What is "stiffens"

Usage examples of "stiffens".

He stiffens in the saddle as he makes out the blurry figure of a bearded officer riding slowly toward him.

Avi, by contrast, stiffens up like a man who stepped on a rusty nail a week ago and has just felt the first stirrings of the tetanus infection that will eventually break his spine.

Rudy stiffens in panic for a moment, thinking that the prisoner sees him.

It's a struggle not to groan out loud when every single one of my injuries acutely reminds me of its presencefrom the fever that's scratching my eyeballs all the way down to the bone bruise that stiffens my right knee.