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 mostly normal, but can be a sign of arthritis which usually improves with rest, medications and exercises. Some people find it weird that whenever they move their heads, they hear a crunching sound in neck. Some think the crackling sound in neck comes from the back of the head where it meets the neck. Others think it comes from the sides or the bottom of the head. While some do not give much thought about it, others are bothered and just want to know what to do about it. Is Crunching Sound in the Neck Normal? It is medically called crepitus, which refers to the grating, cracking, or popping sensation you hear or feel in the skin or joints or skin. The crepitus indicates air accumulation trapped in the compact space of the joint or subcutaneous tissue. When your tendon or muscle slides over a bony surface, a popping sound can sometimes be heard. In the neck, this can happen when your joints glide as you turn your head. This is normal.