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Prevention of acute knee injuries in adolescent female
football players: cluster randomised controlled trial
OPEN ACCESS
Markus Waldén senior researcher 1 2, Isam Atroshi associate professor 2 3, Henrik Magnusson
statistician 4, Philippe Wagner statistician 5, Martin Hägglund senior researcher 4
1Division of Community Medicine, Department of Medical and Health Sciences, Linköping University, 581 83 Linköping, Sweden; 2Department of
Orthopaedics, Hässleholm-Kristianstad-Ystad Hospitals, 281 25 Hässleholm, Sweden; 3Department of Clinical Sciences, Lund University, 221 84
Lund, Sweden; 4Division of Physiotherapy, Department of Medical and Health Sciences, Linköping University; 5National Competence Centre for
Musculoskeletal Disorders, Lund University
Abstract
Objective To evaluate the effectiveness of neuromuscular training in
reducing the rate of acute knee injury in adolescent female football
players.
Design Stratified cluster randomised controlled trial with clubs as the
unit of randomisation.
Setting 230 Swedish football clubs (121 in the intervention group, 109
in the control group) were followed for one season (2009, seven months).
Participants 4564 players aged 12-17 years (2479 in the intervention
group, 2085 in the control group) completed the study.
Intervention 15 minute neuromuscular warm-up programme (targeting
core stability, balance, and proper knee alignment) to be carried out
twice a week throughout the season.
Main outcome measures The primary outcome was rate of anterior
cruciate ligament injury; secondary outcomes were rates of severe knee
injury (>4 weeks’ absence) and any acute knee injury.
Results Seven players (0.28%) in the intervention group, and 14 (0.67%)
in the control group had an anterior cruciate ligament injury. By Cox
regression analysis according to intention to treat, a 64% reduction in
the rate of anterior cruciate ligament injury was seen in the intervention
group (rate ratio 0.36, 95% confidence interval 0.15 to 0.85). The
absolute rate difference was −0.07 (95% confidence interval −0.13 to
0.001) per 1000 playing hours in favour of the intervention group. No
significant rate reductions were seen for secondary outcomes.
Conclusions A neuromuscular warm-up programme significantly reduced
the rate of anterior cruciate ligament injury in adolescent female football
players. However, the absolute rate difference did not reach statistical
significance, possibly owing to the small number of events.
Trial registration Clinical trials NCT00894595.
Introduction
Football (soccer) is the most popular sport worldwide with
around 265 million players, and the number of female players
is growing rapidly.1For example, more than three million young
people play football in the United States alone, and almost half
of them are female2; and in 2006 around 83 000 female football
players from the age of 12 years were registered in Sweden
(population around 9 million).3Unfortunately, knee injuries are
common, and severe injuries such as anterior cruciate ligament
injury constitute a serious problem in terms of long absence
from or termination of football and early osteoarthritis.4-8
In football, the rate of anterior cruciate ligament injury is more
than twofold higher in female players than in male players,9 10
and adolescent girls are especially at risk.10 11 Several parallel
group controlled studies, most targeting adolescents, have
evaluated injury prevention strategies in female football
players.12-21 Only two non-randomised studies in adolescent
footballers showed significant reduction of acute knee injuries
or non-contact anterior cruciate ligament injuries with
neuromuscular warm-up programmes.16 17 However, most studies
were not designed specifically to evaluate preventive effects on
knee or anterior cruciate ligament injury as the primary outcome,
and high quality randomised controlled trials targeting
prevention of acute knee injuries or anterior cruciate ligament
injuries are lacking.22
The main objective of our stratified cluster randomised
controlled trial was to evaluate the effectiveness of a
neuromuscular warm-up programme in reducing the rate of
acute knee injury, particularly anterior cruciate ligament injury,
in adolescent female football players. Our hypothesis was that
the rates of anterior cruciate ligament injury and acute knee
injury would be significantly reduced in the intervention group
compared with the control group.
Correspondence to: M Waldén markus.walden@telia.com
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Research
RESEARCH
Methods
Study design and participants
The study was a two armed, parallel group, cluster randomised
controlled trial. In accordance with most previous studies,12 13 15-21
we chose a cluster design as the intervention was implemented
in a team setting and to minimise contamination between the
intervention and control groups. The study protocol has been
published,22 and the data collection procedures and definitions
used follow international guidelines for surveillance of football
injuries.23 24
We recruited clubs from the female under 14 to under 18 series
(age 12-17 years) in eight regional districts of the Swedish
Football Association (Blekinge, Bohuslän, Skåne, Småland,
Södermanland, Västmanland, Örebro, and Östergötland), all
located in the middle and southern parts of Sweden for logistical
reasons. After obtaining enrolment registries for the 2009
campaign, two authors (MW and MH) contacted eligible coaches
between December 2008 and February 2009. We excluded clubs
that did not respond to the invitation or declined participation,
those that had fewer than two training sessions a week, and
those already using an injury prevention programme.
