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Abstract
Background ‘Boosting’ is defined as the intentional induction of autonomic dysreflexia (AD) by athletes with a spinal cord injury (SCI) at or above the level of T6 for the purpose of improving sports performance. Boosting has been shown to confer up to a 9.7% improvement in race time. Additionally, to compete in a hazardous dysreflexic state, whether intentional or unintentional, would present an extreme health risk to the athlete. For these reasons, the International Paralympic Committee strictly bans the practice of boosting, and has developed a protocol to test for its presence.
Methods Testing was performed at three major international Paralympic events. Education regarding the dangers of AD was provided to athletes and team staff. Testing was conducted on athletes from the relevant sport classes: Athletics (wheelchair racing classes T51/T52/T53) and Handcycling (H1). Key parameters included the athlete's demographics (gender, country of origin), classification and blood pressure measurements. An extremely elevated blood pressure was considered to be a proxy maker for AD, and a systolic blood pressure of ≥180 mm Hg was considered a positive test.
Results A total of 78 tests for the presence of AD were performed during the three games combined. No athlete tested positive. The number of athletes tested, by classification, was: 6 in Athletics T51, 47 in Athletics T52, 9 in Athletics T53 and 16 in Handcycling H1. Of those tested, the average systolic and diastolic blood pressures were 135 mm Hg (range 98–178) and 82 mm Hg (range 44–112), respectively. All athletes were compliant with testing. No athletes were withdrawn from competition due to the presence of AD.
Discussion Testing for the presence of AD in paralympic athletes with SCI prior to competition has been carried out for the first time at three major international paralympic competitions. There have been no positive tests thus far. Knowledge gained during these early testing experiences will be used to guide ongoing refinement of the testing protocol and the development of further educational initiatives.
- Disabled
- Doping
- Injury Prevention
- Physiology
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Introduction
Autonomic dysreflexia (AD), as the term suggests, is a reflex syndrome that is unique to individuals with spinal cord injury (SCI), typically at or above the level of T6.1 ‘Boosting’ is defined as the deliberate induction of AD for the explicit purpose of enhancing sports performance.2 ,3 Athletes with SCI who participate in competitive sports have been known to intentionally induce AD, or ‘boost’, prior to or during a sports competition.
The problem of boosting in paralympic sport
The sympathetic response induced by AD can provide the athlete with SCI an unfair competitive advantage by overcoming the limitations of cardiovascular blunting and a reduced autonomic response to exercise. It is additionally recognised that AD can have catastrophic health consequences, including seizure,4 myocardial infarction,5 cerebral haemorrhage6–8 and death,6 ,8 among others. AD is a medical emergency for individuals with SCI, and remaining in a dysreflexic state for a prolonged period of time is extremely dangerous. Although there are no published reports of an athlete experiencing a severe, adverse health event related to the intentional induction of AD during a major competition, it is known from anecdotal reports and clinical experience that athletes have placed themselves at risk of physical harm by purposefully becoming dysreflexic during training and competition.
Published studies have demonstrated an enhancement in physiological response and sports performance with the intentional induction of AD.2 ,9 ,10 In a prospective, controlled, crossover study of six trained athletes with high-level SCI, Schmid et al9 found that boosting led to a significant increase in the release of adrenaline and noradrenaline during exercise compared with the ‘non-boosted’ state. Additionally, these athletes displayed significantly higher peak power, higher blood pressure and higher maximal oxygen uptake while experiencing AD.9 In a separate prospective, controlled crossover study, Burnham et al2 performed testing on eight highly trained tetraplegic wheelchair racers, all of whom endorsed having intentionally induced AD in the past. Competing in a dysreflexic state was found to provide a 9.7% average improvement in finish times during a 7.5 K wheelchair race.
Although anecdotal and clinical evidence suggest that the intentional induction of AD still occurs, its true prevalence remains uncertain. In a confidential survey of 99 elite athletes with SCI, Bhambani et al11 found that only 56% of the surveyed athletes reported of having knowledge of the practice of boosting. A total of 16.7% of respondents (all males) reported having used AD to improve performance at some point in the past. The authors also surveyed athletes regarding their understanding of the medical dangers of competing in a dysreflexic state and found that 48.9% of athletes felt that this was somewhat dangerous, 21.3% felt it was dangerous and only 25.5% felt it was very dangerous.11 These results indicate that while only a relatively small percentage of athletes with high-level SCI have intentionally induced AD for performance enhancement purposes, there remains a strong need for ongoing educational and deterrence programmes.
