Dorsiflexion is a foot movement upward produced by the activity of the front part of shin muscles. At this position the front part of the foot is higher than the rear part in relation to the horizontal plane. What's good or bad about dorsiflexion activity? Why do we need to talk about it? It is both a simple and complicated problem in running.
Where is it coming from? As I mentioned above, a mechanical part of it consists of moving the foot upward, but how much value does it have for our running for most runners is unclear. We see it often in heel striking runners as a preparation for landing on the ground. By this movement the runner's heel becomes a leading part of the foot during landing. This activity allows one to get a more rigid foot position on the support. What else?
So it is supposed to be a good thing, if the foot is rigid and the heel is striking the ground. But from medical reports and our own experience we know that it is the opposite. Therefore the question again is why we are doing it? Is it a habit, wrong understanding of movement, a lack of skill, or fear? It could be any one of them and all together. But if it's not good, then what are we supposed to do with this problem?
The most important question is: how can we get rid of this problem? If it's wrong learnt movement and a bad habit, then how can we relearn, reeducate ourselves and what it should be instead? The first thing is to change our vision on what landing in running is. What is the purpose of landing and which kind of landing is more beneficial to your running? A beneficial landing means that it allows one to be free from injuries and run faster with less muscle tension and energy spending.
Dorsiflexion is not satisfactory from any of these points of view. That's why your task is to change this habit and improve the skill.
The first step is to change the place for landing - you must land on the forefoot/ball of the foot instead of the heel. Just landing on the ball of the foot alone will reduce the chance of reoccurrence of this habit, but it'll not get rid of it yet.
The next step is to stabilize this landing by using jumping exercises (at the beginning it is very useful to employ jumps with a rope).
Another step is to develop the strength of the front group of shin muscles such as tibialis anterior by doing dorsiflexion exercises with different kinds of resistance: your partner, rubber bands, and free weights. The paradox is that these exercises work remarkably well for reducing dorsiflexion of the feet.
Furthermore this progression will logically lead you to change the paradigm of running - to use the forces of nature instead of your muscles as a leading force in running and will bring you to falling and pulling naturally when Dorsiflexion will get out of the equation.