The hamstrings as a knee flexor are made up of predominately fast twitch fibers that flex the knee (e.g. bring the heel closer to the bottom). This influences the amount of repetitions you should use to train them with. This part of the hamstring also assists in lateral-medial and anterior-posterior stabilization of the knee. This means they are very important anatomically to strengthen to prevent injuries especially Medial Collateral Ligament, Anterior Cruciate Ligament and meniscus injuries. In terms of what strengthening the knee flexors will do for your running, it assists in two of the three aspects: stride rate and stance phase.
Stride rate: As your knee flexors become stronger, they can flex the knee more rapidly after the stance phase or foot strike. This decreases the lever arm of trailing leg and allows the trailing leg to be brought forward faster ready for the next foot strike. By doing this, it will increase stride frequency as now it takes less time for the leg to be brought forward ready for the next foot strike because a) the lever arm is shorter and b) the hamstring is stronger so it can more forcibly flex the knee.
Stance phase: Because the knee flexors aid so much in the stabilization of the knee, research shows this aids biomechanics in the stance phase. The biggest reason increased strength (and especially eccentric) strength in the knee flexors helps is that it takes less time for the leg to stabilize itself when it hits the ground so that foot can push off faster and the next leg cycle can begin. This means you spend less time on the ground running and the stance phase is reduced.
Now just making the knee flexors stronger help these two aspects of your running. But if you want to really improve your running speed and endurance, you might want to use knee flexor (hamstring) strength exercises that match the same biomechanical kinetic chain. For instance, the shortening of the trailing leg’s lever arm as part of your stride recovery is what is known as an open kinetic chain. An open kinetic chain basically means where you apply force to will move. In this case, the runner is applying force through the posterior (or back) of the leg and it is moving. Now in the stance phase on the other hand, the hamstrings as knee flexors are actually part of a closed kinetic chain. Where the hamstrings are applying force (i.e. the ground) does not move. That means for a runner who has poor stride rate they would want to use more open kinetic chain strength exercises like hamstring curl variations:
On the flip side of the coin, for a runner who has a long stance phase, they would use closed kinetic chain strength exercises like GHR variations. Here is a little tip – one way of really picking a poor stance phase up without video footage is the sound of the foot when it hits the ground – if it is loud, the runner has poor eccentric strength in the leg and are more than likely spending too much time on the ground when running.
Now obviously besides providing running efficiency and speed benefits, strengthening the knee flexor portion of the hamstrings will also significantly decrease your risk of lower limb (and especially knee) injuries. In fact, around 80% of all hamstring injuries are distal (meaning closer to the knee) so strengthening this part of the hamstrings will also aid in hamstring pulls. Knee flexor training will also help protect both the medial & lateral meniscus, lateral & medial collateral ligaments and anterior cruciate ligaments of the knee. This is caused by the strong fascial connection the hamstrings have with the just mentioned connective tissue.