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.
Crunches work only the muscles on the front and sides of your abdomen, but it's important to target all the muscles of the core to get more defined absâincluding lower back, hips, and upper thighs, says Lou Schuler, co-author of The New Rules of Lifting for Abs.