Here's the rule to know: If you lie on your back with your legs extended, your abs have the capacity to raise your shoulders about 30 degrees" off the floor. No further. Any exercise that involves movement beyond that 30• range involves muscles other than the abs.
Is it necessarily bad to involve other muscles? In this case, yes. These other muscles, the psoas magnus and psoas parvus, run from the front of the legs, up through the pelvis, and attach to
the lowest six spinal vertebrae. They pull your trunk toward your legs, as do your abs. But unlike the abs, their range of motion is huge: they can flex you forward all the way from a full backbend until your chest touches your knees.
Fig. 3 The psoas muscles
The psoas work most efficiently when your legs are extended and/ or your feet are held–as in Straight–Legged and Roman Chair Sit–Ups.
In this position, the psoas compete with your abs for the first third of the movement, and then take over entirely for the remainder.
Psoas-dominated movements yield very little in terms of ab results—for all energy you put into them. This is the reason you see some athletes rocking through several hundred Roman Chair Sit–Ups, trying for a burn. Any movement you can do that many of is not an efficient muscle conditioning exercise.
Worse, though, is the risk of injury from psoas dominated movements. With each Straight Legged or Roman Chair Sit-Up, the psoas tug at the lower spine. That tug doesn't do much harm as long as the abs remain strong enough to prevent the back from arching. But unfortunately, even if you're in great shape, the abs tire fairly quickly, allowing your back to arch. This causes the vertebrae around the psoas' attachment to grind together. And in a decade or so you may be stuck with permanent lower back pain as a result of disk degeneration.
Kinesiologist have long warned against any supposed "abdominal" exercise where both:
the psoas come into play
the position allows—or worse, encourages your back to arch
Based on these criteria, traditional sit-ups (both Straight–Legged and Roman Chair) must be discarded. Fortunately, there are safer and more effective exercises; these will be explained in the Program Section coming up. Some may be familiar to you, but remember, there's much more to creating the optimum routine than the exercises themselves.