stiff as a board

Stiff as a board is the phrase used for someone whose body feels very stiff and hurts when they try to move it. This is commonly said by someone whose muscles feel stiff after a strenuous exercise whether immediately after it or more often, the next day. As a board is stiff and would hurt when bending it, a person feels much the same way; that if they moved too much, they would break or snap like a board would.