Search strategy

A Medline database search was used to identify relevant articles examining the relationship between obesity and injury risk. The terms “obese,” “obesity,” “overweight” and “BMI” (or “body mass index”) were combined with “injury,” “risk,” “physical activity” and “sport.” The search was limited to non-review papers. Papers specific to children, adolescents and young adults were further examined for relevance.

Is obesity a risk factor for injury?

Most studies examining the effect of obesity on injury risk in children and adolescents use BMI as the criterion measurement.3 4 5 13 14 15 16 17 The use of BMI may be an issue in sports involving large subjects whose muscular development is well above normal for their age and height. In sports such as American football and rugby, a high BMI does not necessarily reflect excessive adiposity. However, Gomez et al18 reported a correlation coefficient of 0.8 for per cent body fat (%BF) and BMI in high school American football linemen. Since linemen are the heaviest athletes in that particular sport, these findings indicate that BMI for the most part does reflect adiposity in these large athletes.

A total of 13 studies that directly or indirectly addressed the role of obesity in injury risk for children and adolescents were reviewed (table 1). Five studies were specific to injuries occurring in American football3 4 5 18 22 Four studies included injuries occurring in all activities, not specifically in sports,13 14 15 17 and one study included students involved in any high school sport16 The remaining three studies were on military recruits19 20 21 where, because the ages of the subjects were 18–22 years, and the basic training setting replicates the stress of sports training, these findings have some relevance for injury risk in young athletes.

Of the 13 studies listed in table 1, nine did not differentiate between types of injuries,3 4 13 14 16 19 20 21 22 one examined all injuries and a subgroup of lower-extremity injuries,18 two studies focused on ankle sprains5 17 and one study was specific to dental injuries.14 Seven studies included male and female subjects while six studies included only male subjects (five on American football, one on military recruits). Of note, only five of the 13 studies cited in table 1 used exposure-based measurements of injury occurrence for examining the effects of BMI.5 18 20 21 22

Eleven of the 13 studies listed in table 1 showed some evidence that either a high BMI or a high %BF was associated with increased risk of injury. For subjects categorised as overweight, obese or in a high-BMI group, the increase in injury risk ranged from 1.4 to 3.9 times the risk for the respective control group. Both studies that found no effect of BMI on injury risk involved 8–15-year-old boys playing American football.3 4 Interestingly, in one of these studies, a low BMI was associated with increased risk in the youngest players (9–10 years old)3 indicating that undersized athletes in a physical sport may be at increased risk of injury in the youngest age groups for competition. Studies on older children playing American football (14–19 years old) showed increased injury risk associated with a high BMI.5 18 22 Kaplan et al22 found obese players to be 2.3 times more likely to sustain an injury than non-obese players. Gómez et al18 found that obese linemen (BMI>29) were 2.7 times more likely to sustain a lower extremity injury than linemen with lower BMIs. Tyler et al5 found that obese players were 3.9 times more likely to sustain non-contact ankle sprains than players with normal BMIs. It appears that the effect of obesity on injury risk in American football may be more apparent in adolescents than in younger athletes.

Of the seven studies that included male and female subjects, two found gender-specific effects. Chau et al15 found an increased injury risk for obese girls but not for obese boys. Additionally, the risk associated with obesity in girls was not specific to sports participation but included non-sports physical activities. In contrast, for military recruits, male subjects in the upper quartile for BMI and %BF had an increased risk of injury, while female recruits did not.21

