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Abstract
Objective The aim of this study was to determine whether there is a difference in the prevalence of tennis injuries between the four most common court surfaces in the Netherlands, including hard court, clay, sand-fill artificial grass and red-sand-fill artificial grass. Natural grass was not included in this study.
Methods This was a repeated cross-sectional study over 6 months, involving members of the Royal Netherlands Lawn Tennis Association (KNLTB). A monthly questionnaire was sent to a random sample of 20 000 KNLTB members, stratified by their club’s playing surface. The questionnaire included questions on court surface, tennis exposure and physical complaints, using the Oslo Sports Trauma Research Centre questionnaire on health problems.
Results A total of 3656 (18%) of the 20 000 invited members completed at least one of the monthly questionnaires [mean age 49 years (15)]. A total of 4047 injuries were reported by 1957 respondents. Of these injuries, 3246 (80%) were overuse and 801 (20%) were acute. There were no statistically significant differences in injury prevalence between groups who played primarily on any one of the four court surfaces. However, players who played on multiple surfaces had a higher injury prevalence, particularly of overuse injuries, than those who primarily played on one court surface. Compared with the other court surfaces, there was a higher prevalence of lower limb overuse injuries when playing on hard court.
Conclusion There is no significant difference in the overall prevalence of injury on clay, hard court, sand-fill artificial grass and red-sand-fill artificial grass.
- Tennis
- Injury prevention
- Epidemiology
- Overuse injury
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Introduction
Tennis is a global sport played on a variety of court surfaces. This is evident at the highest level, where the grand slam tournaments are played on three very different surfaces: the Australian Open and US Open on hard court, the French Open (Roland Garros) on clay and the Championships, Wimbledon on grass. The relative distribution of the different court surfaces differs greatly around the world, for example, in Germany 95% of all tennis court surfaces are clay, whereas in Great Britain and Australia most court surfaces are made of hard court and < 1% are clay or grass.1
There are a number of commonly held beliefs among players and coaches about differing injury risks on the various court surfaces. For example, it is thought that the injury rate is higher on courts with low shock absorption and high friction such as hard court, than on those with high shock absorption and low friction such as clay and grass. However, the current evidence to substantiate these beliefs is conflicting.2–5
The purpose of this study was to determine whether there is a difference in the risk of tennis injuries between the four most common court surfaces in the Netherlands, including hard court, clay, sand-fill artificial grass and red-sand-fill artificial grass.
Methods
Design and participants
This was a repeated cross-sectional study involving members of the Royal Netherlands Lawn Tennis Association (KNLTB) in the period May to October 2014. During this time, a monthly questionnaire was sent to a representative sample of male and female KNLTB members aged 16 years and older. A random sample of 20 000 KNLTB members, stratified by their club’s playing surface, was selected from the KNLTB database. The types of court surfaces were limited to clay, hard court, sand-fill artificial grass and red-sand-fill artificial grass. Sand-fill artificial grass has a longer fibre length and a higher fibre density than red-sand-fill artificial grass.6 In addition, red-sand-fill artificial grass has more sand, with the sand filled in just over the top of the fibres, which enables players to slide more easily. There are very few natural grass courts in the Netherlands, and this surface was therefore excluded from our study. All players’ skill levels were rated using the KNLTB ranking system which ranges from 9 (beginner) to 1 (highest level). This corresponds closely to the International Tennis Number.7
Monthly questionnaires
Invitations to complete an online questionnaire were sent each month by email using online survey software (QuestBack AS, Oslo). The first two questionnaires were sent to all selected players (n=20 000) and over the next 4 months, only to those players who had responded to the first and/or second questionnaires (n=3656). Non-respondents were sent a reminder email after 4 days.
The questionnaire included questions on court surface and tennis exposure (see online for supplementary Appendix 1). In order to record physical complaints, we used the Oslo Sports Trauma Research Centre (OSTRC) questionnaire on health problems,8 which has been translated into Dutch and used in previous KNLTB research.9 The questionnaire has four questions related to physical complaints experienced by the player, including: (1) whether a physical complaint has affected tennis participation in the previous week; (2) whether a physical complaint has affected training volume; (3) whether a physical complaint has affected tennis performance; and (4) the degree of pain experienced. If a physical complaint (hereafter: injury) was reported in any of the four questions, respondents were then asked to provide further information about the injury, including (1) the injury type (acute or overuse), (2) the injury location, (3) the number of days of absence from sport, (4) the need for medical care and (5) the court surface on which the injury arose (for acute injuries). The first questionnaire also contained questions about the number of years of tennis experience, and injuries during the past 12 months. Participants’ age, sex and skill level were taken from the KNLTB membership database.
