ACSM Personal Trainer Certification Practice Exam

During seated leg extensions, the primary muscle engaged is the quadriceps. This exercise specifically targets the quadriceps muscles, which are located at the front of the thigh. The movement involves extending the knee, which is accomplished by the quadriceps contracting vigorously. As the legs are lifted away from the ground in this seated position, the quadriceps work to straighten the leg, thereby emphasizing their role in this exercise.

In contrast, the other muscle groups mentioned do not contribute as significantly to the movement. For example, the hamstrings are involved in knee flexion, which is the opposite of what occurs in leg extensions. Similarly, the glutes primarily engage during hip extension movements and do not play a major role in knee extension. Lastly, the deltoids are shoulder muscles and are not involved in any aspect of the leg extension exercise. Therefore, the quadriceps are solely responsible for the action performed during seated leg extensions, making them the correct answer.