temporal patterns of injury during a rugby season

Temporal Patterns of Injury During a Rugby Season

Jonathan C Alsop 1, David J Chalmers 1, Sheila M Williams 2, Kenneth L Quarrie 1, Stephen W Marshall 3 & Katrina J Sharpies 2

1Injury Prevention Research Unit, Department of Preventive and Social Medicine, Medical School, University of Otago, Dunedin, New Zealand. 2Department of

Preventive and Social Medicine, Medical School, University of Otago, Dunedin, New Zealand. 31njury Prevention Research Centre, University of North Carolina, Chapel

Hill, North Carolina, USA.

Alsop, J.C., Chalmers, D.J., Williams, S.M., Quarrie, K.L., Marshall, S.W., & Sharples, K.J. (2000). Temporal patterns of injury during a rugby season. Journal of Science and Medicine in Sport 3 (2}: 97-109,

The aim of this study was to describe temporal patterns in the frequency, nature and circumstances of injuries occurring among a cohort of 356 rugby players during a club rugby season in New Zealand. It was found that the rate of injury in games decreased significantly over time in both males and females. The reduction in injury rate over the season was more pronounced in some grades, but no differences were found when examined by gender, playing position, age, ethnicity or by health and fitness types. Trends in injury rate were consistent over the rugby season and did not appear to be the result of a bias involving under-reporting of end-of-season injuries. The types and severity of injury remained relatively constant, but the proportion of injuries occurring in back play fell significantly over the season and injuries were more likely to occur in the trunk body region as the season progressed. This study supported the hypothesis that higher rates of injury occur at the start of the rugby season and decrease over the course of the season. This reduction is consistent over time and across player types, and is not attributable either to decreasing injury severity or to increasing player fitness.

Introduction Rugby Union is a vigorous contact sport a)nd, because of its physical nature , injuries are common. In New Zealand, whe1'b Rugby Union is considered by m a n y to be the 'national ' sport, rugby is a maj0}- contr ibutor to sports-related deaths, hospitalisations, emergency depar tment presentat ions and heal th-care costs (Hume & Marshall 1994). The incidence, na ture arid c i rcumstances of rugby injury have been described for several of the rugby playing nat ions including Australia (Davidson, 1987; Seward et al., 1993; Hughes & Fricker, 1994), New Zealand (Dalley et al., 1982; DalIey et al., 1992}, Scot land (Garraway & Macleod, 1995), Sou th Africa (Nathan et al., 1983; Roux et al., 1987; Clark et al., 1990), and Wales (Lewis, 1994). A number of these, and other, studies have documented a decline in injury incidence over time (Sparks, 1985; Lee & Garraway, 1996; Armour et al., ] 997). There is some suggestion in the literature that this decline is due to the data collection methods used and does not represent a t rue decline in incidence 03alley et al., 1982; Roux et al., 1987).

The Rugby Injury and Performance Project (RIPP) was a prospective cohort s tudy designed to identify risk and protective factors for rugby injury (Waller et al., 1994). The pr imary aim of this paper is to describe the temporal pa t terns of injury

97


throughout a club season. Specifically, it was sought to determine whether the incidence of injury in the cohort declined over time, and if this was due to a decline in rugby participation.

Methods In summary, 356 players were recruited from five clubs and four schools in Dunedin, New Zealand, before the start of the I993 rugby season. Male players were drawn from Senior A, Senior B and Colts club teams, and schoolboy first XV's (under 19 and under 18 grades). Female players were drawn from Senior Women's teams and schoolgirl teams (these were the only grades for female players in Dunedin at the time of the study). Following a pre-season assessment, the players were interviewed by telephone once a week, where information was obtained about rugby exposure and injury experience in the previous week. Weekly interviews took place from February through to August, and response rates of 90-95% were obtained in most weeks. A detailed description of the study design has been provided elsewhere (Wailer et al., 1994).

Full data were available for 345 players (258 males, 87 females). Information obtained on each player each week included the number of games played, practices attended and injuries sustained. Data was collected on a per-game (or practice) basis. Players were asked about rugby-related injuries experienced in the past week that either required medical attention or caused the player to miss at least one scheduled game or team practice. For the purposes of the analyses reported here, injury incidents involving more than one injury (8% of incidents) were treated as a single injury incident. In these cases, the most severe injury incident was used.

