Exercise done correctly will support your singing, and maybe your emotional health. I think your singing teacher is worrying that you might tighten your abs too much.
Remember that the diaphragm is part of a system. When you are building the lego truck and you put too tight a rubber band around the wheels, it becomes difficult for the other parts to move. Likewise, as you tighten the muscles around your diaphragm with too many ab crunches, it may take more work to loosen the muscles around the diaphragm so it has proper space to move. (I imagine you are breathing a sigh of relief to take a break from your crunches). Keep breathing – passively!
For example, aerobic exercise is great, strength training is terrific – but not body building. Try combination intervals with low weights and high reps. That will lengthen your muscles, instead of bulking you up – and the stress on your body will be moderate so you have plenty of physical energy and strength for singing.
Yoga is also okay if you do not hold or control your breath while you are holding your postures. The key with singing is to let the diaphragm operate organically and passively. Often yoga teachers ask their students to monitor and control their breath. This is contrary to the requirements your singing body has. Try to do the yoga postures but let your breath be completely free and passive.
Steven Flam is the developer of the groundbreaking teaching technique called BreathingRx For Singers which includes diaphragm target training. For more information or to debate the content of this article please contact him at firstname.lastname@example.org or check out our website at: www.breathingrx.com