Please take a look at the above photo. Both images are taken at the same stage of this runner's gait who's running at 15 kph. The image on the left shows very little knee flexion on the back leg as it begins to swing through. This means that there's little hamstring activation and a lot of demand will be placed on the hip flexors to pull that leg through, which will inevitably lead to fatigue.
The image on the right however shows the same runner lifting that back foot up more, shortening that lever between the heel and the hip and gaining more hamstring activation. This places less demand on the hip flexors as the increased knee flexion helps that hip to move nicely into flexion and therefore into the next step.
So next time you run, have a think about whether your heel is coming up nicely at the back, or are you dragging your feet a little and putting those hip flexors under stress? If you are dragging then consciously try to gradually lift those heels a little. One thing to note though is that you want the lift to be relative to your pace. If you're new to running for example you don't want to be lifting your heels to your back side like Galen Rupp does as that would be inefficient. So work on it gradually, feel comfortable and let me know how you get on.
By Marc Brown