The Ironman triathlon began in Hawaii in 1978 with 50 participants. Since then, the race continues to grow in popularity. Now, each year, more than 50,000 athletes compete in Ironman distance races throughout the world. This race, typically consisting of a 2.4-mile (3.86 km) swim, 112-mile (180.25 km) bike, and a 26.2-mile (42.16 km) run, typically requires extensive training to complete. Athletes typically train for an average of 15 to 20 h·wk−1 (14,23,44). They spend around 6 months preparing for a single race and most continue to be active regularly throughout the year. With near year-round repetitive activity, these athletes are at high risk for overuse injuries. Training in three disciplines simultaneously, triathletes usually have a higher total training volume and intensity than single sport athletes. However, this diversity also allows the athlete to modify training while maintaining cardiovascular fitness when injuries occur (3). Here, we review what is currently known about the epidemiology and risk factors for injuries in Ironman distance triathletes as well as current research on injury prevention. It is hoped that this review guides recommendations for injury prevention as well as identifies gaps in our knowledge that will inspire future research.
Studies looking at the rate of injury among Ironman distance triathletes indicate that injuries are common. In one prospective study of 174 athletes, 87% suffered from some form of overuse injury over a 26-wk training period (1). Another retrospective study of triathletes competing at all distances found that 62% had at least one injury in the past season (36). In a retrospective study of 656 participants in Ironman Europe, almost 75% reported at least one injury since starting triathlons (14). One of the lowest injury rates reported is 37%. This study looked at athletes competing in all distances and only took place over an 8-wk training period (25). A similar rate (36.8%) was seen in a small cohort (18 male athletes) of Ironman distance triathletes on the British National Squad in a retrospective study of self reported injury in the past year (45). One of the highest rates of injury was found by O'Toole et al. (32) who surveyed Ironman Hawaii participants. Ninety-one self-reported sustaining at least one injury in the past year. Additionally, the average number of injuries reported for an individual athlete was 2.9. Difference in rates among these studies could be attributed to variations in study design including length of time athletes were followed or the distance of the race in which they competed. Additionally, most were retrospective studies that are subject to factors such as recall bias. When retrospective and prospective injury rates were compared in a single study, 2.1 times more injuries were reported prospectively and the rate of overuse injuries was 2.4 times higher (49). Most importantly, there is variation among studies regarding the definition of an injury (Table). In two studies in particular, almost 50% of injuries were not evaluated by a medical professional (25,47). It has been shown that changing the definition of injury among the same group of athletes can give differing results for significant risk factors (17).
In addition to a specific definition of injury, it is important to define what constitutes a new injury versus an exacerbation of a previous injury. Shrier et al. (38) present a multistate framework to differentiate a new injury from an exacerbation. They propose that a new injury should be considered whenever an athlete moves from their current state to a less healthy state due to athletic exposure. They acknowledge that athletes often return to activity while continuing treatment for injury. This complicates the definition of healing, where some may define based on return to full activity while others may use signs of tissue healing or return of injured tissue to baseline. Many triathletes continue to train, albeit in a modified routine, after an injury, so further exposure and healing may both be taking place simultaneously (10,32). Therefore, a multistate model for their injuries may be more appropriate.
Characterization of the injuries suffered by triathletes also is variable. However, most studies report that the lower extremity is the most common location for injury. Anderson et al. (1) found that the most common location for acute and overuse problems in Ironman athletes was the knee, followed closely by the lower leg and lower back, and then shoulder. Similarly, knee injuries were most common among British national team members (44). Another study of triathletes across all distances again showed the lower extremity to be the most common site of injury, with ankle and foot problems more common than injuries to the knee (25). Interestingly, back pain was the most common injury reported in a group of Ironman Hawaii finishers (32). Hamstring, calf, and knee injuries were most commonly attributed to running. Achilles injuries and low-back injuries were most commonly attributed to cycling (44).
