In high school, when I weighed something like 130 pounds, I had a Saturday track meet (I think my senior year? - 35+ years ago), got up from the kitchen table that morning and basically collapsed in pain from a sharp lower back pain. Missed that track meet and a few more and walked around like a crooked old man for several days. Have had on and off issues with my back ever since. Have had an MRI, X-rays, etc - nothing unusual was found. Have had periods of relatively normal feeling and other periods where it was very hard for me to walk or stand up straight. Have been to a chiropractor, had cortisone shots - none of it much help. My current doctor believes it stems from stiffness and inflexibility in my legs. I also sit at a desk every day which does not help. I still golf, but I feel the effects of it that night and the next day. I don't think I would have ever been able to play baseball every day (not now or 25 years ago) with the back issues I have had. Obviously Christian Yelich is in much better shape that I have ever been in and has a lot more access to physical therapy and medical analysis. However, it seems like there is a multitude of reasons why backs can be an issue. Its not always as simple as a disk or alignment issue and it seems like its not always easy to diagnose, especially when the pictures show nothing.