Going fast, backwards
I'm susceptible to hamstring niggles/injuries. A search back through my diaries since '04 reveals over 70 mentions of 'left hamstring', so there's something sinister going on there. I think the main cause is asymmetry. Few runners are perfectly symmetrical left to right — run blindfolded on a large flat field and you won't run in a straight line. We use our sight to keep running straight and make automatic adjustments to the stride to do this. Run on a flat sandy beach at low tide and measure your strides — one will be longer than the other, if only slightly. I'm right-handed (legged too — can't high-jump off my left leg) so I have a left hamstring weakness.
I think this hamstring strain is minor in nature. It's responding to rest. In the past I've been able to run through it, or at the most had a week or so off before resuming normal training. I had five days off last week but ran on Thursday and Saturday. Thursday I jogged 3k on the grass track and the hamstring was bearable. On Saturday I raced the Parkrun and the leg felt okay through the first kilometre. I actually ran my fastest first k split (4:43) and was gaining on Jim who ended up running a PB of 23:46. Running up the slight incline onto the bridge my leg started feeling sore and a few hundred metres later it was getting worse so I stopped running. A DNF! Well, after running 1.5k I walked the rest of the course so strictly not a DNF. I've run over 1000 races and had to DNF only five or six times. DNS is definitely preferable to DNF.