Of the four studies that addressed specific injuries, two showed an increased risk of sustaining an ankle sprain in obese subjects,5 17 one found an increased risk of lower extremity injuries in obese subjects and specifically cited ankle and medial collateral ligament sprains,18 and one showed an increased risk of traumatic dental injuries in obese subjects.14 Gomez et al18 found an increased risk for lower-extremity injuries in American football linemen with higher BMI and %BF. The incidence of lower extremity injury for players above and below various BMI and %BF cut-off points was graphed, and in general, injury incidence was at least twice as high in the players with higher BMI or %BF. The authors noted that the incidence of both ankle sprains and medial collateral ligament sprains was higher in higher-fat groups but did not report the specific injury incidences or magnitude of difference between groups. Tyler et al5 found that the incidence of non-contact ankle sprains for obese high school American football players was 3.9 times higher than for players with a normal BMI. Zonfrillo et al17 found that the prevalence of obesity in patients presenting to a paediatric emergency department with an acute ankle sprain was higher than the prevalence of obesity in patients presenting with fever, headache or sore throats with an adjusted odds ratio of 3.26. Lastly, Petti et al14 reported a prevalence of dental injuries of 31% in obese 6–11-year-olds compared with 20% for non-obese children. However, this effect of obesity was not specific to sport but related to a difference in injuries related to indoor play. The results are pertinent to the discussion of obesity and sports injuries because the study noted that prior exposure to injury risk activities, including sports, was actually protective. The authors noted that the non-obese subjects were more active and theorised that they were better adapted to avoid injuries when involved in physical activity.

Direct consequences of increasing rates of childhood obesity on the incidence of paediatric orthopaedic conditions have been documented. For example, the percentage of overweight 13–15-year-old children in Scotland increased from 15% in 1981 to approximately 27% in 2001.23 Obesity is a known risk factor for slipped capital femoral epiphysis, and not surprisingly an increased incidence has been noted in Scottish children over the same time period with an incidence of 3.78 per 100 000 children in 1981 increasing to 9.66 in 2000.23

Proposed injury mechanisms

Six of the studies listed in table 1 discussed potential mechanisms for the effect of obesity on injury risk (table 2). For acute traumatic injuries, there was some consensus suggesting that impairments in postural control in obese subjects was the mechanism for increased injury risk.5 14 15 17 In support of this conclusion, previous studies have shown that obese children have poor postural control compared with non-obese children.24 25 Furthermore, in high school American football players, balance training has been shown to eliminate the increased risk of sustaining an ankle sprain associated with the combination of a high BMI and a previous ankle sprain.26 It remains to be determined if balance training would be effective in reducing the risk of sustaining other types of injuries in obese athletes. However, this type of intervention has been effective in reducing a range of injuries, including but not limited to ankle sprains, in various populations of athletes.27 28 29 31

Impaired postural control in obese children may be due to the challenge of controlling a disproportionately large body mass in activities that require rapid alterations in direction of movement such as occur in most sports. This challenge is compounded by poor physical fitness and low physical activity level. With respect to physical fitness, in weight-bearing activities the obese athlete may have either inadequate strength relative to body mass to safely control body movements or a low cardiorespiratory fitness which may induce fatigue leading to impaired postural control. With respect to physical activity level, a lower prior general physical activity level in obese athletes may compromise neuromuscular control when sports activities are undertaken. The neuromuscular and motor learning adaptations that occur with a high level of physical activity during development may be invaluable to children when they become involved in organised sports.

Potential preventive measures

Reducing injury risk for overweight and obese young athletes may be achieved by interventions that address three specific areas, weight loss, improved physical fitness and improved neuromuscular coordination/postural stability. With respect to weight loss interventions, the primary approach is a combination of diet, exercise and behaviour modification, with pharmacological and surgical treatments being more extreme approaches.30 Diet, exercise and behaviour modification may be more successful in the overweight young athlete compared with the non-athlete given the potential higher level of motivation if the weight loss intervention is seen as a strategy to improve athletic performance. Similarly, achieving improvements in physical fitness may be easier in overweight young athletes than in non-athletes, since there will be specific fitness requirements for particular sports. However, intervention studies aimed at reducing risk of injury in overweight athletes are lacking.

From a practical perspective, an effective intervention to reduce injury risk in overweight and obese young athletes might be to introduce balance training as part of individualised training. McHugh et al26 trained high school American football players deemed to be at risk of sustaining an ankle sprain on a rubber stability pad. The stability training was targeted at players who were overweight or who had a history of previous ankle sprains. The training was incorporated into the players’ weight training programme by adding a dedicated balance training station. In other sports such as volleyball, balance boards have been placed at the side of the court for players to train on during breaks in practice.31 Not only are these types of balance training interventions effective at reducing injuries but also they are inexpensive to implement and have a high compliance. A balance training intervention for overweight and obese young athletes may be effective in reducing the increased risk of injury associated with a high BMI without having to address the wider societal issues of excessive adiposity, poor cardiorespiratory fitness and low physical activity.

