I think perhaps you should look into Ion Channelopathies such as Periodic Paralysis.. hypo hyper and normo kalemic periodic paralysis and Andersen Tawil Syndrome. These are considered rare disorders but I believe they are rarely diagnosed. They are considered rare forms of Muscular Dystrophy. It took me three months of research on the net to come up with my diagnosis of Andersen Tawil Syndrome. I presented my case to my physician based on my research.. a review of my family history and genealogical tree and my connections to known Founders for rare genetic disorders among the very early settlers of New France.. Canada. Stiffness is also part of my clinical picture (myotonia) and by treating my electrolyte disturbances and avoiding fluctuations in my potassium level through diet and supplementation my condition is greatly improved.. Malignant hyperthermia cardiac conditions and paradoxical reactions to medications like sulfa certain antibiotics and anesthesia. Important note is that stiffness for many may be an indicator of Hyperkalemic periodic paralysis in which case too much potassium could be the reason you are suffering from the stiffness.. I was pretty sure my symptoms were the result of a normokalemic nature as my diet was typically low in potassium high in carbs and sodium.. so I experimented with the recommended diet for hypokalemia and in three days my stiffness disappeared flexibility returned and pain virtually disappeared. Since my diagnosis didn't happen until I reached the age of 53 much damage has occurred to my muscles and I now suffer from permanent muscle weakness.