The epidemiology and associated risk factors for muscle strain injuries in endurance athletes

Abstract

Long-distance running continues to increase in popularity owing to its easy accessibility and people’s growing interest in disease prevention. Over the last 25 years, increasing participation in long-distance has been observed. The incidence of running injuries, especially to the lower extremities, has increased as the number of participants in long-distance running increased. Lower extremity muscle injuries (MInj) are multifactorial in nature and certain associated risk factors have been identified in long-distance endurance athletes as potential causes. This study investigated the epidemiology and risk factors associated with lower extremity MInj in long-distance runners participating in the Two Ocean Marathon (2012-2015). Over 4 years 106 743 runners entered the Two Oceans Marathon; 64 740 of the total number entered the 21.1 km and 42 003 runners the 56 km race. In this descriptive cross-sectional observational study of the Two Oceans Marathon (2012 – 2015), 76 654 consenting runners completed an online pre-race medical screening questionnaire of which 62 708 (58.8% of all entrants) were included in the study. A total of 2110 (3.4%) runners reported lower extremity MInj in the last 12 months before race entry. Using univariate and multi-regression analyses, the following categories of factors associated with lower extremity MInj were explored: runner demographics (sex, age, race distance), training/racing history, history of existing chronic disease and allergies. The crude unadjusted risk ratio {prevalence ratios (PRs) and 95% confidence intervals (CIs)} of lower extremity MInj were calculated separately for each risk factor and were reported for all the results. Independent risk factors associated with MInj were explored in the multi-regression analyses and reported as PRs with 95% CI. A main finding of this study was a retrospective annual incidence of 3.4% regarding lower extremity MInj in the lower limbs (n=2110). The anatomical sites at which runners reported most lower extremity MInj (% of all lower extremity MINJ) were the calves (35.4%), followed by the hamstrings (27.4%) and the hips/gluteals (22.8%). Of all lower extremity MInj that reported severity grading (n=1869) 56.6% were in the severe category (grade III-IV) of which 18.7% were severe enough to prevent training or competing. Just less than half (45.5%) reported symptoms of the lower extremity MInj lasting longer than 7 months. The most frequently reported treatment modalities were rest (67.8%) and physiotherapy (66.8%), followed by stretches (50.2%) and strength exercises (42.4%). When comparing race entrants who reported a lower extremity MInj to those who did not (control group) the following factors were associated with a higher prevalence of lower extremity MInj (univariate analyses): male sex (PR=1.27; p<0.0001), older age (> 41 years PR=2.76; p<0.0001), and longer race distance (56 km vs. 21.1 km)(PR=2.09; p<0.0001), increased average weekly training/racing frequency (PR=1.10 for every 1-unit increase, p<0.0001), and increased weekly training/running distance (PR=1.04 for every 5-unit increase, p<0.0001). From the multi-regression analyses, novel independent risk factors associated with lower extremity MInj were increased years of being a recreational runner (PR=1.16 for every 5-year increase, p<0.0001) a higher chronic disease composite score (PR=2.37 for every 2-unit increase, p<0.0001), and a history of allergies (PR=2.08; p<0.0001). To our knowledge the main strength of our study is that it is the largest study conducted on the epidemiology, clinical characteristics and treatment of lower extremity MInj in recreational long-distance runners. In addition, the overall response rate was 58.8% (% of all runner entrants) highlighting the good response rate in our study. Novel independent risk factors associated with lower extremity MInj could be explored using multiple regression analyses. This is the first study to report such independent risk factors. Information gathered in this study enables clinicians and researchers to evaluate potential risk factors and institute appropriate treatment options to better and improve related interventions for lower extremity MInj in long-distance runners. A prospective cohort study design could be used for future studies to establish the cause-and-effect relationship.