Supplementary appendix
Definitions
An injury was defined as any physical complaint sustained by a player that resulted from a tennis match or tennis training, irrespective of the need for medical attention or time loss from tennis activities.10 An acute injury was defined as an injury that developed suddenly with a clearly identifiable cause (in seconds to minutes), and an overuse injury as an injury that developed gradually without a clear identifiable cause (in hours to days).10
Outcome measures
The primary outcome measures were the number of tennis-related injuries sustained, the incidence of acute injury per 1000 hours of tennis training and match play, and the average prevalence of overuse injury. Each month, prevalence measures were calculated for each surface by dividing the number of players who reported an overuse injury by the total number of questionnaire respondents. All measures were calculated separately for each of the four court surfaces. However, players who reported having played < 80% of the time on the same court surface in the previous month were classified separately.
Statistical analyses
Means and SDs were calculated for demographic data. Possible differences in demographic variables between men and women, and between different court surfaces were tested using t-tests for continuous variables and χ2 tests for dichotomous variables.
Differences in the prevalence of overuse injury on the various court surfaces were assessed by comparing the mean prevalence and the 95% CI.
Differences in the incidence of acute injury on the various court surfaces were assessed using survival analyses (Cox regression), using clay as the reference surface. We adjusted this analysis a priori for previous injuries in the past 12 months. Other possible confounding demographic variables (age, gender, skill level, playing experience) were evaluated for each individual analysis, but none were identified.
Results
Participants
A total of 3656 (18%) of the 20 000 invited members completed at least one of the monthly questionnaires. All six questionnaires were completed by 902 (25%) respondents, 581 (16%) respondents completed five questionnaires, 417 (11%) four, 298 (8%) three, 503 (14%) two and 955 (26%) completed only one. Due to incomplete questionnaires, 3320 questionnaires were included in our analyses.
The average respondent age was 49.3 years (SD 14.8), with men slightly older than women (50.1 vs 47.2, p<0.05). Fifty-six per cent of male and 49% of female respondents reported an injury in the previous 12 months (p<0.05). There were no statistically significant differences in skill level or playing experience between male (n=1679) and female respondents (n=1641), or between the respondents on the various court surfaces. Respondent demographics for each court surface are shown in table 1. A majority of respondents had played tennis for >10 years (55%), and there was no difference in playing experience between the respondents on the different court surfaces (figure 1).
Demographic data by court surface
Playing hours per court surface
A total of 117 280 hours of tennis were recorded during the 6-month study. Of these, 101 822 hours were recorded by players who played more than 80% of their tennis on one of the four types of court surfaces (table 1), and 15 458 hours were recorded by players who played on multiple court surfaces. The average number of hours played by each respondent was equivalent for each surface (7–8 hours per month), except for those who played on multiple court surfaces (12 hours per month—see table 1 and figure 1.
Distribution of skill level in singles per court surface.
Injuries by court surface
A total of 4047 injuries was reported by 1957 respondents (‘All complaints’—table 2). Of these injuries, 3246 were overuse and 801 were acute. Medical care was sought by the respondent for 1988 injuries (‘medical attention injuries’). A total of 273 injuries led to absence from sport (‘time-loss injuries’). There were no significant differences in injury prevalence between groups who played primarily on any one of the four court surfaces. However, respondents who played on multiple court surfaces had a significantly higher prevalence of injuries in all body regions than those who played primarily on one type of court surface (table 2).
Absolute numbers and average prevalence of all injuries, time-loss injuries and medical-attention injuries
A total of 3246 injuries (80%) was classified as overuse injuries (table 3). There were no statistically significant differences in the prevalence and severity of overuse injuries between the four different court surfaces. However, respondent players playing on multiple types of court surfaces had a significantly higher prevalence of overuse injuries than respondents who played primarily on one type of court surface, both overall and per body region (table 3).
Absolute number and the average monthly prevalence of overuse injuries (all complaints) in each body region
A total of 801 acute injuries was reported (20% of all injuries) (table 4), including 409 newly incurred and 392 recurrent injuries. The lowest incidence of acute injuries occurred among respondents who played on multiple court surfaces (6.0 injuries per 1000 playing hours; 95% CI 4.7 to 7.2). The highest incidence was found among respondents who played on sand-fill artificial grass (9.7 injuries per 1000 playing hours; 95% CI 8.2 to 11.2). Statistical analysis using Cox regression showed only lower risk of acute injury for playing on multiple court surfaces. No demographic characteristics were found to be confounding variables in this relationship.
Absolute numbers of acute injuries, average monthly severity scores and incidence (number of injuries per 1000 hours of tennis)
Discussion
This study shows that there were few differences in the rate of injury between the four different court surfaces included in this study: hard court, clay, sand-fill artificial grass and red-sand-fill artificial grass. However, players who played on multiple surfaces had a higher injury prevalence, particularly of overuse injuries, than those who primarily played on one court surface. We also found that, compared with the other court surfaces, there was a higher rate of lower limb overuse injuries when playing on hard court.
The overall injury rate among study participants was high, given that at any given point in time, one in three players reported a physical complaint. However, the rate of time loss injuries was low.
