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.
1 Perform each exercise for at least 1 set of 10-16 reps. 2  Do each exercise slowly and focus on good form for each rep. 3  If you have lower back problems during some of the crunches, put a rolled up towel under your lower back for more support.  Skip any move that hurts.