Yoga is one of the most loved activities in the city. It not only helps you to improve flexibility, it can also strength your muscles. This can protect you from arthritis and even back pain.

 
 Here is a list of some yoga courses in Berlin:

http://www.adyayoga.de (you pay what you want!)
http://yellow-yoga.com/ (different options for your yoga class)
http://www.bikram-berlin.de/ (courses around 10 euros)
http://www.sunyoga.de

 
And goods news is that most probably your insurance company will be able to pay up to 80% of your yoga class!..... Just call them and ask them, they often prefer that you continue a healthy life than paying for medicines or a hospital. This is done more often in Germany each year :-)

Good luck finding your yoga course!