1. To stretch and warm one's muscles, so as to become flexible and loose before some strenuous exercise or activity. Make sure you all limber up before we begin practice. I didn't limber up before my run, and now I've got a terrible cramp in my leg.

2. To stretch and warm up someone else's muscles in such a way. In this usage, a noun or pronoun is used between "limber" and "up" in this usage. The team masseuse limbered us up before we headed out to practice. I need to limber up my legs.