Do you remember gym class when you partnered up with someone who held your feet down while you cranked out as many sit ups as you could in one minute? Those were the days. (The days without knowledge of how the core posture muscles work, that is!)
The Psoas, the Abs And Your Low Back
The reason bent knee sit ups aren’t all they are (or were) chalked up to be is that after a certain point in your trunk’s excursion to an upright sitting position, the abs are no longer doing the work. Much of the work to get you off the floor is done by the hip flexor muscles, namely the psoas. The psoas is a deep muscle that attaches on your femur, lays in front of your hip bone and attaches again on the vertebra of your low back. It’s really a wonder muscle, especially for posture and back health. But in this instance, most trainers and exercisers mistake the work of the psoas for the work of the abs. This cheats you of some of the benefits of the ab strengthening you think you’re getting by doing bent knee sit-ups. It can also also mess with your back. American Council on Exercise says that when sit-ups are done from the bent knee position “stress to the low back is present”.
And crunches are not much better, mainly because they emphasis only one, out of 6 total ab muscles (rectus, 2 external obliques, 2 external obliques and transversus). Crunches mainly work the rectus abdominus which is the long vertical muscle in front. Think washboard abs, not a supported low back. Crunches also tend to shorten and bunch up the abs.
The Mind-Body Fitness Approach
Mind-body fitness techniques such as those found in yoga, Pilates and others, teach people to find their deep abdominals and work them with a lengthening contraction (called eccentric contraction). The eccentric contraction occurs when you need to control the body part in relationship to gravity. In other words, it puts on the brakes when you or one of your parts could easily fall or “plop” down. You land gracefully thanks to the eccentric action of (some of) your muscles. The eccentric contraction generates about 4x as much force as the other type. This means if you concentrate much of your strength training on working eccentrically, you’ll get much stronger in less time. You’ll get plenty sore while you’re building this up, though!
