I have a bit of a theory that riding largely/exclusively on the trainer tends to demand a bit less of the postural muscles (as well as the majority of trainer sessions being shorter), so people often decondition these muscles slightly over long periods of largely indoor training. Don’t know how true that is.
I’ve just booked in for a bike fit this weekend, actually, and was asking around for recommendations. Anyway, a riding buddy of mine said (unhelpfully, in the context) that best bike fit he ever had was in a bike shop in Italy about 3-4 years back. He went in to buy a tube and the young lad in the shop (who spoke some English) was marvelling over his (then brand new) Canyon Ultimate with Di2 (shop was proper old school, apparently - 100% steel and Campag).
Mike (my buddy) said to the young lad that it was indeed ‘bellisima’ but he couldn’t get comfortable on it. The youngster, without saying a word, goes into the back of the shop, there’s a shouted conversation in Italian, and he emerges with what looks like his very grumpy grandad in an oil-stained vest. Anyway, Gramps - who speaks no English and does not smile once in the next 30 minutes - tells Mike via the youngster that he needs to see him ride up and down the road.
So Mike pedals in slow 50 yard loops in front of the shop for a couple of minutes until the gets told to stop. The old bloke then tweaks the seatpost height, gets him back on, and repeats the process. He then says something to the young lad in Italian who hops back into the shop, comes out with a new stem, fits it and adds a spacer, gets him back on. Process repeated again. Old boy shifts the saddle back in the clamp a bit, and voila! It’s perfect. Took 20 minutes, the guy didn’t speak a word of English, cost Mike 30 Euros for a stem, and he’s not touched it since. 
