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.
Changes in synovial fluid pressure and moving ligaments and tendons can both cause neck cracking. See Treatment for Neck Pain. However, if the neck cracking noise is accompanied by pain, swelling, or some other concerning symptoms, then it may represent a problem that should be checked by a qualified health professional. See Neck Pain Causes