Apparently the answer is your ankles and calves.
A recent study was brought to my attention by this article. The basis is that you become less efficient as a runner basically because the muscles in your lower leg get tired and your ankles can’t do as much work.
The articles suggests a few ideas about how to work on this problem. But this points to a basic problem. Most of the strength work you might be doing in the gym doesn’t carry over well to real life. Sure, your squats and lunges may be making your quads and butt stronger, but the movements are almost entirely vertical.
Life is lived in the horizontal plane. This is why we see studies that horizontal strength movements – like the hip thrust, kettlebell swing, or SLED TRAINING – are much better at making you faster.
Well, your squats and lunges also aren’t doing anything for your calves. Plyometrics do to some degree, as do loaded jumps or Olympic lifts, but the average runner isn’t doing those.
This is yet another reason that sled training is more beneficial for most people than regular weight lifting. Heavy sled drags definitely work the calves and ankles. And they are way more fun than calf raises or ankle presses. So now we have evidence that sleds will not only help your acceleration, but will very likely greatly benefit your running efficiency and endurance as well.