Peter Attia· MD
in general there's a lot of ways to describe it but um the easiest way is to describe it by the name so fast twitch means that the twitch or the speed of contraction is higher and so those these fibers can contract and squeeze together with through the mechanisms we haven't got to yet we'll get there masas and actin um at a much faster rate having said that the fast switch fibers tend to be larger though not always and certainly not in endurance training individuals and definitely not with aging um that they tend to be almost they almost always are more glycolytically driven and so they're going to have more of the enzymes responsible for anerobic glycolysis they're going to have more glucose in the cell they're going to have less intramuscular triglycerides and they're going to be having they generally have more phosphocreatine slow twitch fibers are fatigue resistant which means these are the ones that they can contract kind of all day long because they don't use as much glucose so they do use quite a bit still they are much better at using fat as a fuel they are uh they tend to have more and larger mitochondria and um the downside is they don't contract with as much velocity in general so that's the the functional that's why we call them twitch