If the incidence of injuries was dependent on only the amount of exposure, a constant weekly injury rate would be expected. However, it was hypothesised that the injury rate would decrease over the rugby season (Sparks, 1985; Lee & Garraway, 1996; Armour et al,, 1997). To investigate this hypothesis the injury rate per 100 player-games (player-practices) was calculated for each week. Anecdotal evidence from the interviewees suggested that there was a greater propensity for players to chose not to report injuries toward the end of the season, due to the increased length of the interviews when injuries were reported. To check trends in the injury rate for this possible bias, sub-sections of the season were examined. No published studies have evaluated the statistical significance of trends in injury rate over a rugby season.

It was also hypothesized that some categories of player groups (e.g. grade) adjust faster than others to the physical and mental demands of rugby. Thus, some categories may have an injury rate that decreases faster than other categories of players. This hypothesis has not been examined previously. Accordingly, trends in injury rates were examined in each level of various demographic and rugby factors, including gender, age, ethnic origin, grade, playing position, rugby-playing experience (the number of years of playing), and whether or not they had played representative rugby during the previous twelve months. In addition to these, various anthropometric and health factors, including height, weight, body mass index, pre-season injury, cigarette smoking status, alcohol use (AUDIT scale, Quarrie et al., 1996), and depressive symptoms (General Health Questionnaire, Tennant, 1977) were examined. Possible differences in injury rate trends were examined by fitness measures, including

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hours of s trenuous physical activity per week, self-rated health status, and ratings of shuttle, sprint, push-up and vertical jump tests (Quarrie et al., 1995). Many of these variables were measured on a continuum and were subsequently categorized into quintiles. No data was collected on the fitness level of the players during the rugby season.

It was also hypothesized that the injury rate would be affected by temporal trends in the number of injuries occurring within certain body sites, game phases and player roles (Davies & Gibson, 1978; Dalley et al., 1982; Davidson, 1987), Data were collected from each injured player on the nature and circumstances of the injury, including the phase of play (serum, line out, maul, kick off, ruck, back play, try seo~ng, tackle), and the players role at the time of injury (ball carrier, tackler, support player). Data were also collected on the body site (head, trunk, arm, hand, upper leg, lower leg/foot) and type of each injury (sprain/strain, bruising/haematoma, other). Trends in the proportion of injuries occurring in each level were examined for each of these factors.

Finally, it was hypothesized that the injury rate would also be affected by temporal trends in injury type and severity. For example, given that a severity threshold is required when identifying injuries, a reduction in average injury severity over the season would lead to a (spurious) reduction in injury rate. Changes in the severity of the injuries occurring during the season were assessed using four dichotomous injury outcomes. For each injury the player was asked whether or not they had received medical attention, whether they continued to play rugby immediately after the injury occurred and whether the injury interfered with what they had planned to do the following day. In addition to these measures, the proportion of moderate (or worse) (AIS>=2) injuries was examined (coded according to the Abbreviated Injury Scale, American Association for Automotive Medicine, 1985).

Because the various grades and individuals within grades did not start their season on the same date, the start of the season was defined for each individual as the week of their first game or team practice. Accordingly, weeks refer to calendar weeks since that date.

The statistical significance of trends in mean number of games and practices was established using weighted least squares regression, The statistical significance of trends in injury rates were assessed by incorporating a linear time (week) factor in a Poisson regression model. The injury rate was defined as the number of injuries divided by the number of games (the offset). Note that for each factor a separate regression was performed, with no other additional factors included. The outcome used for the severity measures was the proportion of injuries in which the outcome occurred (e.g. the proportion receiving medical treatment). A separate binomial regression was performed for each of the four severity outcomes. Poisson and binomial regressions were undertaken using the SAS procedure PROC GENMOD (SAS Institute Inc, 1993).

