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.
It is also a good idea to do some standard crunches to warm up your abdominal muscles. Increasing the Intensity. To perform an abdominal V-Up, lie down on your back on the floor or on a gym mat and then extend your arms behind your head. The back of your hands should touch the floor while your palms face the ceiling.