molecule. 1  Chemistry, Physics. the smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound. 2  Chemistry. a quantity of a substance, the weight of which, measured in any chosen unit, is numerically equal to the molecular weight; gram molecule.
When I do crunches my neck starts to tighten up and it makes me have to rest before doing another set of crunches.
They do separate and have a gap in the middle for your uterus to grow. So it is obviously important to not do any abdo crunches when pregnant! After the baby is born, some people's muscles do not go back easily and they get a 'separated rectus sheath' which just needs some exercises and physio to sort out. It gives people a sort of ointy tummy when they sit up and stuff like that.