I've noticed this problem since I started road running a few months ago, but now I'm also doing a little on the fells it has got much more noticeable.
It seems one of my calf muscles is very tight (the right one) - if I do a standard calf stretch (foot flat, knee bent a little, pushing against a wall), my left leg flexes about twice as far forward as the right. Initially I thought it was something to do with my ankles (one being less flexible than the other) but now I think it must be my calf muscle.
As a consequence, practically every time I run, I end up with one sore calf, although I've only properly strained it the once. I also notice that when I'm ascending, my left leg/ankle is quite 'springy', whereas my right leg/ankle is more solid (not as much stretch in the calf muscle?).
Can't say when this first started, as I've only noticed it when I started running. Could it be a result of having bad sciatica in my right leg a few years ago, and not stretching enough while recovering?
Anything I can do about this? - aside from stretching lots, which I do already, and it hasn't made any real difference (I'm pretty sure the answer is going to be 'go see a physio'). Anyone been to an NHS physio for running type things? Is that a bit cheeky seeing that it's basically recreational? I'm pretty sure my G.P. is a runner (years ago he advised me to start running when I had back problems.... 'it'll increase your pain threshold' he said), so he may be happier than most to refer me.