News flash, runners injure themselves frequently. Furthermore, they keep running when they are injured. One study following 161 runners preparing for either a half or full marathon found 9 out of 10 runners reported illness or a running-related injury leading up to the race. At any two-week period of the 16-week study, 1 out of 7 runners experienced an illness or running-related injury. (source) As any runner will know, most of those injuries did not result in withdrawal from the race. There are three certainties in life: death, taxes, and runners will keep running despite being injured.
I made running a separate article for a couple of reasons. Originally, it was combined with part 1, however, that created a 6000-word article and would have resulted in my expansive readership (don’t burst my bubble) rapidly dwindling. Also, running-related injury prevention is a beastly topic. We are not dealing with a single injury like a hamstring, UCL, or ACL tear — granted, those aren’t necessarily “simple” either.
When assessing the topic of overuse running injuries, you will commonly come across the following: patellofemoral pain, shin splints (medial tibial stress syndrome), Achilles tendinopathy, plantar fasciitis, iliotibial band syndrome, and foot stress fractures. Many injury risk factors for these injuries have been identified, but as you will see, the demarcation of when they become a risk is not clear. With a wide variety of injuries, it is understandable to see long lists of potential sources; however, this does not prevent the witch hunt for a single root cause.
Running is a vast topic that can fill years' worth of clinical education. My hope is this article causes you to question some of the commonly held beliefs around the prevention of running-related injuries.
Finding the right shoes
Most studies assessing running injury risk factors and subsequent injury prevention are cohorts and retrospective observational studies. Like other areas of injury prevention, this is an inherent weakness of trying to determine causal relationships. I will not cover every potential risk factor, but I will highlight the ones most commonly cited by researchers, clinicians, running coaches, and shoe sales personnel.
One of the most commonly cited methods to reduce running injury risk is choosing the right shoe. The Global Athletic Footwear Market is expected to reach $114.8 billion by 2022. Many recommendations and decisions are driven by financial motivators and anecdotal experience. On the clinical and performance side, the typical concerns related to running shoes are foot control, turnover, cushioning, and durability. I will not cover all there is to know about shoes, but I will highlight an interesting review by Nigg et al. published in the British Journal of Sports Medicine. Here is what they concluded:
Specifically, the data regarding the relationship between impact characteristics and ankle pronation to the risk of developing a running-related injury is reviewed. Based on the lack of conclusive evidence for these two variables, which were once thought to be the prime predictors of running injuries, two new paradigms are suggested to elucidate the association between footwear and injury. These two paradigms, ‘the preferred movement path’ and ‘the comfort filter’, suggest that a runner intuitively selects a comfortable product using their own comfort filter that allows them to remain in the preferred movement path. This may automatically reduce the injury risk and may explain why there does not seem to be a secular trend in running injury rates.
After reviewing the individual trials and cohort studies they published, I agree with their conclusion. Choosing insoles and shoes that are comfortable and allow for a natural movement pattern reduces oxygen consumption and neuromuscular recruitment patterns in addition to lowering the injury risk. When assessing pronation and impact forces at foot strike, conventional wisdom fails to hold its ground.
Feel Free to Pronate
Looking at individual foot pronation studies, many are small sample sizes with high degrees of variability in their results. Some of the larger studies indicate pronation may be beneficial for injury prevention. Nielsen et al quantified the foot pronation for 927 novice runners and tracked them for 1 year. The group with the lowest injury incidence held a position of 7–10° pronation. This hardly means we should start training runners to emphasize pronation, but it calls into question the common shoe designs and instructions focused on avoiding pronation.
How Should I Land?
For impact forces at foot strike, shoes are not the only concern. Some shoes will be designed with more cushioning, but this can add weight to the shoe and impact performance, foot positioning, or both. Aside from the shoe design, the foot strike pattern is often assessed and implicated when discussing impact forces, specifically ground-reaction force. The argument in favor of using a forefoot or midfoot strike instead of a heel strike is primarily focused on impact forces. Some will argue it from a performance standpoint, as a forefoot strike increases turnover rate (try sprinting with a heel strike), but I am currently focused on injuries. Another consideration is looking at both peak impact forces and impact loading rates.
