Over the past few months I’ve come to a few conclusions about my preferences in running footwear. In general, I prefer shoes to have either a fairly soft midsole (e.g., Saucony Kinvara, Mirage, and Peregrine; Nike Free), or virtually no midsole at all (Vibram Fivefingers, Merrell Barefoot, Vivobarefoot, etc.). Shoes that have a substantial but rather firm midsole don’t seem to work so well for me (e.g., Saucony Fastwitch 5, Somnio Runaissance, New Balance Minimus Road).
I’ve had a hard time reconciling these rather disparate preferences – there really is not a whole lot in common between the Saucony Kinvara and shoes like the Vibram Fivefingers, so why do both seem to work on my feet?
Lately I’ve been doing a lot of reading about running biomechanics (I guess this isn’t much of a change from the norm…), and in particular I’ve been digging into Benno Nigg’s theories on muscle tuning, as well as the literature on leg stiffness adaptations during running. I’m starting to think that my shoe preferences might be explainable by some of what I’ve read.
In a nutshell, muscle tuning is a hypothesis put forth by biomechanist and long-time shoe researcher Benno Nigg that suggests that we tune our muscular response to ground impact from step to step in order to minimize vibrations that pass through the soft tissues of our lower limbs (see Nigg, 2001 or Nigg and Wakeling, 2001). Every time your foot makes contact with the ground when you run, the soft tissues in your legs (muscles, tendons, ligaments, etc.) jiggle or vibrate relative to the bones that they are attached to. Too much jiggling can damage these tissues and cause soreness, and thus the muscle tuning paradigm suggests that we modify our muscular response to impact in order to minimize the jiggling. In this paradigm, impact acts as an input to tell the muscles what to do. For example, if impacts are hard, we tense up the muscles a bit to reduce the jiggle, and thus we might expect to see greater activity in the muscles.
Where things get a bit complicated is when you consider the fact that the frequency of the vibrations initiated at impact can be variable. Ideally, we want the frequency of the vibrations initiated at impact to be different than the natural vibration frequency of our soft tissues. To understand the concept of natural frequency, Nigg uses the analogy of a wine glass – if you tap a wine glass with a metal spoon it vibrates and makes a sound – this is the natural frequency of the glass. If the vibration frequency at impact is too close to the natural frequency of your soft tissues, a phenomenon called resonance occurs and jiggling is maximized (I think I’ve got this correct – some of it is a bit beyond my comprehension still!). This is where shoes and running surfaces come in – they can alter the interaction between the foot and the ground, just as adding liquid to a wine glass can change the vibration of the glass and the sound produced. Variations among shoes and surfaces can alter the vibrations that pass up the leg, either for the better or for the worse. A similar logic can be applied to compression gear – one of the potential benefits of compression gear is that it can reduce the jiggle and any damage that might come along with it.
The goal under the muscle tuning paradigm would be to find a shoe that is tuned properly to the natural frequency of your individual body – in doing so, you can theoretically minimize the jiggle and maximize comfort and efficiency. A hard midsole or surface will tend to increase vibration frequency, whereas a soft midsole/surface will tend to have the opposite effect. Somewhat paradoxically, in his book Biomechanics of Sports Shoes (2010), Nigg states that the tenets of the muscle tuning paradigm suggest that a light/well-trained individual would be better suited by a softer shoe (presumably since they have a higher natural frequency in their tauter soft tissues), whereas a heavier/poorly trained individual would be better suited by a firmer shoe. Got it?
Another thing that happens as we run is that we modify the stiffness of our lower limbs in order to maintain a relatively stable displacement of our center of mass (see Ferris et al., 1998 for an excellent summary). In this sense the mass of our body can be viewed as a weight, and our lower leg can be viewed as a spring that supports that weight – this is known as the mass-spring model of running. Moving the center of mass (COM) up and down is energetically costly, and the body seems to have an inherent desire to control the displacement of the COM in order to keep it as stable as possible regardless of what is happening underfoot. If viewing a runner from the side, this can be thought of as a relatively stable up-down position of the head and trunk regardless of the type of surface the runner might be passing over at any given moment.
To give you an extreme example of this, think about what would happen if you were to run across a hard gym floor and onto a trampoline. What would your legs do? Most probably, when you hit the trampoline your legs would remain extended to a much greater degree at each joint than they were while running over the hard floor – in other words, your legs would be straighter on the trampoline. The springiness of the leg is a function of its joints – the hip, knee, ankle, and in some cases the bones and connective tissues of the forefoot and arch. As a general rule, leg stiffness will increase on softer surfaces, and will decrease (i.e., the joints will bend more) on hard surfaces. It is possible, however, that there may be a cost to extremes of stiffness in either direction. Too straight a leg could increase impact through the bones (think of the jolt you get when stepping unexpectedly off a curb), whereas a leg with extreme bending at the joints could work the muscles of the leg a lot harder (i.e., a wasteful, inefficient gait).
