Which muscle group is primarily targeted when laying down with a leg straight up?

When lying down with a leg straight up, the primary muscle group being targeted is the hamstrings. This position places the hamstrings in a stretched state, especially if the leg is held vertically or at an angle, effectively engaging these muscles as they work to stabilize and control the position of the leg. The hamstrings are responsible for knee flexion and hip extension, and when the leg is elevated, they must maintain tension to keep the leg in that position against gravity.

Additionally, while some engagement from the glutes and hip flexors may occur during this exercise, the primary action and emphasis are on the hamstrings due to their role in managing the weight and position of the elevated leg. The quadriceps are less involved in this specific position, as their primary function is knee extension, which is not a dominant action when the leg is straight up. Thus, the hamstrings are the correct answer for the muscle group primarily targeted in this scenario.