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.
1 Keep the head in this position as long you can to stretch the back of the neck, then release. 2  Joint Movement-When the neck joints are not freely moving, grinding noises may be heard. 3  Muscle tightness can also cause neck pain. 4  Gently stretch your neck to ease the tension in the neck muscles.