We randomised all clubs that agreed to participate into an
intervention or control group. We used a computer generated
list of random numbers to randomise clubs stratified by district,
whereby all teams from the same club were assigned to the same
group. We chose stratification by district to account for
variations in league systems (for example, by competitive or
age level) between districts. One author (IA) who was blinded
to the identity of the clubs did the randomisation. The
randomisation procedure aimed to achieve a balanced number
of clubs in the intervention and control groups, considering that
the clubs had different number of teams. Randomisation was
revealed after recruitment of the final club to ensure concealment
of allocation.
Intervention
Our intervention was a neuromuscular warm-up programme
(Knäkontroll, SISU Idrottsböcker, Sweden, 2005) containing
exercises focusing on knee control and core stability similar to
other programmes in, for example, team handball at that time.25
The six exercises were a one legged knee squat, a pelvic lift, a
two legged knee squat, the bench, the lunge, and jump/landing
technique.22 Each exercise is subdivided into four steps of
progressive difficulty and a pair exercise (table 1⇓). The
exercises were preceded by 5 minutes of low intensity running
and took about 15 minutes to complete after familiarisation.
The intervention clubs were instructed to do the exercises during
the warm-up at two training sessions a week throughout the
whole season. All players started on the first level of difficulty
and proceeded to the next level when exercises were performed
with good control as assessed by the coach.
Study procedures and outcomes
We recruited 68 physiotherapists (study therapists) and eight
physicians (study physicians) to serve as medical support to the
clubs. The study therapists assisted coaches with data collection
and recorded injuries in the two groups. The study physicians
assisted the study therapists in diagnosing injuries. Two authors
(MW and MH) held instructional regional meetings for coaches
in both groups in each district, briefing them about the study
procedures. Thereafter, a study therapist gave practical
instructions about the exercises to one coach and a player from
each team in the intervention clubs. Each coach also received
a CD-ROM and a leaflet describing the programme. Coaches
who could not attend the meetings were given the same verbal
and written information by their study therapist before the start
of the trial, and coaches in the intervention group also received
the practical exercise instructions. During the practical
demonstration of the exercises, study therapists instructed the
coaches carefully on how to clear players to progress to the next
level of difficulty—that is, when all repetitions were performed
with good neuromuscular control, mainly focusing on core
stability and proper knee alignment. Control group coaches
were instructed to train and play as usual throughout the 2009
season without any changes.
We collected data during the competitive season (1 April to 31
October 2009). Injury surveillance included a baseline
questionnaire, players’ attendance, and injury registration. The
baseline questionnaire included name, social security number,
anthropometrics, menarche, previous acute knee injuries, current
knee complaints, and family history of anterior cruciate ligament
injury. The coach registered individual playing time (registered
as minutes of actual participation for each player) and absences
(due to acute knee injury or other reasons) for each training
session and match during the season on a computer based
player’s attendance form and emailed the data monthly to the
study therapist and authors (MW and MH). If a player had
additional playing time with a district or national team or another
team within the club, this was also registered on the attendance
form. Participation in other sports or leisure time activities was
not recorded. The coach reported acute knee injuries to the study
therapist, who evaluated the injury in person and documented
it on an injury report form. A recordable acute knee injury was
one that occurred during football training or match play, had a
sudden onset, and led to a player being unable to participate
fully in future training or match play (excluding contusions).22
A severe injury was one that caused absence of more than four
weeks. We regarded injuries as non-contact if they occurred
without contact with another player or object (excluding the
playing surface).22 We defined an anterior cruciate ligament
injury as a first or recurrent partial or total rupture of the
ligament either in isolation or associated with concomitant
injuries to the knee joint.5If an anterior cruciate ligament injury
was suspected or the diagnosis was unclear from the study
therapist’s examination, the player was referred to a study
physician for further evaluation. Players with possible anterior
cruciate ligament injury were offered examination with magnetic
resonance imaging. Use of magnetic resonance imaging was
not mandatory to confirm other knee injuries if anterior cruciate
ligament injury could be ruled out clinically. If any information
was missing or was unclear, we contacted the coach or player
for further clarification.
The coaches, players, and study therapists were not blinded to
group allocation, but the study physicians who assessed the
primary outcome were. Two authors (PW and IA) who were
not involved in the intervention or data collection did the
intention to treat analysis with group identity concealed.
Compliance
The coaches in the intervention group documented the execution
of the warm-up programme on the players’ attendance forms
for each training session. Study therapists made two
unannounced visits to each intervention team to monitor
compliance and execution of the programme. We instructed the
therapists to observe the beginning of the training session out
of sight of the on-field coach. After having observed the
execution of the warm-up programme, the therapists corrected
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RESEARCH
any training errors. Except for these two unannounced visits,
no other supervision of coaches was carried out.
