Which of the following exercises targets both the glutes and back extensors in Reformer 3?

The Long Box Swan exercise effectively targets both the glutes and back extensors. This movement involves extending the spine while simultaneously engaging the posterior chain, which includes the gluteal muscles and the muscles of the lower back. As the upper body lifts away from the box, the glutes activate to help stabilize the pelvis and maintain proper alignment, while the back extensors work to support the spinal extension.

In contrast, other options may not engage both muscle groups as effectively. For example, Leg Circles focus more on hip mobility and don't significantly activate the back extensors or provide the same level of glute engagement. The Spine Stretch Forward primarily targets the abdominal muscles and hamstrings with minimal involvement of the glutes or back extensors. Lastly, while Plank on Forearms does engage the core and some back extensors, it doesn't specifically target the glutes in the way that the Long Box Swan does. Therefore, the Long Box Swan stands out as the most effective exercise for simultaneously working the glutes and back extensors together.