26 September 2014

Killer shoes - The importance of footwear traction in mountain trail running

Catching my breath I finally took the time to turn around and look out over the valley. There were still thick veils of mist lower down, but the rain had finally abated after constantly falling since last night. Looking out over the snow-clad mountains on the other side of the valley I could see the first stars of the new night breaking through the rapidly vanishing clouds. I turned off the headlight and let the night surround me. It was warm without any wind and I was surprised that I anyway could feel soft movements from the thick wet grass around my ankles. Slowly turning around towards the slope I touched the wet grass with my hands. I leaned forward a little bit to let wetness and cold of the grass caress my warm face. “Swimming in a green waterfall of grass”, I thought. Suddenly the adrenaline started to rush through my body again and I pulled up my head and turned on my headlamp.  Looking up over the grassy slope I thought I could see solid rock that must be part of the ridge. “Not far away now”, I thought as I moved my right foot up towards what looked like a more solid tussock. Feeling the rubber of my outsole dig into the soil and fine gravel under the grass I put my weight on the foot and followed after with my body. “Just one step at a time, Peter, just one step at a time”. After another step upwards I stopped again and looked down the slope. I could not see Otto, but knew he must be close on the other side of the little spur I had crossed further below. “It might be possible here, Otto” I screamed. “It might finally be possible”.

It was the second night of Petite Trotte à Léon (PTL) and we were standing on a horrible wet grassy slope of Croix de Tousse at an altitude of around 2800 meters high over the treeline on our way up towards the crossing of Côte 2922 to Petite Vélan. I still do not know exactly how we could have ended up off route and by how much. When I looked at the GPS it one minute said that we were only 10 meters from the route, the next it said 40 – a huge difference in a terrain like this. Not able to rely on the GPS we tried for a long time to find our way on the map – which of course was not detailed enough as the elevation curves did not catch how steep parts of the slope was and we often found ourselves stuck and had to climb down again. I also still do not know exactly how steep it was –  definitively too steep as we had to climb using both feet and hands and trying to find fixing points for them so part of the slope was probably a grade 4 scrambling.

Although the climb upwards in the slippery wet grass on this crazy steep slope was one of the scariest things I have ever done in my life, it was nothing compared to when we had to descent to try to find another route upwards towards the ridge. It was here I perhaps for the first time realized that your life in the mountains might depend on your shoes. Both Otto and I had of course so called trail running shoes that according to the manufacturer should be suitable for running in the mountains. However, while my fairly new La Sportiva Bushido shoes had a fairly good grip on the wet grass, Otto’s slightly more worn shoes of another brand had what looked like almost no traction. Numerous times during our wayfinding on the less step parts of the grassy slope his feet suddenly lost their grip and flew up and he fell hard backwards. Each time I was really scared that he should break his back, a leg, or worse, hit his head or not being able to arrest the fall after touching ground and rolling down the slope. It was certainly a close call several times.  In retrospect, we should of course have tested our shoes at various conditions before the race, and steep grassy slope we learned painfully during PTL are quite common in the Alps if you are not following any normal path or trail. Paradoxically, Otto had in contrast to me actually tested his shoes in the mountains in the northern part of Sweden just before the race, but obviously not on wet grassy steep slopes, while I think I was just lucky with my shoe selection. We learned later on in the race that the solution was for Otto to put on his crampons each time we crossed wet grassy and muddy terrains, the crampons certainly do work well on other surfaces besides ice, but this night we were probably too scared and shocked to come up with this simple solution.

