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.
Once you get used to the regular crunches, modify the basic crunch to get an even more effective tummy exercise! 1  Lie down on the floor with your hands behind your head. 2  Now bend your knees as you would do in crunches, keeping your feet on the floor.