My physical therapy answer: No. The action of a sit-up does not help your core. But of course it can't be a straight answer. Disclaimer: if you contract your tummy muscles and draw them toward your spine, then yes, sit-ups are a way in which you can train your core. But the movement of a sit-up doesn’t.
Why? The motion of a basic sit-up heavily involves hip flexion. You start in a position on your back and when you finish, your abdomen is close to your thighs. That’s hip flexion. Your abs don't do hip flexion. Whether your thighs are moving toward your stomach or the reverse, the motion is still hip flexion. Sit-ups, especially when your feet are pinned down, is a hip flexor exercise. Nobody needs that. Maybe in rare cases. But rarely. RARELY!!
Do you have frequent low back pain? If yes, then that is a second reason not to do sit-ups. But there are many different ways to get a core workout and many that don’t run the risk of worsening your back problems.
For those of us who have low back pain, disk issues, instability, or recurrent sprains, generally our low backs do not like compression/pressure. When you fire your hip flexors, the major ones pull on your spine because they directly attach to it. That compresses your spine.
For many of us, our weekdays include driving in a car then sitting all day. The result: shortened and tightened hip flexors because you are sitting in 90˚ of hip flexion all day. If your hip flexors are tight, stretch them and foam-roll them – don’t do resistance training with them. They are likely already pulling your spine forward.
So how do you train your core? I’ll have later entries for a discussion on core as it involves many muscles. Right now, let’s discuss your tummy muscles, because that’s usually what people think about.
Feel free to do crunches, improve your diet, do cardio: that make you have a 6 pack. You’ll look great in the park in the summer. Sadly, your 6-pack and “hard to reach obliques” do little to nothing to stabilize the center of your body.
You DO, however, have one front muscle that you’ll never see that is very important for spinal stability. That’s your transverse abs. AKA your T.A. or TvA. It is under your other 3 layers of abs and wraps around you like a cummerbund. Through fascia in your back, it attaches to your spine and when you contract it, it stabilizes your spine and also helps take the workload off your deep back muscles which may be tired and fatigued.
Look at these pictures -->
As little as 4 lbs of force created by the T.A. can be effective in increasing stability. 4 lbs! That’s easy – so all you have to do is turn it on, and you can do that!
If you are doing crunches on the floor or on a physioball, keep your spine in neutral, draw your belly button toward your spine, breathe, and raise your upper trunk ever so slightly. That will train your T.A. If in a prone or side-plank, draw your belly button toward your spine. That will train your T.A. If doing stability work on a BOSU, draw your belly button toward your spine. That will train your T.A. Yes, there is a pattern emerging here.
More to come on core....
Dr. Jesse Roles is a Doctor of Physical Therapy and owner of RiNo Physical Therapy, Inc. in Denver.