The GHD sit-up 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.
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This second point is consistent with our belief that the most powerful, functional, and developmental contractions of the trunk are isometric, not isokinetic. Our favorite ab exercises are predominantly stabilization or isometric exercises. The GHD sit-up, the L sit, and the overhead squat share this stabilization role. The lack of trunk flexion in these moves hides their potency from the uninitiated.
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Our experience with athletes and static hip flexion work like the L sit and more dynamic exercises like the GHD sit-up have led us to several conclusions:

• The hip flexors’ purchase and strength suggest their importance to functional movement. One expert calculated that they are capable of generating many times the force that the abs can. To think that muscles with that much mechanical advantage should not be used to that advantage is ridiculous.

• Most modern athletes are hip flexion weak and it affects most performance.
• Weak hip flexors assure weak abs—especially weak lower abs—and no amount of crunches can compensate. (It seems that every gym has an abs class instructor who has a prominent lower abdominal pooch. Ask her to hold one knee up while standing on the other leg and to resist your pushing the knee downward with a couple of fingers. It’s easy to push the knee down, and it shouldn’t be).
• Without static contraction/stabilization exercises, the abs never learn to perform their most critical, functional, role—midline stabilization. What about the danger to the lower back purported to be induced by strong hip flexor work? We have neither induced nor seen this damage. We do however have some hunches as to how this might have occurred in communities where roman chair sit-ups and traditional military PT sit-ups were in wide acceptance.
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  First, in military and law enforcement PT where the sit-up was king, it was and is essentially a biphasic movement. With feet anchored and knees bent, this sit-up comes up with a slight pause in the middle of the action. Look at video of someone doing these sit-ups and you’ll see the pause. What is happening is that the upper back makes solid contact with the ground under the upper abs and so they can flex the trunk and fulcrum off the contact point. As the sit-up continues, the middle abs flex the torso but the lumbar curve surrenders without finding resistance and at full middle rectus contraction the spine is neutral and not flexed. The contraction occurred with no real load; the belly and back just sank closer to the ground. This stalls the sit-up but the pelvis and low back have solid contact with the ground so the hip flexors complete the movement. The natural, biphasic, one-two count of the military sit-up is a repeat of upper abs throwing the movement to the hip flexors where they complete the movement. Upper abs, hip flexors. Upper abs, hip flexors. No effective middle ab work. This deficiency of middle ab work, and consequently strength in the middle rectus, and the violence of the toss from upper to lower abs may have presented unhealthy wear and tear on the lumbar spine. This understanding came, in part, from some brilliant work by Koch, Blom, and Jacob in producing the ”Ab Mat”.
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  Second, in watching people perform sit-ups on the GHD we note that very few employ the full complement of hip flexors in sitting up. The hip flexors include the iliopsoas and rectus femoris. The iliopsoas originates at the lumbar spine and attaches to the femur. In the sit-up it pulls the athlete to seated by the lumbar spine, potentially creating nettlesome shear forces on the spine. Rectus femoris is the top piece of the quadriceps and it both extends the leg and flexes the hip. Rectus femoris originates at the pelvis and attaches to the patella via the patellar tendon. In the sit-up rectus femoris pulls the athlete to seated from both the pelvis and the iliac spine. The activation of rectus femoris during the GHD sit-up does two important things. First, it adds significant force to the movement. The acceleration of the torso to upright is so forceful when rectus femoris is engaged that our trainers can detect its use or lack of participation from their peripheral vision. What it adds to the movement is obvious in speed and acceleration of the torso. Second, rectus femoris reduces the shear force on the lumbar vertebrae by pulling from the pelvis and iliac spine instead of the lumbar spine. When coaching the GHD sit-up, we cue for the athlete to sharply extend the legs while coming up. The difference is obvious to everyone watching when rectus femoris kicks in. Those who have identified the GHD sit-up and other feet-anchored sit-ups as a source of low-back pain seem to always pull with the iliopsoas alone and never use rectus femoris. Those who’ve had problems with low back pain from GHD or roman chair sit-ups will find considerable relief by training to use the full complement of hip flexors in performing sit-ups.
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  Third, all too often communities that have held the sit-up in high regard have typically neglected hip extension work. Military and police physical training has historically been enamored of the sit-up. It is one of the yard sticks by which police and military fitness is traditionally measured. In most of these programs there are no squats, no deadlifts, no good mornings, no stiff legged deadlifts, and no back or hip extension exercises. The posterior chain in these communities typically sees no work other than running or perhaps burpees. What this imbalance of regular hip flexion exercises with little hip extension and no full-range hip extension portends for injury we don’t know. The imbalance can’t be a good thing, however. Regardless, we see our back and hip extension drills on the GHD to be indispensable to lower back health.

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Our newcomers start out on the GHD machine under close coach surveillance to make sure that they can execute the full range of motion without collapsing. Athletes who are unable to perform a proper GHD can sub with the AbMat until they develop enough core strength to perform a GHD sit-up.
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Greg Glassman cautions against “unbridled bouts on the GHD for newcomers to avoid the Matt Weaver cantaloupe syndrome” (after performing twenty-one reps of GHD sit-ups with a full range of motion, hands reaching back to the floor, he was unable to sit up the next morning – fluid swelled Matt’s abs and within a week had drained into the inguinal canal and filled his scrotum). The GHD can absolutely not be taken lightly, and has the potential to completely sideline otherwise very strong athletes for a few days to a week.