Not being a professional athlete and not having the possibility to test several shoes in mountain conditions I started to look at what data and research that is available about the traction of outsoles of trail running shoes. Is there any comparative data available? Any standards in how to report the traction on different surfaces? Sadly, I did not find much. Almost all major trail shoe brands, like the one Otto was wearing, are making bold claims on their webpages about how good the traction is in all sorts of terrain. Just reading some of these claims make me skeptical – how could a shoe with good grip on both dry and wet rock also be good for mud, wet grass and even wet wood? Some brands, actually for instance Otto’s again, show rather impressive research data on their webpage regarding traction. Obviously, these data did not correlate well with how the shoes actually performed out on the real mountain and it makes me question the value of research performed by the manufacturer itself when there is no independent control of the data. The same thing probably hold true for at least part of the shoe reviews posted by bloggers and others who have received the shoes for free. Furthermore, Otto’s shoes were not as new as mine, but the outer soles were still well defined. How is the traction of the shoes affected by wear – when is it time to buy new ones? If readers of this blog could point me towards solid comparative independent data on these features of trail running shoes I would be very happy, but I doubt such data exist.

What I did find was a list of recommended shoes at skyrunning races on the homepage of the International Skyrunning Federation (see figure below). In their recommendation they rate the shoes as good or very good for the various types of skyrunning disciplines. It is not described how their selection and rating of shoes were being done, but I assume it involved some kind of testing of the shoes. Of note, Otto’s brand is not among the recommended in this list and it would have been good to know if they had tested the shoes and found them not meeting the standards for inclusion or if this just means that they have not tested them. Obviously, I also found a number of trail running shoe tests in various running magazines, but while features such as weight, drop, presence of rock protection and the width of the toe box naturally are objective measures differing between shoes, traction is for almost all trail shoes tested scored high in these tests and there is in most cases difficult to differentiate the shoes from each other depending on this.  Almost no test takes into account the performance on the varying surfaces in the mountain environment such as for instance dry and wet rocks, loose talus and gravel, wet grass, mud and wet roots.
List from ISF of recommended shoes 2014
Looking at the scientific literature about shoe – surface traction I could not find any study of trail, mountain and sky running shoes or even normal athletic running shoes. More surprisingly, I could neither find any good studies regarding traction of climbing footwear, only one study about traction in ladders shoes (Chang et al “Available friction of ladder shoes and slip potential for climbing on a straight ladder” Ergonomics 2005; 48: 1169) and a general article about the slip-resistance of different types of rubber (Manning et al “The effect of roughness, floor polish, water, oil and ice on underfoot friction: current safety footwear solings are less slip resistant than microcellular polyurethane” Appl Ergon. 2001; 32: 185). In other sports the traction developed at the shoe – surface interface has been extensively studied as it has been shown to influence injuries. Many studies have focused on the rotational traction torque forces and friction on various surfaces as higher forces in numerous studies have been associated with higher joint loading and in particular knee and ankle injuries (see for instance Wannop et al “Footwear traction and lower extremity noncontact injury” Med Sci Sports Exerc 2013; 45:2137). There are plenty of publications about this in for instance soccer (De Clercq et al “Cutting performance wearing different studded soccer shoes on dry and wet artificial turf” Footwear Science 2014; 6: 81; Schrier et al “Shoe traction and surface compliance affect performance of soccer-related movements” Footwear Science 2014; 6: 69; Smeets et al “Torsional injuries of the lower limb: an analysis of the frictional torque between different types of football turf and the shoe outsole” Br J Sports Med 2012; 46: 1078; Severn et al “Science of synthetic turf surfaces: investigation traction behavior” J Sport Engin Technol, 2011; 225: 147), American football (Wannop et al “Footwear traction and lower extremity noncontact injury” Med Sci Sports Exerc 2013; 45: 2137; Iacovelli et al “The effect of field condition and shoe type on lower extremity injuries in American Football” Br J Sports Med 2013; 47: 789);  tennis (Clarke et al “The development of an apparatus to understand the traction developed at the shoe-surface interface in tennis” J Sport Engin Technol, 2013; 227: 149; Damm et al “The effects of surface traction characteristics on frictional demand and kinematics in tennis” Sports Biomech 2013; 12: 389) and golf (Worsfold et al “Kinetic assessment of golf shoe outer sole design features” J Sports Sci Med 2009; 8: 607; Worsfold et al “Low handicap golfers generate more torque at the shoe-natural grass interface when using a driver” J Sports Sci Med 2008; 7: 408; Worsfold et al “A comparison of golf shoe designs highlights greater ground reaction forces with shorter irons” J Sports Sci Med 2007; 6: 484).