The statistical significance of differences in injury rate trends between levels within factors were established using a Type 3 contrast (PROC GENMOD) on the 'factor' xweek interaction term. This term was incorporated into a Poisson regression model where both main effect terms ('factor' and week) were present, Poisson regression models included an overdispersion parameter in order to account for correlation introduced by making repeated observations on the study participants. A separate regression was also performed for each of the twenty

99

Temporal Patterns o f l n j u r y During a Rugby Season

factors examined, with no additional explanatory variables included. Th roughou t this paper 'b' is used to denote the time trend estimate, '95% CI' to denote the 95% confidence interval for the estimate, and 'p' to denote the statistical significance of this estimate (against a null hypothesis of zero trend).

Results Over the club season concerned, the RIPP cohort accumula ted a total of 4 ,403 player-games and 8,653 player-practices, during 5,877 player-weeks of follow- up. During this time 462 injuries in games and 107 injuries in practices were sustained. The nature and c i rcumstances of these injuries have been described elsewhere (Bird et al., 1998). Twelve game injuries and three practice injuries were excluded from the analysis, as these injuries occurred prior to the s tar t of the season. Male players were followed for a mean of 18 weeks and female players for a mean of 16 weeks. Males played, on average, 14 (standard deviation=5) games and attended 28 {s.d.= ]0) practices, while females played, on average, I 0 (s.d.=5) games and attended 16 {s.d.= 10) practices. The median n u m b e r of games played in a week was I for both males and females. The median n u m b e r of practices at tended in a week was 2 for males and 1 for females. For males, more t h a n one game was played in 6.6% (298/4525) of player-weeks, while for females, more than one game was played in 6.5% (89/1352) of player-weeks. The average follow- up time was similar for each of the grades within genders.

2.5

2 -

1.5

0.5

1.6 1.4 1.2

E g 0.8 ~= 0.6

0.4- 0.2-

Male •

% /

I I I I I I I

2 3 4 5 6 7 8

Female

_ _ Practices I I I I I I I t I I I I I I I I

9 10 11 12 13 14 15 16 17 18 192021 2223 2425 Weeks since start o f s e a s o n

\

_ _ Practices I I I 1 I I I I I I L I I 1 1 I I I

2 3 4 5 6 7 8 9 10 11 12 13 14 1.5 16 17 18 19 20 Weeks since start o f s e a s o n

Figure 1:

lO0

Mean number of games and practices by time since start of season, for mates an~ rereads, RIPP ~ , 1993 season.


Exposure Figure 1 shows the mean number of games played and practices attended each week for males and females. The mean number of games played by males remained roughly constant over the season, and no significant trend was found (b=-0.0028, 95% CI=-0.0091,0.0035, p=0.365). The slight rise at the end of the season arose from a small number of players who all played at least one game per week during that period. The mean number of games played per week at the start of the season was estimated (via the weighted regression model) to be 0.82, falling to 0.75 by the end of the season. This represents a small decline compared with the decline in the mean number of practices: 1.75 at the start of the season, down to 1.42 at the end (b=-0.0137, 95% CI=-0.0202,-0.0071, p<0.001): There was a slight rise in the mean number of games played by females over the season (b=0.0109, 95°/6 CI=-0.0094,0.0311, p=0.276), and a slight reduction in the number of practices attended (b=-0.0091, 95°/6 CI=-0.0268,0.0086, p=0.295).

The large mean number of games played in weeks 11 and 12 was due to a Senior Womens grade tournament held d u l ~ g this time. Note also that the 'Easter ' holiday occurred during weeks 7, 8 and 9 for most players. No schoolgirl grade player was followed for more than 13 weeks.

tnJu~ The mean num ber of game injuries per male player per week dropped significantly 03=-0.0022, 95% CI=-0.0037,-0.0008, p=0.005} over the season (see Figure 2).

=

0.20

0.15 _

0 . 1 0 -

0 . 0 5 -

0 . 0 0

Male

I I I i I I I I I I I I I I | I I I I I I I I

2 3 4 5 6 7 8 9 1011 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 20 2122 23 24 2:~

Weeks since start of season

Female 0.09

0.08 0.07 ' 0.06-

0.05 -

0 . 0 4 -

0.03 0.02

0.01

0,00

/ I t • _ _ Practices

\ / ,

3 4 5 6 7 8 9 lO I I 12 13 14 15 16 17 18 19 20

Weeks since start of season

Figure 2:. Mean number of injurfes W time since star of season, for n k ~ and females, RIPP cohort, 1993 season.

