Sometimes when you get an injury, for example shoulder pain in the Right when benching or shoulder pressing, and you do a rehab program for it. For the side that was painful, Right side. You improve scapular control, via strengthening lower trapezius in shorter length and thus the range of motion of the scapular into posterior tilt, better adduction, and Upward Rotation, and then you go back to doing the bench and the Right shoulder pain is gone. You carry on progressing with the weight one the bar, and all of the sudden the other shoulder , Left one, or elbow start hurting. Why is that? It might be because actually both sides had deficiency in correct movement, muscle length-tension-strength imbalance, but the Right one was initially a bit worse than the Left shoulder. So initially it was getting a little bit more impingement etc, and was being irritated regularly causing pain. Then after you improved how that side moved, you potentially now made it move better and through fuller range then the ‘good’ other side .So now the Right is performing better than the Left. Now when coming back to the bench, your Right is able to tolerate the same weight the Left side was able to, and then you can progress further. However, the new added weight now is well tolerated by the Right side as it is now moving better, the Left side was never lifting this new weight as this was always restricted by pain in the Right side, so it’s a new territory for the Left. And because it had slightly off biomechanics/control etc, now it’s being tested, taken for a spin on a faster track, and the wheels are not liking it. The solution…improve the Left shoulder as well, WHILE maintaining the Right. It might be a little game you have to play now and again, and not only at the shoulder, but other parts of the body. Remember, the body changes all the time, adjusting to what it’s exposed to, ALWAYS and ALL THE TIME. If you don’t stress it, that’s a signal to get weaker, if you stress it, it improves. It’s not looking for long-term view ‘correctness’ (that’s your job), it’s looking for shorter term efficiency.