Saturday, 14 December 2013

Foot strike under a microscope

Foot strike plot borrowed from here 
I want to show that it's nearly impossible to perceive a running step exactly as it is happening and furthermore the idea of deliberately controlling your stride is perhaps harder than it sounds.

To begin at the end, it takes between 150 and 250 milliseconds from the moment your foot touches the ground until the tips of your toes leave during take-off. The time of your foot strike is proportional to your speed. As you run faster, your footsteps take less time. But even at a slow jog, the time it takes for a sin

gle step is short. For instance, assume you are running at 180 steps per minute. If each leg is taking 90 steps per minute then each stride cycle lasts 0.667 seconds. The time of each kick is at most half this time (because the opposite foot has to return to the start positing in the same amount of time) so that your foot landing, takeoff, and mid-air float last about 333 milliseconds.

A quick test: Try standing with one foot barely ahead of you while the second extends behind you enough to have your heel off the ground and toes touching. If you are about average height your rear leg will be extend behind you about 60 or 70 centimetres. For someone running a 56 minute 10k means (3 meters per second), taking 180 steps per minute means you are moving forward 1 meter per stride. Even at this modest pace, to move a full meter in one step you must leap into the air each stride momentarily. Of the 333 ms your foot is moving backwards, your foot is actually in contact with the ground for about 200-230 milliseconds*.

One running step does not take much time. Now here is why it is hard to consciously change our stride as it is happening: A nerve impulse takes about 20-25 milliseconds to receive the signal from your foot. Your brain requires at least another 80 milliseconds to process this sensory information, plus more time to send a new signal back to the source. Our reaction times are even longer, as the require both travel time for nerve signals and the mental machinery to back it up and work out to about 150 ms. Running motions are always quick. I tried running a little after writing this intro, and found myself surprised how weird it is to try and fully, consciously perceive your own running foot fall. Try it for yourself; the more you think about it the weird it gets.

Running motions, step by step

Let's discuss some of the events that take place in a single running stride. It all happens so fast that our brains cannot consciously absorb it. But if we slow down time, we can imagine what happens from the point where the foot makes contact with the ground until the opposite foot lands. In 1/3rd of a second there's more going on than you might expect.

t = 0: Depending running technique, either the forefoot or heel first make contact with the ground. Sensory nerve impulses are now travelling to the brain at around 100 m/s. At this instant your brain has no sensation of what has happened. The force on your foot is zero but rising fast.

t = 25 ms: The foot is now experiencing a rapid increase vertical forces, already amounting to about 1.5 body weights. Impact forces have not yet distributed to other parts of the foot. Your brain has just received notice of touchdown.

t = 50 ms: If our runner was a heel striker, some of the impact forces are now been transferred to the arch (mid foot) and forefoot, while a forefoot landing has transferred some of the force to the heel. Impact forces are now at 2 body weights. Though the brain is unconsciously controlling all these motions our runner is not fully aware of what is happening.

t = 110 ms: The foot strike is already half over. Landing forces are being absorbed while the push-off phase has begun. Impact forces have peaked at 2.4-2.9 body weights and this force has been almost entirely transferred to the forefoot. Much of this excess weight is now to relaunch the body and to overcome the forces of gravity. Our runner is only now fully conscious of her own foot strike. Only now can our runner send signals back to the legs and feet to make some gait alteration.

t = 150 ms: It is now the push-off phase and the body is being thrust into the air by the leg and foot muscles. The tip of the forefoot is behind our runner because of her rear leg extension. As the body accelerates into the air, vertical forces are dropping. This is also earliest moment once can modify their running motion based on the sensation of the impact forces.

t = 220 ms: The foot and leg are in a follow-trough motion behind the body and g-forces are now zero. During these past 220ms the opposite leg has been moving forward, preparing for its own landing phase. You brain is now aware of what has taken place but it's too late to do anything about it!

t = 220 to 333 ms: If running around 180 steps per minute then our runner will be spending 110 ms or so in the air before the opposite foot touches the ground. Thereafter the running cycle begins anew. During this short "break", your brain is processing visual information as it is clear that one cannot do much to alter a foot's landing once it has begun. In fact most conscious decisions about your footfall take place during this time!

To conclude, your foot makes contact with the ground in the blink of an eye (quite literally, as eye blinks last between 100 and 400 milliseconds). From the time when your foot lands on the ground and when you actually feel the force of landing (and register it mentally), your foot strike stride is half over. In other words to make any conscious changes to your stride, you must send those signals before your foot has landed. This is why your eyes are such an important running tool, to tell your feet how to react before you've stepped onto a new surface. Is this information useful regular runners? I would argue yes, because it reminds us that most running motions are unconscious, even at the most casual level. Though it would seem rather obvious, running with your eyes closed is surprisingly difficult even on relatively flat terrain (whereas skiing and biking, whose motions last longer, would be less difficult to do blindfolded over the same terrain).

How do you improve you running form? It can't be easily done in full conscious effort. It's better to hone one's skills through better muscle memory by praising as many unconscious movements as often as possible. That's one reason easy running is so useful, as it trains you to run more on autopilot compared with the more focused tempo or training runs. Really it's just another way of saying you should practise well and practise often.

*Or else find the distance your leg extends behind you while standing AND the distance you cover per step running. If you divide the first value by twice the second then you have the fraction of time your foot spends on the ground. The percentage should be somewhere in the range of 17-35%. Doing this calculation you might be surprised to discover a faster turnover leads to more time on the ground. However if you increase your overall speed, both the relative "touchdown" phase and the absolute contact time go down.

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