Identify a leg muscle that crosses more than one joint.

The rectus femoris is a muscle that crosses both the hip and the knee joints, making it a multi-joint muscle. It originates from the ilium part of the pelvis and extends down to the patellar tendon, which connects to the tibia at the knee. This dual crossing allows the rectus femoris to be involved in two primary actions: hip flexion and knee extension. Its ability to function across multiple joints is critical during movements such as running, jumping, and kicking, where both flexion of the hip and extension of the knee are required simultaneously.

In contrast, the vastus lateralis is part of the quadriceps group but only crosses the knee joint, functioning solely to extend the knee. The soleus, a muscle located in the back of the lower leg, is involved in plantarflexion of the ankle and also only crosses one joint. The gluteus maximus primarily acts on the hip joint, aiding in hip extension, but does not cross another joint in function. Hence, the rectus femoris stands out as the only muscle in this list that crosses and acts on more than one joint.