Hi everyone. I've been riding fixed gear every day for a little under a year now. I typically ride to work (4 miles) and my route involves a pretty long climb (maybe 1/4 of the whole ride). I don't have any pain when I've been riding for a little while, but I notice that I can get some pain in my right knee when I first start riding. I notice it only when I'm back pedaling, but it goes away entirely after a few minutes of riding.

I was wondering if anyone could recommend me some warm ups I could do for my knees before riding. Whether they be stretches, warm up exercises, etc.
Thanks in advance.