@Martial Garo--Santos from a medical stand point it is really exiting to hear this and I‘m craving to solve a mystery here. From a sports perspective it really sucks to hear the complexity of this problem. Since it‘s not a local problem (MRI was clean) it is only right to look deeper into it (historia morbi, context factors, etc.). What‘s interesting to me that a lower back pain is going into the arm since it‘s not directly connected neurologically. I can think of a lot of connecting chains throughout the body that all wouldn’t really show in an MRI scan. It might be interesting to look into the lymphatic system since it seems to be a onesided issue and that‘s „the only“ system that would work onesidedly. Wishing you the best of luck on this one, feel free to keep me updated!!