Secular Trends in the Performance of Children and Adolescents (1980–2000)
It is widely believed that the performance of children and adolescents on aerobic fitness tests is declining. To test this hypothesis, this meta-analysis compared the results of 55 reports of the performance of children and adolescents aged 6–19 years who have used the 20m shuttle run test (20mSRT). All data were collected in the period 1981–2000.
Following corrections for methodological variation, the results of all studies were expressed using the common metric of running speed (km/h) at the last completed stage. Raw data were combined with pseudodata generated from reported means and standard deviations using Monte Carlo simulation. Where data were available on children and adolescents from the same country of the same age and sex, but tested at different times, linear regression was used to calculate rates of change. This was possible for 11 (mainly developed) countries, representing a total of 129 882 children and adolescents in 151 age × sex × country slices.
There has been a significant decline in performance in the 11 countries where data were available, and in most age × sex groups, with a sample-weighted mean decline of 0.43% of mean values per year. The decline was most marked in older age groups and the rate of decline was similar for boys and girls.
There has been a very rapid secular decline in the 20mSRT performance of children and adolescents over the last 20 years, at least in developed countries. The rate of decline is not related to the change in the country’s relative wealth, as quantified by per capita gross domestic product (GDP).
It is widely believed that the aerobic test performance of children and adolescents has declined over the last few decades. Sedentary technologies, the easy availability of energy-rich food, and declines in community-based physical activity have been implicated. However, despite a great deal of anecdotal and lay speculation, there have been very few studies of secular trends in the performance of children and adolescents on aerobic fitness tests. This is partly due to the wide variety of tests used (and methodological differences even when the same test has been used), and partly due to the lack of conduits and incentives for data sharing.
Since its description in 1984 by Léger et al., the 20m shuttle run test (20mSRT) with 1-minute stages has been widely used to assess the aerobic fitness of children and adults. This test consists of a number of ‘stages’ (also called ‘levels’), each lasting about 1 minute and comprising a number of 20m ‘laps’ (also called ‘shuttles’), paced by beeps on a cassette or compact disk (CD). At each stage, the required running speed increases. Each stage includes seven or more laps, depending on the required running speed and exact protocol used. The test has been shown to be a reliable and valid field test to estimate maximal oxygen uptake (V̇O2max).[4, 5, 6] The 20mSRT has probably been the most widely used test to assess the aerobic fitness of children and adolescents.
Léger et al.’s, original 1-minute protocol, which starts at a speed of 8.5 km/h, and increases in speed by 0.5 km/h each minute.
The protocol used by the Eurofit, the Australian Coaching Council, the British National Coaching Foundation, and the American Progressive Aerobic Cardiovascular Endurance Run (PACER) system, among others. In this protocol, participants start at a speed of 8.0 km/h, the second stage is at 9.0 km/h, and thereafter increases in speed by 0.5 km/h each minute.
The Queen’s University of Belfast protocol, which starts at 8.0 km/h, and increases in speed by 0.5 km/h each minute.
In addition to variation in protocols, there has been variation in how results have been reported. Individual results have been reported as the number of completed stages, the running speed at the last completed stage, the number of completed laps (or stages plus laps), the number of minutes the test lasted, or as an estimated maximum oxygen uptake (V̇O2max) based on regression equations.[4,13]
There have also been inconsistencies when group results have been presented. Consider the case where, using the Léger et al. protocol (protocol 1 in table I), individual A has completed 5 stages and 4 laps, a total of 44 laps, and individual B has completed 6 stages and 7 laps, a total of 56 laps. If we express our results as the average number of completed laps, the average will be 50. If we express our results as the number of stages completed (5 for A and 6 for B), we will report an average of 5.5 stages, which may be translated as 45 laps.
These methodological issues complicate attempts to meta-analyse or cumulate the results of the many 20mSRT studies which have been conducted, often on very large samples of children and adolescents. In addition, in many cases, these studies have been commissioned by government agencies or sports organisations, and the results have not been published or even analysed. This paper attempts to collate all available studies which have used the 20mSRT with children and adolescents, and expresses the results using the common metric of running speed at the last completed stage. Our intention was to chart the evolution of performance where tests have been conducted on children and adolescents of the same age and sex in the same country across different years. Data on secular trends in performance may complement the surprisingly few published studies[14, 15, 16] and many anecdotal reports suggesting precipitous declines in the performance of children and adolescents. Comparison of secular trends between countries, sexes and age-groups may also help to elucidate mechanisms and correlates of changes in test performance.
