Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
stiffen [ˈstɪfn] vi (body) напряга́ться (напря́чься*perf) (joints, neck) не сгиба́ться (impf)
my muscles have stiffened у меня́ свело́ мы́шцы
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.