Before you read this, make sure you understand that getting stronger by lifting weights will not mean you are going to get too big or you will lose speed.  This blog post here and this article here explains why this is not the case (with references) and why lifting weights will actually make you quicker!  We should also understand the dynamics of running.  There are basically three parts to running: the stance phase (which is the amount of time you spend on the ground with each foot strike), the stride length and the stride rate (or frequency).  To run faster, you have to either a) decrease the stance phase, b) increase the stride length or c) increase the stride frequency.  This applies to every distance from a 100m sprint to a ultramarathon.  The first group of muscles we are going to talk about in regards to their importance to running and why they need to be as string as possible are the hamstrings and in particular when they flex the knee.

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 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 moves.  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:

Prone Hamstring Curl

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.

Razor Curl

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.

As you can see, training the knee flexors can help you run faster and stay injury free if you are doing a lot of running training.  Keep an eye out for Part 2 in this series where I will talk about how the hamstrings as hip extensors impact your running.  And if you ever wanted to run a marathon (or even half marathon), make sure you check out my Run A Marathon In 8 Weeks program by clicking here.