What I did find, however, was a number of scientific studies how the gait and motor pattern is changing when running on a slippery or uneven surface (for instance Sterzing et al “Running on an unpredictable irregular surface changes lower limb biomechanics and subjective perception compared to running on a regular surface” J Foot Ankle Res 2014; 7: A80; Chang et al “Contribution of gait parameters and availability coefficient of friction to perceptions of slipperiness” Gait Posture 2014; Epub ahead of print; Voloshina et al “Biomechanics and energetics of walking on uneven terrain” J Exp Biol 2013; 216: 3963; Gates et al “Kinematic strategies for walking across a destabilizing rock surface” Gait Posture 2012; 35: 36; Cappelini et al “Motor patterns during walking on a slippery walkway” J Neurophysiol 2010; 103: 746; Fong et al “Greater toe grop and gentler heel strike are the strategies to adapt to a slippery surface” J Biomech 2008; 41: 838). Not surprisingly, it appears runners on a slippery surface lower their limb positions and increase the stiffness of the legs in order to keep the center of mass (COM) more aligned with the supporting limbs and also use a gentler heel strike and relies more on the toe grip. These gait changes require more energy and I have previously written a blog post about the positive training effects of running on snow and sand and the same thing could probably be applied to wet grass and mud.

I also found a very interesting recent article looking specifically at the footwear traction and kinematics of walking at different directions in slope terrain (Wannop et al “Footwear traction and three-dimensional kinematics of level, downhill, uphill and cross-slope walking” Gait Posture 2014; 40: 118). In this study, ten participants walked along in various directions across a 19° inclined walkway. Quite surprisingly to me, the required traction at touchdown in order to not slip was similar at different sloped levels compared to level walking and the authors speculate that the increased likelihood of heel slipping during hiking down a steep slope therefore potentially is due to the presence of loose material (rocks and dirt), rather than the overall lack of traction. Not surprising, traverse walking cross the slope required the most rapid foot-floor eversion, which the authors speculate could place the hiker at higher risk of injury with a misstep or if there was a slight slip. I think the same results probably would be achieved in running as well.

While it is interesting to know that you change your gait when running on a slippery surface or on a steep slope, and thus probably could train your ability to do so, it is clear that it is not enough and that in order to avoid a fall during a mountain and sky running you also need shoes with good traction. In the lack of good independent comparative data about the traction of various shoes I think you still have to rely on trial and error yourself or talking to other runners or, if they have a blog or have written a review, reading about their experiences. I was reassured before PTL when I saw that Jared Campbell had used La Sportiva Bushido when he won and completed Barkley Marathons earlier this year. However, I think shoe selection is something very personal and of course there are other important factors than only traction. I am very happy with my selection of shoes for PTL. Last year I was running TDG in La Sportiva Helios and, although the traction was great and they were really light and comfortable, they did not offer the required protection for the last part of the race, while the Bushido did that during PTL. Still, for a shorter mountain or skyrunning race I would probably select Helios or Salomon S-Lab Sense 3 Ultra, and for a muddy trail race I would probably choose Salomon S-Lab Fellcross 3. For training I rely much on several old Asics shoes I have in my closet when running on roads, Salomon S-Lab XT models in mixed terrain and recently more frequently also on Hoka Hoka Rapa Nui shoes. But, we are soon entering a new season with new shoes coming out from the different brands so next year the selection will probably be different. Lastly, I think disclosure is important in a blog post like this and this year I have not received any shoes for testing and have not been sponsored in any way.
You also have to like the look of your shoes -
after all this is what you see most of the times in the later stages of a long race
(Picture from TDG 2013)

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