Which of the following muscle groups is least likely to be engaged during the Long Box group series?

The quadriceps are least likely to be engaged during the Long Box group series, primarily because the movements within this series focus more on posterior chain activation rather than anterior chain engagement. The Long Box series on the reformer typically involves exercises that emphasize spinal extension and the muscles responsible for that action, such as the back extensors and muscles around the shoulders, which assist in stabilizing and supporting movement.

In contrast, the exercises performed in this series do not require significant knee extension or flexion, which primarily recruits the quadriceps. While there may be some minor activation depending on the specific exercise and body positioning, the primary focus is on the back extensors and shoulder muscles to facilitate movements such as rowing or pulling, which are integral to this series. Therefore, the quadriceps are not a major player in this context.