Singing loudly has nothing in common with shouting.
It does, however have a lot in common with yawning. Essentially you need to have a wide open airway and, as stated above you have to use the diaphragm to move the air. The key muscles, strangely, are your abdominals which act like the bellows on a squeezebox. These need to be strong and responsive. leeneia has probably given you the safest advice - remember you're dealing with muscles and these are easily injured.