According to personal trainers, sports medicine personnel and various newspaper and fitness magazine writers, in order to maintain good posture when exercising, especially when doing strength exercises you should contract the abdominals. According to these sources, you must contract the abdominals to prevent arching of the lower back to keep the spine in safe posture. Contraction of the abdominals is said to be the key to preventing lower back injury.
However, the information presented is questionable. Foremost is that the abdominals are not postural muscles. Contracting the abdominals does not ensure good posture and in fact, disrupts good posture. Understand that when you contract the abdominals you flatten the lower back. In essence you remove the arch which is critical to maintaining a strong and effective spine.
The erector spinae muscles of the lower back are the key muscles that maintain the erect body position. They are postural muscles and should be under contraction on all exercises requiring good standing posture or for maintaining effective normal curvature of the spine. Thus the recommendation to contract the abdominals when doing exercises such as the squat, lateral arm raises or overhead presses is erroneous.
In fact, volitionally contracting the abdominals on any exercise other than those for strengthening the abdominal muscles, is most often not called for. In almost all cases it will disrupt your ability to do the exercise correctly and effectivelly — unless you relax the abdominal muscles as you do the exercise.
Keep in mind that when you concentrate on keeping the abdominal muscles contracted, it will be impossible to have strong contraction of the erector spinae muscles of the lower back. As mentioned above these are the key muscles that maintain proper spinal posture and should be under contraction during execution of all exercises in which the spine must be maintained in a strong and safe position.
Thus, once again it is critical that you closely examine what you read and not take everything as being sound information. It may come from a reputable source but it should still be questioned. Checking the facts should be your major responsibility since most magazine and newspaper editors are not qualified in the areas in which they write.. As a result, the problem of having more misinformation continues to grow.
For more information on this and similar topics see Biomechanics and Kinesiologyy of Exercise, Build a Better Athlete, and Sports: Is It All B.S.?