Generally, runners have weak hamstrings. Most don't run fast enough to really strengthen them. How many times have you went into a weight room and seen a long distance athlete train hamstrings or posterior chain work? From my experience, hardly ever outside of a group of elite runners at an olympic training center in Chula Vista, CA. But, that type of attention to detail is one of the reasons they are there.
Sprinters can sometimes run too fast and strain them. They have the opposite problem as most of us. They may train them, but train them improperly, or think they have trained them enough to run mid 10's in the 100m.
The hamstrings are made up of three muscles, semitendinosus, semimembranosus and the biceps femoris. The biceps femoris is actually two muscles, long and short, and have two different nerve innervations. The adductor magnus, a huge muscle, is often times referred to as the fourth hamstring and should be trained as such.
Charles Poliquin believes that the hamstrings are mostly fast-twitch and should be trained with 3-6 type reps. His take on coming back from a hamstring injury is first-rate in my opinion. Usually you do a lot of reps so that fatigue doesn't come into play. He reasons that it is the fast-twitch fibers that get strained. As such, it is these fibers that need to be strengthened using heavier weights and lower reps.
Franz Bosch, a biomechanist, wrote the book Running: Biomechanics and Exercise Physiology in Practice. This is a great book! I highly recommend you own it if you work with runners and sprinters.
He believes before weights or reps are thought about, focus on coordination. This simply means the exercise is done technically perfect - correct tension is maintained on the hamstrings at all times, control over the speed when reactive work is done, and any synergistic muscles that work with the hamstrings are working correctly. The big take away is the hips and knees must be working together properly. He believes that that the majority of exercises should be done standing, knees extended, with the weight shifting though the hips as this stresses the hamstrings the best.
As a general rule, if you're standing on two legs doing an exercise, it is the dorsal muscles (erectors) that are the limiting factor. When you switch to one leg, it becomes the hamstrings. So one-legged variations are a big theme if you want to attack the hamstrings. Just try a single leg back extension. You will feel the difference.
Here is Oregon Strength Coach Jim Radcliffe demonstrating one of these qualified exercises. I read Jim say he once got on a plane and took the red eye back just to hear Franz Bosch present for an hour. Anyone that has seen Oregon play football knows that his athletes are fast and explosive.