Subject Specific vs Systematic Responses to Interventions

In yesterdays post on the quest for the ideal heel drop in a running shoe I used a study to illustrate a point  about subject specific responses of 2 subjects in the study. A comment was made about this being cherry picking. This is not what cherry picking is and rather than continue to respond in the comments, I thought I would write a post to expand on the concept for a wider consideration.

Lets say there was a lab based study done on a particular intervention (it could be a foot orthotic or running shoe design feature; or a running form change; a stretching exercise; or strengthening activity) and to keep it simple lets say the study measured the impacts of that intervention on one parameter. Lets say the parameter measured was related to load in a particular tissue. So this means that if the intervention can be shown to reduce loads in that particular tissue, then that intervention might have the potential to be a good treatment for those who have a problem with that tissue.

If this hypothetical study found that there was a systematic reduction in the loads with the intervention, great. The recommendation is going to be that this intervention should be possibly considered in those who need the load reduced in that particular tissue. (I won’t get into the issue here about you can’t reduce the load in one tissue without increasing it in another; I wrote about that here).

However, If the hypothetical study did not find a systematic reduction in the loads with the intervention then the traditional recommendation is going to be that this intervention should probably not be used for reducing load in that tissue. Traditionally this is probably where the story ends, but more attention is started to be given to the subject specific responses to interventions, even when the systematic response is a non-response.

If you look at the data in most of the studies that find no systematic differences, some subjects did get a response in one direction and some subjects did get a response in the other direction, so the mean response, was “no response” (ie no mean or systematic differences). So if we pretend to look at the data in our hypothetical study we might find that some subjects did get a reduction in load in the tissue with the intervention and some subjects did get an increase in load; and some subjects would not have changed; hence the mean response of no systematic differences. This means that the traditional recommendation from this sort of study might mean that some people that can be helped with the intervention are going to miss out (which is a bad thing); but also those that could also be hurt by the intervention are also going to miss out (which is a good thing).

What is needed is, even though there was no systematic or mean differences, why did some go one way and some go the other way? What are the indicators or “clinical tests” that might be able to predict the response in which direction?

Looking at, understanding and explaining the subject specific responses is going to go a long way to find the answer to the why one size does not fit all. If we can determine the indicators or “clinical tests” that predict the response in which direction we can give better advice on which intervention to achieve what we think needs to be achieved.

A good overly simplified example to raise here would be the question of do running shoes control pronation? If you look at the data in the studies that show they don’t, there are some individuals that have less pronation and some individuals that have more pronation, but the mean response was no change in pronation with the running shoe. So is the conclusion that running shoes do not control pronation valid?

Does that make sense?

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7 Responses to Subject Specific vs Systematic Responses to Interventions

  1. simon Bartold December 2, 2013 at 1:43 am #

    The premise that running shoes can control pronation has been one of the great perpetuated myths of the running shoe industry over the past 20 years. Unfortunately it is a concept so ingrained in the minds of the consumer, the manufacturers have shamefully exploited the notion, despite zero evidence to support the concept since at least 1999, when I first went into print saying that the concept of “motion control’ was erroneous and should be scrapped!

    • Kevin A. Kirby, DPM December 2, 2013 at 4:38 am #

      However, running shoes with dual density midsoles can generate increased external subtalar joint moments over shoes with midsoles of a uni-density material. This may in fact help reduce the symptoms in runners who have pronation-related injuries or may help prevent pronation-related injuries in runners who are susceptible to such injuries. If not the term “motion-control”, what should be call shoes with this midsole design: “Rearfoot Supination Moment Inducing Shoes”??



  2. dingle December 2, 2013 at 12:47 pm #

    What sort of injuries are you suggesting are pronation related Kevin?

  3. Kevin A. Kirby, DPM December 4, 2013 at 4:28 am #

    It is assumed that injuries such as medial tibial stress syndrome, patellofemoral syndrome, posterior tibial tendinitis, posterior tibial tendon dysfunction, sinus tarsi syndrome, and distal plantar fasciitis are related to excessive magnitudes of subtalar joint (STJ) pronation moments since 1) therapies (i.e. foot strappings, foot orthoses) that increase the external STJ supination moments usually cause a decrease in patient symptoms, and 2) biomechanical modelling shows that excessive STJ pronation moments causes increased stress to the specific anatomical structures which become injured in these pathologies.

    That does not mean that excessive STJ pronation moment causes these injuries in all runners or that these injuries are only caused by excessive STJ pronation moments or that all pronated feet will develop these injuries. What it does mean is that when a runner develops one of these injuries, the podiatrist or other foot-health clinician can quite predictably and successfully treat these injuries by increasing the external STJ supination moments acting on the foot with either modified over-the-counter foot orthoses, custom foot orthoses, by other shoe modifications or by foot strappings.

    I have been using “motion control” running shoes to enhance the treatment of running/walking injuries in thousands of patients over the past three decades of being a sports podiatrist. The dual density midsole design of these shoes effectively will increase the medial shift in center of pressure on the plantar foot due to the differential displacement of midsole materials at heel stike and early support phase of the running gait cycle. This differential displacement of running shoe midsole material will, in turn, increase the external STJ supination moments by shifting the center of pressure medially.

    “Motion control” running shoes are not for everyone. However, when these shoes are selectively recommended for the right runner with the right mechanics, they can be very effective at allowing many runners to stay uninjured during their training miles.


    Kevin A. Kirby, DPM
    Adjunct Associate Professor
    Department of Applied Biomechanics
    California School of Podiatric Medicine

  4. dingle December 5, 2013 at 4:16 am #

    ´Assumed’ …. really? MTSS etc is assumed to be related to pronation.

  5. Kevin A. Kirby, DPm December 5, 2013 at 3:56 pm #

    Yes….really….assumed….based on the following research:

    Positive association found between navicular drop test and MTSS in three prospective studies and in one case control study, linking MTSS to increased foot pronation

    Bennett JE, Reinking MF, et al: Factors contributing to the development of MTSS in high school runners. J Ortho Sports Phys Ther, 31:504-511, 2001.
    Yates B, White S: The incidence and risk factors in the development of MTSS among naval recruits. Am J Sp Med, 32:772-780, 2004.
    Bandholm T, et al: Foot medial longitudinal arch deformation during quiet standing and gait in subjects with MTSS. J Foot Ank Surg, 47(2):89-95, 2008.
    Raissi GR, et al. The relationship between lower extremity alignment and MTSS among non-professional athletes. Sports Med Arthrosc Rehab Ther Technol, 1(1):11-18, 2009.

    In your experience of treating athletes, Dingle, with MTSS, what do you propose to be the cause of this injury?? Or, Dingle, are you just another minimalist runner who doesn’t have any experience treating athletes but has lots of opinions that are based on their experiences as a runner??

  6. dingle December 5, 2013 at 4:05 pm #

    Well first of all i never assume anything of a client.
    Secondly i run in what ever shoe i like.
    Thirdly what does what i run in have to do with anything?

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