As with the sit-up, Horton says the rectus abdominis—the group of muscles on the front of the stomach, responsible for spinal flexion and the six-pack shape—is doing most of the work.
“But any sit-up is going to get everything in your core involved—it’s just a question of how much,” says Horton. “The rectus abdominis is going to be your prime mover, it is a spinal flexion movement, but your core and your hip flexors will all engage.”