Statistical methods
The primary outcome was the rate of anterior cruciate ligament
injury; secondary outcomes were the rates of severe knee injury
and any acute knee injury.22 We did the pre-trial sample size
calculation without accounting for clustering and based it on
an annual/seasonal incidence of anterior cruciate ligament injury
of 1.15% in female adolescent football players.22 With a power
of 80% and an estimated reduction of anterior cruciate ligament
injury by 50% in the intervention group, we aimed to recruit
8118 players (4059 in each group).
The unadjusted intention to treat analysis used Cox regression
with robust standard error estimation to estimate the rate ratio
between groups and corresponding 95% confidence interval, in
accordance with Lin and Wei.26 We used the robust standard
error estimate to account for the correlation of outcomes within
each club. We used Wald’s test to calculate P values. We used
the same method to do subgroup efficacy analyses for compliant
players (defined as having carried out at least one intervention
session a week on average), adjusting for the pre-specified
covariates age, menarche, previous acute knee injury, match
frequency, and match play with other teams. As a sensitivity
analysis, we also analysed the primary outcome by using a
shared frailty Cox regression model with club as the frailty
effect. We checked for possible deviation from the proportional
hazards assumption of the Cox regression model by using the
non-proportionality test on the basis of the Schoenfeldt residuals.
We calculated the absolute rate reduction with 95% confidence
interval for the primary outcome as a crude estimate of the rate
difference by using the Mantel-Haenszel method according to
Greenland and Rothman.27 This corresponds to a standardised
rate difference with weights that depend on the number of player
hours in the intervention and control groups of each stratum. In
contrast to the Cox regression, this method makes the
simplifying assumption that the rate is constant over time in
each group. We calculated the number needed to treat for the
primary outcome as the inverted absolute rate reduction. We
estimated the intra-cluster correlation coefficient for the primary
outcome by using generalised estimating equations assuming
an exchangeable correlation structure, Poisson distribution, and
a log link function. We stratified all analyses by district because
of the stratification of the randomisation procedure. We did no
interim analyses. We used mean (standard deviation) to make
between group comparisons for anthropometric data and
exposures. We calculated time to medical attention as median
(interquartile range). All tests were two sided with a significance
level of <0.05. We used Stata version 11.0 for analyses.
Results
Study population
The target population consisted of 414 eligible clubs that we
approached for recruitment; 309 clubs (range 1-5 teams in each
club) accepted and were randomised into intervention and
control groups (figure⇓). Shortly after randomisation, 18 (6%)
clubs were dissolved before entering league play owing to a
shortage of players. Excluding these clubs, the dropout
frequency was 21% (intervention 16% (23/144 clubs), control
26% (38/147 clubs)). We obtained consent for 4564 players,
and 65 players declined participation. The players in the two
groups who completed the study were similar in terms of
baseline characteristics (table 2⇓).
Acute knee injuries
During 278 298 football hours (intervention 149 214 hours,
control 129 084 hours), 96 knee injuries (intervention 49, control
47) in 92 players (intervention 48, control 44) were recorded.
Twenty-one anterior cruciate ligament injuries occurred in 21
(0.46%) players, of whom 7 (0.28%) were in the intervention
group and 14 (0.67%) were in the control group (table 3⇓).
Fifty-seven severe knee injuries (22 collateral ligament or
capsular sprains, 21 anterior cruciate ligament injuries, 7 patella
dislocations or subluxations, 6 meniscus or chondral lesions,
and 1 tibia plateau fracture) occurred in 57 players (intervention
26, control 31). No injury occurred during the execution of the
warm-up programme.
The medical support supplied to the clubs evaluated 86 (90%)
of the 96 injuries. Eight injuries were treated at a local
emergency department without notification of the study therapist
at occurrence of the injury. Two injured players did not seek
medical attention at the time of injury, and the diagnoses were
based on study therapists’ examinations and players’ interviews
after return to play. The median elapsed time from injury to
examination by the study’s medical support was 4 (interquartile
range 2-8) days; for anterior cruciate ligament injuries it was 3
(2-6) days. Magnetic resonance imaging or arthroscopy was
done for 20 of the 21 anterior cruciate ligament injuries and for
36 of the 57 severe injuries.
Effectiveness of neuromuscular warm-up
programme
The unadjusted Cox regression according to intention to treat
showed a statistically significant 64% reduction in the rate of
anterior cruciate ligament injury in the intervention group
compared with the control group (rate ratio 0.36, 95%
confidence interval 0.15 to 0.85, P=0.02) (table 4⇓). The
sensitivity analysis, using a shared frailty Cox regression model,
yielded almost identical results (rate ratio 0.36, 0.14 to 0.91,
P=0.03). We detected no statistically significant deviation from
the proportionality assumption for the Cox regression model
(P=0.232). The estimated intra-cluster correlation coefficient
for the primary outcome was negative (−0.0026) and hence
given the value 0. The absolute rate difference for the primary
outcome was −0.07 (95% confidence interval −0.13 to 0.001)
per 1000 playing hours in favour of the intervention group, and
the corresponding number needed to treat was 14.