The pathophysiology and clinical presentation of AD
Owing to autonomic dysfunction, athletes with a high-level SCI have limited physiological potential for improvements in cardiac output and maximal oxygen uptake during times of strenuous exercise.12 ,13 The loss of sympathetic cardiac innervations often results in a maximum heart rate between 110 and 130 bpm as determined by sinoatrial activity.12 ,14 ,15 This phenomenon is commonly referred to as ‘cardiovascular blunting’, and dramatically inhibits sports performance.16
When a noxious stimulus is applied below the level of neurological injury, a sympathetic response is activated by spinal reflex pathways within the sympathetic chain, running from spinal cord levels T1–L2. Given the presence of SCI, the brain cannot provide central inhibitory control of this reflex to the sympathetic chain below the level of neurological injury. This results in an uncontrolled and sometimes massive release of adrenaline and noradrenaline, with subsequent effects such as systemic vasoconstriction (particularly of the splanchnic bed), acute hypertension, headache, diaphoresis, skin flushing, piloerection, nasal congestion and often a feeling of aggression or anxiety. An acute rise in blood pressure is sensed by the carotid baroreceptors, which relay an afferent impulse to the vasomotor centre of the brainstem and activate the parasympathetic nervous system. Given that vagal innervation to the heart is not interrupted by SCI, this often results in a relative, and seemingly paradoxical, bradycardia in conjunction with the acute hypertensive state. Systemically, this parasympathetic counter response is invariably overwhelmed by the dramatic release of catecholamines, and the individual is left acutely hypertensive until the noxious stimulus is removed.1
In athletes, AD is typically induced by providing a noxious stimulus below the level of neurological injury, such as purposeful distention of the urinary bladder. Urinary bladder distension is most often achieved by drinking large volumes of fluid prior to an event and then clamping a bladder catheter so that urine cannot drain. While this is one common means of intentionally inducing AD, there are anecdotal reports of other methods of applying a noxious stimulus to an insensate region, such as tying a tight strap around one's leg, breaking a toe or sitting on a sharp object.17
The diagnosis of AD is a clinical one that is based on various signs and symptoms such as hypertensive emergency, bradycardia, flushing, piloerection and diaphoresis.1 The definitive identification of an athlete who in a dysreflexic state is very challenging as the majority of these findings, with exception of blood pressure and heart rate, are fairly subjective. To date, there does not exist a clear-cut, objective test or proxy marker that will allow for the identification of AD with high sensitivity and specificity.
The development of an education and deterrence programme for the intentional induction of AD
In order to promote safe and fair competition and in accordance with the IPC Handbook,18 the IPC Medical Committee initiated an Operational Management Plan19 to test for the presence of AD at major international competitions beginning in 2008. The objectives of this programme were threefold: (1) to provide education regarding the dangers of AD and protect the health of paralympic athletes, (2) to deter the practice of boosting and (3) to ensure fair play in paralympic sport.
The IPC Medical Committee educated athletes, coaches, team physicians and other officials about the IPC Operational Management Plan on the intentional induction of AD through the provision of written information sent to all participating National Paralympic Committees (NPCs) in advance of each of the three games. Additionally, information was provided in person at the team physician meetings as well as at the Chef du Mission and sport technical meetings prior to the start of each of the three games. Materials included the information about the testing protocol as well as educational information regarding potential adverse health consequences related to AD.
For the purposes of testing, boosting was defined as the presence of AD within 2h from the official start of competition. Although there are many clinical signs and symptoms that correspond with the presence of AD, testing in a games-time environment necessitated the selection of an objective measure that could provide definitive evidence of the presence of AD. For this reason, the measurement of systolic blood pressure (SBP) was utilised as a test for the presence (or absence) of AD. A SBP of ≥180 mm Hg was set as the threshold to constitute a positive test. For purposes of preventing potential false-positive tests, athletes were required to submit medical documentation of hypertension if they were known to have elevated blood pressures on an ongoing basis due to frequent, unintentional exacerbations of AD or due to underlying arterial hypertension.
The aims of this paper are to describe in detail the International Paralympic Committee's (IPC) Operational Management Plan as it relates to testing for the presence of boosting, and to report the outcomes of testing at three major international paralympic events. Future directions for the IPC's ongoing testing and deterrence efforts are then presented.
Methods
Data collection was performed as a portion of a larger policy implementation strategy set forth by the IPC. Data collection, including clinical testing, occurred at three major international events: the Beijing 2008 Paralympic Games, the Guadalajara 2011 Parapan American Games and the London 2012 Paralympic Games.
Measuring blood pressure
Blood pressure testing was performed utilising a single, automated sphygmomanometer (ReliOn Auto Inflate Digital Blood Pressure Monitor #HEM-8705-WM) with a digital display that was easy to read by the athlete, his/her representative and a member of the IPC Medical and Scientific Department. A second automated sphygmomanometer was available for use as a back up in the case of failure of the first machine. Both automated sphygmomanometers were calibrated prior to testing by comparing automated readings to the reading of a manual sphygmomanometer. This occurred prior to each testing session. Appropriate blood pressure cuff sizes were utilised for each athlete's test. The IPC Medical and Scientific Department authorised members of the IPC Medical Committee or health professionals from the Organizing Committee to undertake the actual blood pressure examinations. Blood pressure measurements were conducted in a standard fashion via the inflation of a blood pressure cuff circumferentially around the arm ∼5 cm proximal to the elbow.
