Have to keep limber

[chrysanthemum]

The void in Mark's life those first few days was almost unbearable. "Uncle Mark, you're doing too much," Lucy told him as he worked ceaselessly at one small task after another. "Have to keep limber," he said, smiling. He didn't about Ricky's being gone.
Will you please tell me the meaning of Have to keep limber?

 

[Rover_KE]

Concise Oxford English Dictionary © 2008 Oxford University Press:
limber1

▶adjective - supple; flexible.

Rover