Author: Dr. Nate Jones, PT
Running is one of the most common forms of exercise in the world. People engage in it to improve their health, lose weight, for fun, and to participate in an enormous variety of competitions. It is an activity available to almost everyone, has very few barriers to entry, and includes a wide variety of health benefits.1 However, running has an immensely high injury rate compared to other ways of exercising, and running related injuries are one of the key reasons why people discontinue running.2 This article series is going to explore why running has a high injury rate, which specific aspects of running contribute to certain common injuries, and how best to avoid said injuries.
Why Does Running Have A High Injury Risk?
In order to discuss running injuries, it is imperative to define what an injury is. Studies disagree on the exact definition, but Buist et al put forth one that makes sense: “any musculoskeletal pain of the lower limb or back causing a restriction in running (mileage, pace, or duration) for at least 1 day.”3
There are many studies investigating the rate of running related injuries in multiple populations, including: novice runners, recreational runners, elite runners, male runners and female runners. A meta analysis from 2015 looking at 13 different studies found that the number of injuries per 1000 hours of running ranged from 2.5 injuries in long distance track athletes, all the way to 33 injuries in novice runners.4 There are studies showing an injury rate of 25% in novice runners training for a 4 mile event,3 a 20% injury rate in novice runners performing a graded training protocol based on the 10% rule (more on this later),5 an 85% injury rate in runners training over the course of a year and half for a 15k run, then a 25k run, then a marathon,6 and many more.
So why does running have such a high injury rate? There are dozens if not hundreds of studies examining this very issue dating back to at least the early 1980s, and the scientific community has yet to come to a consensus.
We have research examining:
For every study reporting one variable as a risk factor for injury, there’s another one showing either no effect or beneficial effect of that same variable. Trying to sift through the studies and find a way to prevent injury can be very frustrating, and it may appear overwhelmingly difficult to come away with specific, actionable items. However, the huge body of seemingly contradictory research is not, in fact, contradictory at all - it just creates a very complicated pattern and model of injury.
What it boils down to is this: running injuries are multifactorial and extremely dependent on the individual runner. Training history and tolerance to specific loads are the key factors (recreational runners and military recruits who played ball sports as children are much less likely to experience stress fractures7,8). We cannot say that running with a forefoot strike versus a rearfoot strike reduces injury rates for everyone - in specific individuals it may reduce injury risk, but in others it may increase it. If somebody has a low load tolerance in their Achilles tendon (i.e. the Achilles tendon is weak) and switches from a rearfoot strike to forefoot strike, then they do not allow for sufficient adaptation time in their Achilles tendon, therefore they are much more likely to experience an Achilles tendon injury. However, if a different person has an extremely strong Achilles tendon but has weaker knees, switching to a forefoot strike may put them at less risk of knee injury while not increasing their Achilles tendon injury risk.
The concepts of training history and load tolerance are not unique to running. However, the injury rate of running is higher than many other exercise modalities. There are two main factors at play here that are fairly unique to running:
In other words, our muscles, connective tissue, and bones must undergo a very high amount of stress in order for us to get a benefit from running, and we don’t notice the stress as much as we do in other activities.
This is in part because running is a highly repetitive impact activity - think about a car slowing down from 60-0mph over the course of 10 seconds versus slowing down over a couple seconds after slamming on the brakes - the car slows the same amount, but the rapid deceleration will cause the occupants and the car to experience much higher forces. Most runners tend to experience at least 2 times their body weight through each leg with every step,8 and this can increase up to 5.5 times body weight with sprinting10. However, the high force happens very quickly so we don’t tend to interpret our sensation of it as such.
Why is our interpretation of running stress lower than with other activities?
It is likely due to the multifactorial nature of pain and fatigue. Pain has been written about extensively elsewhere, but the short version is that pain is always a creation of the brain to try and protect the body from what the brain believes to be a threat based on the available evidence it has gathered. The evidence for a threat usually comes in part from signals from the body (nociceptors, sensory inputs, and other change and load detecting structures), but it is up to the brain to interpret those signals based on the learned context of the situation. Fatigue is similar, in that the brain creates the emotional fatigue response based on inputs to the brain from the body when the brain interprets those signals as a threat and decides to protect the body from the activity it’s participating in.11 Inputs from the body to the brain that can contribute to fatigue include (but are not limited to) body temperature, fuel stores in muscle cells, and tissue damage12, and these are interpreted in the context of previous training experiences, perceived effort, stress level, mood, breathing rate13, and much more. Keep in mind that this is a very simple overview of these topics; pain and fatigue are very complex phenomena.
Exercise itself can reduce the chance of a mechanical stimulus being interpreted as painful during and after exercise14-16 and chronically over time.17 Interestingly, endurance athletes may be better able to tolerate a stimulus perceived as painful than strength athletes.18
In addition, there is a growing body of evidence that shows that the feeling of exhaustion that leads to stopping an endurance exercise session is more closely related to the perception of effort versus local muscle fatigue.19-21 The act of running uses tendons such as the Achilles tendon and IT band as elastic springs to store and return energy (the faster the run, the more we depend on elastic energy22), thus decreasing energy demands and rate of energy expenditure of the muscles involved. This means that even though mechanical stress through the musculoskeletal system can be very high, signals from the running muscles and loaded tissues to the brain may not be very intense compared to other activities. This is due to a large percentage of the energy requirement being covered by elastic energy return, and therefore the brain may not pay as much attention to the signals due to exercise’s effect on mechanical pain thresholds.
Finally, the use of shoes may decrease sensory information from the body to the brain - the fat pad in our heel contains many nerve endings (called Pacinian corpuscles) that respond to impact and vibration.23 When people run barefoot, they automatically shift their foot strike to decrease impact through the heel,24 and barefoot runners tend to run less mileage and have a resulting decrease in number of injuries.25 This is likely due to self-limiting based on improved perception of impact, as when running volume is controlled for, injury rates become similar.
This reasoning can be supported indirectly by looking at injury rates of novice runners versus more experienced runners and at the injury rate of non-impact athletes transitioning to running. Novice runners are significantly more likely to experience an injury.4 This is likely in part because as runners become more experienced, they adapt to the high forces of running and their tissues are better able to tolerate the high loads. In comparison, non-impact athletes (swimming, biking) who may already have decent cardiovascular capacity but whose musculoskeletal systems are not yet adapted to the load of running have been shown to have a higher injury risk with a transition to running, as the stimulus required to improve their fitness is higher than their musculoskeletal systems can tolerate and adapt to.26
One point that should be made clear before moving on from why running injuries happen is that stress and mechanical load are not bad things. They are requirements to adapt to exercise - our bodies need the right amount of stress and load not only to become stronger and more fit, but also just to maintain themselves and not become weaker. Running is a high impact activity, and for the reasons previously outlined this may result in a higher than normal injury risk, but if the load of running is managed appropriately through (in order of importance) proper progression to allow for adaptations to occur, accessory strength work to improve tissue capacity, and individual adjustments to running form, injury risk should significantly decrease.
If you want an actionable guide to keep yourself training longer without any injury set backs, check out our PDF guideline here:
References in order