GHD stands for Glute-Hamstring Developer. It could also be called the Ab-Quad Developer. It all depends on which way you are facing when you sit on it.
We’ll come back to that. First, let’s talk about sit ups in general.
As our overall understanding of anatomy and spine function has developed we have moved away from the sit ups you did in elementary school.
Do not do sit ups on flat ground. Even the Army doesn’t do that anymore.
In this set-up, the head initiates the movement with a snap forward, the abdominals hold the spine in a flexed forward position and the small hip flexor muscles do all the heavy lifting of the torso. The hip flexors pull hard on the spine and you end up with an inefficient sit up and pain in your low back.
For a much more detailed account of this, please read this.
Enter the AbMat. Anyone who has been to Second Wind has probably used an AbMat for sit ups, but never really thought much about why. They just know it feels better.
The fat part of the mat goes against your low back, feet together and knees spread. The spine extension you get when laying back on the mat creates a full range of motion for your abs to contract. The spread knees takes the hip flexors out of the line of action, so the abs will have to do the pulling.
Once the contraction begins in the abs, the lumbar spine now has something to push against, so it does not go into unnatural flexion, which causes low back pain. It is this support of the low back which is the most vital role the AbMat plays.
Again, it’s all explained in better detail here.
Finally, the cumbersome machine in the corner. With the GHD, the goal is to perform a sit up while keeping the spine completely stable (our definition of core strength) and using the combined forces of the hip flexors and quadriceps to powerfully flex the trunk.
To perform a GHD sit-up, there is some leg flexion in the descent. Then, the leg extends dramatically and pulls the athlete to seated position. Conversely, if the athlete does not extend the leg to come to seated, the primary movers are the hip flexors, but specifically the psoas, a muscle that is connected to the lumbar spine, thus pulling on the low back. However, by engaging the quads, specifically the rectus femoris, we pull from the pelvis, which offers enormous mechanical advantage and leverage.
The two sit-ups, GHD and AbMat, complement each other beautifully. One is dynamic in
the hips and static in the trunk; the other is dynamic in the trunk and static in the hip. In
conjunction with the L-sit (static in the trunk and hip), they develop a formidable capacity
in the midline.
For obvious reasons, it’s hard to use one GHD in a class setting, but once you learn how, please feel free to use it before or after class and during Open Gym.
Next time: GHD Hip extension.
Further Reading: Three Important Ab Exercises, by Greg Glassman