This is a tricky one to nail down whether it's true or false, so thought I'd pitch it out there. I have read, once in a while, that poor muscle tone comes with the AS condition. I know that more neurological physical manifestations come with it, such as awkward gait and posture, appearing "ret*d" to a layperson while running, and such, but I wouldn't consider muscle tone a neurological manifestation. Yet we see these stereotypes in popular culture, like Sheldon from "3rd Rock" and they're always like string beans. (for the height issue, BTW, most Aspies I've met - male and female- are at least average height, so that's not an issue).
Being an Aspie, I have noticed that it seems more difficult for me to put on muscle - I found through a gym coach in my late 20s that part of it was my technique - and though I've worked out an average of over 200 days a year from the age of 26 to 33 (I'm 37 today), I always found that friends/acquaintances who worked out at a similar frequency got better results. I had pretty good diet too with lots of chicken and fruits/veggies. I'm not small by any means, I'm 5'10" and 202 lbs. But I still find it's not the same gains for the amount of effort I put in, so perhaps there is some impaired muscle growth element apparent in Aspies, I don't know. Maybe the poor muscle tone "symptom" is more a byproduct of Aspies having less interest in going to the gym because of various factors e.g. lack of buddy to go with, lack of money for membership, getting overwhelmed by all the lights/sounds/crowds? I'm not sure, but I would tend to discount the muscle tone as a "symptom" of the condition, strictly speaking.