In an effort to better understand injury patterns in triathletes, a few studies have examined which of the three individual events athletes report sustaining the most injuries. Egermann et al. (14) found that Ironman competitors reported the most injuries during cycling. However, they did not differentiate between overuse and acute injury, the latter of which are more common with cycling. Zwingenberger et al. (49) looked at injuries reported by athletes of all distances and found cycling and running to have similar injury rates, at 43% and 50%, respectively, and a much lower rate of 7% in swimming. Other studies have found that running results in the most injuries despite the fact that athletes spend the most training time cycling. Burns et al. (7) found that the majority of injuries were reported during running both in preseason and in competition season among a group of Australian athletes. Similarly, most knee injuries were attributed to running (12). Korkia et al. (25) found 65% of injuries among 155 British triathletes occurred during running. However, as athletes train for all events simultaneously, it may not be accurate to attribute injury to a specific one of the three events.
Identifying factors associated with increased risk for injury is important to guide recommendations for injury prevention. Looking at athletes of all distances, there was no association between sex, age, and physical characteristics such as height, weight, and body mass index (BMI) and injury incidence (14,25,44). Similarly, there was no association between competitive distance or difference between recreational or elite athletes and injury incidence. This same study found no association between weekly total training distance, training time, or the number of weekly workouts and injury incidence over an 8-wk study period (25). In a group of athletes who participated in Ironman Hawaii, training distance per week, average training pace, and average training time per week were not associated with injury risk (32).
Positive associations between training and injury also have been found. Among a group of 35 male Olympic and Ironman distance triathletes on the Great Britain National Squad, running injuries were found to correlate with total run training time in the week before starting to taper for their National Championship over a 5-yr study period. Overuse injuries in the Ironman distance athletes also were found to correlate with the amount of time spent on speed training. In particular, Achilles tendon injuries were correlated with the distance covered running hills and negatively correlated with time spent on long runs. Low back injury also was associated with time spent on speed cycling (44). These results suggest that athletes should limit speed work close to a race and instead focus on distance at a more moderate pace. However, it is difficult to generalize these recommendations as the association was based on training data from only 1 wk among an elite group of athletes. In a retrospective survey of 164 triathletes, neck pain has been shown to correlate with total years in sports and the number of previous sports-related injuries. Neck pain was not shown to be associated with age, BMI, or the number of races in which an athlete had participated. It also was not associated with training duration, assesses as an average of weekly time spent training (42). In a study of 128 Australian triathletes, a supinated foot type was associated with injury during a 10-wk competition season but not during the 6-month preseason (8). In this same population, longer running distances in preseason training were associated with higher risk of competition season overuse injury, but there was no association between total training time per week and injury (7). This highlights the importance of adequate follow up as there can be a lag between a training error and the resulting injury. Retrospective data from Ironman athletes found a positive relationship between reported weekly training hours and muscle and tendon injury. However, they did not differentiate between overuse and acute injury. They also found that having a coach did not affect injury incidence. This is concerning because a coach would be expected to help structure a training plan that would properly prepare an athlete for their race while limiting the risk of injury (14). In another study of 258 triathletes of all levels, athletes training 8 to 10 h on average over 3 yr were found to have the lowest percentage of injuries. Athletes training less than 8 or more than 15 h· wk−1 had a statistically significantly increased injury risk. They found that an intermediate time spent training for running and cycling individually also was associated with the lowest risk of injury. Time spent swimming was not associated with injury (36). Villavicencio et al. (43) found that the number of triathlon in which the athlete had participated and previous sports-related injuries were predictive of low back pain. Similarly, Burns et al. (7) found that previous injury was associated with increased injury risk during competition season. This suggests that adequate recovery may not have been properly determined and achieved.
In summary, the current research on injuries in triathletes has been focused mostly on prevalence and characterization of injuries. This leaves a gap for triathletes looking for guidance on injury risk and most importantly, injury prevention as they train and compete. Recent research in training and injury risk in other sports and in athletes in general can provide some insight and lead to future studies to determine if these principles can be successfully applied to triathletes, in particular those training for and competing in Ironman races.