1.1 Data Sources
Studies were located by searching online databases (Sports Discus, Medline, AustROM, CINAHL, Digital Dissertations, Current Contents) using the following keywords: shuttle, shuttle run, MSFT, 20MST, 20mSRT, beep test, multi-stage. These keywords were use in combination with the following modifiers: child, children, pre-adolescent, adolescent, adolescence, pubescent, pubescence boy, girl, young, youth and infant. When published reports were obtained, all relevant references contained in the studies were followed up. Finally, attempts were made to personally contact the authors of each report to ask if they knew of further studies, and to clarify details of their own study.
the authors reported their results in terms of V̇O2max only, which could not be transformed to the common metric we adopted, namely running speed at the last completed stage (see section 1.2);
results were reported for large, undifferentiated age ranges (e.g. 12–17 years) or combined boys and girls into a single group;
the test protocol used was unknown;
more complete data were provided by the study authors;
the study reported data, or subsets of the data, which had been reported in other located studies;
permission to use unpublished data was withheld.
Age was recorded as the estimated mean age of all children and adolescents in the age × sex slice for each study. In most studies, age was reported as age at last birthday, so we assumed the mean age was 0.5 years after the last completed year (e.g. unless otherwise reported, if a study reported testing 12 year olds, we assumed the mean age was 12.5 years). Three studies covered a range of ages, with a maximum span of 3 years. In these cases, the midpoint of the age range was taken as the sample age. For example, for a study which reported measuring 10–11-year-old children (i.e. anywhere between just 10 and almost 12 years), the sample age was taken as 11.0 years.
1.2 Data Treatment
To compare studies, it was important to express the results using a common metric, to know which protocol was used, and to standardise the method of analysis. We chose running speed (km/h) at the last completed stage as the common metric across studies. While this allowed us to cumulate studies, it incurred a small cost in terms of comparability. This is because a running speed at the last completed stage of say 9.0 km/h translates into different test histories according to the protocol used. In protocol 1, the participant who finishes at 9.0 km/h will have run one previous minute at 8.5 km/h. In protocol 2, they will have run one previous minute at 8.0 km/h. In protocol 3, they will have run one previous minute at 8.0 km/h, and one minute at 8.5 km/h. It also means that the minimum possible speed will vary between protocols — 8.5 km/h with protocol 1, and 8.0 km/h with protocols 2 and 3. However, because very few participants — particularly among the older age groups — complete only one or two stages, the differences in the early stages would serve largely as different types of warm-up, and the effect would have ‘washed out’ by later stages.
Where the protocol used was uncertain (i.e. when the protocol described did not match that of the reference cited, when the same author reported using different protocols in different studies, or when the protocol was not mentioned) we contacted the study authors for clarification, and also obtained a copy of the actual cassette or CD used in the study. It is important to understand that testers are usually instructed to write down the last stage (or last minute) value they hear called on the cassette. Therefore the way they report performances will differ according to whether the stage number is called at the beginning or end of the stage (that is, whether ‘Stage 1’ is called at the beginning or the end of the first stage), and whether elapsed time is called on the full minute, or each half minute. There are a number of commercially available cassettes, and many in-house versions. Having the actual cassette used allowed us to correct for these methodological differences, and also to be sure of the protocol used. Personal communication with the study authors also allowed us to check suspected typographical errors, obtain more detailed information (e.g. finer resolution within age groupings), clarify analytical methods, and verify which cassette was used. Several researchers also provided us with the original raw data.
Where raw data were available, the running speed at the last completed stage for each participant was calculated according to the protocol used. If the study reported results as average running speed at the last completed stage, we first determined whether the cassette used called ‘Stage 1’ at the beginning of the cassette, or at the end of the first stage. If ‘Stage 1’ was called at the end of the first stage, we retained the reported value. If ‘Stage 1’ was called at the beginning of the first stage, we subtracted 0.5 km/h (the speed increment from stage to stage) from the reported mean speed.
