For me, it’s usually my low back that felt fatigued in a low position. I was complaining of chronic low back pain years ago, and a massage therapist had me do some poses and made range of motion observations of me. She thought the issue was actually short and weak hamstrings, and lack of strength and mobility in the hips. On the bike I lacked the core strength to hold myself up which led me, I think, to overuse my back to keep my torso up, and overweighting my hands.
I started doing challenging yoga workouts on tape (yes, on tape) that made a life changing difference for me in core and posterior strength, and flexibility. The power yoga types (Rodney Yee Power Yoga for the win!) are subtle but produced results for me. For gym exercises, single legged glute bridges and especially slow, heavy reverse dead lifts help the posterior chain (similar to the bent over row position). Side planks will work the forearm strength, core and hips. Front of the core gets too much attention.