We found no significant rate reductions for secondary outcomes
(table 4⇓). The adjusted subgroup analyses of compliant players
(1303 players in 112 intervention group clubs, 1967 players in
106 control group clubs) showed a statistically significant 83%
rate reduction in anterior cruciate ligament injury (rate ratio
0.17, 0.05 to 0.57, P=0.004), as well as significant reductions
for secondary outcomes in the intervention group compared
with the control group (severe knee injury rate ratio 0.18, 0.07
to 0.45, P<0.001; any acute knee injury rate ratio 0.53, 0.30 to
0.94, P=0.03). Finally, exploratory analyses of non-contact
anterior cruciate ligament injuries showed a reduction in rates
favouring the intervention group. The reduction was statistically
significant only for the adjusted subgroup analysis of compliers
(intention to treat analysis rate ratio 0.40, 0.13 to 1.18, P=0.10;
adjusted subgroup analysis rate ratio 0.26, 0.07 to 0.99,
P=0.049).
Discussion
The principal finding in this cluster randomised controlled trial
was that a 15 minute neuromuscular warm-up programme
reduced the overall rate of anterior cruciate ligament injury in
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RESEARCH
adolescent female football players by 64%. Additionally, we
saw a preventive effect for severe knee injury and any acute
knee injury in players who completed the programme at least
once a week (compliers).
Prevention of knee injury
Of the previous controlled studies that evaluated neuromuscular
training programmes in adolescent female football players,12 14-20
only two non-randomised studies have shown preventive
effects—one on hospital treated acute knee injuries and the other
on non-contact anterior cruciate ligament injuries.16 17 However,
most previous studies were too small to show preventive effects
on knee injuries, and this, together with poor compliance by
players, might have been an important reason why they failed
to show an effect. We found a reduction in the overall rate of
anterior cruciate ligament injury by almost two thirds according
to a stratified Cox regression model, and the result was
replicated in a sensitivity analysis. The number needed to treat
was 14, meaning that for every 14 players who receive the
warm-up programme one less anterior cruciate ligament injury
would occur per 1000 playing hours. However, as the absolute
rate difference according to the Mantel-Haenszel method did
not reach statistical significance, the number needed to treat
should be interpreted with caution.28 We believe that the main
reason for the discrepancy between the statistically significant
rate ratio from the Cox regression model and the non-significant
absolute rate difference according to the Mantel-Haenszel
method is that this last method, besides the constant rate
assumption, can exhibit lower statistical precision.27 We chose
the Mantel-Haenszel method in spite of this drawback, as it is
recommended to avoid possible bias when the number of events
is low.27
In accordance with previously evaluated neuromuscular
programmes, the exercises evaluated in this study targeted leg
and core muscles, balance, landing technique, and proper knee
alignment. The rationale for including such exercises in
prevention programmes for anterior cruciate ligament injury is
that lateral trunk displacement and excessive dynamic knee
valgus have been found to be risk factors for this type of
injury.29 30 Because most anterior cruciate ligament injuries in
football players are believed to result from non-contact injury
mechanisms,5neuromuscular preventive programmes have
primarily targeted non-contact injuries. However, having proper
neuromuscular control might also help to protect the knee from
injury after a gentle push to the trunk or a fair shoulder to
shoulder tackle. We therefore chose to include all anterior
cruciate ligament injuries regardless of mechanism of injury as
the primary outcome in our trial. In an exploratory analysis of
non-contact anterior cruciate ligament injury, we found no
significant effect (P=0.10) even though the point estimate (rate
ratio 0.40) suggests a reduction of the same magnitude as for
the primary outcome. Interestingly, five of the anterior cruciate
ligament injuries occurred during football training, all of which
were non-contact in nature and occurred in the control group.
This suggests that non-contact anterior cruciate ligament injuries
during training may be preventable with the neuromuscular
training programme. The finding is consistent with a previous
study on female collegiate football players, in which the
preventive effect was seen only for training related anterior
cruciate ligament injuries.13
Strengths and limitations of study
To our knowledge, this study is the largest randomised
controlled trial of injury prevention in sports to date, involving
more than 4500 players. Some important strengths of the trial
include cluster randomisation of clubs to avoid contamination
between the intervention and control groups, individual playing
time registration with no missing data for analysed clubs, careful
monitoring of compliance, blinded analysts, and no involvement
of the authors in the development of the programme.
Additionally, the medical support supplied through the study
enabled quick and qualified examination of injuries by
experienced sports medicine practitioners, and the study
physicians who verified the primary outcome were blinded to
group allocation.
