Which of the quadriceps is a two-joint muscle?

The rectus femoris is considered a two-joint muscle because it crosses both the hip and knee joints, thereby acting on both. This muscle originates from the anterior inferior iliac spine (AIIS) and the superior part of the acetabulum, and it inserts into the patellar tendon, which attaches to the tibia. Its role as a two-joint muscle enables it to facilitate both knee extension and hip flexion.

In contrast, the vastus intermedius, vastus medialis, and vastus lateralis are all classified as one-joint muscles. These muscles primarily function to extend the knee and do not cross the hip joint, meaning they do not contribute to hip movements like flexion. This distinction helps in understanding the different roles of the quadriceps muscles in activities involving the legs, such as walking, running, or jumping, where the rectus femoris can engage in actions affecting both joints.