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Abstract
Background The half-marathon (21 km) race is a very popular mass community-based distance running event. It is important to determine risk factors for medical complications during these events, so that prevention programmes can be developed.
Objective To determine risk factors associated with medical complications during 21 km road running events.
Design Prospective study.
Setting Two Oceans half-marathon (21 km) races.
Participants 39 511 starters in the 21 km race.
Methods Medical complications (defined as any runner requiring assessment by a doctor at the race medical facility or a local hospital on race day) were recorded over a 4-year study period. Medical complications were subdivided according to the system affected and by final diagnosis. A Poisson regression model was used to determine risk factors for any medical complication and more common specific complications.
Results Independent risk factors for medical complication during 21 km running were older female runners (women >50 vs ≤50 years; p<0.0001) and year of observation (2008 vs 2011; p=0.0201: 2009 vs 2011: p=0.0019; 2010 vs 2011: p=0.0096). Independent risk factors for specific common medical complications were: postural hypotension (women, slow running pace), musculoskeletal complications (less running experience, slower running pace) and dermatological complications (women).
Conclusions Older female runners are at higher risk of developing medical complications during 21 km road running races. Environmental conditions in a particularly cold climate may also play a role. Less running experience and slower running pace are associated with specific medical complications. Medical staff can now plan appropriate care on race days, and interventions can be developed to reduce the risk of medical complications in 21 km races.
- Running
- Epidemiology
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Introduction
The Two Oceans 21 km race takes place annually during late summer in Cape Town, South Africa. This is a relatively new category of race in the Two Oceans race series, and the number of entrants has increased dramatically over the past decade. Currently, the entries are limited to about 15 000 runners per year and are also oversubscribed. Importantly, entries to the 21 km race are open to experienced and novice runners of all ages >16 years and require no qualifying time. These races can therefore attract entrants who are not regular exercisers (‘weekend-warriors’).
It is well established that vigorous physical activity, such as distance running, may also be associated with a number of medical complications, including life-threatening medical complications in a variety of systems.1 In a large prospective study over a 4-year period, we documented the incidence and nature of medical complications during the 21 km race.2 These data showed an incidence of 5.14 medical complications per 1000 runners starting the 21 km race (1 in 195 runners starting the race) and an incidence of serious life-threatening medical complications of 0.51 (1 in 1961 runners starting the race).2 In this study, we also showed that the most common medical complications in 21 km runners (per 1000 starters) were dermatological complications (1.29/1000), followed by postural hypotension (1.06/1000), musculoskeletal complications (0.96/1000), serious exercise-associated muscle cramping (sEAMC; 0.25/1000) and gastrointestinal complications (0.23/1000).2
However, we are not aware of any systematic analysis to determine the independent risk factors associated with medical complications in 21 km running events. There are some data indicating that age,3 ,4 gender,4 ,5 running intensity (running pace),6 ,7 running experience8 and environmental conditions9 are possible risk factors for some specific medical complications during endurance exercise. However, these and other potential risk factors for medical complications have not been studied in 21 km runners. This information is vital to (1) plan medical care during these popular community-based events and (2) to design prevention programmes and thereby reduce the risk of medical complications (serious and less serious) during recreational running.
Therefore, the aim of this study was to identify independent risk factors that are associated with medical complications during 21 km running. In addition, we wanted to determine which risk factors are associated with specific, more common medical conditions in different organ systems in 21 km runners.
Methods
Type of study
This is a prospective study in a 4-year cohort (2008–2011) of runner data that were collected from the race registry and medical admissions data.
Participants and demographics
This study forms part of a series of studies that were initiated to determine the incidence and risk factors for adverse medical events in exercising individuals, and to then develop strategies to reduce this risk—the Strategies to reduce Adverse medical events For the ExerciseR (SAFER) studies.10 More specifically, this study is a component of a large prospective study that was conducted in all the participants of the Two Oceans Marathon races (consisting of a 21 km and a 56 km ultra-marathon) over a 4-year period (2008–2011), and the details of the study methodology have been fully described.2 In summary, during the 4-year study period, a total of 39 511 21 km runners started the races (78.3% of all registrations for the 21 km) and only runners who started the race were included as participants in this study. Starters consisted of 21 028 men and 18 483 women; all race entrants are required to be 16 years or older and there is no upper limit to the age. The demographics of all the 21 km race starters by gender, age groups (<30, 31–40, 41–50 and >50 years) and by year of participation in the 4-year study period is depicted in table 1.