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This corresponds to an average decrease of 0.2% per week. In addition, the mean number of practice injuries also dropped significantly (b=-0.0012, 95O/o CI=- 0.0018,-0.0006, p=0.008). There was a sharp rise in the mean number of game injuries sustained at the end of the season, especially during weeks 22 and 23. No significant trends were identified in the mean number of game injuries per female player per week (b=-0.0014, 95°/6 CI=-0.0033,0.0005, p=0.142), nor the mean number of practice injuries per female player per week (b=0.0003, 95% CI=- 0.0008,0,0013, p--0.617).

Injury Rate The weekly game injury rate, for both genders, is Shown in Figure 3. Also shown in this figure are the predicted values obtained from the regression of injury rate against week. The male injurY rate peaked at both the start and end of the season. Overall, there was a statistically significant decrease in the male game injury rate over time (b=-0.0254, 95% CI=-0.0434,-0.0076, p=0.005). This corresponds to an average decrease of 2.5% per week. Higher injury rates at the start and end of the male season would result in a concave curve. This was tested for (using a quadratic term in the regression) and was not significant. Thus, the decline in injury rates over the season remained relatively consistent over time. The female injurY rate showed more variation, but also a significant decrease over time 0a=- 0.0628, 95% CI=-0.1184,-0.0090, p=0.024), an average decrease of 6.1% per week. The female rate was generally lower than the rate for males. No significant concave (quadratic) behaviour in injurY rates was found for females. There was a statistically significant decrease in the male practice injurY rate over time 0a=- 0.0517, 95% CI=-0.0884,-0.0163, p=0.005). The female practice injurY rate was highly variable, and showed no significant trend over time (b=0.0421, 95% CI=- 0.0724,0.1594, p=0.467).

Arbitrarily chosen sub-sections of the season were then examined. The male game injury rate decreased significantly {b=-0.0366, 95% CI=-0.0564,-0.0170, p<0.001) during the first 20 weeks. The female injury rate did not decrease

25

] Male A 20-] . . .Predicted(Male) / \

/ Fen'kale I li

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Weeks since start of season

Figure 3: Game injur/ rate W time since start of season, RIPP cohort, 1993 season.

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Factor Number Chl.square*" p-value of levels

sex (male/female) 2 1.66 0.197 Age (<17, 18-19, 20-22, 23 and over) 4 2.61 0.455 Ethnicity (European, Maori/Pacific Islander) 2 2.06 0.151 Grade (5 male grades, 2 female grades) 7 17.28 0.008 POsition (forwards, backs) 2 1.08 0.299 Number of previous years of play* 5 2.39 0.664 Representative play in last 12 months (yes/no) 2 0.23 0.628 Height (m). 5 1.89 0.757 Weight (kg). 5 1.84 0.766 Body Mass Index (kg m-2) * 5 2.35 0.671 Injury experience (none, previous, current) 3 1.81 0.405 Smoker (non-smoker, ex-smoker, current smoker) 3 1.62 0.444 Alcohol use (AUDIT <8, AUDIT>=8) 2 1.51 0.219 Depressive symptoms (GHO=O, GHO=I) 2 0.41 0.522 Strenuous physical activity (hours per week). 5 5.89 0.210 Self-rated health status (not too good, good, very good) 3 4.52 0.104 Shuttle test* 5 3.44 0.487 Sprint test. 5 3.66 0.454 Push-up test* 5 1.71 0.788 Vertical jump test. 5 6.43 0.170

* OuinUles used ** TYPE III contrast

Table I: Trends in injury rate by

35

)0=

!5-

!0-

5_

i0-

5_

0-

Colt

A - ' -Predicted(Colt ) A

/ Vg . o, o,M,,0 / \

I I I I I 1 I I I I I I I I I 1 I I I I I I I 2 3 4 5 6 7 8 9 l0 II 12 13 14 15 16 17 18 19 20,21 22 23 24 25

Weeks since start o f season

Figure 4: Game inju~/rate ~/~rne s~ce ~ n of season, by mar grade, ~PP cot~, 1993 season.

