The next time you watch elite marathoners racing in a pack, pay attention to their form. In particular, try to identify one feature of their form that's shared by the entire group. It might be difficult.
While most of them will exhibit a midfoot landing pattern, some will be heel strikers. Some will have a high stride rate (about 180 steps per minute), but the taller ones will probably have a lower cadence. And though the majority will scoot along the ground with minimal vertical displacement, one or two will be rather bouncy.
There is one characteristic that all elite runners share and would become immediately apparent if a recreational runner was added to the mix. Put a 2:40 marathoner next to a bunch of 2:10 marathoners and you'll notice that all of the elites look incredibly relaxed compared to the slower runner. And as we'll see in a moment, it's not necessarily because the slower runner is less fit.
The ability to run relaxed at aggressive speeds, or "relaxed smooth ease," as I like to call it, is the cornerstone of skillful running. While the best runners are the most relaxed, all runners are capable of becoming more relaxed.
You can become a better runner without acquiring a midfoot landing pattern or increasing your stride rate or reducing your vertical displacement. You can't become a better runner without injecting relaxed smooth ease into your stride.
I'm not the first coach to make this point. In a 1962 article for Sports Illustrated, Arthur Lydiard, arguably the greatest running coach in history, wrote, "Forget about form. If a joker throws his arms around, that's fine, so long as he is fit and relaxed. Then he runs smoother and easier, and form takes care of itself. We want the chap who can run for two or three hours and come back looking as fit as he did when he went out."
So, what exactly is relaxed smooth ease, and how do you get it? Only recently have scientists identified the precise nature of the phenomenon that Lydiard observed. What the naked eye perceives as relaxation in a runner's form is now measurable by accelerometers as control entropy—a subtle variation in a runner's movements from one stride to the next.
It turns out that, contrary to what you might expect, experienced runners tend to have more variability in their stride than beginners do. In fact, the same pattern holds in all sports: more experienced and skilled athletes exhibit a greater degree of "play" in their movement patterns from one repetition to the next, while less seasoned and skilled athletes move more robotically.
The source of this play, or variability, is the brain. Control entropy is a physical manifestation of a brain that is passively listening to the body more than it is actively telling the body what to do. Brain imaging studies have revealed that there is less activity in the brains of highly skilled athletes when performing their sport than there is in less skilled athletes. Essentially, the more skilled an athlete is, the more he or she performs on "autopilot."
When you run, your body gathers information about its interaction with the environment (mainly the ground) and sends it to your brain, which then processes this information and uses it to identify and retain more efficient movement patterns. Your brain must be passive and receptive in order for this unconscious learning process to proceed effectively. And the more your brain learns from your body, the quieter it becomes. Efficient running is a matter of running with minimal muscle activation, and muscle activation requires brain activation. You need a quiet brain to produce a relaxed, economical stride.