Long-term consequences of injury

The increased risk of injury in obese and overweight children and adolescents may be compounded by a more prolonged recovery. Timm et al32 found that long-term ankle morbidity after an acute ankle injury was much more prevalent among children with a higher BMI. Obese and overweight children were more than twice as likely to have persistent swelling and pain. Approximately 33% of overweight and obese children had persistent swelling and pain compared with only 15% of children with lower BMIs. Marchi et al33 previously reported that more than 20% of children sustaining acute ankle sprains can be expected to have permanent sequelae associated with the injury (permanent defined as problems persisting at 12-year follow-up). Increased morbidity following acute injury in overweight and obese children has the potential to create a vicious cycle; injury risk is increased by the combination of a high BMI, poor cardiorespiratory fitness and low physical activity, recovery is compromised by being overweight, prolonged recovery further decreases fitness, and the forced relative inactivity further increases BMI.

Considering that obesity and low cardiorespiratory fitness are risk factors for many chronic illnesses, such as heart disease and diabetes, forced inactivity due to the prolonged effects of injury in obese children may have dire consequences. Therefore, interventions that focus on the specific rehabilitation needs of overweight young patients are critical. In this regard, aggressive early management of swelling may be an important target for treatment. Persistent inflammation following joint injury has deleterious effects on articular cartilage.34 Overweight and obese children have persistent swelling and pain following ankle injury,32 and this might reflect a prolonged inflammatory response to injury. Considering that obesity is a strong risk factor for the development of osteoarthritis35 and that childhood obesity is associated with elevations in systemic inflammatory markers such as C reactive protein and interleukin-6 (IL-6),36 37 effective management of postinjury joint inflammation may be more important in overweight young patients with acute joint injuries. Physical modalities for controlling joint swelling, such as cryotherapy and electrical stimulation combined with compression and elevation, may need to be applied more frequently or for longer durations for the overweight patient. Additionally, progression of weight bearing or other joint compressive loading may need to be delayed. However, it may be equally important to avoid prolonged joint immobilisation in order to avoid deleterious effects on articular cartilage.38 Furthermore, immobilisation has been shown to have a proinflammatory effect, while continuous passive motion has been shown to have an anti-inflammatory effect.39 Therefore, for joint injuries, such as ankle sprains, overweight and obese patients may benefit from (1) increased provision of supervised physical therapy emphasising modalities that control swelling, (2) longer periods of non-weight-bearing with slower progression of partial weight-bearing and (3) continuous passive motion or other low load cyclic joint motion.

Growth and development in the overweight young athlete

An additional concern for the overweight young athlete is that being overweight is associated with elevations in systemic inflammatory markers36 37 and that proinflammatory cytokines can inhibit anabolic hormones associated with growth and development.40 In non-obese wrestlers, intense training results in elevations in inflammatory cytokines and depression of anabolic mediators.41 42 These effects are reversed when intense training ceases.42 The effect of obesity on the anabolic hormonal response to exercise and training in children has not been studied extensively. Acute exercise-induced elevations in growth hormone are attenuated in obese children versus normal weight children,43 but it is unknown if such effects impair adaptations to exercise training.

Further research

Large-scale prospective studies are needed to quantify the relationship between obesity and injury risk in youth sports. Studies with sufficient sample size and diversity can quantify sports-, gender- and injury-specific effects. Additionally, time-based measures of athletic exposures are needed to accurately document injury incidence, and as a basis for analysing risk factors and injury-prevention interventions. Once the magnitude of the problem is established for a particular sport and particular types of injuries, specific injury-prevention interventions can be designed and tested. The possibility that overweight young athletes have a protracted recovery from injury and are at increased risk for residual symptoms needs to be examined.

Conclusions

Based on a growing volume of diverse literature, it appears that overweight and obese adolescents are at increased risk of injury in sports and general physical activity. The exact mechanism of increased risk has not been established, but a general consensus is that impairment in postural stability is an important factor. Balance training is a viable intervention to reduce the injury risk associated with being overweight or obese.