Archive for the ‘Statistics’ Category

Life Expectancy in Hunter Gatherers

June 28, 2011

A common misconception that exists today is that the lifespan of early humans was extremely short. Often people quote numbers like 30 years as the average life span of early hunter gatherers and farmers. A look at modern day hunter-gatherer societies is the best way to examine the likely life span of early humans.

The study longevity Among Hunter-Gatherers: A Cross cultural Examination covers this topic with a look at the human life span in several different cultures. The study puts forward the hypothesis that there is a prototypical pre-industrial mortality profile in humans. Or in other words, there is a normal human lifespan that can be seen across many cultures and levels of pre-industrial development.

Our conclusion is that there is a characteristic life span for our species, in which mortality decreases sharply from infancy through childhood, followed by a period in which mortality rates remain essentially constant to about age 40 years, after which mortality rises steadily in Gompertz fashion. The modal age of adult death is about seven decades, before which time humans remain vigorous producers, and after which senescence rapidly
occurs and people die.

The Analysis of Injury Rates in Running

June 8, 2011

In the previous post we have covered both the evolutionary and biomechanical bases for running and in particular barefoot running. The evidence thus far has pointed towards barefoot running being not only possible but the preferable mode of running. This evidence has for the most part centered around the factor of injury prevention.

Due to the wide amount of variation between runners in factors such as speed and mileage, injury rate is the only common metric for effectiveness of a mode of running that can easily be measured. To add to this, injury is a very common occurrence that has only a few significant predictors or direct causes.  This is to say that injuries tend to occur at a regular rate in all runners. This makes it a great tool to judge the effectiveness of a given technique as any change from the the statistical average would be easily apparent. Two polls taken roughly thirty years apart showed similar injury rates of 60% and 66% per year (Runners World). Reports of average injury rates today vary widely between 19.4% and 92.4% per year (Incidence and determinants of lower extremity running injuries in long distance runners: a systematic review), but the mode rate is around 50% per year. In other words, half of the running population gets injured in some way every year, and this estimate may very well be on the low side.

Injuries in this context are defined as any damage or disorder that causes significant change or even cessation of ones normal running routine. The large focus in most studies is on chronic injuries, or injuries that are not the result of sudden trauma and persist over long periods of time. Acute injuries are described as the result of sudden trauma and non persistent.  Over use injuries largely fall into the category of chronic injury. Examples of these types of injuries would be: stress fractures, Iliotibial band syndrome, Patellofemoral Pain (runners knee), Achilles tendinitis, and plantar fasciitis to name just a few. The significance of these types of injuries is that the cause is usually systemic rather then the result of any one particular action.  Systemic causes would include things such as personal physiology, running form, shoe type, and running surface.

The big question yet unanswered among all of the studies done on this subject is still why the injury rate of running  is so high. To give some perspective on this we could imagine what would happen if another species shared this rate of injury. Lets imagine that one out of every two cheetahs in the wild got injured at least once a year, like humans. We all know how important running is to the survival of these cats, so its not hard to imagine that an injury might be the difference between life or death. We know that cheetahs do not have this high of an injury rate by the simple fact that they are still alive as a species. An injury rate, and subsequently a death rate of 50% would kill the species off faster then they could reproduce. Given the evidence I provided in the previous section about the importance of running to early humans, it would seem that there is a discrepancy between the paleoanthropological narrative and the modern day demonstration.  If we hold the running man theory to be correct, then the only conclusion to be made is that a 50% injury rate is not inherent to our species. If it was in fact inherent to humans we would simply not been a viable species.

The question that immediately follows then would be what causes this injury rate if not the simple act of being a human. A number of factors can be eliminated easily. “Running injuries. A review of the epidemiological literature.” examined relevant medical literature and came to this conclusion:

[factors] Significantly not associated with running injuries seem age, gender, body mass index, running hills, running on hard surfaces, participation in other sports, time of the year and time of the day.

The association between running injuries and factors such as warm-up and stretching exercises, body height, malalignment, muscular imbalance, restricted range of motion, running frequency, level of performance, stability of running pattern, shoes and inshoe orthoses and running on 1 side of the road remains unclear or is backed by contradicting or scarce research findings.

None of the factors listed above show a high correlation with increased running injuries. In fact one of the factors, age, is found as a protective factor of injury in some studies.

A keystone study in this field (Is your prescription of distance running shoes evidence based?) did a systemic review of relevant medical literature and came to this conclusion in regards to modern running shoes, or in the terminology of this study “Pronation control, Elevated Cushioned Heel” PECH shoes:

Biomechanical and epidemiological studies have raised significant questions about the
capacity of running shoes incorporating either cushioning, heel elevation or sub-talar
control systems to prevent injury and have identified their potential to cause harm. We
identified no clinical trials which assessed the impact of the PECH design, which
incorporates all three of these features, on either running injury rates, running
performance or runner’s global health and wellbeing. Until such evidence becomes
available, PECH running shoes must be considered unproven technology with the
potential to cause harm. As such, the prescription of PECH shoes to distance runners is
not evidence based.

