I've been using this equation for years and only just noticed that it spells fun. It doesn't really of course since that's a mu not a u and fun doesn't have an equals sign in it but you see what I mean. The thing I do for fun, rock climbing has a lot of friction in it (on a good day) but we don't make as much use of Newton's second law as in this clip. Credit where credit is due this is Brodie Pawson and the original video is on facebook . The person filming didn't get the whole of the run in so I clipped two different runs together, not so successfully, but it shows the whole sequence.

So, here's the physics. To run up the wall you need enough friction to balance your weight. Friction is proportional to the normal force which is the force between the feet and the wall, this is equal to the mass of the runner x acceleration so to increase the force the runner must the accelerating (or decelerating if you must). To get the acceleration the runner runs at the wall then uses it to slow down, the faster he does this the greater is the normal force and therefore friction. This doesn't last long, maybe enough for 3 steps so he uses the other wall. Someone asked on the facebook post, "do you need two walls"? The answer is yes