If the study reported results as the average number of stages completed (or as average minutes), we first clarified whether the cassette called the stage number at the start or end of the stage, and whether the cassette they used reported half minutes as well as full minutes. These factors will affect the conversion of completed stages or minutes to running speeds. Testers are asked to record the last stage (or minute) value called on the cassette. Consider a participant who completes 6 stages and 8 laps. If the cassette uses only full stages called at the end of each stage, this performance will be reported as 6 stages. If the cassette also uses half stages, the performance will be recorded as 6.5 stages. If the cassette uses full stages called at the start of each stage, the performance will be reported as 7 stages. Therefore, if the testers used cassettes with stages called at the start of each stage, we subtracted 1 from the reported stage value. On average, testers using cassettes with half-minute calls will record 0.25 stages longer than those using full stages, so in this case we subtracted 0.25 from the reported stage value. In all cases where cassettes with half-minute calls were used, we verified procedures with the authors or associates. Having calculated the adjusted number of stages (or minutes), we converted this to a running speed according to the protocol used, as shown in table I. For example, a reported average of 5.5 stages would equate to 10.75 km/h for protocols 1 and 2, and 10.25 km/h for protocol 3. Finally, stages are not always precisely 1-minute long. Some studies called elapsed time on the minute, while others called stages at the end of stages (which might differ by a second or two from the exact minute).1
1.3 Statistical Analysis
It was necessary to combine data from different sources to calculate rates of change. To combine data sets, we used Monte Carlo simulation to generate pseudodata. This technique attempts to ‘recreate’ the unavailable raw data by using a random normal generator to produce datapoints based on reported means and standard deviations (SDs). It assumes that distributions are approximately normal, which was true of the raw data sets that were available.
One study, which constituted 0.7% of all datapoints, reported median rather than mean values. In this case, we substituted the median values for means. To check the validity of this procedure, we compared the means and medians for all those studies where both were reported. The mean-median difference was on average 0.3% of the mean value.
Two studies did not report SDs, and in another the reported SDs were improbably large. In these cases, constituting 1.3% of all datapoints, we estimated SDs based on the sample-weighted coefficients of variation (CVs) from all other studies. CVs varied with mean running speed and with sex, so separate regressions were calculated relating CV to mean running speed for both sexes. These were used to estimate the CV, and hence the SD, for those studies where values were missing. The mean coefficient of variation was 8.5 ± 2.1%.
Raw data were available from seven studies, comprising 21 817 or 17% of all datapoints. For those studies where raw data were not available, pseudodata were repeatedly generated until the calculated mean differed from the reported mean by less than 0.5%, and the calculated SD differed from the reported SD by less than 2.5%. Pseudodata were then merged with raw data where necessary before analysing the combined data set.
Where two or more data sets were available from the same country for children and adolescents of the same age and sex but measured at different times, linear regression (with year of test as the predictor variable, and running speed at the last completed stage as the response variable) were used to determine the rate of change of performance. This was then expressed as a percentage of the mean value for all datapoints in the regression. For each of these 11 countries, an overall rate of decline for boys and girls was calculated using the sample-weighted mean percentage change across all age × sex slices.
An unpaired t-test was used to compare rates of change in boys and girls. A one-group t-test was used to determine whether the mean of calculated rates of change differed from zero.
Changes in performance were calculated in 11 countries, of which ten (Australia, Belgium, Canada, France, Greece, Italy, the Netherlands, Northern Ireland, Spain and the US) were developed economies, and one (Poland) a transitional former socialist economy. In these countries, changes were calculated in a total of 151 age × sex × country slices, covering an average span of 7.5 years (range 1–14.5 years). Each calculation involved as few as two to as many as 12 separate studies. Of the 151 changes, 106 were negative (declines in performance), and 45 positive (improvements in performance). The sample-weighted mean change was −0.046 km/h per year, or −0.43% of the mean running speed per year. This was significantly different from zero (p ≤0.0001). The 95% confidence interval for the overall sample-weighted mean was −0.46 to −0.40%. Considering only the statistically significant changes (p ≪ 0.05; 96 of the 151 calculated changes), the sample-weighted mean was −0.062 km/h per year, or −0.57% per year.
