Which of the following is not a muscle that acts on two joints?

The soleus muscle is not considered a muscle that acts on two joints. Its primary function is to facilitate plantarflexion of the ankle when the knee is bent or straight, but it does not cross the knee joint. It lies deep to the gastrocnemius, which does cross both the knee and ankle joints, allowing it to participate in movements at both of these locations.

In contrast, the gastrocnemius, biceps brachii, and rectus femoris are all biarticular muscles, meaning they span and exert influence across two joints. Specifically, the gastrocnemius crosses the knee and ankle joints, the biceps brachii crosses the shoulder and elbow joints, and the rectus femoris crosses both the hip and knee joints. Understanding the role and anatomical positioning of these muscles is crucial for comprehending their functions in movement and exercise.