(TLDR - saddle side-profile shape, not just position, may be quite influential in causing or reducing lower back pain, I'm suggesting flatter saddles may be better for some of us, in a way that's not been widely discussed that I'm aware of)
I do get back pain on longer hilly rides, depends on the bike though. I'm more of a gear pusher than a spinner and until fairly recently I couldn't quite touch my toes (I can now but it took a period of regular stretching after a lifetime of not bothering). That combo of low cadence and/or climbing strength with low-ish flexibility seems to be a prime cause of lower back pain - the common description is of hip flexors and hamstring tension pulling/rotating the hips out of natural position and the tension in the lower back comes from resisting that. Add a lower drop-bar bike position and lower back tension can be a common problem.
First phase of this causing me problems was ~8-9 years ago, doing 'hilly 100' type sportives. A chat with a triathlete about bike position suggested that maybe a closed thigh/hip/upper body angle might be the cause. I swapped to an inline post (on a 73.5 STA) and found it did delay or eliminate the problem. First step in realising how important saddle position is.
A while later and I'm not riding a fast road bike and I'm not a fan of sitting too far fwd over the BB as it just shifts weight onto my hands. I'm riding project frames with adjusted geometries and wondering how much of what feels right is based on muscle memory and adaptation and how far I could adapt myself to be comfortable on a less 'normal' bike layout. Cue a period of all kinds of aches and stretching plans... The stretching worked though, backs up that point about hip flexors. Mid-ride I can get back tension pain, get off and stretch for a moment and then carry on. But it's a PITA (PITB?) to break a ride like this. And it was showing up in a stiff back during sleep or in the mornings which worried me.
Looked at things from the start. Saddle position is the base point for long-distance bike fits, that has to be right for anything else to work out. My saddle is a comfy old fave, no problem there. But I move around a fair bit on the bike I'm currently riding. 50-50 time in the drops or the hoods, perhaps >60% in the drops some rides. On and off-road. That affects my hip angle... as (probably) does the shape of my saddle? It's the WTB/Spoon type of shape with a pronounced flat S-curve when seen from the side. That curve does affect comfort, so would it affect position? Seems natural that it would.
Popped a long, flat Arione-copy shape saddle on the bike 3 weeks ago. I seem to settle onto the saddle slightly differently based on using hoods or drops, before I tended to settle onto the same comfy spot on the saddle. I don't know if I'm actually any more efficient but I either had a few good days or I feel as if I may be getting a more effective pedal action in the drops now. Best bit - no more back pain since then. No stiff back when getting up. It stopped almost there and then.
Conclusion?
Any angle or 'scoop' to the saddle can bias our seating position to a particular area of the saddle but different positions (ie drop bars or anything with fore-aft grip range) may need different sit bone / hip angle positions ie the ability to make small adjustments naturally. A scooped design may bias that position to a sweet spot on the saddle. That saddle shape may work to encourage the right hip angle in some riders / on some bikes, for others it may be creating tension as the body tries to compensate for being a few degrees out of ideal alignment. A flat saddle may reduce or eliminate that effect. It allows me to sit fore-aft +/- 1" or so as needed and the saddle itself isn't encouraging any hip rotation since the surface is pretty much all at the same angle to the seat post and BB.
While this may suggest that my saddle wasn't in the right place to start with, I've ridden enough set ups and tinkered with them all enough to know it wasn't far off and the general range of position I've used hasn't made any difference in this respect (aside from knee pain when it was too far back). It's certainly got my interest in bike fitting up again though - I can't assume the set up I have is optimum, just self-selected through experience.
Will report back to see what effect it has on the proper all-day distance rides (did a local 13-15hr mix-terrain route on this bike recently, was off and stretching a fair bit over the last 4hrs so will aim to do that again soon to see). Hopefully it works longer-term and isn't just muscle-memory jolt that's created a change. I hadn't really considered saddle influence in this way and anything that helps keep us comfy for longer helps us go further
