There is a location called Meditation Garden where you will learn proper stretching and yoga techniques so muscles won't stiffen whether you are working with the game or not.
Coblation-channeling uses heat to shrink and stiffen the soft palate and it uses molecular disintegration to remove some of the obstructive tissue.
As a child grows the eustachian tubes widen and stiffen, allowing air to enter and fluid to drain from the middle ear more efficiently.
Diabetes will also affect the joints of the feet causing difficulty in walking as the feet stiffen.
When these two powers quarrelled after the peace of Nicias it remained loyal to the Spartans; but the latter thought it prudent to stiffen the oligarchic government against a nascent democratic movement.