But having good posture aka a flat back isn't about 'good' or superior flexibility is the point often lost. An inflexible person can have good posture on a road bike. I am not particularly flexible. The point is, a more aggressive torso angle can be achieved by rotating the pelvis forward and it takes extraordinary flexibility to achieve an aggressive torso angle with pelvis rotated back because you are asking your vertebrae to do the impossible....high relative angulation. This position causes strain and injury. I have said before that the biggest reason that riders don't rotate their pelvis forward is because of weight on their balls. The key is to find a saddle with perineal relief to allow rotating the pelvis forward so you can ride with a straight back and aggressive torso angle to get low out of the wind and put strong force to the pedals. Good posture whether walking, typing at a computer or riding a bike is a recipe for the best back health.