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.
Please read this other page before doing any neck muscle stretches. If you can hear a crunching noise in your neck when you move it, any stretches will need to be prescribed by your physiotherapist. Mobilizing any tight joints may need to be done individually, to avoid further stretching hyper mobile ones.