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.
Plus, if you don't do them right, you won't see results. First of all, if you're still doing sit-ups, stop. There's a reason why people do crunches now instead. Traditional sit-ups can pull at the lower back and neck muscles, or even kick the hip flexors into action when the abs are what you want to feel working.