The caused of stiff muscles is: when your muscles get tight from like a hard workout or after you got injured. it is hard to move when your muscles are stiffed. resting may help your muscles relax-this is also an example of muscle tension. Stiff muscles often go away you can stretch or exercise your muscles to make it feel better. Even after a tough workout, you should always rest and stretch so you can ease the pains thats going to happen when you go asleep.