Demographics of all the 21 km race starters (gender, age groups and year of participation)
For purposes of conducting research on these runners, this database of runners is registered with the Research Ethics Committee of the University of Cape Town (REC ref: 030/2013) and permission to investigate the incidence, nature and factors associated with medical complications during endurance running in the Two Oceans races was obtained from the Research Ethics Committee of the University of Cape Town (REC ref: 009/2011; REC ref: 441/2012).
Medical data collection
Details of the medical data collection procedure have been reported.2 In summary, accurate and comprehensive medical data on all medical complications were recorded during the 4-year study period. A ‘medical complication’ was defined as a medical condition that required medical care on race day that was of sufficient severity to warrant a medical assessment by a doctor, either in the medical facility at the end of the race, or at one of the referral hospitals (for runners who were assessed by medical staff on the route). Medical complication data entered into the database were recorded in a standardised format and included the system affected (cardiovascular, respiratory, fluid, electrolyte and acid base imbalance, central nervous system, thermoregulatory, musculoskeletal, gastrointestinal, dermatological, metabolic, renal and other) and the specific final diagnosis.
Incidence of medical complications
The crude incidence of any medical complications was calculated as medical admissions per 1000 runners who started the races, and this has already been reported by year of observation and race type.2 These incidences were also reported in subgroups of runners by race type, system affected and final diagnosis (for the 5 most common medical conditions).2 In this paper, the incidence of medical complications will additionally be reported by age groups (<30, 31–40, 41–50 and >50 years), previous running experience in the Two Oceans 21 km races (≤1 previous medals, ≥2 previous medals) and running pace (<6, 6–7 and >7 min/km).
Statistical analysis of data
All data were entered into an Excel spreadsheet (Microsoft 2010) and then analysed using the SAS (V.9.3) statistical program (SAS Institute Inc, Cary, North Carolina, USA). The medical complications data were analysed with a Poisson regression model, using a robust error estimator (log link function). This cohort consists of correlated data as 29% of runners ran this race more than once during the 4-year period. The correlated structure was accounted for by using an unstructured correlation matrix. This was to estimate the incidence rates (IRs) and CIs. Group comparisons and 95% CIs for these IRs and differences were also obtained. Regression analyses were conducted to determine risk factors associated with the development of any medical complication during 21 km running and then also for the five most common medical complications that occurred. Risk factors investigated included year of race, gender, age group, running experience category and running pace category. The year of race was included in the analyses as a proxy for environmental conditions on race day to determine if it was a risk for medical complications. The mean (SD) temperature, humidity, rainfall, cloud cover, wind speed during the race period (06:00 to 12:00 on race day) and calculated WBGT index for each year of race have already been reported.2 The five most common complications were postural hypotension, sEAMC, gastrointestinal complications, musculoskeletal complications and dermatological complications. The adjusted model for any medical complication was reported as adjusting for all five risk factors (being present in the model), irrespective of whether they were significant. Owing to the small number of reported specific complications, the adjusted models for these only included the significant risk factors.
Results
Incidence and risk factors associated with the development of any medical complication during 21 km running
The incidence (5.14/1000 runners starting the race; 95% CI 4.48 to 5.90) of any medical complication during a 21 km race by year of observation (as a proxy for environmental conditions), gender, age group, running experience and running pace is depicted in table 2.
The incidence (per 1000 runners starting the race: 95% CI) of any medical complication during a 21 km race by year of observation (environmental conditions), gender, age group, running experience group and running pace group
Univariate regression analysis
The crude unadjusted analysis showed that there was a significant difference in the incidence of any medical complication during 21 km running by gender (women>men; p<0.0001). Although there was no overall significant difference in the incidences between the age categories (p=0.1173), the following individual comparisons were significant: the >50 age group had a higher incidence versus (1) 16–30 (p=0.0444), (2) 31–40 (p=0.0139) and (3) 41–50 (p=0.0234). There was also a significant difference in the incidence by year of observation (environmental conditions; p=0.0308), with a higher incidence in 2011 (colder ambient temperature of 11.5 (2.1)°C) compared with both 2008 (ambient temperature of 18.2 (1.9)°C; p=0.0376) and 2009 (ambient temperature of 17.1 (1.4)°C; p=0.0030). There was also a significant difference in the incidence by running experience category (p=0.0231), with a higher incidence in runners with the least experience (≤1 medal) compared with runners in the more experienced category (2+ medals; p=0.0228). Finally, there was a significant difference in the incidence by running pace category (p=0.0421). Runners in the slowest pace category (>7 min/km) had a higher incidence of any medical complication compared with runners in the intermediate pace category (6–7 min/km; p=0.0189) and runners in the fast pace category (<6 min/km; p=0.0384).
