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.
When you perform abdominal exercises, you often target your upper or lower abdomen for work. While standard crunches or curls will put stress on the upper abs, reverse crunches or hip lifts will blast the lower abs. You may think youâre working two muscles, but the abdominal muscle, or rectus abdominis, is technically a single long muscle.