such as the knee extending, the hamstrings, including the biceps femoris, act as antagonists to the quadriceps femoris, the major muscle group responsible for straightening the knee. Similar agonist and antagonist dynamics occur at the hip joint during movements such as hip flexion, where other muscle groups take over the primary action and the hamstrings provide opposition to control the movement.
The biceps femoris also helps with lateral rotation of the thigh at the hip joint when the knee is flexed. This muscle is essential for the stabilization and movement of the lower limb, playing a vital role during activities involving the legs such as walking, running, and jumping.