Thank you so much @tfcoates. Pardon me for replying late.
Yeah, we may grow stronger by training over several generations and our skeleton is rather strong to sustain aerodynamic loads, but not light enough. We aren't strong enough in the sense that we cannot produce enough power for flight, our respiratory system isn't efficient enough, neither do we have enough muscle mass in pectoralis to produce enough lift by flapping. The muscles present in our body should be redistributed. Our legs need to be less muscular but strong enough to sustain loads during takeoff and landing. In birds, the breast muscles weighs around a third of its body muscles, and technically, our chest muscles should weigh more than a third of our body weight. I may be wrong but It seems rather unlikely to happen even after several generations of training. I am really happy that you seem curious, Thank you again..... :)