What muscle group is the primary mover during a crunch?

The primary mover during a crunch is the abdominals, particularly the rectus abdominis. When performing a crunch, the action primarily involves the contraction of the abdominal muscles to lift the upper body towards the knees, which is the key movement in the exercise. The focus on this muscle group helps in flexing the spine, which is essential for the crunch's effectiveness in targeting the core.

Other muscle groups may assist in the crunch, but they are not the main performers of the movement. The obliques, for example, play a supporting role, especially if there is a rotation involved in the movement. The rectus femoris, which is part of the quadriceps, is primarily involved in knee extension and hip flexion rather than in the flexion of the spine. The pectorals are primarily engaged in upper body movements involving the arms and shoulders and do not significantly contribute to the crunch motion. Thus, the abdominals are correctly identified as the primary movers in this exercise.