Although it's not part of the official diagnostic criteria for Aspergers, low muscle tone is nevertheless an condition strongly associated with Aspergers.
Why is it so Confusing?
When most people hear about "low muscle tone" they assume that it has something to do with bodybuilding - I certainly did at first. This is particularly confusing since many newly diagnosed aspies are children and it's quite uncommon to see any child with a well-developed set of muscles. Most parents will either simply ignore the condition or assume that a bit of outdoor activity, eg: playing soccer, is required.
The other confusing thing is that there are adult aspies out there who regularly attend the gym and who have "better" muscles than many NT people. How do they fit the criteria?
I saw a great line on a web site discussing Hypotonia. "Your kid seems perfectly good with their muscles - they are strong, they run round with boundless energy, but they have trouble doing things. You have likely been given the standard explanation about your kid's muscles being floppy and you just cannot see it in your kid."
Defining Low Muscle Tone
Low muscle tone refers mainly to the distribution of muscles on the body, their initial state, speed and stamina. The affected muscles can be "trained" but that training won't come from sport or from and normal gym/weight training. It comes from some very specialized training - and it won't be 100% effective. In young children, the problems of low muscle tone will reduce in severity as they get older - up to about the age of 10, though aspies will likely continue to adjust and compensate for the rest of their lives.
Low muscle tone is often described as "floppiness". This is because the muscles are supposed to help support the skeleton and are supposed to prevent certain types of movement. Since the muscles aren't particularly tight, people with low muscle tone often experience "hypermobility", the ability to move limbs into awkward positions.
As children, aspies often find that they are able to easily perform feats which require flexibility but not strength or balance, such as splits, backbending and shoulder rotation. They may display unusual flexibility in other joints such as fingers.
The Bad News about Low Muscle Tone
Such flexibility comes with a price and aspies are usually quite uncoordinated and clumsy. In running, this contributes to the famed "unusual gait". It's easy to imagine that low muscle tone only affects the big muscles but this isn't the case, it affects all activities requiring muscles including most notably, speech, pencil grip and writing.
When sitting or standing for long periods, aspies tend to slump quite a bit. Sometimes, they will stand with their legs crossed in what appears to be an uncomfortable fashion. My mother was constantly trying to correct this stance and while I'm reasonably aware of it at work, I still find myself standing that way regularly. I'll point out now that although this looks uncomfortable, this is actually a very comfortable stance for aspies.
Aspies often sit with their head and shoulders rolled forward and will frequently lean on walls, furniture, door frames and desks. Parents of aspie children will probably be very familiar with being "leant on".
Low muscle tone does not prevent aspie children from enjoying themselves, they can run and play with other children without feeling any ill effects. The problem is that, they're a bit slower and they tire easily. This means that team sports, like soccer are often not well suited to aspies. In the case of my son, we turned to scouting as an alternative to soccer.
Dangers inherent in Low Muscle Tone
While the slumping and leaning behaviours aren't necessarily great posture, they're not particularly dangerous to the aspie unless the position is adopted for very long periods without proper breaks.
I have first-hand experience with this problem as I've had episodes of "overuse syndrome", a kind of RSI, with my hands, arms and shoulders from sitting at my computer for too long. It took quite a while for OH&S to work out that the issue wasn't with my hands, or even with my workspace. It was simply due to excessive time spent in an unsupported position.
Fixing the Problem
As I said earlier, the fix isn't normal weight training, it's physiotherapy and specialized muscle training. There is also a need for awareness and constant correction of one's position. In my case, the muscles most needing training were small ones high on my back. Correcting my keyboard "slump"moved my arms and shoulders back into less damaging positions. I have to be constantly aware of my position and correct it thoughout the day. I also do a bit of stretching and strengthening work on them at the gym.