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.
Okay, the first potential reason that people suffer from neck pain during abs exercises is poor form. When sit ups fell out of favor, crunches became the primary abs exercise in fitness routines, but most people dont know how to do crunches correctly.