Should you brace your abs in posture? 

It is commonly claimed that one should "engage the core" implying clenching the abs in posture. But is this true?
Clenching the abs will rigidify the trunk and prevent proper breathing. It will also flex the trunk, causing hyperextension of the hip joint, stretching and weakening the hip flexors, as well as lumbosacral flexion with subsequent malcompressive [anterior] disc shearing. Not a great strategy.
As I have said many times, we need to maintain proper extension in the lumbosacral spine in posture, without clenching the abs! But, in this regard, many patients dread having to let the "womb" hang out. It is therefore important to understand that we need to learn how to engage the transversus abdominus in posture rather than bracing the obliques and/or rectus abdominus. This can be accomplished by gently "sucking" the lower abdomen in, ensuring that the abdominal wall is not rigid upon palpation. If it is rigid, then the patient is still using the RA or obliques. Using the transverse abdominus in posture is also important when treating abdominal hernias.
This way, we will be able to maintain proper abdominal stability without compromising breathing or pelvic positioning.