Again, we run into conflicting evidence, however, overall it appears the following occurs. Relative to heel striking, forefoot striking reduces both the peak impact forces and the impact loading rates. Heel strikers can potentially even the playing field with a cushioned heel, but there may be other consequences, such as increased stride length, decreased turnover, and changes in loading patterns. It is likely the cushion will impact the peak impact forces more than the loading rates, due to the nature of the heel strike and using the heal as a braking mechanism. Some studies indicate loading rates are more concerning than peak forces for injury occurrence.
One argument made against peak forces is the difference between sprinters and distance runners. While sprinters have significantly higher peak forces than distance runners, they do not have greater injury rates. The mileage and duration of running, however, is much lower for sprinters. Regardless, it appears forefoot striking does limit forces. The question is similar to the one concerning lifting and low back pain: how much stress is too much?
Conventional wisdom attributes increased risk of injury to the knee, hip, low back, and shin for heel strikers. The risk shifts to the Achilles tendon and plantar fascia when moving to a forefoot strike. But does the research support these claims? Maybe. Milner et al. demonstrated runners who have suffered from a tibial stress fracture display greater instantaneous and average vertical loading rates. Daoud et al. concluded runners who use a heel strike are more oft-injured than those using a mid and forefoot strike, but the sample size was small and in trained individuals (Division 1 Cross Country Collegiate Athletes). With differing workloads and intensities, a history of training, and access to coaches and dieticians, it is difficult to apply these findings to the general runner. The sample size also limits the ability to effectively break down injury types but here is what Daoud et al. concluded:
As hypothesized, the set of predicted rear-foot strike (RFS) injuries (hip pain, knee pain, lower back pain, tibial stress injuries, plantar fasciitis, and stress fractures of lower limb bones excluding the metatarsals) were between twofold and fourfold more frequent in RFS than in fore-foot strike (FFS) runners, with significantly lower rates of mild and moderate injuries in FFS runners (P = 0.0121 and P = 0.0014, respectively), and a significantly lower rate of moderate plus severe injuries in FFS runners (P = 0.0058). In contrast, the incidence of injuries predicted to be higher in FFS runners (Achilles tendinopathies, foot pain, and metatarsal stress fractures) was not significantly different between the two groups.
In studies with larger samples — keeping in mind they are still observational studies with significant limitations — the results are mixed. Adding a study by Hamil et al. to Milner and Daoud — ranging the sample sizes from 341 to 1203 runners — there were still no significant differences in overall injury rates between rearfoot and forefoot strikers. When breaking down by injury type, two of the studies found midfoot dorsal pain, metatarsal stress fractures, Achilles tendinitis, and post tibial tendinitis were the most common injury type but there was no difference in rates between strike pattern. Overall, cohort studies do not support the claims of specific foot strike patterns leading to specific injuries. Like ACL, UCL, and hamstring injuries, single factors rarely explain the incidence of injuries.
What about randomized control trials?
The nature of randomized control trials leads researchers to ask specific questions with a narrow focus. Thus, many studies only assess a single risk factor, such as the ramp of training mileage. Buist et al. published a randomized control trial in the American Journal of Sports Medicine in 2008 titled “No Effect of a Graded Training Program on the Number of running-related Injuries in Novice Runners: A Randomized Controlled Trial.” While the title gives the author’s conclusion away, the details should still be noted.
The authors list four primary factors related to running injury: lack of running experience, previous injury, running to compete, and excessive weekly running distance. As we have seen, most studies assessing injury risk factors and injury prevention strategies are cohort studies. With running injuries primarily being chronic, overuse developments opposed to an acute injury, it is difficult to run a trial. The longer a trial carries on, the more potential influence from external sources. For the Buist trial, they had an impressive 532 participants. There are issues of selectivity bias, as the recruitment was conducted via local media advertisement, but the size of the trial still carries benefits.
