Hypermobility Syndrome/double jointed/Overflexing/hyperextensive call it what you will and ASD seems to go hand in hand; I've heard so many parents comment on the uber-bendiness of their child's joints, the high number of dislocations and sprains.
Couple that with the often high pain threshold of ASD and you can get a dislocation or inflamed joint that barely registers on the kid's horizon for the day.
And it certainly doesn't get in the way of all that physical activity they get into.
Until something gives.
My own Aspie teen has frequent cramps, the urge/need to crack his knuckles and 'pop' whichever joints are bugging him.
When he was younger we woke up to find he'd flexed his hip joint until it trapped a nerve in the night.
First warning we had was he just wouldn't get out of bed the next morning.
No tears, no tantrums, no yelling, no pain.
He just wouldn't weight-bear.
Medical intervention saw the problem solved without surgery (thank Gawd!).
Which I keep in mind, wishing he'd chew his fingernails instead of flexing his fingers all over the place.
When hubby tracked down some family members he discovered a strong family trait of scoliosis of the spine, largely associated with the hyperflexing ability each member had in childhood.
In fact, every family member in one part of his family have a marked curvature of the spine.
High rates of arthritis and other associated joint conditions, like Carpel Tunnel syndrome, RSI, etc, are common.
And after his first double footy match he almost bathed in Deep Heat.
I can see I'll have to buy shares in that one!