Contract the abdominals. I feel confident that you will see this recommendation with just about every exercise that is being described for you to execute. It appears with regularity in many newspapers as well as fitness magazines.
For reasons that I have been unable to determine, the proponents of contracting the abdominals state that is necessary to maintain good posture, protect the spine, prevent arching of the back, create greater force and most importantly, for safe and effective execution of the exercise. But yet, I have not seen any substantiation for these statements.
For almost all exercises I do not recommend contracting the abdominals. The only time they should be contracted is when you are doing abdominal strengthening exercises. If they contract during other exercises it usually happens automatically without you having to initiate the contraction. Not doing this can lead to injury.
For example, I recently read an article stating that you should contact the abdominals when doing the squat exercise. But if you do this you will be forcing the spine into flexion — a very unsafe position in the squat. For back safety, is necessary to contract the erector spinae to maintain lumbar lordosis, i.e., an arched back.
This fact is not opinion. It is based on scientific studies that show the erector spinae is under strong contraction during execution of the squat while the abdominals are relaxed. It should also be noted that the erectors are involved in posture as they are responsible for maintaining the upright position. The abdominals do not have a similar function.
The main function of the abdominal contraction in a non-abdominal exercise is to create and maintain intra-abdominal pressure. This in turn does help protect the spine – together with strong contraction of the erectors and breath holding – when it is subjected to extremely high loads. With relatively light loads, this is not necessary.
Thus for any standing or seated exercise, the best recommendation is to contract the erector spinae and not be concerned with contracting the abdominals. Doing this will keep you in a safe and comfortable position when doing the exercise.