As I expected, people correctly covered and addressed this to great effect. I love this group 
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WRT to back shape on the bike, those pictures are clearly extremes and not ones I would aim to get for any fit outside of rare exceptions.
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Generally speaking, you want to place a rider in a position that allows them to achieve a comfortable reach to the bars that also lets their back rest in a “natural” position. I placed natural in quotes because what is natural is unique to every person.
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We want to let that rider specific back shape happen in most cases vs trying to force someone into a shape that is “ideal” or meant to mimic someone else. Placing the rider in a position that is somewhere between those crazy extremes shown is most often beneficial.
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As mentioned above, most people will end up with some amount of forward back curve and pelvic rotation that is between those wild extremes in the pictures. That “Correct” example is clearly a convex mid-back shape that is NOT something I’d expect for all but a rare few riders.
- This is possibly related in the sense that it is about your body, but totally unrelated in the general sense of position on the bike. You need to handle strength training in the context of proper technique and practices, without direct consideration of your bike setup that is a totally separate process.
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