It is more common in autism and so is hypertonia. But can exist for many reasons, by itself, brain damage, genetic defects ect.
I myself am very inflexible and as a child could honestly barely raise my legs.
My joints are both hyper flexible and have this painful habit of dislocating.
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Diagnosed autistic level 2, ODD, anxiety, dyspraxic, essential tremors, depression (Doubted), CAPD, hyper mobility syndrome
Suspected; PTSD (Treated, as my counselor did notice), possible PCOS, PMDD, Learning disabilities (Sure of it, unknown what they are), possibly something wrong with immune system (Sick about as much as I'm not) Possible EDS- hyper mobility type (Will be getting tested, suggested by doctor) dysautonomia