Bike fit / correct riding position

3mm can be the gap between life and death regardless of being healthy or not, I had the saddle set a little too far back previously when changing saddle, and was wondering why it felt hard to pedals and had a sore back, 4mm forward it's sorted.

This is merely an observation that changing a bicycle position by as little as 4mm can be felt. The slightest change can, of course, effect muscle use. Nothing, however, here indicates that one position is better than the other. It can well be Coke Classic versus New Coke. Most cyclists--- including many elite ProTour riders--- have strong backs and comparatively weak stomach muscles. The slightest shift can rapidly be felt in back soreness. We also have a very inconsistent leg muscle development. That is why we need to--- and often don't--- exercise our abdominal muscles and, among other things, stretch our hamstrings. The computer geeks amongst us have it as a double whammy as when we're not on our bikes we're sitting in chairs.