This trial also has some limitations. Firstly, the estimation of
the sample size was not based on a conventional calculation for
cluster randomised controlled trials, as valid data for rates of
anterior cruciate ligament injury based on playing time, and
corresponding intra-cluster correlation coefficients, in adolescent
female football players were lacking in the literature.22
Consequently, estimating the intra-cluster correlation coefficient
and the inflation factor a priori was difficult. Secondly, we did
not recruit the estimated sample according to our sample size
calculation, as the number of clubs that were excluded before
randomisation (105 clubs), that were dissolved shortly after
randomisation owing to shortage of players (18 clubs), or that
dropped out during the season (61 clubs) was higher than
expected. A lower than expected seasonal incidence of anterior
cruciate ligament injury (0.67% v1.15% according to pre-trial
sample size calculation) also contributed to a limited number
of anterior cruciate ligament injuries being recorded. However,
we believe that the number of players included achieved the
purpose of the study, even though we acknowledge that a larger
sample would have added further strength to our study.
Thirdly, a possibility of selection bias exists because clubs that
initially chose to participate (and intervention clubs that
continued through the whole study period) may have been more
motivated to follow the neuromuscular training regime than
clubs that did not respond to the invitation, declined
participation, or dropped out during the study. Although
providing a reason for leaving the study was not necessary
(according to research ethics), a plausible factor contributing
to the high dropout frequency might be unwillingness to
complete the individual playing time registration, as this puts
substantial strain on the recorders. In line with findings in other
recent trials on adolescent football players,12 19 20 the dropout
frequency was higher in the control group, possibly indicating
disappointment with group allocation. Fourthly, we have no
data on the number of players in the non-participating clubs, as
reliable statistics on the number of eligible players in each
district in the target age groups were lacking. However, on the
basis of the average squad size of the participating clubs, we
estimate that approximately 6500 players were randomised in
the trial.
Fifthly, the study therapists had several assignments, including
monitoring compliance and evaluating knee injuries, and they
were thus not blinded to group allocation. However, recruitment
of additional physiotherapists specifically to examine the knee
injuries was not possible. Sixthly, we set the cut-off threshold
for players’ compliance at performance of at least one
intervention session a week. This is arguably rather low, but it
was based on the fact that most clubs had two weekly training
sessions; and a recent Norwegian trial on adolescent female
football players reported average attendance of around 60%,
and the average player’s compliance in the intervention group
in that study was 0.8 sessions a week.19 All compliance data
were reported by coaches on the players’ attendance forms, but
we were unable to validate the coaches’ reports except for the
two unannounced visits by the study therapists. Furthermore,
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RESEARCH
the progression of the exercises was done under the supervision
of the coach without any guidance by the study therapists during
the season, and the extent to which the intervention clubs used
the programme with fidelity and progressed through the
programme as intended is uncertain. We chose this approach to
resemble a real world situation, where no such external
professional resources would be available to youth football
clubs.
Finally, we had no control over whether any of the control group
clubs performed exercises similar to our intervention programme
during the study period, despite coaches having stated at study
inclusion that they did not do any such exercises and the fact
that they were instructed to carry out their usual training as
normal during the season without any changes. However, any
possible bias introduced by such contamination of the control
group would lead to underestimation of the preventive effect
found in our study.
Implications and applicability
The number of female football players, a high risk group from
a knee injury perspective, is growing rapidly worldwide.
Successful preventive measures in football could thus have a
substantial effect on the injury burden and also dramatically
diminish the costs associated with treatment. The direct costs
of surgery and hospital care for anterior cruciate ligament
reconstructions are high,31 32 and additional costs are associated
with non-surgical treatment, postoperative rehabilitation, and
disability claims. Consequently, if almost two thirds of the
anterior cruciate ligament injuries in adolescent female football
players were avoided, this would substantially reduce the need
for surgery and rehabilitation with large health economic
benefits. Future efforts should include cost-benefit analyses of
neuromuscular training programmes, as well as implementation
research identifying motivators and barriers (among coaches
and players) to adoption of such training programmes.
Whether our results can be generalised to other age groups, male
football players, or other team sports is uncertain. However,
similar neuromuscular programmes have shown preventive
effects in other cluster randomised controlled trials for training
related anterior cruciate ligament injuries in female collegiate
football players,13 as well as on lower extremity injury in
adolescent female football players,19 team handball players
(mainly female),25 and adult female floorball players.33
Therefore, assuming that the programme might be also effective
in other football populations as well as in other sports is
reasonable.
We thank the study assistants from the Swedish Football Association
(Per Renström, professor, Annica Näsmark, physiotherapist, and Anneli
Gustafsson, football coach) for their general help with the study, and
Jonas Ranstam, senior bio-statistician, for statistical advice. We also
thank the coaches and all players who participated in the study, as well
as the study therapists and study physicians.
