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Objectives--Although the nature of rugby injury has been well documented, little is known about key risk factors. A prospective cohort study was undertaken to examine the association between potential risk factors and injury risk, measured both as an injury incidence rate and as a proportion of the playing season missed. The latter measure incorporates a measure of injury severity.
Methods--A cohort of 258 male players (mean (SD) age 20.6 (3.7) years) were followed through a full competitive season. At a preseason assessment, basic characteristics, health and lifestyle patterns, playing experience, injury experience, training patterns, and anthropometric characteristics were recorded, and then a battery of fitness tests were carried out.
Results--A multiple regression model identified grade and previous injury experience as risk factors for in season injury, measured as an injury incidence rate. A second model identified previous injury experience, hours of strenuous physical activity a week, playing position, cigarette smoking status, body mass index, years of rugby participation, stress, aerobic and anaerobic performance, and number of push ups as risk factors for in season injury, measured as proportion of season missed.
Conclusions--The findings emphasise the importance of previous injury as a predictor of injury incidence and of missing play. They also show the importance of considering both the incidence rate and severity of injury when identifying risk factors for injury in sport.
(Br J Sports Med 2001;35:157-166)
Keywords: injury; epidemiology; risk factors; cohort; rugby union
Participation in physical activity and sport is often recommended as a means by which the risk of contracting many of the "diseases of the sedentary", such as coronary heart disease and cancer, can be reduced. [1,2] Recognition of this protective effect has led to programmes designed to promote the benefits of participation in sport and physical exercise and increase participation rates.  Little is known, however, about the risks and costs of participation in sport and other physical activity, partly because of a lack of epidemiological research. [4-7] Calls have been made for the application of epidemiological methods to the investigation of risk factors for injury resulting from sport and physical activity.  The undertaking of such studies has been hampered to some extent by methodological issues such as difficulties in setting up injury surveillance systems, defining sports injury, and the complexity of data analysis in cases in which participants sustain multiple injuries during a season of play. [8,9]
The multiplicity of factors that may contribute to injury from sporting activity, and the complexity of the relations among them, mean that identifying causal mechanisms poses a challenge to epidemiologists. [5,6] Potential risk factors have been classified into those intrinsic and those extrinsic to the sportsperson.  Intrinsic factors are specific to the individual, and include age, sex, anthropometric characteristics, fitness, psychological characteristics, health status, and injury history. Extrinsic factors are those external to the individual and include the nature of the sport, environmental conditions, and equipment. 
Most previous research attempting to investigate risk factors associated with sports injury has used the incidence rate as the outcome variable. [11-13] Studies that identify risk factors for sports injuries and recommend interventions based only on injury incidence rates may be missing an important part of the impact of injury on players--that is, the severity of the injury. Measuring the proportion of the season missed as the result of injury is one method of generating a proxy measure of injury severity.  The identification of risk factors associated with the effect of the injury on subsequent participation may be as important in understanding how to reduce the burden of injuries on sports participants as identifying factors associated with the injury incidence rate.
A recent prospective study into intrinsic risk factors for injuries resulting from physical activity identified previous injury and exposure time as being more important predictors of injury incidence rate than psychological, psychosocial, physiological, and anthropometric measures.  A study of army trainees found that greater age, higher cigarette consumption, previous low physical activity levels, high or low flexibility, and low levels of aerobic fitness were associated with a higher risk of injury. [12 13]
Rugby union football (rugby) is a vigorous contact sport, which enjoys particular popularity in Australia, Britain, France, New Zealand, and South Africa. The nature and incidence of rugby injuries have been well documented, with cervical spine injuries receiving particular [14-22] attention. In New Zealand, the combination of a large player base (about 120 000 in a population of 3.8 million people) and a high incidence of injury  result in rugby being the largest contributor to sports injury costs borne by New Zealand's mandatory injury compensation scheme administered by the Accident Compensation Corporation (ACC). 
From what is known about rugby injury, it appears that there is a higher incidence at higher grades, although the incidence rate of particular injuries-for example, spinal cord injuries-- does not always follow this pattern. [14-16 21] The types of injury a player is likely to sustain are also related to playing position--for example, those in the front row positions are more at risk of cervical spine injury during scrums than those in other positions. [20 21] The tackle appears to be the phase of play associated with the greatest risk of injury overall. 
Given the limited information available on risk factors for sports injury in general and the lack of analytical studies on rugby injuries, the New Zealand Rugby Injury and Performance Project (RIPP)  was undertaken to examine a wide range of extrinsic and intrinsic factors postulated to be associated with rugby injury. The purpose of this paper is to explore the associations between potential risk factors for rugby injury, as assessed before the season, and both injury incidence during the season and the proportion of the season missed because of injury, using information obtained from the RIPP.