Multiple regression analysis
In the adjusted model (table 3), there was a significant interaction effect between age category and gender (p=0.0257), which indicates that the trend for the incidences in the age categories were different for men and women. More specifically, women older than 50 years had a higher risk of a medical complication compared to women younger than 50 years of age (p=0.0001). This was not true for men older than 50 years of age, who did not have a higher risk of medical complications compared with younger men. An additional independent factor associated with an increased risk of developing any medical complication in 21 km running was year of observation (2008 vs 2011, p=0.0201; 2009 vs 2011, p=0.0019; 2010 vs 2011, p=0.0096).
The incidence (per 1000 runners starting the race: 95% CI) of any medical complication during a 21 km race for the adjusted model
It is noted that the higher incidence was in 2011, and this was a race that took place in colder environmental conditions (table 4).2
Environmental conditions on race day for each year
Incidence and risk factors associated with the development of postural hypotension during 21 km running
In our study, the incidence (per 1000 race starters) of postural hypotension was 1.06 (95% CI 0.79 to 1.44; 1 in every 943 race starters). The incidence (per 1000 runners starting the race) of postural hypotension during a 21 km race by year of observation (environmental conditions), gender, age group, running experience and running pace is depicted in table 5.
The incidence (per 1000 runners starting the race: 95% CI) of postural hypotension during a 21 km race by year of observation, gender, age group, running experience group and running pace group
Univariate regression analysis
The crude unadjusted analysis showed that there was no significant difference in the incidence of postural hypotension during 21 km running by year of observation (environmental conditions; p=0.2232), age category (p=0.4387) and category of running experience (p=0.8817). However, there was a significant difference in the incidence by gender (p=0.0118), where women had a higher incidence of postural hypotension. Further, there was also a significant difference in the running pace category (p=0.0141). Runners in the slowest category (>7 min/km) had a higher incidence of postural hypotension compared with runners in the intermediate pace category (6–7 min/km; p=0.0077). There was no significant difference between the runners in the fastest pace category (<6 min/km) compared to the intermediate pace category (6–7 min; p=0.2027) and the slowest category (>7 min/km; p=0.1006).
Multiple regression analysis
In the adjusted model, the only independent factors associated with an increased risk of developing postural hypotension in 21 km running were female gender (p=0.0394) and slow versus intermediate running pace (>7 vs 6–7 min/km; p=0.0139).
Incidence and risk factors associated with the development of sEAMC during 21 km running
In our study, the incidence (per 1000 race starters) of sEAMC was 0.25 (95% CI 0.14 to 0.47; 1 in 4000 runners). The incidence (per 1000 runners starting the race) of sEAMC during a 21 km race by year of observation, gender, age group, running experience and running pace is depicted in table 6.
The incidence (per 1000 runners starting the race: 95% CI) of serious Exercise Associated Muscle Cramping (sEAMC during a 21 km race by year of observation (environmental conditions), gender, running experience group and running pace group
Univariate regression analysis
The crude unadjusted analysis showed that there was no significant difference in the incidence of sEAMC during 21 km running by gender (p=0.4078), year of observation (p=0.2675), category of running experience (p=0.5732) or running pace category (p=0.5273).
Multiple regression analysis
In the adjusted model, there were no risk factors associated with an increased risk of developing sEAMC in 21 km running, as the numbers were too small.
Incidence and risk factors associated with the development of gastrointestinal complications during 21 km running
In our study, the incidence (per 1000 race starters) of gastrointestinal complications was 0.23 (95% CI 0.12 to 0.44; 1 in 4348 race starters). The incidence (per 1000 runners starting the race) of gastrointestinal complications during a 21 km race by year of observation, gender, age group, running experience and running pace is depicted in table 7.
The incidence (per 1000 runners starting the race: 95% CI) of gastrointestinal complications during a 21 km race by year of observation (environmental conditions), gender, age group, running experience group and running pace group
Univariate regression analysis
The crude unadjusted analysis showed that there was no significant difference in the incidence of gastrointestinal complications during 21 km running by gender (p=0.8925), year of observation (p=0.6646), age category (p=0.7741), category of running experience (p=0.6891) and running pace category (p=0.8818).
Multiple regression analysis
In the adjusted model, there were no factors associated with an increased risk of developing gastrointestinal complications in 21 km running, as the numbers were too small.
Incidence and risk factors associated with the development of musculoskeletal complications during 21 km running
In our study, the incidence (per 1000 race starters) of musculoskeletal complications was 0.96 (95% CI 0.70 to 1.32; 1 in every 1042 race starters). The incidence (per 1000 runners starting the race) of musculoskeletal complications during a 21 km race by year of observation (environmental conditions), gender, age group, running experience and running pace is depicted in table 8.