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significantly (13=-0.0551, 95% CI=-0.1242,0.0131, p=0.114) dmSng the first 15 weeks. As an additional check, the injury rates for only the first 12 weeks were examined. The male game injury rate decreased significantly ('o=-0.0362, 95% CI=-0.0722,-0.0004, p=0.048} during this period, but the female game injury rate did not (b=-0.0576, 95% CI=-0.1452,0.0283, p=0.191). Thus, it appears that the reduction in the male game injury rate was more consistent in terms of statistical significance than was the female injury rate. However, within both males and females, the magnitudes of the trends for the regressions were approximately equivalent and all indicated a downward trend in game injury rate. Similar results were obtained for the practice injury rates for both genders.

Whether or not the drop in injury rate differed significantly between various groups within the cohort was examined. Of the twenty demographic, rugby, anthropometric, health and fitness factors the only factor for which the trends in injury rate varied significantly across levels was the grade (Table 1). Specifically, the male game injury rate decreased significantly faster in the Colt grade, compared with the other males grades (see Figure 4).

Nature and Circumstances of:Injury It was hypothesized that any trends found in injury rates would result from changes in the type, site and severity of the injuries, changes in the players role, or changes in the phase of play in which the injury occurred. Few statistically significant trends in the proportion of injuries occurring within levels of these categories were found (see Table 2}. The proportion of injuries occurring in the lower leg/foot region decreased significantly over the season, while, the proportion occurring in the trunk region increased significantly. No significant changes by player role or type of injury were identified. The proportion of injuries occurring in 'back play' decreased significantly over the season. At the start of the season approximately 13% of all injuries occurred during 'back play'. This proportion declined to around 5% by the end of the season. No significant trends in the proportion of injuries occurring in other phases Of play were identified.

Of the 432 coded injuries, 322 (75%) were coded as AIS=I and the remaining 110 (25%) as AIS=2 or 3 {only three injuries were coded as AIS=3). None of the four injury severity indicators showed significant trends in the proportion of injuries that resulted in these outcomes (Table 3). When disaggregated by gender there were no significant differences in slope for any of the severity indicators. The proportion of Colt grade players, however, who stopped playing immediately after the injury occurred decreased significantly faster compared with the other male grades (p=0.009). No significant differences in the other three injury severity indicators were found when examined by grade.

DiSCUSSiOn Although previous rugby studies have collected information on exposure to games and practices for the purpose of determining injury rates, few have controlled for exposure (Roux et al., 1987; Nathan et al.i 1983). In this study, the mean number of games played and practices attended by males each week remained roughly constant over the season. Aside from the effects of the Senior Women's grade tournament during the middle of the season, the mean number of games and practices attended was relatively constant over time for females also. The mean number of injuries per male player declined significantly for both practices and games. No significant downward trends were found for females. This may have

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Factor

injury ~pe

Body Site

Player Role

Game Phase

Level

Sprain/Strain Bruising/Haematoma

Slope (b) 95% Confidence (:hi. p-value Interval square

Lower Upper Limit Limit

186 0 .0145 -0.0169 0.0459 106 -0.0223 -0.0595 0.0149

0.82 0.366 1.3B 0.240 0.01 0.911 0.33 0.567 7.60 0.006 0.02 0.876 0.02 0.918 0.39 0.532 4.18 0,041 0.96 0.328 1.47 0,226 0.04 0.850 0.77 0,381 0.11 0.743 0.53 0,468 0.22 0.639 1.53 0.216 0.12 0.732 5.81 0.016 0.30 0.586 0.78 0.377 0.60 0.439

other' Head Trunk Arm Hand Upper leg Lower leg/Foot

158 0 . 0 0 1 8 -0.0307 0.0343 101 -0 .0110 -0.0484 0.0264 74 0.0580 0.0168 0.0992 11 0 .0081 -0.0932 0.1094 49 -0 .0026 -0.0526 0.0474

111 0 . 0 1 1 4 -0.0243 0.0471 73 -0 .0464 -0.0909 -0.0019

Other Ball carrier Tackler Support player Other Scrum Line Out Maul RUCk BaCk Play Try Scoring Taclde Other

31 -0 .0319 -0.0958 0.0320 165 -0.0201 -0.0526 0.0124 98 0 . 0 0 3 6 -0.0338 0.0410

