The most common riding mistake is rounding the shoulders and upper back, while jutting our head too far out in front. This is called forward head carriage. Then there is the looking down part. Hummm...the same position as when perusing Facebook our mobile phones.
Add reins in hand, the horse leaning on the bit, pulling us even farther forward. Think about it. It is the same as resistance training. We are actually building muscle and muscle memory to maintain this position. So, sitting up correctly is actually the exact opposite of what we are training our bodies to do.
Over time, our back muscles become chronically overstretched becoming thin and weak. Too weak and too inflexible to hold the torso and head in correct alignment. Meanwhile the chest muscles become strong and shorten. Thus, our default posture is a slouch.
Of course it doesn't stops there. When our upper body is slouched forward, the tail bone also slides forward and tucks too far under - as opposed to sitting on the seat bones. This is called "posterior pelvis". It is no wonder equestrians have back pain!