All this time you thought those thousands of sit-ups were going to build you a six pack… only to have me tell you that you might have been wasting your time.
(And chances are, you have no business doing any abs work – unless your diet is great, you are doing compound weight training, and are already fairly lean.)
The truth is that sit-ups only work your abs indirectly. All that time that you spent doing those sit-ups was really a greater benefit to your hip flexors than for your abs. Were you not trying to build up tough hip flexors with your sit-ups? If not, I’m sorry to be the one to bring you the bad news. I’ll explain what all of this means.
How your abs work:
Muscles work through contraction. To put it simply, when a muscle contracts, it becomes shorter in length as the fibers pull together. So let’s think about what this means for your abdominals. What happens when your abdominals contract?
Again, when you abdominals contract, the muscles of your stomach area pull together and essentially become shorter, thereby moving the skeletal structure they are attached to. This contraction of the abdominals shortens your abdomen and pulls your rib cage and your shoulders toward your hips (see the picture up top).
Pulling your shoulders toward your hips is exactly the type of movement we see in the various types of stomach crunch exercises. A proper crunch will keep your lower back flat to the ground, and as your shoulders are pulled toward your hips, your spine will also inevitably bend and flex. This is what we call spinal flexion.
Crunches aren’t for sissies though. The range of motion for these is quite small, so they don’t look as impressive as a full sit-up, but a well-executed crunch will place intense stress on your abs and create a burn.
Focus on really pulling with the stomach muscles and squeezing them at the top of the movement for a moment. Like any other exercise, adding weight is important, but we’ll talk about ways to do that another day.
Why sit-ups don’t work (very well):
Here’s the bad news: all of that great spinal flexion and pulling with the abs doesn’t happen very much when you do sit-ups. Think about what’s happening when you do sit-ups. Your body is bending at the hips to pull your torso upward. This is called hip flexion, and it doesn’t use your abs nearly as much.
The beginning of the sit-up movement is the most effective part for your abs. The sit-up begins by pulling with the abs for about the first 30-45 degrees of the movement. That’s the good news.
The bad news is that the rest of the movement is done by your hip flexors. The hip flexors are a little group of muscles that help connect your femur (leg bone) to your back bone/ pelvis (check out the picture on the left).
Once your abs have completed the first 30-45 degrees of movement in your sit-up, they then begin to act only as stabilizer muscles. They hold your upper body in that flexed position isometrically while your hip flexors do the dirty work to pull your body upward at the hips. So your abs are indirectly involved in the movement, and this explains why they can get a little sore when you do sit-ups, especially if you have weak abs.
Don’t believe me? Try it for yourself right now. Stand in a doorway and hold on with one hand for stability. Place your other hand on your abs to see how much they flex. Now, raise your leg straight up in front of you. You will feel tightness in the top of your thigh, but not your abs. Leg raises and sit-ups are essentially the same movement in reverse.
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