140 0 .0149 -0.0184 0.0482 47 0 . 0 0 8 4 -0.0420 0.0588 30 0 . 0 2 2 6 -0.0384 0.0836 9 -0 .0275 -0.1426 0.0876

51 -0.0321 -0.0829 0.0187 71 0 . 0 0 7 4 -0.0349 0.0497 36 -0 .0804 -0.1459 -0.0149 9 0 . 0 3 0 0 -0.0778 0.1378

161 0 .0145 -0.0176 0.0466 83 0 . 0 1 5 9 -0.0243 0.0561

~ /e 2: Trends/n/niu~, rate across/eve/s of Pacts.

Factor'

Received medical attention Discontinued rugby playing Interference with planned activities Moderate injury (AIS=2)

n Slope (b) 95% ~ Chl. p-value Interval square

Lower Upper limit Umit

405 0 . 0 2 6 9 -0.0264 0.0802 0.98 0.323 143 0 . 0 0 7 0 -0.0265 0.0405 0.17 0.682 206 0 .0096 -0.0216 0.0408 0.37 0.546 110 -0.0047 -0.0402 0.0308 0.07 0.795

*Note that factors are not mutually exclusive.

Table 3: Trends in sever#l/factors.

been a result of the low number of injuries sustained by this group, resulting in reduced statistical power.

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There was a significant reduction in the game injury rate for both males a n d females over the season, with peaks frequently occurring near the start of an individual's season. This indicates that the incidence of injury is not solely dependent upon the amount of exposure. The sharp peak in the male injury rate over the last few weeks of the season may have been due to competition finals being played at a greater intensity than games played earlier in the season. The drop off in the male game injury rate at the very end of the season may be spurious, due to the small numbers of players involved in rugby at this time. Although the practice injury rate was more variable than the game injury rate (for both genders), there was a statistically significant reduction in the practice injury rate in males. This may have been due to a reduced intensity in practices towards the end of the season.

It was found that the reduction in the game injury rate was steeper in the Colt's grade. This was coupled with a significant decrease in the proportion of injured Colt's grade players who did not continue playing after injury. As the season progressed there may have been a greater willingness (or pressure) for these players to keep 'in the game'. No other statistically significant differences in injury rate trends were observed amongst the twenty measures.

These findings confnnn earlier reports of a declining incidence of injury through the rugby season (Sparks, 1985; Lee & Garraway, 1996; Armour et al., 1997). This decline has most commonly been explained in terms of lack of preparation or 'match fitness' at the beginning of the season (Roux et al., 1987; Hughes & Fricker, 1994; Lee & Garraway, 1996). This study has shown, however, that the level of fitness at the start of the season had no effect upon how quickly the injury rate declined. It may be argued that the reduction in the injury rate of players who are unfit at the beginning of the season {but steadily gain match fitness) occurs faster than the reduction in the injury rate of players who are fit at the beginning of the season. Such a difference was not observed in this study, thus the 'match fitness' hypothesis can be ruled out as a possible explanation for the decline. Wekesa et al. {1996) identified a reduction in the number of injuries d u n g a Rugby World Cup qualifying round, and argued that "the motivation to win mus t have decreased over time". This hypothesis cannot be ruled out .

An alternative explanation of the declining incidence of injury is that a number of the studies reporting such patterns have relied on the so called 'pyramid' approach to data collection (Dalley et al., 1982; Roux et al., 1987}, with reporting dropping off over the course of the study. Recent research by Garraway and Macleod {1995}, however, suggests that the pyramid system can work successfully providing there is intensive monitoring by paid staff. In the present study, information on rugby exposure and injury occurrence was obtained using weekly interviews conducted by paid telephone interviewers. The response rate was generally high and did not decrease over the study period. In addition, the downward trends in injury rate were consistent over the whole season. Thus, the statistical significance of these reductions was not due to a sudden drop in the injury rate at the very end of the season. A slow 'drop-off in the reporting of injury starting at the beginning of the season, however, would result in a declining injury rate. It is unlikely that such a pattern occurred in this study, with a sudden drop in injury reporting towards the end of the season being a more likely response of players.