Contributors: MW, IA, and MH were responsible for the conception and
design of the study. MW and MH coordinated the study and managed
all aspects, including data collection. HM provided technical support
and was responsible for database management. PW did the analyses,
which were planned and checked with the other authors. MW, IA, and
MH wrote the first draft of the manuscript. All authors had full access
to all data and contributed to interpretation of the findings and critical
revision of the manuscript. MW and MH are the guarantors.
Funding: The study was financially supported by the Swedish Football
Association and the Folksam Insurance Company. This research also
received grants from the Swedish National Centre for Research in Sports
and from the Hässleholm Hospital. None of the sponsors had any role
in the design or conduct of the study; the collection, analysis, and
interpretation of the data; or the preparation, approval, and submission
of the manuscript. No author or related institution has received any
financial benefit from research in this study.
Competing interests: All authors have completed the Unified Competing
Interest form at www.icmje.org/coi_disclosure.pdf (available on request
from the corresponding author) and declare: MW, HM, and MH had
financial support from the Swedish Football Association and the Folksam
Insurance Company; MW and MH received research grants from the
Swedish National Centre for Research in Sports and IA received a grant
from the Hässleholm Hospital for the submitted work; MW and MH have
been paid by the Swedish Football Association for constituting the
medical staff of the Swedish male under 19 national team; no other
relationships or activities that could appear to have influenced the
submitted work.
Ethical approval: The study was approved by the regional ethical review
board in Linköping (M197-08). Players and parents or guardians gave
individual written informed consent.
Data sharing: No additional data available.
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2 United States Youth Soccer Association. Largest youth sports organization celebrates
35th anniversary. 2009. www.usyouthsoccer.org/aboutus/History.asp.
3 Swedish Football Association. Rekordmånga fotbollsspelare i Sverige [Record number
of football players in Sweden]. 2006. http://svenskfotboll.se/arkiv/tidigare/2006/11/
rekordmanga-fotbollsspelare-i-sverige/.
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BMJ 2012;344:e3042 doi: 10.1136/bmj.e3042 (Published 3 May 2012) Page 5 of 11
RESEARCH
What is already known on this topic
Knee injuries are common in football regardless of the playing level, and adolescent female players are more susceptible to anterior
cruciate ligament injury than their male counterparts
Several parallel group controlled studies have evaluated different injury prevention strategies in adolescent female football players
However, few studies have shown statistically significant reductions of knee injury or anterior cruciate ligament injury with neuromuscular
training
What this study adds
A 15 minute neuromuscular warm-up programme reduced the overall rate of anterior cruciate ligament injury by 64% in adolescent
female football players
Players who carried out the programme at least once a week (compliers) additionally had lower rates of severe knee injury (>4 weeks’
absence) and any acute knee injury
Neuromuscular training should be part of the warm-up programme for young female football players
24 Hägglund M, Waldén M, Bahr R, Ekstrand J. Methods for epidemiological study of injuries
to professional football players—developing the UEFA model. Br J Sports Med
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injuries in youth sports: cluster randomised controlled trial. BMJ 2005;330:449.
26 Lin DY, Wei DJ. The robust inference for the Cox proportional hazards model. J Am Stat
Assoc 1989;84:1074-8.
27 Greenland S, Rothman KJ. Introduction to stratified analysis. In: Rothman KJ, Greenland
S, Lash TL, eds. Modern epidemiology. Lippincott Williams & Williams 2008:273-4.
28 Altman DG. Confidence intervals for the number needed to treat. BMJ 1998;317:1309-12.
29 Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. Deficits in neuromuscular
control of the trunk predict knee injury risk: a prospective biomechanical-epidemiologic
study. Am J Sports Med 2007;35:1123-30.
30 Hewett TE, Myer GD, Ford KR, Heidt RS Jr, Colosimo AJ, McLean SG, et al.
Biomechanical measures of neuromuscular control and valgus loading of the knee predict
anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports
Med 2005;33:492-501.
31 Brophy RH, Wright RW, Matava MJ. Cost analysis of converting from single-bundle to
double-bundle anterior cruciate ligament reconstruction. Am J Sports Med 2009;37:683-7.
32 Janssen KW, Orchard JW, Driscoll TR, van Mechelen W. High incidence and costs for
anterior cruciate ligament reconstruction performed in Australia from 2003-2004 to
2007-2008: time for an anterior cruciate ligament register by Scandinavian model? Scand
J Med Sci Sports 2011; doi: 10.1111/j.1600-0838.2010.01253.x.
33 Pasanen K, Parkkari J, Pasanen M, Hiilloskorpi H, Mäkinen T, Järvinen M, et al.
Neuromuscular training and the risk of leg injuries in female floorball players: cluster
randomised controlled study. BMJ 2008;337:a295.