3.1 Methodological Issues
Ideally, a study of this sort would use large raw data sets from random samples taken in different countries over regular time intervals. This was clearly not possible in the present case. The studies used a variety of sampling procedures. Some used large stratified random samples on a regional or national basis, while others used convenience samples, occasionally from a single school. Others used proportional sampling based on education systems or geographical areas. In many studies, there was inevitably a degree of self-selection. The participants may have come from different ethnic groups, and from groups differing in their exposure to physical activity. These different sampling procedures obviously raise issues of representativeness — a problem common to almost all cumulation studies. Some of these factors may produce unrepresentatively high values, others low values. It is likely that the sheer number of datapoints in this meta-analysis will dampen irregularities arising from sampling inconsistencies. In any case, the present study offers the most comprehensive picture to date of recent secular trends in the performance of children and adolescents on aerobic tests in the developed world.
The pseudodata method will produce samples similar to the original data sets, providing the original data sets are normally distributed. If the original data sets were skewed, the reported means will overestimate (positive skew) or underestimate (negative skew) the actual medians. However, unless there are systematic changes in skewness over time, the trends in performance scores will not be biased. The few raw data from the studies analysed here where comparisons could be made did not show any consistent change in skewness. Increasingly positive skews in 1600m run times in 10–12-year-old Australian children between 1985 and 1997 have been found, and such changes in skewness could produce artefactual declines in performance using the methods we have employed, or indeed whenever means are compared. However, when the Dollman et al. 1600m values were expressed as speeds rather than times, the distributions were much closer to normal, and there was very little distributional shift between 1985 and 1997. It is hard to quantify how much potential shifts in skewness would affect the results in this study. In the Dollman et al. study, the decline in performance calculated using mean values was at most 10% greater than when it was calculated using the medians.
A review of 13 studies[4, 5, 6,11,13,49,52,53,65, 66, 67, 68, 69] of the validity of the 20mSRT in normal, untrained children (relative to peak V̇O2) showed a sample-weighted average coefficient of determination (r2) of 0.5. This shows that a large part of the variability in 20mSRT performance can be explained by the variability in peak V̇O2. In addition, the coefficient of determination varied with age, with r2 values of 0.2 and 0.7 in 7- and 17-year-old children, respectively. However, it is important to remember that factors other than peak V̇O2 also contribute to 20mSRT performance, particularly in younger children where motor skills and cognitive ability are likely to play a major role. Running efficiency, anaerobic capacity, motivation and social dynamics are all likely to be important. Validation studies of the 20mSRT typically involve children and adolescents across a range of ages tested cross-sectionally. Factors contributing to longitudinal variability within the same age group may be quite different. However, we really have no evidence of systematic changes over time in these other factors. It is the changes in performance, rather than underlying mechanisms, which are the primary focus in this paper.
It is known that many other factors can affect 20mSRT performance. These include environmental differences, clothing and running surfaces, test familiarisation and instructions, and the purpose and context of testing. These were not always reported in the studies analysed here. However, we have no evidence of systematic changes in these factors over time, so while they certainly affect variability in test results, they are unlikely to affect the overall trends.
3.2 Comparisons with Other Studies
There have been few studies which have directly and systematically addressed the issue of secular declines in aerobic test performance, although it is widely believed that children and adolescents are becoming fatter and less fit. Using data from published studies, government reports and personal communications (see next paragraph), we have been able to compare aerobic fitness tests covering a 20-year span from 1980–2000 using tests other than the 20mSRT. They have been conducted in seven developed countries on children and adolescents aged 7–19 years, and have involved over one million study participants. They present a very consistent picture: all have found declines in aerobic performance across all age × sex slices, averaging 0.2–1.1% per year.
Dollman et al. compared a national survey of the performance of Australian children in 1985 to matched measurements taken on 1463 10–11-year-old South Australian children in 1997. Over the 12-year period, performance on the 1600m run test declined by 0.5–0.8% per year. A similar study of 2450 7–10-year-old Tasmanian children found a decline in 1600m run performance between 1985 and 1995 of 0.02–0.4% per year. A recent New Zealand study examined changes in 550m run performance in 5579 children from a single intermediate school (ages 10–14 years) over a 9-year period from 1991–2000. The average decline in performance was 0.2% per year for both boys and girls.
Analysis of data from mass testing programs run by the Japanese Ministry of Education, Science and Culture shows that distance run performance (1000 and 1500m) decreased by 0.4% per year (range 0.3–0.6% per year) in 12–17-year-old Japanese adolescents between 1985 and 1998. In South Korea, tests on over 260 000 10–17-year-old children and adolescents conducted from 1983–1985 and again in 1998 show an average decline of 1.1% (range 0.8–1.8%) per year in 600–1000m run performance. A second South Korean survey using the 1200m run test, found a decline of 1.1% per year (range 0.8–1.4%) in 11 636 12–16-year-old adolescents tested between 1988 and 1998.
