"The lesson we’ve drawn from the GHD sit-up is that in spite of the primacy of hip flexors over trunk flexors, or the abs, in this sit-up it recruits the abs powerfully in two ways. First, the movement takes the trunk from hyperextension to full flexion, albeit with negligible load. (No crunch can match this range of motion.) Second, the role of the abs in this sit-up is powerful and largely isometric—i.e., they stabilize the torso from undue extension."