Training Load and Injury
A recent consensus statement from the International Olympic Committee examined training load in all types of athletes (39). They defined training load as the total burden that is applied as a stimulus to a human biological system. This can include both sport and nonsport, physical, and psychological stresses. Many different methods have been proposed to monitor the load applied to the athlete, defined as external load, and the body's response, defined as internal load, in all types of sporting activity. Some are straight forward such as monitoring distance covered, time spent in training, frequency of competitions, or number of repetitions of a single activity or motion (39). Global Positioning System devices have been used to monitor both distance and velocity (2). Power also can be used either as an average or to look at time spent in different zones and has been most extensively used in cycling (21). Monitoring of internal load is achieved through perceived exertion scales, psychological questionnaires, and heart rate. Psychological stressors have been linked to injury risk in many team and individual sports (39). This association can be quite complicated. For example, it was recently shown that exposure to a stressful situation reduces the pain threshold in triathletes. However, the reduction was less in those with the greater stress response (16). Recreational athletes also must balance their training with careers, family, financial, or educational pressures (3). Although many of the tools to monitor internal load are self-determined, Saw et al. (35) have recently shown that subjective measures can accurately reflect an athlete’s well-being and response to acute and chronic workload. Most endurance athletes determine their training volume based on either distance or time. Incorporating some form of monitoring internal load may help to more accurately monitor the true load experienced by the athlete and could be a consideration in future studies.
It is recognized that not only the total training load but also appropriate changes in training volume as well as adequate recovery are important for injury prevention. A recent systemic review highlighted the association between training load and injury incidence in many team sports, with increased loads putting athletes at higher risk of injury. This risk is present up to 4 wk after the increased load (13). However, other studies support higher training loads as protective against injury in team sports and in runners (33,34,41). The goal of training in any sport is to use the training load as a stimulus to produce adaptations in the body allowing it to sustain greater loads in the future. Injury occurs when the load applied either acutely exceeds the tissue’s capacity to maintain integrity or chronically exceeds the tissue’s ability to repair between episodes of stress. Therefore, it is important that load be applied and increased in a way to produce positive adaptation and prevent injury and overtraining (13). Athletes in many team sports have been shown to be at increased risk for injury when their weekly workload was rapidly increased (39). Gabbett (15) has proposed the acute:chronic workload ratio as a way to monitor progression of training. Acute workload is representative of the total workload for the athlete in 1 wk, and chronic workload is an average of the weekly totals over the past 4 wk. This ratio has been investigated in cricket, rugby, Australian football, and soccer players. Findings show that athletes in these sports are more resistant to injury at a higher chronic workload and that they can tolerate higher workloads when this ratio remained below 1.5, ideally 0.8 to 1.35 (18,19,26,29). This has not yet been investigated in triathletes.
In all studies evaluating the association of training with injury in triathletes, the training volume was evaluated as a single average over a time interval that varied among the studies and was sometimes not defined. With the current literature, a temporal relationship between changing training volume and injury cannot be evaluated. Using an acute:chronic load measure with endurance athletes may help better define a link between training and injury, define training errors, and ideally promote strategies for injury prevention.
Recovery also is important in injury and overtraining prevention. As mentioned, with training, a load is applied to the body producing positive adaptations allowing the ability to withstand increased stress over time. However, just as when the load is too great, when the recovery is insufficient, performance declines. This deficit of recovery and performance decline has been defined as overreaching (27). If overreaching continues without proper rest and recovery, overtraining syndrome develops (27). Symptoms of overtraining syndrome include fatigue, mood changes, and decreased performance (9). Fatigue has been linked to changes in running mechanics associated with injury (40). After a 45-min exhausting run, 21 runners of all levels showed a decrease in heel lift, increased time of foot contact with the ground, and increased load transmission to the upper body. Many of these changes were more pronounced in less experienced runners (40). These kinematic changes have been associated with both patellofermoral pain and lower extremity stress fractures (11,48). Similarly, in a group of soldiers going through U.S. Army Basic Combat Training, muscle fatigue was found to be predictive of overuse injury (30).