Using one-quarter to one mile (402–1609m) runs, Updyke and Willett reported an average decline of 1.1% (range 0.7–1.9%) per year in the aerobic performance of 6–17-year-old American adolescents between 1980 and 1989. In Italy, 11–14-year-old children were tested in 1981 and again between 1995 and 2000 (Buonaccorsi, personal communication) using a 1200m run test. A total of 2499 children were tested. On average, performance deteriorated at the rate of 0.9% per year (range 0.4–1.5%). Distance run tests conducted in Poland in 1989 were repeated in 1999. A total of 274 014 children and adolescents aged from 7–19 years were tested using 600–1000m runs. The average annual decline in performance was 0.7% (range 0.3–1.1%). These results are consonant with those found in the present study. The rate of decline of performance has, in biological terms, been very rapid.
3.3 What is Causing the Observed Decline in Running Performance?
Performance fitness in running can be reduced by lower aerobic fitness, or increased fatness, or both. Children and adolescents are certainly getting fatter. Cumulated results from 28 reports of changes in body mass index (BMI) in 5–16-year-old children and adolescents since 1980 from the US and Australia show that BMI has increased at a median rate of 0.6% per year — comparable to the rate of decline in aerobic performance found in the present study. There have been similar trends in skinfold thicknesses and body mass. Increased fatness may be the result of increases in energy intake, decreases in energy expenditure, or both. Decreases in energy expenditure are also likely to be associated with reduced vigorous activity, and hence a lower level of aerobic conditioning.
energy intake appears to be relatively stable;
longitudinal studies suggest that activity levels of children and adolescents are declining;
there appears to have been an increase in inactivity (e.g. television viewing).
Data on secular trends in energy intake are sparse and inconsistent, largely due to sampling and methodological variation. Few data are available on specific trends in the energy intake of children and adolescents, and fewer still on changes in the distribution of intakes, and on changes in saturated fat consumption as part of total intake. However, some ‘snapshots’ are available. The mean energy intake of 14–15-year-old British children declined by about 20–30% between the 1930s and the 1980s, while there was a 20% reduction in the average intake of UK residents between 1970 and 1990. Based on food balance equations, the Food and Agriculture Organisation reported that per capita energy availability in developed countries did not increase between 1980 and 1998. Other studies, however, have shown opposite trends. Using apparent consumption data, Harnack et al., for example, estimated that in the US, per capita food availability had increased by 15% between 1974 and 1990.
Not all studies have found a strong link between fitness and physical activity in childhood.[84,85] However, inter-subject variability in fitness is associated with many factors aside from physical activity, including nutritional status and genetic constitution. Furthermore, the quantification of physical activity is inevitably limited in time and perhaps type, and fitness changes may be the result of long-term physical activity patterns. There are also few reliable data on secular trends in physical activity patterns in children. One area where quantifiable data are available is in children’s use of transport. Between 1975–1976 and 1989–1994, the percentage of 5–10-year-old children in Britain walking to school fell from 71– 62%, while the percentage travelling by car rose from 15–28%. Between 1985 and 1993, the average yearly distance walked by all British children aged under 15 years fell from 395– 317km, and the average yearly distance ridden on bicycles fell from 61–45km. In the US, Department of Transportation data show that between 1977 and 1995 there was a 37% decline in the number of trips made by children on foot or by bicycle. Comparison of data from surveys in 1985 and 1997 showed a decline in the number of organised sports 10–11-year-old Australian children reported playing (from a median of 2 to a median of 1 for boys, and from 1–0 for girls) [Hill AM and Olds TS, unpublished observations]. In the US, enrolment in high school physical education has fallen from 42% in 1991 to 27% in 1997. Heath et al. estimated that between 1984 and 1990, the percentage of US high school students participating in 20 minutes or more of vigorous physical activity three or more times a week declined from 62% to 37%. Given the consistency of declines in the performance and physical activity in children, it appears that both school-based physical education and government intervention strategies are failing, highlighting the pressing need for new approaches.