The incidence (per 1000 runners starting the race: 95% CI) of musculoskeletal complications during a 21 km race by year of observation (environmental conditions), gender, age group, running experience group and running pace group
Univariate regression analysis
The crude unadjusted analysis showed that there was no significant difference in the incidence of musculoskeletal complications during 21 km running by gender (p=0.6901), year of observation (environmental conditions; p=0.3011) and age category (p=0.9940). There was a significant difference in the incidence of musculoskeletal complications by pace category (p=0.0248). Runners in the slowest pace category (>7 min/km) had a higher incidence of musculoskeletal complications compared with runners in the intermediate pace category (6–7 min/km; p=0.0110).
There was a significant difference in the incidence of musculoskeletal complications by category of running experience (p=0.0324). There was a higher incidence of musculoskeletal complications in runners with the least experience (≤1 medals) compared with runners in the more experienced category (2+ medals; p=0.0379).
Multiple regression analysis
In the adjusted model, the independent factors associated with an increased risk of developing musculoskeletal complications in 21 km running were least race experience (≤1 vs 2+; p=0.0315) and slower running pace (>7 vs 6–7 min/km; p=0.0259).
Incidence and risk factors associated with the development of dermatological complications during 21 km running
In our study, the incidence (per 1000 race starters) of dermatological complications was 1.29 (95% CI 0.98 to 1.70; 1 in every 775 race starters). The incidence (per 1000 runners starting the race) of dermatological complications during a 21 km race by year of observation (environmental conditions), gender, age group, running experience and running pace is depicted in table 9.
The incidence (per 1000 runners starting the race: 95% CI) of dermatological complications during a 21 km race by year of observation, gender, age group, running experience group and running pace group
Univariate regression analysis
The crude unadjusted analysis showed that there was no significant difference in the incidence of dermatological complications during 21 km running by year of observation (environmental conditions) (p=0.3191), running experience category (p=0.4104) and running pace category (p=0.6923). There was no overall significant difference between the age categories (p=0.2581), except for a significant difference in the >50 age group compared with the 41–50 age group (p=0.0400). There was a significant difference in the incidence of dermatological complications by gender (p=0.0481). Female runners had a higher incidence of dermatological complications compared with male runners (p=0.0481).
Multiple regression analysis
In the adjusted model, the only independent factor associated with an increased risk of dermatological complications in 21 km running was female gender (p=0.0481).
Summary of risk factors associated with the development of medical complications during 21 km running
Independent risk factors associated with any medical complication, and more common specific medical complications (postural hypotension, sEAMC, gastrointestinal, musculoskeletal and dermatological) in 21 km runners, are depicted in table 10.
A summary of risk factors for any medical complications and more common medical complications during 21 km running
Discussion
In this study, the incidence and risk factors for any medical complication, and specific common medical complications during 21 km running, are reported. In addition, a Poisson regression model was used to determine risk factors that may be associated with the development of any medical complications and more specific common medical complications during 21 km running. Risk factors that were entered into the model were: year of observation (environmental conditions), gender, age, running experience and running pace.
The first main findings of this study were that older women and the year of observation (colder environmental conditions) were risk factors for the development of any medical complication during 21 km running. Second, we identified risk factors associated with specific common medical complications in 21 km runners as follows: postural hypotension (female gender and slow running pace), musculoskeletal complications (less running experience and slower pace) and dermatological complications (female gender). We could not identify specific risk factors associated with the development of sEAMC and gastrointestinal complications in 21 km runners, because of the small numbers.
In this study, runners in the slowest running pace category had a higher risk of developing postural hypotension and musculoskeletal complications. The most likely explanation for this observation is that the slower running pace is related to the development of symptoms during the race. In our study, we could not study a cause-effect relationship between these parameters, as we did not have data on the onset of the complication during the race or changes in running pace during the race. Our model did take into account year of observation age, gender, running pace and running experience, but we did not have any data on other variables that may influence running pace, particularly training history. A slow running pace may also be indicative of inadequate training, and this variable, together with other determinants of running pace as risk factors for medical complications such as postural hypotension, would have to be explored in future studies.
In this study, older women were at increased risk of developing any medical complication. More specifically, women were at increased risk of developing postural hypotension and dermatological complications. Only in two previous studies of runners were the risks of medical complications assessed by gender. In marathon runners, no differences in the incidence of any medical complications were observed in female runners compared with male runners,11 but during a multistaged 240 km ultra-marathon, female runners were at slightly higher risk of developing medical complications.4 Only one study in ultra-marathon runners has reported risk factors for dermatological complications, and in that study female runners were not at higher risk of dermatological complications.4 As in other studies,4 ,11 most of the dermatological complications in our study were skin abrasions, friction injuries and cuts from minor falls (data not reported).
