Running injury: the truth about YOUR risk

Most training programs and training strategies focus on improving performance. Yet they often overlook one of the most fundamental components of the athletic process, because even the perfect training program isn’t of any use if it can’t be completed due to injury! Unfortunately, runners get hurt, and injury is one of the most significant obstacles to achieving one’s athletic goals. Staying healthy and injury-free must therefore remain a priority.

Beyond running competition

Beyond competitive runners, the risk of pick up a running-related injury is relevant to any athlete in a sport involving running. Many of the injuries non-running athletes experience are related to the running component of their sport. In other words, just because someone doesn’t run competitively doesn’t mean the same dynamics aren’t at play. Because injury is so prevalent in running, and because being injured is so detrimental to long-term progress, it’s important to understand the factors that can lead to a running injury.

While biomechanical factors across many different joints have been related to injury prevalence in one way or another, it’s easy to lose sight of the ‘big picture’ risk factors, which have a dramatic impact. Two of those factors are simple: body mass and previous injury history. While there has preliminary research suggesting the importance of these two factors, ideally we need a large prospective study on this topic, which would be extremely valuable for clarifying the role these factors play in the incidence of running injuries.

New research

The good news is that a group of Swedish researchers have performed an 18-month prospective cohort study to investigate the influence of body mass and running related injury history on the prevalence of future running related injury⁽¹⁾. There were two major strengths to this study. The first was that the study was a prospective study. In other words, they recruited the subjects, identified their characteristics, and then the study was performed. This approach contrasts with less accurate retrospective studies, where a group of subjects are questioned about past events, and conclusions are drawn from that data.

There are many benefits of prospective studies. The two most prominent ones are the clear temporal relationship and a reduced recall bias. Because it’s easier to establish exactly when certain events occurred, it’s easier to demonstrate that a given outcome followed a given exposure. This makes it easier to demonstrate a causal link between two events. And because data is being collected as it happens, it’s more likely to be accurate compared to having subjects try to recall events from the distant past. Together, these factors improve the quality of the data and strengthen the relationship between the studied variables.

The second major strength of the study was the number of subjects that participated in the study. With large numbers of subjects, it is easier to detect small changes in outcomes, and the results are less likely to be biased by subjects with extreme characteristics or experiences. It’s also possible to divide the subjects into smaller subgroups while still retaining a high number of subjects within each subgroup. This makes it possible to achieve more precise findings as there are often important differences between these subgroups.

What they did

This study included 6,861 subjects who were recruited from all over the world. The only requirement was that the potential subject needed to be familiar with English and they needed to be using a Garmin wearable device to track their training. Because the subjects were from all over the world, rather than one country or city, some of the subtle biases found in specific locations were eliminated. This was a further strength of the study.

Prior to the beginning of the study, the subjects were categorized into one of two groups. The subjects either had or had not sustained a running-related injury in the last 3 months. This was determined by a ‘yes’ or ‘no’ response to the following question:

Have you had any running‐related problems during the PAST THREE MONTHS? (pain, ache, stiffness, swelling, instability/giving way, locking, or other complaints that affect any of your normal running activities)

3,684 subjects had been injured and 3,177 had not been injured according to their answers to this question, so it was a relatively even split between injured and non-injured runners.

The subjects were also categorized by their body mass index, which is calculated as the body mass in kilograms divided by the height in meters squared. This provides a measure of one’s size relative to their height. As one would expect the mass of an individual 2.0 meters tall to be larger than an individual that’s 1.50 meters tall, this metric provides a way to compare the body masses of these two individuals. The subjects were categorized into 4 groups based on their body mass index: less than 19, 19-25, 25-30, and over 30.

These two categorizations lead to the creation of eight groups that would be studied. For each of the four body mass groups, there was a group that was previously injured and a group that was not previously injured. These eight groups were then tracked for the following 18 months to determine which, if any, group(s) were more likely to sustain an injury, and how quickly those injuries would emerge. To keep of track of injury status throughout the 18-month period, the subjects were sent a questionnaire at the beginning of each week that included the following question:

In the past week, have you had a musculoskeletal injury, or have you experienced a problem to muscles, tendons, or bones that is fully or partly caused by running?

The subjects responded with one of three possible answers. They could answer that they were ‘injury-free’, meaning that they had no issues with injury. They could also answer that they were ‘uninjured, yet with problems’, which mean that the subject may be experiencing pain or irritation, yet they were able to continue with their running activity (volume, frequency, intensity). Finally, they could answer that they were injured, meaning that they were experiencing pain and irritation, and this resulted in a reduction in running activity.

For each group, the ‘cumulative injury incidence proportion at 1000 km’ was calculated. This is the percentage of the subject group that is likely to achieve an injury after 1000 km. Although 1000km is a relevant accumulated mileage for most runners, especially over an 18-month follow up period (around 13kms per week), the researchers also created ‘Kaplan-Meier’ plots which showed how the injury risk changed after different cumulative volumes.

The researchers also compared the risk difference between groups. As a BMI of 19-25 is considered normal, and not being injured is also consider normal, this was used as the reference group. All other groups were then compared against this reference. As an example, if a given group had a risk difference of 20%, they were 20% more likely to be injured than the reference group.

What they found

When the results were calculated, between 40 and 70% of runners would experience a running related injury after 1000 kilometers of running. As compared to the reference group, those who had experienced a previous running injury had a 20% or more increased likelihood of sustaining an injury (see figure 1). Within both the previously injured group and the non-injured group, the risk of running related injury increased with increased in body mass index (figure 1). Both body mass index and previous injury history are risk factors for future injuries, and the effect is worse when these factors are combined. Unsurprisingly, for all runners, risk went up the more they ran.