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Why running shoes don’t work with Steve Magness: Part 2 | Boddicker Performance

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Why running shoes don’t work with Steve Magness: Part 2

by on Feb 5th, 2010

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Today is a continuation of Steve Magness’s great article looking at the role of impact forces on controling the mechanics of running.  Enjoy.

-Carson Boddicker

Impact Forces: The picture gets cloudier:

But it’s not as simple as described above.
In an interesting study by Scott (1990) they looked at peak loads on the various sites of likely injury for runners (Achilles, knee, etc.). All peak loads occurred during mid-stance and push off. This led to an important finding that “the impact force at heel contact was estimated to have no effect on the peak force seen at the chronic injury sites,” and led to speculation that impact force did not relate injury development.

Further complicating the impact force idea is that when looking at injury rates of those running on hard surfaces or soft surfaces, there appears to be no protective benefit of running on soft surfaces. Why is this? Because of something called pre-activation and muscle tuning which will be discussed below.

Supporting this data, other studies have shown that people who have a low peak impact have the same likelihood of getting injured as those with a high peak impact force (Nigg, 1997). If you want to complicate things even further, impact seems to be the driving force between increased bone density.

As a coach or trainer this should make sense. The bone responds to the stimulus by becoming more resistant to it, IF the stimulus is not too large and there is enough recovery.

Underestimating our Body: Impact forces as feedback:

Back to the question I asked earlier: How can impact forces not change based on shoe sole softness and why isn’t running on hard surfaces lead to more injuries?

The problem is, once again, we underestimate the human body! It’s an amazing thing, and we never give it the credit it deserves. The body adapts to the surface that it’s going to strike, if you give it a chance. The body adapts to both shoe and surface adjusting impact forces via changes joint stiffness, the way the foot strikes, and a concept called muscle tuning.
An example of this can be seen with barefoot running, the diminished proprioception (sensory feedback) of wearing a shoe negates the cushioning of the shoe. Studies using minimal shoes/barefoot have shown that the body seems to adapt the impact forces/landing based on feedback and feedforward data. When running or landing from a jump, the body takes in all the sensory info, plus prior experiences, and adjusts to protect itself/land optimally As mentioned above, it does this through a variety of mechanisms. Thus, you stick some cushioned running shoe on the bottom of your foot and the body goes “Oh, we’re okay, we don’t need to worry about impact as much, we’ve got this soft piece of junk on our foot
One concept that needs to be further discussed is muscle tuning. It’s a concept recently proposed by Nigg et al. in 2000. He sees impact force as a signal or a source of feedback, as I stated earlier. The body then uses this information and adjusts accordingly to minimize soft tissue vibration and/or bone vibration. His contention is that impact force is not the problem, but rather the signal. Muscle tuning is essentially controlling these vibrations via a variety of methods. One potential mechanism is pre-activation. Pre-activation is activation of the muscles prior to impact. In this case it serves as a way of muscle tuning to prepare for impact and in addition can alter muscle stiffness, which is another way to prepare for impact. Pre-activation has been established with multiple EMG studies.
Shoes not only impact this, but surface type does too. As mentioned previously, the change in running surface did not impact injury rates. Why? Probably because the body adapts to running surface. In an interesting study measuring muscle activity, O’Flynn(1996) found that pre-activation changed based on surface. To prepare for impact, and presumably to minimize muscle/bone vibration, when running on concrete pre-activation was very high, when running on a soft track, not so much.

What all of this means is that the body adapts via sensory input. It has several different adaptation methods. A shoe influences how it adapts. The shoe is not doing anything to alter cushioning, it is simply altering how the body responds to impact. It’s a significant mindset jump if you think about it. Here’s the summary:
The type of shoe and material of the shoe changes impact NOT because of alignment of the lower leg or because of changes in cushioning. Instead it changes impact characteristics because it alters the sensory feedback
In conclusion on the cushioning concept. Well, what are we trying to cushion? Heel impact forces have not been shown to relate to injuries, in fact in one study low impact runners had a 30% injury rate compared to a 20% injury rate in high impact runners. Shoe midsoles do not change, or marginally change impact forces anyway. So, not only may cushioning not be the answer, the shoes might not even be doing their job. But what about those shoe cushioning studies showing improved cushioning with their new midsole?! Well, the majority of that testing is done by using a machine to simulate the impact forces that you experience during running. That means, yes it may cushion an impact more, but it doesn’t take into account the role of the body adjusting impact based on feedback.
The reason cushioning doesn’t work? Because the body adapts based on feedback and feedforward information. These results prompted one notable researcher(Nigg,2000) to call for the reconsideration of the cushioning paradigm for running shoes.
Barefoot running?

Quickly, this topic could not be complete without a brief mention of barefoot running. An interesting thing to note is that the initial peak impact force is absent in barefoot running when compared to running with shoes. What this means is that, the impact forces look like (A) for shoes and (B) for barefoot. That initial little blip in A is the initial impact force. There is a hypothesis that this initial impact force is related to injuries.