In our study, older age in women was a risk factor for developing any medical complications, but was not associated with any common specific medical complications. Older age may therefore be a risk factor for other less common medical complications in 21 km runners. We did not have sufficient sample sizes in the other less common categories of medical complications, and this aspect would have to be explored in future analyses using larger cohorts.
A further finding in our study was that less running experience was a risk factor for the development of musculoskeletal complications. This is perhaps not surprising, as runners with less experience (frequently novice runners) may not have prepared adequately for the race and suffered from minor musculoskeletal injury. A prerace running injury in these runners may also have been a factor, and this would have to be explored in future studies.
Finally, the year of observation (the year in which the ambient temperature was colder than in other years) was predictive of any medical complication, but not of any of the more common specific medical complications. We included this variable in the model because of the potential variation in environmental conditions that may occur from year to year. Our data show that, in general, environmental conditions on race day (ambient temperature, humidity, rainfall and wind speed) were quite similar in the 4-year study period, with the exception that in 2011 the ambient temperature was about 5°C cooler than in other years (table 4). The highest incidence of medical complications was recorded in this year. As reported in one previous study, we also show that colder environmental conditions can increase the risk of medical complications,11 and this information can assist race organisers in the planning of medical care. We do recognise that factors other than environmental conditions may also have played a role in the higher incidence of medical complication that we reported in 2011. These would have to be explored in future studies.
The strengths of this study are, first, to the best of our knowledge, this is the largest prospective study until now to determine possible risk factors for any medical complication and specific common medical complications during 21 km running. Second, we had data on all the runners who registered and started the races in the 4-year study period. Third, the medical data we recorded on all medical complications were comprehensive and accurate. One of the limitations of our study was that we could not include all the possible intrinsic and extrinsic risk factors for medical complications in our model as these data were not available. In future studies, additional prerace medical and training history, as well as history of previous medical complications, would have to be included to determine other possible risk factors for medical complications in 21 km runners. Furthermore, in the older age categories, the number of participants was smaller, and this may limit the precision of the findings. More specifically, the precision of the incidence of medical complications for the subgroup women older than 50 years was lower than that of the other subgroups due to the relatively smaller samples in this subgroup.
The practical clinical implications of these data are that, for the first time, 21 km runners who are at higher risk of developing any medical complication and some common specific medical complications can now be identified. Apart from assisting medical staff to plan appropriate care on race days, interventions, including runner education programmes, can now be developed to reduce the risk of medical complications in 21 km runners.
What are the new findings?
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To the best of our knowledge, the Strategies to reduce Adverse medical events For the ExerciseR (SAFER) III study is the first study to systematically document risk factors for all medical complications and specific common medical complications during running in a population of 21 km runners.
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Older women and the year of observation (colder environmental conditions) were risk factors for the development of any medical complication during 21 km running.
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Independent risk factors associated with specific common medical complications in 21 km runners are as follows: postural hypotension (female gender and slow running pace), musculoskeletal complications (less running experience and slower pace) and dermatological complications (female gender).
How might it impact on clinical practice in the near future?
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Clinicians taking care of distance runners can now start to identify subgroups of 21 km runners who are at higher risk of developing any medical complication, and specific common medical complications during races.
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Data from this study will form the basis for further clinical studies to first develop and then test the effects of intervention strategies to reduce the risk of adverse medical events during 21 km running.
Acknowledgments
The authors would like to acknowledge all the medical staff for their service to the athletes participating in the Two Oceans Marathons over the years, the race organisers for their support of the project and all the athletes for their interest and participation in the races.
Footnotes
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Contributors KS participated in study planning, data collection, data interpretation, manuscript (first draft) and manuscript editing. MS was responsible for the overall content as guarantor, study concept, study planning, data collection, data interpretation, manuscript (first draft), manuscript editing and facilitating funding. WD participated in study concept, study planning, data collection, data interpretation and manuscript editing. SS and EJ participated in study planning and data analysis including statistical analysis, data interpretation and manuscript editing.
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Funding Clinical Sport and Exercise Medicine Research Fund (partial funding); IOC Research Center (Cape Town) (partial funding); Medical Research Council (partial funding, statistical analysis).
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Competing interests None.
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Ethics approval Human Research Ethics Committee, Faculty of Health Sciences, University of Cape Town.
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Provenance and peer review Not commissioned; externally peer reviewed.