I really don't know how to tell the difference in most gymmies. I can tell the difference in the extremes, really fast twitch and really slow twitch, but in most cases, it's hard to tell, especially when they are really new to gymnastics. My dd is an extreme, fast twitch ectomorph.... If you blink when she tumbles, you miss it. Muscles upon muscles, not just fit and not in a cute way. Her guns are larger than mine. Harlem pants fit like leggings. Flexibility is a constant struggle. Lots of amplitude in leaps, but struggles to get 180 (except in straddle). Lots of times she has too much power and not enough control. Literally steps into a robhs full so she has more control of the landing. Best bud, extreme slow twitch, long and lean, legs for days. You can see every shape of her exceptional form in tumbling. Lots of control, not much power. She really has to get a good punch for her double. Takes longer to 'get' higher skills, but when she does, they are flawless and I've never seen her lose a skill. Other best bud, small and petite! Amazing extension, beautiful lines, hyper leaps! Just gorgeous! Tumbles well. Couldn't tell you slow or fast twitch, but absolutely amazing when she hits. Her issue.....focus. She's the type of gymnast that a coach would get overly frustrated with. Absolutely capable physically, but maybe not ready mentally and emotionally. Beautiful nonetheless!!! If we could mold the three together......