The glute-ham developer, or GHD for short, is an often deserted item of equipment that lurks in the corner of many a CrossFit®️ gym, functional fitness space and strength facility. Most people, the author included, only tend to visit this torture device in bouts, and writhe in pain for a few days after doing so. There are many movement variations possible on the GHD, but today we shall focus on the sit-up.
The GHD sit-up is an exaggerated range of motion flexion about the hip, from a hyperextended position, to almost fully flexed. Contrary to popular understanding, it recruits the hip flexors, not just the rectus abdominus (abdominals). Chief among the muscles recruited is the rectus femoris; the only one of the quadriceps to cross both the knee and the hip joint. By aggressively extending the knee at around parallel, the rectus femoris is pre-stretched in its knee extensor role, allowing it to be more powerfully recruited into its hip flexion role.
This movement, while impressive to watch, carries hidden dangers if attacked too wholeheartedly without building capacity over time. It taxes the entire midline, and the extended range of motion, and large eccentric component to the movement lead to high levels of soreness for several days after training. Taken to its extreme, this can present as Rhabdomyolysis (or Rhabdo for short).
The linked video shows how to measure the device to fit your frame, and the caption explains how to increase load over time, and what you should have checked off before you tackle the GHD sit-up.
Keep your eyes peeled for more uses for the GHD, and more technique tips.