If your arms are bent when addressing the putt, this will create tension in your stroke.
It is important to have the correct putter length and set up position with arms relaxed and straight, but not stiff when putting so you can have a nice, smooth stroke. The Fundamentals of Putting will help you understand the importance of this.
franklygolf 
I got a Frog. I got the book. I experimented with length, grip, and style (settled on left hand low as a right hander), and now my putting is better than it has been in 45 years of playing golf.