I've been having this issue for probably a few years now and I wonder if disproportionate conditioning may have to do with it. My abs seem to rarely take any stress, and my shoulders tend to be the most strained body part when exercising (whether it be L-sits, planche progressions, hanging leg raises, etc.) I was also wondering about my shoulder, because when I do upper body exercises, from my shoulders to my lats tend to be the only parts that seem to get strained. I wouldn't consider my issue severe in any sense, but it bugs me because my shoulders tend to fatigue faster than anything else. Any advice?