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.
There are numerous types of crunch exercises available; however crunches on the ball have been proven to give some of the best results. To focus exercise ball crunches on your lower abs, position the ball under your mid to low back, place your hands behind your neck and place both feet flat on the floor. Arch your back to lower your shoulders down to the ball â this is the key to this exercise that helps to focus on your lower abdominals.