SUBJECTS AND STUDY DESIGN
The RIPP was a prospective cohort study in which rugby players were recruited at the beginning of a rugby playing season and then followed on a weekly basis until the end of the same season. At the beginning of the 1993 rugby season, 356 rugby players (258 men and 92 women) were enrolled in the RIPP. Players were recruited into the study through five rugby clubs and four secondary schools. The study design, basic characteristics, anthropometric and physical performance attributes, alcohol use, and patterns of previous injury of the RIPP cohort members have been reported elsewhere. [8 25-29] This paper examines risk factors for the male players only; the sample of female players was too small to allow reliable estimates of the various risk factors to be calculated independently.
Players completed a preseason assessment that involved completing a single self administered questionnaire and undergoing a series of anthropometric and physical fitness assessments. The assessment lasted approximately 2.5 hours.  Table 1 gives the factors measured at the preseason assessment. The previous injury experience of players was classified as follows: players who had not sustained an injury in the previous season (no injury in previous 12 months), those who had been injured during the previous season but were not currently injured at the preseason assessment (previous season), and those who reported that they were injured at the time of the preseason assessment (preseason injury). Players were asked to estimate how many hours a week, in the previous four weeks, they had spent in "strenuous physical activity" (not further defined). Alcohol use was assessed using the Alcohol Use Disorders Identification Test (AUDIT).  The AUDIT is a 10 item questionnaire used to screen people for hazardous or harmful alcohol consumption. A score of 8 or more is taken to indicate an alcohol use disorder. Competition anxiety was measured by the Sport Competition Anxiety Test (SCAT).  The SCAT is a 15 item measure of competitive trait anxiety. Scores range from 10 to 30, with a higher score indicating a higher level of anxiety. Psychological wellbeing was measured using the General Health Questionnaire (GHQ).  The GHQ is a self administered screening questionnaire which measures general psychological distress. The 12 item version of the GHQ was used.
The anthropometric and physical fitness measurements were taken upon completion of the questionnaire. The methods used are described in detail elsewhere.  Anthropometric measurements taken from the players included height (m), body mass (kg), and skinfolds from six sites (triceps, subscapular, suprailiac, abdomen, mid thigh, and medial calf). The sum of skinfolds from these six sites was used as a measure of body fatness, as was body mass index (BMI) (kg/[m.sup.2]). The physical fitness measures examined were a 20 m multistage shuttle run for aerobic endurance,  number of push ups performed at a constant cadence until this could no longer be maintained, vertical jump height (best of three), sprinting 30 m from a 5 m running start, and a set of repeated high intensity shuttles. [28 29 33 34] Weekly endurance training load was calculated from the players' reported intensity, duration, and frequency of aerobic training. Time and resource constraints precluded the measurement of strength and flexibility.
IN SEASON FOLLOW UP
The in season follow up consisted of a weekly telephone interview conducted with each player. Nine trained interviewers collected information about rugby exposure (number of games and practices attended, warm ups, grade, playing position, involvement in foul play, use of protective equipment) and any injuries sustained (site, type, description of how the injury occurred, medical attention received, treatment, whether the injury caused the player to miss any games or practices, whether the injury was the result of foul play). An injury event was defined as one that caused the player to either miss at least one game or scheduled team practice, or to seek medical attention.  During most of the 23 weeks of the club season, interviews were completed with 90-95% of players. Overall, 90% of attempted interviews were completed. 
Two outcome measures were chosen to examine the influence of the preseason factors on injury experience during the season. These were the injury incidence rate (IR) and the proportion of the season a player missed because of injury (PM). IR (per 1000 player hours) was calculated as follows: IR = 1000 x (number of injury events that occurred during games/(1.33 x number of games played)). Each game of rugby lasts 80 minutes, hence the multiplication by 1.33 to convert games played to hours of play.
To calculate the proportion of the season missed, players who did not miss any games were considered to have completed the season with no missed time (PM = 0). For the purposes of calculating PM, any practices they missed because of injury in a particular week were ignored. PM was calculated as follows: PM = number of weeks in which game(s) were missed because of injury in games/number of weeks in which one or more games was played. For weeks in which there was no follow up interview, the entire week was ignored. Although this would have the effect of inflating PM, given that 90% of attempted interviews were completed, this effect was not expected to be large or to introduce any systematic bias.
These two outcome measures were chosen to provide complementary information. The two measures are not independent, but both were used to examine whether specific risk factors were related to different measures of injury. IR provides a measure of the number of injuries sustained per unit of exposure to rugby games, whereas PM provides a proxy measure of injury severity. 
A total of 258 male rugby players completed the questionnaire and at least part of the physical assessment. The univariate associations between potential risk factors and the two outcome measures (IR and PM) were examined first. Categorical variables were created from continuous variables by assigning players to quintiles (for instance, on the basis of their performance on each of the fitness tests). The relative risk (RR) of sustaining a greater incidence of injuries or of …