They are also both REALLY extreme! Hunching yourself over like in the first picture is going to restrict breathing and extending like the second is just a recipe for all kinds of disasters caused by unnecessary muscle tension needed to hold the position!
As above, those pictures are horrible for cycling. You want a naturally rounded back somewhere between those extremes. How rounded is going to vary by individual, but I don’t think anybody should look like your second picture with an extended back.
That’s a different issue IMO. You do want a straight/extended back for deadlifts for safety reasons. Lots of YouTube videos on how to lift with good form. Use a mirror if you’re not already. And start light and get form nailed before you start lifting heavy enough to injure yourself with poor form.
This is likely a mixture of weak core and poor mobility. dead lifts will help with core strength, but can you do a proper deadlift with no weight? what does your squat look like? can you get in a deep squat with your heels on the ground and your arms down? In that position can you straighten your back? Then can you raise your arms above your head? There are lots of mobility related workouts/stretches available on youtube and elsewhere for this kind of stuff.
As I expected, people correctly covered and addressed this to great effect. I love this group
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.
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.
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.
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.
You can submit a question for consideration on the TR podcast page:
Worth noting that one of the more “successful” cyclists of the last generation had a horribly arched back on the bike, both road and TT. But that was his natural position…trying to change it to a flatter back would likely have gained no benefit.