Accepted: 23 March 2012
Cite this as: BMJ 2012;344:e3042
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BMJ 2012;344:e3042 doi: 10.1136/bmj.e3042 (Published 3 May 2012) Page 6 of 11
RESEARCH
Tables
Table 1| Details of neuromuscular warm-up programme used in intervention group
Repetitions/durationInstructionsExercise
Slow movement with smooth turn, horizontal pelvis and non-supporting foot in front of body with slightly flexed
hip and knee
One legged knee squat:
3×8-15 repsHands on hipsLevel A
3×8-15 repsHold ball over head with straight armsLevel B
3×5 repsHands on hips; mark with non-supporting foot just above ground at 12-02-04-06 o’clock positionsLevel C
3×8-15 repsBend down while holding ball and let ball touch ground outside supporting foot; make diagonal movement upwards
and raise ball over head with straight arms on contralateral side
Level D
3×5-10 repsTeammate stands slightly oblique in front of you and ball is pressed between lateral sides of feet of non-supporting
legs
Pair exercise
Supine position; lift pelvis from ground while keeping back straightPelvic lift:
3×8-15 repsBoth feet on ground and hands across chestLevel A
3×8-15 repsOne foot on ground and contralateral leg flexed in hip and knee 90° with both hands on kneeLevel B
3×8-15 repsOne foot on football and contralateral leg flexed in hip and knee 90° with arms on ground alongside bodyLevel C
3×8-15 repsOne foot on ground and other in air; keep upper arms on ground with elbows flexed 90°; push away supporting
foot and land on contralateral foot
Level D
3×8-15 repsTeammate stands with flexed knees and supports heel of one of your feet in her hands; hands across chest and
lift pelvis
Pair exercise
Slow movement with smooth turn, back in straight position and feet shoulder-wide apart with soles in contact with
ground
Two legged knee squat:
3×8-15 repsHold ball in front of body with straight armsLevel A
3×8-15 repsHands on hipsLevel B
3×8-15 repsHold ball over head with straight armsLevel C
3×8-15 repsSame as level C but continue movement and rise up on toes after returning to starting position and stay briefly
in that position
Level D
3×8-15 repsTeammate stands next to you approximately 1 m away, facing opposite directions; hold ball between you with
one hand and other hand on hip; apply slight pressure on ball while performing knee squat
Pair exercise
Lift body and keep it in straight lineThe bench:
15-30 secProne position; support on knees and on lower arms with elbows kept under shouldersLevel A
15-30 secSame as level A but with support on tip of feetLevel B
15-30 secSame as level B, but move foot to side and back to starting position; alternate sidesLevel C
5-10 repsLie sideways with support on foot and lower arm with elbow kept under shoulder and other hand on hip; lift hip
off ground and stay briefly in that position with good control before slowly returning to starting position
Level D
15-30 secTeammate stands behind you and holds your feet or lower legs; lift the body and walk forward by using hands on
ground
Pair exercise
Take deep step with marked knee lift and soft landing; rear knee should not touch groundThe lunge:
3×8-15 repsHands on hips; move forward with each stepLevel A
3×8-15 repsHold ball in front of body with straight arms; rotate upper body while stepping forward and position ball laterally
of front leg; move forward with each step and alternate sides
Level B
3×8-15 repsHold ball over head with straight arms; perform forward lunge and push back with front leg and return to starting
position
Level C
3×8-15 repsHold ball in front of body with straight arms; perform sideways lunge and return to starting positionLevel D
3×8-15 repsTeammate stands in front of you 5-10 m away; perform forward lunge while making throw-in with ballPair-exercise
Make jump with soft landing; stay briefly in landing positionJump/landing:
3×8-15 repsStand on one leg with knee slightly bent and hands on hips; make short forward jump and land on same foot;
jump backwards to starting position
Level A
3×8-15 repsStand on two legs shoulder-wide apart with hands on back; make sideways jump and land on one foot; alternate
sides
Level B
3×5 repsTake a few quick steps on same spot and make short jump straight forward landing on one footLevel C
3×5 repsSame as level C, but change direction and jump to one side (90° turn); alternate sidesLevel D
3×8-15 repsTeammate stands in front of you approximately 5 m away; make two legged jump while heading football and land
on two legs
Pair exercise
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RESEARCH
