Which of the following is considered an open kinetic chain exercise?

Leg (knee) extension on a machine is classified as an open kinetic chain exercise because it primarily involves movement at a single joint (the knee) while the distal segment (the foot) is free to move in space. In this exercise, the hip remains stationary, and the focus is on extending the leg against resistance provided by the machine.

Open kinetic chain exercises are characterized by this independence of movement, allowing for isolation of specific muscles, making them effective for targeting and strengthening individual muscle groups. The leg extension specifically targets the quadriceps, facilitating controlled motion and muscle engagement which is distinct from closed kinetic chain exercises, where the limb is fixed and movement occurs at multiple joints simultaneously.

In contrast, the other choices involve multi-joint movements with the feet in a fixed position, making them closed kinetic chain exercises. Squats and deadlifts engage multiple muscle groups across several joints, while push-ups involve coordinated movements of the arms, shoulders, and core with the feet planted, illustrating the differences between these types of exercises.