Barefoot Running: Springs, Dampers, and Pressure
I'm a little off topic with this post - more structural mechanics than fluid mechanics. Normal service will resume shortly, but in the mean time, I have to admit it - I'm a recovering barefoot runner. Recovering in the sense that no matter what I've tried up to now, whenever I run fast or long in barefoot shoes (oxymoron I know, bare with me) I end up with calf muscles that are so sore I can't run for days afterwards. Well I thought it was way past time to mine my engineering knowledge for some insight, and this is what I came up with.
My natural tendency when I run on soft surfaces or in typical cushioned running shoes is to strike the ground with my heel first and let the shoe or ground (grass or sand) absorb most of the impact of each stride I take. As soon as I remove the cushioning when I try to run in so-called barefoot running shoes on a hard surface then the tendency is to strike the ground first with my forefoot. I found it extremely painful to strike with my heel first and so I don't.
With my new found forefoot-first strike I use my calf muscles as a damper to absorb (and ultimately dissipate) the impact energy. Then I use my calf muscles again to push off and launch my next stride as before. So my poor calf muscles have no letup. They are working twice as much as they are used to and I believe at least 75% more than they need to. I think this extra work is the cause of my barefoot problems.
What if I try landing flat footed, i.e., heel and forefoot at the same time? The first thing to note is that my heel is now taking part in the initial impact with my forefoot. These two contact points allow the arch of my foot to load up and store energy like a leaf spring. Springs are designed to store and later release energy. I think this process then helps relieve my calves of their damping duties during initial impact allowing my foot arch to store some of the impact energy ready for release at the start of my next stride. So less damping for my calves (equal to less stress, less work and hopefully less soreness) and an added bonus is the return of some energy from my foot arch to propel my next stride.
Another bonus of the flat footed landing is the larger contact area, which given the same downward force as before means the impact pressure (force / area) is reduced by up to a factor of 1.75 (assuming my heel is 0.75 the area of my forefoot). I say another bonus because it reduces the pressure on my forefoot and therefore the risk of injury and if I encounter a stray stone (which is inevitable) then it is less of a problem given that the pressure is that much lower than for my forefoot-first strike.
Of course you may ask why bother with this barefoot approach to running if your current cushioned running shoes are not causing you any problems? Well that is a whole other story...