A deep, full-range sit-up off the glute ham machine.
Control the descent and do not bounce out of the bottom, build the range over time.
Anchoring your feet in the glute ham machine turns a sit-up into a full-range core lift, your torso travels back over the pad into a genuine stretch on the abs before you crunch back to vertical. That extended range loads the abs in a lengthened position no floor sit-up can reach, which is exactly where the growth stimulus is richest. It is an advanced trunk builder, best treated with the same respect as any loaded lift, controlled descents and range you build over weeks.
Range. On the floor your torso stops at the ground, on the GHD it keeps travelling back past neutral into a deep ab stretch. That extra range makes it far harder and far more productive, and also why you should build up to it gradually.
They are demanding, not inherently bad. The risk comes from bouncing out of the bottom or diving into huge range on day one. Control the descent, keep the range modest at first, and extend it as your trunk gets stronger.
Start small, 2 or 3 sets of 5 to 8 controlled reps, even if that feels easy on the day. The deep stretch causes serious soreness in the unprepared, so let volume climb over weeks, not within one session.
