A stronger hip flexor, for running.

Let's get the obvious out of the way first - the posterior chain exists, matters and has to be trained to be a faster runner. But to neglect the anterior chain is to invite leaks in the kinetic chain, performance decrements and injury. After evaluation reveals an iliopsoas active insufficiency, the goal is to improve raw strength of the iliopsoas, and rectus femoris, as hip flexors - in a linear fashion, without the adductors and TFL contributing by adding rotation. In other words - straight hip flexion, no rotation across the midline. My preference is to couple this with strong hip extension static stability on the other leg. 
So, first, inhibit TFL and adductors with some local soft tissue therapy, for example, a foam roller or spikey ball to the TFL and adductors. 
Then, a core-activated hip extension stretch. 
Next, quadruped rock against a ball. 
Then perform core-activated hip flexion. 


This same drill can be done similarly by leaning against the wall, pushing into the wall, to generate trunk stability (core activation), and pulling into hip flexion with bands attached to your ankles, secured to something behind you. Remember that wall position from these drills? 
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You could also do a supine version of core-activated resisted hip flexion on a stable stance leg, as per this video. 
Naturally I want to bring this activation, and resistance, to reactive strength, in a pattern, so we can do this drill.
Then we have to get you into run movement prep drills. 
High knee march
Leg cradle
Quad stretch with hip internal rotation.
Quad stretch to toe touch

The hip lock drill should be added here. I've had some athletes with, well.... let's face it... too much muscle slack. And they want to be good at running. And they want to be good at force production and force absorption and force reutilisation... in other words - jumping, landing, quick contact rebounding. So, they've developed so many breakages in their kinetic chain that many things suffer.... from their big toe to there lower back, well.... even further up. And when it comes to muscle slack, it takes quite a bit of strength training to stiffen up such slackness.
Let's repeat that. It takes strength training to stiffen them up. Tendons, ligaments, connective tissue. (They're usually very good at contorting, and have great potential at yoga) But in the meantime, we can begin re-programming the type of kinetic chain robustness that is required to not leak energy out in running. Here's a drill that does just that.
And it's such a great drill because we get "performance" of the motor pattern we want (co-contraction in the hip and pelvis) without having to coach it. We simply cue the athlete to achieve a result, which is to keep one knee up and push a weight up as much as they can on the stance leg. To push that weight up, you have to automatically co-contract everything in the stance hip/pelvis area.
In motor learning terms, it's intrinsic knowledge of result in order to improve something somewhere else.
This position of co-contraction is called the Lock Position. It's absolutely crucial in running.
If the free hip is not going up, you're lost. In hurdling, it's the main quality they need. Watch Sally Pearson from 3:18 in this youtube clip to see it's relevance.
The position of the pelvis is a result of good movement. So what do we teach first? We teach them the end position - the lock position. And the body will self organise towards this. It's a common theme in cueing of many movements, but in this specific case - running and jumping, the clear outcome should be that the hip and pelvis are in the lock position in stance.
This can be such a crucial drill for those any issue around the hip, knee and foot.
Followed by A march, B march, A skip, B skip, bounds and runs. 

Voila. 

​Rinse and repeat.