There is stronger evidence in relation to increasing inactivity both in children and in adults. Some studies on the association between television watching and levels of obesity have found that as the hours of television watched increases, physical activity levels decrease and obesity increases, both in adults and children. However, other studies have been unable to find such links. With increasing television ownership, viewing time has increased: television viewing has doubled in Britain compared with the 1960s. In the US, large use-of-time surveys have been conducted amongst American adults every 10 years since 1965. Reported time spent televiewing among adults has increased from 1.5 hours per day in 1965, to 2.1 hours per day in 1975 and 1985, and 2.3 hours per day in 1995.
Both increases in energy intake and decreases in energy expenditure have been characterised as products of increasing affluence. Reduced active commuting, increased access to sedentary and labour-saving technologies, a trend towards mediatisation and vicarious experience of vigorous activity, and the disintegration of community-based organised sport with increased household and job mobility have all been implicated. Given this, then beyond a certain threshold — associated with freedom from hunger and disease — we would expect to see aerobic performance declining with increasing affluence. To test this hypothesis, we obtained historical data on national gross domestic product (GDP) for the 11 countries where changes in performance levels were calculated. The relationship between the per annum percentage change in GDP (in $US) over the measurement period, and the per annum percentage change in 20mSRT speed was not significant. There was a weak positive relationship (p = 0.01) between the absolute annual rate of change of GDP (in $US) and changes in performance — higher rates of increase of GDP in absolute terms were associated with lower rates of performance decline. It should be noted, however, that there were fairly consistent increases in GDP in all the countries in this study in the period from 1980–2000 (2.5 ± 1.1% per annum), so the resolution is quite low.
The decline in performance was not different between boys (sample-weighted mean change = 0.46% per year) and girls (0.41% per year). Nor was the rate of change related to the mid-year of the measurement period over which the change was calculated, suggesting that the secular decline has been relatively constant over the last 20 years. However, there were differences in the rate of decline across age groups. Declines were quite consistent in children, whereas declines in adolescents became increasingly larger. It is hard to know how to interpret this pattern. It may signal the cumulative effect of exposure to environments that are ‘toxic for exercise’, or the presence of environmental factors which have a greater effect on older rather than younger children, or more optimistically — but less probably given evidence of the constancy of the trend — activity-reducing factors which were stronger in the past than they are now.
Methodological drift has led to the results of 20mSRTs being largely incommensurable. A single test protocol should be used (Léger et al.’s protocol 1), or at least the protocol used should be accurately reported.
More care should be taken in the standardisation and reporting of factors such as environmental conditions, running surfaces, clothing, pre-test instructions, and test familiarisation. Studies should be conducted to assess the effect of these factors on performance.
A standard multilingual test package should be made available, including a CD, cassette, instruction booklet, reporting forms, and data summaries.
Results should ideally be expressed as running speed at the last completed one-minute stage, and certainly not as estimated V̇O2max values only.
When summary statistics are provided, they should be broken down into 1-year age and sex slices based on age at last birthday.
The year of measurement should be reported.
Both mean and median values should be reported.
Researchers should be encouraged to make their raw data available, and an Internet-based data repository should be established.
Finally, mixed longitudinal studies with standardised sampling procedures should be initiated in as many countries as possible.
It should be noted that a reported value of, for example, 3.2 stages (using protocol 1) may mean either 3 stages and 2 laps (i.e. 3 stages + 2/8 of a stage = 3.25 stages) or 3.2 stages. This was checked with the study authors where it was unclear.
The authors would like to thank the following people for allowing them access to their data: Scott Baker and the South Australian Sports Institute, Georges Baquet, Serge Berthoin, Alberto Buonaccorsi, Sara Mulkearns and Jeff Walkley and the Australian Council for Health, Physical Education and Recreation. Thank you also to the following people for their kindness in providing extra information about their studies: Natalie Balagué, Mario Bellucci, Michael Booth, Colin Boreham, Jan Borms, Valerie Burke, Dean Cooley, William Duquet, Juan García, Giorgos Georgiadis, Beth Hands, Deborah Hoare, Johan Lefèvre, Matt Mahar, Craig Mahoney, Denis Massicotte, Lars McNaughton, Tony Okely, Ryszard Przeweda, Chris Riddoch, Javier Rivas, Willem van Mechelen and Emmanuel Van Praagh. No sources of funding were used to assist in the preparation of this manuscript. The authors have no conflicts of interest that are directly relevant to the content of this manuscript.
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