Another possible cause for the observed decline in injury rate was a reduction

106


in injury severity over time, rather than any increased avoidance of injury itself. For example, a transition from 'moderate' injuries to 'minor' injuries over the season could have meant that the 'minor' injuries were not reported as they did not fall within this study's definition of injury. The four measures of injury severity used here exhibited no significant temporal trends, so this factor can be dismissed as an explanation for the decline in injury rates. The Abbreviated Injury Scale (AIS) is not particularly appropriate for sport injury, due to the relatively 'minor' severity of the injuries involved. However, in this study it allowed for an initial investigation of temporal trends in injury severity. In addition, the AIS has been widely applied in numerous other studies of injury.

A further indicator of severity is the number of games missed due to injury, This measure is problematic, however, in that the number of games lost is a fimction of the time of the season in which the injury occurred. For example, injuries occurring in the last match of the season may be quite severe, but will have no effect upon subsequent game playing (as there is none!). In addition, in this study, weeks in which injuries occurred were often followed by periods of holidays, for which no exposure information was collected. It is likely, however, that the number of games missed is highly correlated with the injury severity indicators used here (de LoEs & Goldie, 1988; van Mechelen et al., 1996). That is, for a player to miss a game through injury, the injury is likely to have been moderately severe to have interrupted play, have required medical attention, and disrupted activities planned for the following day.

Of interest was the finding that the proportion of injuries occurring in 'back play' dropped markedly over the season. This may have arisen from a reduction in the amount of 'back play' over the season, due to deteriorating ground conditions. The increase in the proportion of injuries occurring in the trunk body region may have arisen due to harder tackling as the season progressed, or, perhaps, as the result of a cumulative effect of multiple tackles occurring in this region. The decrease in the proportion of injuries occurring in the lower leg body region cannot be accounted for at this stage.

The temporal nature of re-injury could have had some affect on the pattern of weekly injury rates. If it is believed that previously injured players have a higher risk of injury than those without previous injury, as has been reported elsewhere (Requa et al., 1993; van Mechelen et al., 1996), higher rates of injury towards the end of the season would be expected. However, the opposite pattern in injury rate was observed. An explanation for this may be that injured players may choose not to play again, thereby not exposing themselves to further risk. This explanation is not particularly likely, given the enthusiasm of players to return to play.

Finally, the reduction in injury rate over the course of the season may have arisen as a result of a survival effect. 'Injury-prone' cohort members would have been injured in rugby earlier in the season, compared to other members. Thus, after the start of the season, the 'injury-prone' cohort members would have reduced exposure to rugby. However, the magnitude of any survival effect may have been small, given that most cohort members quickly returned to rugby after injury. In addition, no differences in injury trends were found in terms of any of the anthropometric, health or fitness factors that were measured.

Conclusions By ruling out many possible explanations, we are led to the conclusion that the

107


observed reduction in injury rate over the season was very likely a real effect. The reduction in injury rate was quite consistent across player categories and with regard to pre-season health and fitness. While player grade affected the decrease in injury rate, it appears that the overall decrease was attributable to some unmeasured factor{s). The identification of these factors remains paramount. There are some methodological issues, however, which need to be addressed in any subsequent study in order to dismiss alternative explanations for the temporal patterns that were observed.

Acknowledgments The Rugby Injury and Performance Project was funded by a grant from the Accident Rehabiitation and Compensation Insurance Corporation [ACC). The Injury Prevention Research Unit is funded jointly by the Health Research Council of New Zealand and the ACC. Sheila Williams is funded by the Health Research Council of New Zealand. The views expressed in this paper are those of the authors and do not necessarily reflect those of the above organisations. The authors wish to acknowledge the important contribution of their co-investigators throughout this project: Anna Waller, Yvonne Bird, Jean Simpson, Barry Wilson, David Gerrard, Martin Toomey, Philip Handcock, Kenneth Hodge, and Michael Feehan. The authors are grateful to Shaun Stephenson for his assistance in revising the Final manuscript. The authors also wish to thank the following people and organisations for their contribution to this project: the players, interviewers, Otago Rugby Football Union, and New Zealand Rugby Football Union.

References American Association for Automotive Medicine {19851. The Abl)mviated l- j~, T Scale, Arling~ton

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temporal patterns of injury during a rugby season

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