Pulled Muscle

Two old men were sat on a bench outside a nursing home having a chat. "How are you, Richard?" asked George.
"I'm not feeling too good today, I'm utterly exhausted," replied Richard. "I've pulled a muscle, and it's killing me."
"I'm surprised that a pulled muscle makes you feel so tired," said George.

Richard yawned and said, "Well, it does if you pull it a hundred times in one night."