Our findings of few differences in injury rate between court surfaces are in accordance with an earlier Dutch study that compared the incidence of injuries on clay and sand-fill artificial grass.4 They found no significant difference in injury incidence between the two court surfaces, but this study was underpowered compared with the current study. Breznik and Batagelj5 investigated the percentage of retirements during tennis matches on different types of court surfaces in the period 1968–2010. The highest percentage of retirements was found on hard court (2.62%) and clay (2.78%) and the lowest percentage on grass (1.65%) and carpet (1.86%). This suggests a similar rate of injury on hard court and clay, which is in agreement with our findings. However, since we did not include natural grass and carpet due to a lack of availability of these court surfaces in the Dutch outdoor season, we cannot draw any conclusions about these two surfaces.
Our results are in contrast with those of Bastholt,3 who compared the number of medical treatments of players on the Association of Tennis Professionals (ATP) tour on tournaments on grass, hard court, clay and carpet. The risk of treatment in his study was lowest on clay and highest on grass and hard court. However, it is difficult to draw any conclusions based on this study as the number of participants and medical staff available at each tournament was not adjusted for.
Overuse injuries
We found a significantly higher rate of overuse injuries of the lower extremities on hard court than on the other three surfaces. Nigg and Denoth,2 who conducted one of the first studies on injuries and court surface, had results similar to ours. They performed a retrospective survey among 1003 tennis players and found a significantly lower frequency of pain and injuries of the lower extremities among players who played on clay and artificial grass compared with players who played on synthetic court, hard court and carpet.
This might be explained by the characteristics of the court surface, namely the friction coefficient and shock absorption. Hard court has a higher friction coefficient than clay, sand-fill artificial grass and red-sand fill artificial grass. On court surfaces with a low friction coefficient it is possible to slide, resulting in longer braking distance and lower peak load on the lower extremities.11 On court surfaces with a high coefficient of friction sliding is much more difficult, resulting in shorter braking distance and higher peak loads. In addition, clay, sand-fill artificial grass and red-sand-fill artificial grass have better shock absorption than hard court. Therefore, prevention strategies may include using a tennis shoe with good shock absorption (extra cushioning or padding) and an outsole with a low friction coefficient on hard court.
Multiple court surfaces
Players who played on multiple court surfaces had the highest rate of overuse injury in all body regions. This could either be due to the fact that they played more hours per week of tennis, or because regularly changing surfaces increases stress on the body, with players having insufficient time to adapt to the new surface (different ball bounce and ball speed, different sliding characteristics).12 This may be a group that particularly needs injury prevention focus, in particular load management.13
Methodological considerations
This study included a large representative sample of the Dutch recreational tennis population, and through stratified sampling, we have acquired a study sample equally distributed over the different playing surfaces of interest. As such, the conclusions of the current study apply to the largest population of tennis participants for whom tennis is an enjoyable healthy and social event. The findings may, however, not apply to competitive tennis players with a higher skill level who play (many) more hours of tennis, and who are therefore more exposed to the potential risks of the court surfaces. Also, the court surfaces of interest were limited to clay, hard court, sand-fill artificial grass and red-sand-fill artificial grass, and other surfaces such as grass and carpet were not included. This choice was based on the most common surfaces recreational players in the Netherlands play on, but limits the conclusions to those surfaces alone.
Although response rates were relatively low, this is what could be expected from an open invitation in such a large recreational sports population. Nonetheless, some selection bias cannot be ruled out while players with a current physical complaint may be more likely to respond to the questionnaire than someone who is symptom-free. However, the distribution of respondents over the court surfaces was equal, which enables comparison of the injuries by court surface.
Through the use of the OSTRC questionnaire on health problems,8 9 we have been able to gather information on all health complaints incurred by the respondents, not limited to time loss injuries alone. Naturally, it provides a more comprehensive overview of the health problems incurred by recreational tennis players. It should be borne in mind, nonetheless, that this registration method does not allow for determining and analysing acute injuries according to the standard method of incidence calculations. Incidence rates of acute injuries are based on all reported acute injuries. This figure may therefore contain recurrent injuries and in some cases, double reporting may have occurred in consecutive months. The acute injuries may therefore provide a slight overestimation of the actual risk of acute injury. This problem is the same for every surface and a comparison between surfaces is therefore still valid.
Conclusion
The most important outcome of this study is that there is no significant difference in the overall rate of injury on clay, hard court, sand-fill artificial grass and red-sand-fill artificial grass.
There was a higher rate of lower limb overuse injury among hard-court players, and the overall injury rate was highest among those who played on multiple court surfaces. These groups should be targeted with injury prevention strategies.
What are the findings?
The most important outcome of this study is that there is no significant difference in the overall rate of injury on clay, hard court, sand-fill artificial grass and red-sand-fill artificial grass.
There was a higher rate of lower limb overuse injury among hard-court players.
The overall injury rate was highest among those who played on multiple court surfaces.
How might it impact on clinical practice in the future?
This study may increase awareness of the appropriate footwear needed for different playing surfaces.
This study highlights the importance of proper load management to prevent overuse injuries in tennis.
Acknowledgments
The authors thank Ank Boullart for helping design the online questionnaires, selecting and recruiting study participants and collecting all the data.
Footnotes
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.