Ironman triathletes invest significant time in their training. Review of the literature presented here suggests that increased training load, when not appropriately applied, can be a risk factor for injury. However, increased training volume is necessary to improved race performance (24). Therefore, it is important that an athlete is able to plan their training properly to achieve their goals while limiting the risk of injury. Unfortunately, the training principles most often followed by athletes and coaches are based on expert opinion rather than research. For example, the 10% rule, stating that training volume should be increased at a rate of no more than 10% per week, is followed by many (22). However, Buist and colleagues (6) showed that novice runners who followed a more aggressive training plan had no increased risk of injury. Additionally, Nielsen et al. (31) found that the increased risk of overuse injuries was present only when novice runners progressed their training volume 30% or more per week. Both studies used novice athletes, surprisingly little is known about proper training load progression in more experienced endurance athletes who already have a training base. Moreover, it is even more difficult to determine proper progression for a triathlete training for three disciplines simultaneously. Another popular training principle is the idea of periodization, a yearly training cycle that is divided into smaller cycles designed to meet specific performance goals, compete, and then recover. This regimen was popularized by Bompa and Haff (5) based on exercise principles and results of Olympic athletes in the 1950s. Many modifications have been made through the years since its introduction, however, there is little evidence to support their effectiveness or influence on injury prevention in the general population (20).
Recently, a more comprehensive approach to injury risk has been proposed that may have particular merit to assessment of triathletes. Meeuwisse et al. (28) propose that the relationship between risk factors and injury is not a linear relationship. Multiple risk factors interact to determine susceptibility to injury, and these factors can change over time. Furthermore, most athletes continue to participate or return to participation after injury. Therefore, injury should not be considered an endpoint, but rather, it feeds back to modify the athlete's risk profile. Bittencourt and colleagues (4) expanded this theory. They propose looking for regular patterns associated with injury determinants rather than evaluating isolated risk factors. The interaction of risk factors can produce an overall predisposition or protective profile that then leads to injury or adaptation. The results then feed back as determinants. Widnt and Gabbett (46) reviewed how workload can fit into an injury risk framework. They propose a model where an athlete's internal risk factors combine to form their predisposition to injury. Application of a workload exposes them to external risk factors creating susceptibility to injury. The result of the workload can be positive, increasing fitness, or negative, increasing fatigue. These results then modify the athlete's internal risk. If there is an inciting event while the workload is applied, injury may occur. Additionally, Shrier (37) emphasizes the importance of choosing the appropriate statistical analysis to define a causal relationship rather than just an association between a risk factor and injury, as injury prevention strategies should be based on addressing a true cause of injury. Evaluating injury risk in the sport of triathlon is complicated in that athletes are simultaneously training for three events that involve repetitive movement putting them at risk for overuse injury. These models of approach to injury may help design studies that can better evaluate the contribution of certain intrinsic and extrinsic factors to an athlete's predisposition to injury. For example, evaluating how internal load is related to injury risk or looking at the risk of injury associated with changes in workload, such as the acute:chronic workload ratio, may identify relationships that can be used by athletes to prevent injury.
The sport of triathlon is relatively new, but participation continues to grow. This sport presents a unique challenge as athletes must train for three different events simultaneously. Studies looking at the epidemiology and risk factors for injury among triathletes have shown variable results. This could be due to differing definitions of injury, athlete populations, and length of time during which the study takes place. Specifically, studies evaluating the association between training and injury have all used a single average objective value of training load. There are many tools that have been validated to assess training load and recovery, which, if applied to triathletes, may help better understand and define associations between training and injury and ultimately guide recommendations for injury prevention.
The authors declare no conflict of interest and do not have any financial disclosures.
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