Often people refer to the sit-up and the crunch as the same thing but in-fact there is a big difference between the two.
So Whats The Difference?
The basic difference is that when you do a sit up, you sit all the way up. Your back is all the way off the floor and you should be sitting upright. When you do a stomach crunch you only raise your body up far enough for your arms to touch your legs, or for your shoulders to just leave the ground a little. Your lower back remains on the floor. The stomach crunch eliminates any involvement by the hip flexors and makes the crunch an effective isolation exercise for the abdominals.
How To Do The Stomach Crunch
The movement begins by curling the shoulders towards the pelvis, with hands beside the neck, head or crossed over the chest (not behind the head). You then generally pull yourself up with the use of your abdomen halfway to your knees or with your shoulders just off the ground. The difficulty of the crunch can be increased by lying on a declined bench and/or holding a weight on the chest or behind the head.
- Read my article on how to perform the stomach crunch
How To Do The Sit-Up
This movement begins by lying on the floor on your back. Your knees should be bent and your feet flat on the floor. This helps to reduce stress on the back muscles and spine. You then lift your back from the floor using your stomach muscles until everything down to you bottom is not touching the ground. People generally place their hands behind their head for the sit-up to help bring their body all the way up.
- Read my article on how to perform the perfect sit-up
The sit-up is still widely used in defense training, martial arts, sporting clubs and general exercise routines. However modern physical trainers and experts argue that the sit-up can lead to spinal damage.
For the full sit-up, research suggests that the abdominals only flex for about the first 30 to 45 degrees of movement which is basically equal to lifting your shoulder blades off the ground (in other words performing the stomach crunch). As mentioned previously the sit-up involves the use of the hip flexor muscles as well as the stomach muscles. Beyond 30 degrees of movement, the strong hip flexors start to take control.
This then places pressure on the lower spine by the hip flexors. The leverage exerted risks compression of the lumbar intervertebral discs and it is due to this reason that the stomach crunch is often recommended over the sit-up.
An Alternative To The Sit-Up
The Janda Sit-Up attempts to prevent the hip flexors from being overactive during a sit-up. This type of sit-up will never totally remove the hip flexors but will weaken their involvement. To perform this exercise you lie in a normal sit-up position with your feet flat on ground. However as you lift up you keep your arms extended forwards and forcefully tighten the hamstrings and buttocks. This causes the hip flexor involvement to be reduced in a process called reciprocal inhibition. This means that the opposite muscles to the contracted ones will relax. Here is a video of the Janda Sit-Up:
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