We've all done it right, gone to do some sit-ups and put our feet under a bar, bench, radiator or anything to fix our feet in place so we can perform a true 'sit up'.
Restricting your feet for a sit-up exercise
It's a common sight in most gym's, I know I've seen it many times, however. If you're doing sit-ups like this, you're doing them wrong. Fixing the feet enables you to pull harder with the hip flexor, in turn, this puts increased stress on your lower spine, essentially this allows you to sit up without bending your spine. What happens from this action is that the effort required from the hip flexors is increased while the effort required from the abdominals is decreased, precisely opposite to why most people would perform the sit-up exercise with restricted feet.
Overall the sit-up is one of the least effective abdominal exercises, because, during the sit-up exercise your abdominal muscles spend very little time under tension, even when performed unrestricted. It's your hip flexors which start the movement, followed by some abdominal activity before finishing with momentum making the exercise ineffective at stimulating the rectus abdominals, our primary core muscle and the one responsible for flexion of the spine gets minimal stimulus.
The same is accurate with straight leg sit-ups which put enormous pressure on the lower spine, requiring a considerable effort from your biomechanically disadvantaged hip flexors. This will cause lower back discomfort and likewise stimulate the abdominals for a minimal amount of time during the movement.
So, what can you do?
If you're guilty of the above, your probably wondering what it is you should be doing. To start with as mentioned, sit-ups are an in-effective exercise and there are plenty of other exercises especially for beginners which are much more useful such as the crunch or abdominal curl exercise.
The Crunch or Abdominal Curl Exercise
The crunch is an excellent exercise which keeps the abdominals under tension for much longer, it's performed with feet unrestricted and legs at 90 degrees reducing the involvement the hip flexors can have in the exercise activity.
How to perform a Crunch Exercise
To start, lie on your back in the supine position with your legs bent and feet flat on the floor about hip-width apart. Cross your arms over your chest, so your fingertips are pointing to or almost touching your shoulders. From this position contract, your abdominal muscles lifting your head shoulders and mid-back off the floor, as you contract you will be shortening the distance between your ribcage and your pelvis. Hold this position for a second or two at the top and slowly resist as you lower yourself back to the floor.
Bent Knee Crunch Exercise Video
Remember not to thrust your upper body up off the floor; if you feel discomfort in your neck, you can place your tongue on the top of your mouth to reduce this.
Progressions: Hold your arms our above your head
Regressions: Place your hands on top of your thighs and crunch up pushing your fingertips towards your knees.
There are instances when a sit-up exercise with restricted feet might apply to an individuals training plan, and indeed, more advanced exercise might require fixation of the feet or a similar exercise pattern with straight leg raise which need a stable base level of core stability.