A recent study by Squadrone et al.(2009) compared running shoes, barefoot running, and running in Vibram Five Fingers. They demonstrated reduced impact forces, shorter ground contact and stride length, but increased stride frequency while running barefoot (and in Vibrams) as compared to running with shoes. This is not unexpected, but shows that running shoes do in fact alter our normal strides. An interesting point is the reduction in stride length but increase in stride frequency. Shoes tend to promote this longer stride at a consequence of ground contact times and frequency. This happens because of changes in feedback signaling, increased likelihood to land on heel stretched out, increased weight, all of which lead to longer times on the ground. It’s interesting to note that elite runners all have short ground contacts and high frequencies (as demonstrated by the often quoted Daniels study of 180 strides per minute).
Tying this to the discussion above on the body controlling things based on sensory information, when running barefoot, there is a higher degree of stiffness in the lower leg. Increased stiffness can result in an increased SSC (stretch shortening cycle) response, resulting in greater force on the subsequent push off (2001). Dalleau et al. demonstrated that pre-activation causing increased stiffness improved Running Economy. In his study, the energy cost of running was related to the stiffness of the lower leg (1998)

Another recent study found that knee flexion torque, knee varus torque, and hip internal rotation torque all were significantly greater in shoes compared to barefoot. What does all of this mean? Potentially, this means more stress on the joints in this area. Jay Dicharry put it best when he said:
“The soft materials in modern running shoes allow a contact style that you would not use barefoot. The foot no longer gets the proprioceptive cues that it gets unshod. The foot naturally accommodates to surfaces rapidly, but a midsole can impair the foot’s ability to react to the ground. This can mute or alter feedback the body gets while running. These factors allow a runner to adopt a gait that causes the elevated forces observed above.”

The one thing that non-barefoot/heel strike proponents use to dismiss midfoot striking/barefoot running is the Achilles tendon. They say, correctly, that the load on the Achilles is higher in midfoot striking runners. The Achilles is meant to take a large load. The problem is we’ve weakened the Achilles through years of wearing shoes with their elevated heels. Essentially, we’ve created the Achilles problem with the shoes meant to prevent it. The Achilles is designed to operate in a rubber band like fashion. . During impact such as the braking or contact phase of running, the achilles tendon stores energy and then subsequent releases that energy via recoil during the take off phase of running. The Achilles, can store and return approximately 35% of its kinetic energy (Ker, 1987). Without this elastic storage and return, the oxygen uptake required would be 30-40% higher! So, in terms of performance why are we trying to minimize the tendonous contribution? It’s like giving away free energy.

Running shoes do not utilize the elastic storage and return as well as barefoot or minimal shoes. More energy is lost with shoes than with barefoot running (Alexander and Bennett, 1989). In addition, in some models of shoes, the arch is not allowed to function like a spring. The arch of the foot can store around 17% of kinetic energy (Ker, 1987). Given these results, its not surprising that running barefoot when compared to running with shoes is more efficient. Several studies have shown a decreased VO2 at the same pace with barefoot running, even when weight is taken into account. This should be no surprise as I mentioned above, without elastic recoil VO2 requirement would be 30-40% higher. Running in a minimal shoe allows for better utilization of this system.

So, the take away message is that shoes change natural mechanics to one that creates mechanical changes that are not optimal for running fast (decreased stride frequency, increased ground contact, decreased stiffness of the system, decreased elastic contribution, and on and on).

Tying it together with elites:

Looking at elite athletes, when racing and training, they generally have higher turnover, minimal ground contact time, and a foot strike that is under their center of gravity. Since the majority of elites exhibit these same characteristics while racing, it makes sense that this is the optimal way to run fast. So, why are we wearing footwear that is designed to increase ground contact, decrease turnover, and promote footstrike out in front of the center of gravity? I have no idea.

Conclusion:

In conclusion, I’m not some fanatic saying everyone ditch shoes now. Chances are you’ve been running in shoes for 20+ years. Your bodies done some adapting during that time. You’ve got to gradually change if you want to undue some of the changes.

The purpose of this article wasn’t to talk about the benefits of barefoot running. Instead it was to point out the problems with Running Shoe classification. It’s based on a cushioning/pronation paradigm that simply is not as true as they want us to believe. That paradigm needs to be reevaluated. It’s not founded on good science but rather initial ideas that made sense with no science behind them, but upon further review may not stand up to testing. A recent study found that using the good old shoe classification system that everyone uses, had little influence on injury prevention in a large group of Army Basic Training participants (Knapik, 2009). They concluded that selecting shoes based on arch height (like all major running magazines suggest) is not necessary if injury prevention is the goal. I guess that means the systems broken…

Where do we go and how do we fix it? I have no idea. Sorry, no genius answers here. My inclination is that we aim for letting the foot function how it is meant to function, or at least come up with some shoe that may alter foot mechanics but while still allowing feedback/functionality of the body. The first step is looking at the foundation on which running shoes are built upon, the motion control, stability, and cushioning paradigm. My take is that it needs to be reevaluated. I’m going to end with something I’ve already said, but it’s an important concept to get across:

The body is more complicated and smarter than we give it credit.
The type of shoe and material of the shoe changes impact or stride characteristics NOT because of alignment of the lower leg or because of changes in cushioning. Instead it changes impact and stride characteristics because it alters the sensory feedback. The brain is a wonderful thing.’

Be sure to check out Steve’s Blog for more great information!

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Leave a Comment »1 Comment
  • Mark October 6, 2010

    Good stuff.

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