It is the intended purpose of this series of post to demonstrate that proper running form should reduce injury and that running barefoot encourages proper form. It is true that many proponents of barefoot running will claim that it almost eliminates injury all together. On this point however the actual data is very thin. Only one study could be found that included barefoot and “minimalist” runners in the sample group. This study (Relationships among Shoe Type, Foot Strike Pattern, and Injury Incidence) is, at the time of writing, still unpublished and has not gone under peer review. I did manage to make contact with the researcher heading the study, Don Goss, and he supplied me with the poster presented at ACSM in Denver this year.  These preliminary results where as follows:

•1 yr injury incidence rate was 53.9% for all runners combined.

•1 yr overall risk of injury for those wearing traditional running shoes (55.4%) was greater than for those running barefoot or wearing minimalist running shoes (46.3%, relative risk = 1.19, X²=6.39, 1df, p=.01).

•Relative risk calculations indicate risk of foot injury among barefoot runners was 2.81 times greater than for those wearing traditional running shoes (X²=17.9, 1dF, p<.001).

•Incidence of knee injuries was not significantly different among foot strike groups (X²=2.19, 2dF, p=.34) or between traditionally shod and barefoot/minimalist runners (X²=1.24, 1dF, p=.27), however, relative risk calculations indicate risk of knee injury among traditionally shod runners was 1.3 times greater than for the barefoot/minimalist runners.

These numbers do support the case for barefoot and minimalist running as a means of injury prevention, even if the margins are very small compared to  the lofty claims of many proponents of barefoot running. 46% injury rate still seems to be too high to fit with the evolutionary narrative though.  It would seem that based on this, footwear alone is not the cause of the high injury rate in runners. Running barefoot may help reduce the risk of some injuries, but it also increases the risk of others. There are two factors that I hypothesize could be causes of the still very high injury rate. First is the exclusive use of minimalist footwear rather than true barefoot running. Second is the comparatively late adoption of running in life and the even later and more recent adoption of the barefoot mode of running.

This study also examined a similar point in asking whether people had changed their shoe type or foot strike pattern recently.

•35.2% (520) of runners taking the survey changed their shoe type in the last 2 yrs. Of those, 63.6% (331) also changed their foot strike pattern.

•33.4% (530) of respondents changed their foot strike pattern in the last 2 yrs. 63.9% (339) of those also changed their shoe type.

Adoption of a new mode of running can be difficult, and if not undertaken properly can lead to injury. I put forward the possibility that barefoot and minimalist runners make up a disproportionate amount of  those who have either changed their foot wear and strike pattern. I base this hypothesis on two factors: The reported changes in shoes and strike patterns and the recentness of the barefoot/minimalist movement. 63% of those who changed their shoe type also changed their strike pattern. A change in strike pattern coinciding closely with a change in footwear fits the profile of adopting minimalist shoes, as these shoes require a very different strike pattern then conventional shoes. This was confirmed by Don although he could not give me exact number at this time.  In addition, the book ‘Born to run” which is credited with the popularization of the barefoot movement was published in 2009 and there fore coincides closely with this two year period. If a large portion of barefoot and minimalist runners have only adopted this mode of running with in the past two years, then there is significant reason to expect an elevated injury rate over that of habitually barefoot people.

Use of minimalist shoe may also be responsible in part for the elevated rate of injury. In the study barefoot and minimalist runners are grouped into one category. While running in minimalist shoes such as vibrams is very similar to running barefoot, it is not exactly the same and lacks a few key qualities. Muted sensory feed back and limited range of motion in the foot are two major factors that set any shoe, minimalist or traditional, apart from true barefoot running. As discussed in the previous post, proprioception and flexion in the foot are important to reduce impact and subsequently protect against injury. Any limiting of these factors has the ability to increase the risk of injury. Ironically, barefoot runners seems to be in the minority among the barefoot/minimalist community. Because of this, interpretation of the statistics must take into account the uneven representation of barefoot runners.

Ultimately there is not enough information on the subject of barefoot running to come to a conclusive understanding of its effects on injury rates. There is similarly little to no information on traditional running shoes that indicates they could reduce the rate of injury. Based on the preliminary findings of one yet unpublished study, running shoes perform worse than barefoot or minimalist shoes in protecting against injury.

It is my hypothesis that habitually barefoot people who have a life long history of running are at the least risk of running injury, but as of yet no conclusive research exists to support this idea.