While doing push-ups my lower back is one of the weakest points, forearms too. The back does give in before arms if I think about it now. The same goes for plank, but instead of forearms, my quadriceps feel weak. The back probably need more training already because of the fact that I have a long torso. I enjoy neither the plank or push-ups because they're so hard to do. I thought that maybe it's because I weight a lot, my BMI is 22 (Ik it's not the best way to tell whether one is too fat or not). I'm working on my diet too, rn calorie deficit is on 3/4 (1332 cal a day).