I have been promising these thoughts for a while. When you look at all the studies on running economy, the bulk of them are either showing that heel striking is more economical or that there is no difference between forefoot/midfoot vs rearfoot striking in running economy. The evidence does not support the rhetoric and propaganda from the fan boys for the one size fits all. However, if you look closely at the data in these studies there is a lot of variability from one runner to another which means for some runners forefoot/midfoot striking is probably more economical and for some runners heel striking appears to be more economical. Why is that? When you look at all the studies on injury rates between heel striking vs midfoot/forefoot striking or barefoot/minimalist vs shod running, the bulk of them are showing no differences. The evidence does not support the rhetoric and propaganda from the fan boys for the one size fits all. However, there is no doubt that some individuals who changed the way they run are getting less injuries and there is equally no doubt that some get more. Why is that? When you look at all the research on the kinematic and kinetic effects of running shoes and foot orthotics on runners maybe that the mean change is x% with a p value of 0.0x, but when you look at the data on individuals the responses are usually in both directions and are subject specific. Why is that? Clinically, we have all heard the cliché that we treat individuals and not means or p values.
There are possibly a number of potential reasons for this individual variability and I will explore just one of them: joint axes variations. This concept has occupied a lot of my time and underpins the work I have done previously on supination resistance. The reason I think joint axes variations are so important is that a variation in the position of a joint axis affects the lever arm that a tendon has to the joint to move it. That in turn affects the load in that tendon (ie injury risk) and how hard the muscle has to work to move that joint (ie economy of action).
To explain further, I will start with an oversimplified example or model as a thought experiment to illustrate what I mean (this example is actually wrong and not supported by the evidence for reasons I will get to after the example, but for now lets pretend that its right as it does help with understanding; this is what philosophers of science call an instrumentalist theory and those that read my 2000 paper on the MTJ know all about that!).
The variation in the length of the calcaneus (heel bone) is well documented and there is quite a variation from one person to another. As the achilles tendon attaches to the back of the calcaneus, this means that there is going to be quite a variability in the distance between that insertion and the axis of rotation of the ankle joint. These individual differences in lever arms are going to have hypothetical effects. Those with a longer calcaneus are probably going to be able to get the heel off the ground really easy because of that longer lever arm (ie basic physics), and keep it off the ground without a lot of effort. Are these the runners that can easily forefoot strike? Additionally, the load going through the Achilles tendon will be lower as the calf muscles do not have to work so hard. Are these runners at less risk for achilles tendon problems?
Conversely, in those who have a shorter calcaneus, the calf muscles are going to have to work a lot harder to get the heel off the ground due to the shorter lever arm. Are these the runners that are more uneconomical when forefoot/midfoot striking? Are these the runners that are greater risk for Achilles tendon problems? Should these runners keep heel striking?
While this oversimplified thought experiment is helpful to understand the concept of how variations in lever arms to a joint axis may impact on injury risk and running economy, the reality is much more complex. This model is not supported by the evidence. Two studies ( Scholz et al, 2008; Raichlen et al, 2011) have looked at the issue of running economy and length of the calcaneus and both found the opposite of what the above thought experiment or model would predict. The reason for that is what a potentially more sophisticated version of the model could explain and is related to the energy return that occurs from the achilles tendon during running and the impact of that on running economy. Those with a shorter calcaneus are going to probably store and return more energy and this is possibly a greater contribution to running economy.
Lets look at another joint that I know more about, the subtalar or talocalcaneal joint. The joint moves about an axis of rotation that has substantial variation. Here are the diagram from Manters study in 1941:
In the transverse plane the axis varied from 7 to 47 degrees from the midline.
In the sagittal plane the axis varied from 20 degrees to 68 degrees from the horizontal plane.
This variation from person to person is massive and has to have implications for the way that each individual functions dynamically. There are many different permutations of variations of subtalar joint axis variations (and don’t forget that this happens in the context of variations in the position of the joint axes of all the other joints in the foot … who said foot function was easy to understand?). Lets just take one of those permutations: the transverse plane variability and the variations that will occur in the lever arm to the posterior tibial tendon. If the axis is more medial (ie closer to the 47 degrees), the posterior tibial tendon is going to have a shorter lever arm and is going to have to work a lot harder if it had a longer lever arm is the axis was closer to the mean of 23 degrees. This is obviously going to have significant implications for function, running economy and loads going through the tendon. The posterior tibial muscle is probably the most powerful invertor, supinator or anti-pronator of the foot. If it has a shorter lever arm to the joint that this motion occurs at, its not going to be able to do it job very well and be at a much greater risk for injury. Every case of posterior tibial tendon dysfunction I have seen had a medial subtalar joint axis (and yes I know the plural of anecdotes is not data). In all the runners that I have seen since being aware of the forefoot/midfoot vs rearfoot strike issue, I have never seen a forefoot/midfoot striker with a more medial subtalar joint axis (and yes I know the plural of anecdotes is not data). The reason is obvious, as the posterior tibial tendon also passes behind the ankle joint axis, so also helps get the heel off the ground. If its already working at such a high level due to its shorter lever arms to the more medial subtalar joint axis, its not going to be able to contribute much to getting the heel off the ground – this is going to affect injury risk and running economy. One study has correlated achilles tendon pain to variations in the position of the subtalar joint axis, presumably because of the variations in lever arms that the achilles tendon will have to the variations in the position of the joint axis. This variation could also go a long way to possibly predict why some runners respond to ‘motion control’ features in running shoes and some do not.
Also keep in mind that with the variation of each joint axis, each tendon that crosses it will have differing effects depending on the lever arm. The extreme example is the function of the anterior tibial muscle that inserts in the the area around the base of the first metatarsal and medial cuneiform. I like asking the students is the anterior tibial muscle an inverter or everter of the foot. They almost always say inverter as that is what there anatomy texts say. But look at the top diagram above and where it inserts. The anterior tibial muscle is going to be either an inverter or everter of the foot depending on where the subtalar joint axis is – surely that has got to have significant effects on the function of the foot?. Can you appreciate how complex that this is getting and how one size is not going to fit all.
Hopefully all of this makes sense. I have only scratched the surface of the topic. I only talked about the axes of rotation of two joints that are modeled as hinge joints. There are a lot of other joints in the foot and lower limb. I recall reading a while back a study in the cerebral palsy literature and I apologize profusely for not providing the reference, as I just can’t find it again! In the study they used MRI to look at the distance between muscle insertions into the pelvis and the center of the hip joint. There was a massive variation in the lever arms between the subjects which had implications for the surgical management of gait problems in cerebral palsy. There is no reason to doubt that the same variation exists in runners and that is going to have implications for running form and how easy or harder it is going to be or not be to change the proximal control of gait.
There is so much to do, not only in the modelling of this, the developments of “clinical” tests to better predict responses to these variations and research on the concept. From my perspective: God put me on this earth to accomplish a certain number of things – right now I am so far behind, I will never die.