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.
Form [edit]. A crunch begins with lying face up on the floor with knees bent. The movement begins by curling the shoulders towards the pelvis. The hands can be behind or beside the neck or crossed over the chest.