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 your core by lifting your upper torso and challenging the muscles of your abdomen. Performing crunches incorrectly may cause neck pain and discomfort either during the exercise or as the day goes on.