What is the correct back position on an MTB? While searching for my lower back problems I come across the suggestion that you should have a straight back on your bike to avoid pain, one example:



But on the internet you find a lot of "professional riders" that sit with a pretty rounded back and on a short frame, for example: 



Ok, she is climbing and this is maybe just a temporary position.  

source: https://www.orbea.com/img/blognew/in-text/Pendrel%202012%20en%20baja.jpg



source: https://hips.hearstapps.com/hmg-prod.s3.amazonaws.com/images/slides/still-going-0-1440612429.jpg?resize=980:*



source: https://www.orbea.com/img/blog/large/PENDREL_WORLD_CUP-20140906164951.jpg

Looking at their elbow bent they are all on pretty short frames, haven't checked the dates of the pictures. But a "straight back" sounds like a good idea even though frame geometries change over time.  

Question: what's the right back position?
A straight back might avoid pain there, but it's going to create quite a lot somewhere else.

In your first rounded back/straight back comparison pictures you can see that the rider's pelvis is upright and the ischial tuberosities of the pelvis  - the 'sit bones' are on the saddle. In the second picture the rider's pelvis is rotated forward putting pressure on the soft tissue of the perineum. That picture makes me wince just looking at it. Chronic pressure on the perineum can lead to all kinds of complications, including numbness and erectile dysfunction (if you are a male).