All participants had not run in the previous 12 months or sustained an injury in the previous three. The two randomized groups received differing training instructions. The graded training group received a 13-week schedule which gradually increased running time and decreased walking time for the week. The control group received an 8-week schedule with the same pattern, except the changes were more dramatic. Each group started with 30 minutes of self-selected pace running and 30 minutes of walking in the first week. At week 12, the graded group finished with 90 minutes of running and no walking, while at week 7, the control group completed 95 minutes of running and 5 minutes of walking. The final week for both groups, the week before a planned 4-mile race they were all preparing for, consisted of 30 minutes of running and no walking (a de-load week).
Some issues immediately come to mind. This is not only looking at a 10% grade increase — the oft-recommended amount — but also looking at a difference in overall training volume. Another issue is compliance. While all runners logged their miles, without direct oversight, fudging of numbers may have occurred. Running speed and shoe type were not controlled for either. As we have explored previously, these can all impact injury risk. As these are novice runners, the population likely consists of many shoe types and running styles, some that may fall in their own individual category of uniqueness Some athletes, particularly the novices, may have recently changed shoe type or running style based on a recent Runners World article or comment from the shoe sales rep. This all highlights the difficulty in developing randomized control trials for injury risk assessment.
At the end of the study, the difference in injury risk between the groups was statistically insignificant. This was not a reliance on “significance” either, the difference truly was small. The graded group had 30 running-related injuries (RRI) per 1000 exposure hours and 20.8 RRI per 100 runners at risk compared to 38 RRI per 1000 hours and 20.3 RRI per 100 runners for the control group. The exposure hours are certainly different, but it is difficult to draw firm conclusions. In the end, we are left with more questions than answers
Training load is key, but how much is too much?
There are many risk factors that are attributed to running, but few are substantiated in the research. I have not covered all of the common personal factors -age, sex, height, weight, genetic imprinting — or training factors — weekly/running days, total mileage on a pair of running shoes, injury history, training environment — but as you review the literature, it is likely no single factor is the magic variable to focus on. When educating and designing a program with an emphasis on injury prevention, a multimodal approach is key. We also must realize injury prevention can never be guaranteed.
Some food for thought to leave you with is how we measure training load. Runners often determine their training volume strictly by the number of miles they run. This is akin to measuring your volume in the gym by the total number of sets you lift only. A mile is not a mile. What was the speed, the variability, the external conditions (e.g. temperature), and the gradation (e.g. hills vs. flat)? What other training is being completed (lifting weights, swimming, yoga, etc.), and what are the recovery strategies (sleep, diet, rest)? The intensity of the training must be considered for both improving performance and preventing injuries. Volume is important, but it is only part of the equation.
Moving Beyond Weekly "Distance": Optimizing Quantification of Training Load in Runners - PubMed
It may be difficult to change the running culture's obsession with weekly distance, but advanced and emerging methods…
Running is a prime example of weighing risks and rewards. The health benefits and risk for injury are both substantial. There is a time and place for debating the best mode of exercise and it is not within this post. You can certainly make the argument you can obtain peak fitness and health without running, just as you can rack up the mileage for optimal health. There is the enjoyment factor to take into account as well. If you decide running is to be a part of your life and workout regimen, then understanding the risks for injury can help minimize the chances of them occurring, but it won’t eliminate them. The same goes for any athletic endeavor. There will always be a risk for injury, but the benefit often far outweighs the risk.
Zach is a physical therapist, researcher, and educator whose mission is to challenge the way clinicians think and close the information gap between clinicians and laypersons. Receive a biweekly newsletter containing his latest blog posts, recent research, and articles on critical thinking and healthy living, and recommended books at zacharywalston.com.
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