1. Your leg force counteracts the force on the pedal. Your whole BODY moves forward/upwards on the bike as a result. YOu do NOT require powerful back muscles even for devastating cycling power. It's totally false that the back muscles have to counteract all the leg forces, or even a small percentage of it. If that were true, Bradley Wiggins would have monstrous ridges of back muscles to counteract his 450watt output. The power put out is counteracted by your whole body+bike moving, NOT your back. Track and short distance sprinters do benefit from a strong back as they can have greater engagement of their arms to add power to the legs, but for most other cycling, you're doing it wrong if you're generating significant forces from your arms pulling up rather than your legs unless it's the absolute final sprint.

- Ab work does NOT strengthen your back for extension motions. Abs are for abdominal flexion and have nothing to do with the back. Some motions like deadlifts engage multiple muscles groups including back and abs, but the main load is still on the back extensors, not the abs for all extension exercises. I'd see a new chiropractor if he says you're working your back extensors while doing ab exercises. Weighted cruches are NOT going to improve your deadlift or back extension motion, no matter what.

- Pros often do weight training as an added supplement to their riding (or running if you're a runner.) Often it's because they can't safely add further cycling training load to their regiment because they're maxxed out. None of them will substitute weight workouts for bike workouts. Very different than the amateur age-grouper who's not close to maxxing out their bike time, and if anything, needs to bike,bike,bike if they want to get better on the bike. Lifting weights will not help them compared to more quality bike time. I certainly wish that all the people I race against would take 3 hours of their 10 hours per week of training and devote it to weight training instead of doing quality bike work for those 3 hours - that would be a dream come true! Even 1 hour spent on a subthreshold 1hr ride instead of weight training per week will be noticeable after a few months of training at 10hrs/week volume.