Table 2| Baseline characteristics and exposure data. Values are numbers (percentages) unless stated otherwise
Control group (n=2085 players)Intervention group (n=2479 players)Variable
Characteristics
109121No of clubs
157184No of teams
14.1 (1.2)14.0 (1.2)Mean (SD) age (years)
Age (years):
116 (6)159 (6)12
650 (31)783 (32)13
594 (28)741 (30)14
424 (20)471 (19)15
208 (10)227 (9)16
93 (4)98 (4)17
163.8 (6.6)163.5 (6.8)Mean (SD) height (cm)
53.3 (8.4)53.3 (8.6)Mean (SD) weight (kg)
1474/1967 (75)1726/2321 (74)Menarche
161/2011 (8)215/2396 (9)Previous acute knee injury
16/2011 (0.8)23/2396 (0.96)Previous ACL injury
504/2011 (25)583/2396 (24)Current knee complaints
295/1976 (15)361/2325 (16)Familiar disposition of ACL injury
Clubs’ exposure
41.6 (11.3)39.8 (12.1)Mean (SD) training sessions per season
20.1 (7.2)20.0 (8.0)Mean (SD) matches per season
1.8 (0.3)1.8 (0.3)Mean (SD) training sessions per week
0.9 (0.3)0.9 (0.3)Mean (SD) matches per week
Players’ exposure
30.2 (12.3)28.8 (13.1)Mean (SD) training sessions per season
16.4 (7.6)16.5 (8.5)Mean (SD) matches per season
1.3 (0.5)1.3 (0.5)Mean (SD) training sessions per week
0.7 (0.3)0.7 (0.3)Mean (SD) matches per week
45.4 (18.8)44.0 (20.9)Mean (SD) training hours per season
16.5 (9.2)16.2 (9.6)Mean (SD) match hours per season
68.7 (21.0)69.0 (21.7)Mean (SD) training attendance per season*
488 (23)706 (28)Players with match play with other teams†
ACL=anterior cruciate ligament.
*Calculated as percentage of eligible team training sessions player participated in during season.
†Players who played matches with district or national team or another team within club during study inclusion.
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RESEARCH
Table 3| Characteristics of anterior cruciate ligament (ACL) injuries recorded in trial
ACL
reconstruction
Associated
lesionsType of tearMRIContactLimbActivityAgeGroupInjury date
NoMCLTotalYesYesLeftMatch14Control13 April
NoMCL and LMTotalYesYesLeftMatch14Control18 April
YesNoTotalYesNoLeftTraining16Control18 April
NoMCLPartial†No*NoLeftTraining16Control22 April
YesNoTotalYesNoLeftMatch14Intervention10 May
NoMMPartial‡No*NoLeftTraining15Control11 May
NoLMTotalYesYesLeftMatch14Control17 May
YesLCLTotalYesNoRightMatch15Control25 May
NoNoPartial¶No§YesLeftMatch14Control9 June
YesNoTotalYesNoLeftMatch15Intervention17 June
YesNoTotalYesNoLeftMatch15Intervention7 Jul
YesNoTotalYesYesRightMatch14Intervention25 July
NoNoTotalYesYesRightMatch14Control31 July
YesNoTotalYesNoLeftTraining16Control6 August
NoMCLTotalYesNoRightMatch15Control8 August
YesNoTotalYesYesLeftMatch17Intervention9 August
YesMMTotalYesNoRightTraining14Control10 August
YesMMTotalYesNoRightMatch13Control29 August
YesNoTotalNo**NoRightMatch16Intervention11 September
YesMCL and LMTotalYesYesRightMatch16Control20 September
NoNoPartial††YesNoLeftMatch15Intervention4 October
LCL=lateral collateral ligament; LM=lateral meniscus; MCL=medial collateral ligament; MM=medial meniscus; MRI=magnetic resonance imaging.
*Subacute arthroscopy instead of MRI.
†Tear of anteromedial bundle with increased anterioposterior translation and negative pivot shift diagnosed during clinical examination under anaesthesia and
arthroscopy.
‡Partial ACL tear with increased anterioposterior translation and negative pivot shift diagnosed during arthroscopy for locked knee due to bucket handle tear of
MM.
§Injury not treated by medical support of study.
¶Partial ACL tear with increased anterioposterior translation and negative pivot shift diagnosed at repeated clinical examination in subacute phase.
**Player declined initial MRI, and diagnosis was confirmed later during ACL reconstruction.
††Partial ACL tear with increased anterioposterior translation and tendency to positive pivot shift diagnosed at repeated clinical examination in subacute phase.
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RESEARCH
Table 4| Effectiveness of neuromuscular warm-up programme in adolescent female football players according to intention to treat. Values
are numbers (percentages) of injured players unless stated otherwise
P valueRate ratio (95% CI)*Control group (n=2085 players)
Intervention group (n=2479
players)Injuries
0.020.36 (0.15 to 0.85)14 (0.67)7 (0.28)Anterior cruciate ligament injury
0.180.70 (0.42 to 1.18)31 (1.49)26 (1.05)Severe knee injury
0.710.92 (0.61 to 1.40)44 (2.11)48 (1.94)Any acute knee injury
*Rate ratios from unadjusted analyses, stratified by district, using Cox regression with robust variance estimation to account for correlation of outcomes within
each club.26
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RESEARCH