The mechanism by which these stiffness adaptations are accomplished is not entirely understood (at least by me), but it seems to happen on the fly, and we are able to make the necessary changes prior to stepping on a new type of surface if we know that a change in surface properties is coming (e.g., see this paper by Ferris et al., 1999). Conversely, unexpected changes can cause trouble, and in his book Nigg tells an interesting story about how he was able to help reduce injury rates among Cirque du Soleil performers by advising them to redesign their stage from a compliant surface supported at regular intervals by hard beams (e.g., and unpredictable hard/soft surface) to a uniformly hard surface (i.e., a predictable surface). He told the story in reference to muscle tuning, but it seems it could equally apply to stiffness adaptations.
So how does all of the above relate back to my running shoe preferences?
I suspect that each of us has a certain range of leg stiffness that is tolerable for us while we run, and that we each have slightly different natural frequencies for the soft tissues in our legs. Our individual tolerances are probably determined by our bony and muscular anatomy, body weight, training status, and a variety of other factors. Thus, what makes a given shoe comfortable for one individual and not for another may tie back to factors such as these.
If I had to guess, I probably do very little forefoot running in the Saucony Kinvara – most of the time I’m probably landing either flat or slightly toward the heel. What this does is largely remove the forefoot and ankle as springs in the chain to cushion impact (the ankle and arch will still likely play some role via pronation and subsequent arch compression), and replaces them with the soft, compressible midsole of the shoe and probably greater bending of the knee. Because the midsole of the Kinvara is soft, the exchange works for me, and I can run comfortably in the shoes. Interestingly, I have found that my Kinvaras are less comfortable now that I have put over 200 miles on them, and I think that this might be because the midsole has broken down and no longer has the same degree of softness that it once did. This probably leads me to adapt my leg stiffness slightly when I run in them, leading to slightly greater flex at the joints. This in turn, might be sensed as a bit more effort required to run in them. Just speculation, but it makes logical sense.
Now, a different thing happens when I run in a shoe like the Vibram Fivefingers (VFFs). In the VFFs I think I do a lot more forefoot striking, and thus I incorporate the ankle/Achilles tendon as an additional shock-absorbing spring in the chain of the lower limb (probably the forefoot to some degree as well). This is an adaptation to the lack of cushioning in the shoes – my muscles, tendons, and ligaments do the work instead of a soft midsole. The end result though is the same – I can run just as comfortably in VFFs on hard surfaces as I can in my Kinvaras, I just use different mechanisms (incorporation of the ankle and arch via a forefoot strike) to keep the displacement of my COM more or less constant. This also might explain why people see resolution of knee problems or initiation of ankle/foot problems when they run in shoes like the Vibrams – there is less reliance on the knee as a spring, and greater reliance on the ankle and foot.
Now what of shoes with a firm midsole? If I had to guess, a shoe with a relatively thick, firm midsole like the NB Minimus Road feels bad to me because the firmness of the midsole combined with the firmness of the road lead me to have to bend my knee and possibly my hip to a greater extent in order to maintain displacement of my COM (there’s little give by the shoe or the ground surface). If I forefoot strike in a shoe like the Minimus, this might not be a problem, but I generally have trouble forefoot striking in any shoe that does not have an ultrathin, flexible sole (e.g., Vibrams, Merrell Barefoot, Vivobarefoot, etc.). This might also explain my reaction last summer to running in my Kinvaras on a rubberized track – I didn’t enjoy it much because I had the strong feeling that the soft shoe on a soft surface just didn’t mesh well together – perhaps my legs had to stiffen beyond their normal comfort zone to handle the doubly-soft surface underfoot.
One idea that all of this suggests to me is that maybe I should stop fighting what my feet/legs want to do in a given pair of shoes. The human body is very intelligent and perceptive, and it seems to be pretty darned good at figuring out what to do given the conditions it is presented (and it can adapt on the fly!). Maybe trying to force a forefoot strike in a shoe like a Kinvara is the wrong approach? Maybe shoes do matter as much or more than form in that they dictate what form you body will want to use in any set of shoe/surface circumstances? Maybe I should just go with the flow, give my body the shoes that it likes, flip on the autopilot switch, and enjoy my runs?
I’m not sure whether stiffness adaptations or muscle tuning is more important, or if they go hand in hand (I have focused moreso on the former in this post). All of this is mostly just me thinking out loud and mulling publicly some of the thoughts that have been going through my head of late. I’m not sure how any of this might be applied in a practical sense short of running in lots of different shoes and seeing what works best (I’m lucky to have been able to do this!), but I’m curious as to whether others might have had similar experiences and/or thoughts on these topics. Feel free to share in the comments!