Two days ago I pulled a hamstring. It's not bad, I can walk without pain, but I do feel it intermittently. Probably a 3-4 on a scale of 1-10. Yesterday I did a short easy run on the treadmill and some stretches and had no problems, but the pain is still there. So, should I stop running for a few days to let it heal, or continue with gentle exercise?
A good general rule for whether you should run or not is that if your injury results in you having to change your normal running stride in any way whatsoever, then you shouldn't run. If you do you'll risk injuring something else that is compensating for your changed form.
Out of curiosity, how did you injure your hamstring? If you pulled it while running, then you likely have a problem with your running stride. Hamstrings can get over-stretched when your legs extend too far away from your body's center of mass, and this seems to happen a lot when people try to go too fast and stretching their stride out to compensate for a low cadence or lack of power in their feet/lower legs, or they were trying to run downhill at a pace that their legs could not adequately control. Be careful of either of these.
As you mentioned in your post, when running easy you don't feel a lot of pain in your hamstrings; this is because when your stride is not over-stretched and you land under your body your hamstrings aren't stretched too far. Ideally, you should be able to run in this fashion no matter how fast you're moving. I followed you in another thread where you mentioned wanting to run a half-marathon; keep this easy stride in mind as you increase your distances.
Best of luck, and feel better soon!
I would definitely recommend taking some time of the interval training until your hamstring is better. When you feel better, ease back into the intervals, starting out slower and being mindful of how far your legs extend out with your stride. I would also recommend that you do some hill training, either outside or on the treadmill (set the incline to 5-6). You can either do intervals or try the "never-ending hill" run, depending on what your focus for the day is (speed/endurance). This way you can work on strengthening your leg muscles as well as your aerobic fitness, and running uphill also forces you to have a shorter stride, which is something you'll want to work on anyway. It's a great combination workout that will touch on multiple areas of your fitness.
Good luck, and run healthy!
I know this is a settled thread, but I also noticed you didn't mention which hamstring, the right or the left. If it is the right hamstring, driving in the US can be hard on right hamstring recovery, because of the way the car seat traps the hamstrings on that side when you work the brake and gas pedal. Cruise Control helps, but only on long drives.
Earlier in the summer, I met a standout runner who complained to me about his right hamstring, and who had reached the same conclusion about the car seat. He pulled his seat back to elongate the muscle, but I suggested pushing the seat up as far as possible to elevate the knee and hamstring above the seat. Since then, I've placed an old running shoe before the gas pedal so I can prop my heel on it to enhance the effect.
Just yesterday, I saw a fellow marathoner while training at a local park. She was walking with a hitch, so we talked, and she said her right hamstring had been bothering her since a marathon we had both run in 2005. I didn't know this was why she hadn't competed at that level in the years since.
Granted, there are reasons why the hamstrings can become compromised in training, particularly the right when running hard laps in the usual counter-clockwise direction, but there is no doubt they are starved for oxygen after a workout or race, and that is often when we slip them into the constraints of a car seat for the long drive home. In my friend's case, that was a four-hour drive home after that 2005 marathon - probably the same reason my right hamstring bothered me more after the same race last year, when it wasn't a problem during the race itself.
When you add up the total time most of us have to spend sitting each day, the poor hamstrings don't have a chance, once they get to needing real help. All the remedial steps in between can't make up for the lack of circulation in those chairs and seats. This of course, applies whether it is on the right or the left.
To help restore circulation, I cradle my right hamstrings with my right hand while I drive, using gravity to get some deep pressure into the knotted spots in the center. This may seem counterproductive at first, but pressure skillfully applied will produce vasodilation in the affected tissue, when room is allowed for the circulation to work its magic. This strategy has gotten me through many a hamstring issue, despite all the chairbound insult my busy life can deliver.
As I talked with my friend in the park, another therapist/trainer with elite running experience joined us. We discussed the fact that some hamstring pains have nothing to do with the hamstrings themselves, but can be symptoms of sciatica from spinal or gluteal causes. We joked about charging her 300 bucks an hour to fix her problem, then shared some tips about using tennis balls and other focused aids to work the areas herself. It's true there are many potential causes of pain felt in a particular area, and some are helped in ways that seem to have nothing to do with the pain itself.
So, watch your training, but also everything you do when you are not training. It may have more to do with your pain than anything else.