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.
If practiced often, neck cracking is also responsible for less mobility in our neck. Each time your neck is cracked it results into wear and tear of the cartilage present in your vertebrae. This can also lead to neck arthritis in the near future. Its advisable to perform neck cracking under expert supervision.