Athlete selection criteria
Athletes with high-level SCI were tested at specific, predetermined events within the sports of Athletics (wheelchair racing, classes T51/T52/T53) and Handcycling (class H1). In Athletics, testing was conducted at the 100 m, 200 m and 400 m events. In Handcycling, testing was conducted at the time trial event. The selected events and classifications were those in which athletes with high-level SCI compete.
The presence of SCI in each athlete was confirmed by the evaluation of medical data obtained at the time of the athlete's classification. After the competition schedule was finalised, individual athletes were selected for testing at random from the start lists. The Operational Management Plan also allowed for athletes to be targeted for testing if a specific concern regarding boosting was raised. Additionally, a representative of the IPC Medical and Scientific Department could request that an athlete undergo testing at any time during the duration of the games. Athletes could be selected for testing on more than one occasion.
Testing procedure
Athlete notification
As per the protocol, athletes were notified of selection for testing by a member of the IPC Medical and Scientific Department no greater than 2 h prior to competition. For all testing encounters, the athlete was given the opportunity for a third-party representative (eg, coach, team manager, team physician, trainer) to accompany him/her for testing. Upon notification, the athlete was asked to report immediately to the dedicated testing location at the venue, accompanied by his/her selected representative. Every effort was made to ensure that the process of testing did not interfere with the athlete's preparation for the competition.
Test implementation
At the time of testing, the procedure was explained to the athlete and his/her representative. The athlete and examiner(s) mutually agreed on which arm would be used for the blood pressure measurement. The examiner(s) recorded the following information on the official testing record: (1) athlete's name, (2) athlete's gender, (3) athlete's nationality, (4) accreditation number, (5) date of the event, (6) venue of competition, (7) event entered for competition, (8) athlete's classification, (9) time at which the event begins and (10) the arm that was used for testing (right or left). The athlete's blood pressure and heart rate were then measured and recorded. A copy of the documentation form is provided in online supplementary appendix A.
According to the Operational Management Plan, if the first SBP measurement was <180 mm Hg, testing was finished and the athlete was allowed to compete. If the first SBP measurement was ≥180 mm Hg, the athlete would be chaperoned and a second blood pressure test would occur ∼10 min later. If the second SBP measurement was <180 mm Hg, testing would be finished and the athlete would be allowed to compete. If both the first and the second SBP measurements were ≥180 mm Hg, and the athlete had not previously produced medical evidence of pre-existing hypertension or frequent unintentional exacerbations of AD (see The development of an education and deterrence programme for the intentional induction of AD section), the Technical Delegate for that event would be informed to withdraw the athlete from the competition. At the time of testing, additional clinical observations were made providing information regarding the presence of a dysreflexic state. These included the presence (or not) of flushing, piloerection and diaphoresis. However, given the difficulty in objectively quantifying these observations, they were not utilised for the purposes of testing. If an athlete was found be hypertensive, yet below the threshold criteria for a positive test, the team physician was notified and advised to monitor the athlete's blood pressure outside of competition for the athlete's health and safety purposes.
After testing was complete, the following information was also recorded on the official testing record: (1) the athlete's comments or concerns regarding the testing process, if provided, (2) the signature of the athlete, (3) name and signature of the athlete's representative, if present and (4) the name and signature of the authorised examiner(s) (see online supplementary appendix A). From start to finish, the time required to test one athlete and complete the documentation paperwork was generally <3 min.
The testing location was chosen so as to minimally interrupt the athletes’ preparation for the event. In Athletics, a testing station was placed directly outside the first call tent. In Handcycling, testing took place near the start line, immediately after the equipment technical check. Testing was performed in a semiprivate fashion (either behind a curtain or within the call tent). Given the non-invasive nature of the test, the current Operational Management Plan does not require private testing.
Results
Athlete characteristics
A total of 78 tests were conducted on 56 athletes from 24 countries. Fifty men and 6 women were tested (table 1). On these 56 athletes, a total of 67 tests were carried out on men, and 11 tests were carried out on women. The breakdown of athletes tested by sport classification was 6 athletes (8%) in Athletics T51, 47 athletes (60%) in Athletics T52, 9 athletes (12%) in Athletics T53 and 16 athletes (20%) in Handcycling H1.
Athlete characteristics
Blood pressure measurements
There were no positive tests for boosting according to the criteria set forth in the IPC Handbook18 and IPC Operational Management Plan.19 In all cases, testing was conducted within 5–45 min prior to the start of the competition. Across all sport classifications, the average systolic and diastolic blood pressures were 135 mm Hg (range 98–178) and 82 mm Hg (range 44–112), respectively (table 2). When calculated within each sport classification, the highest average systolic and diastolic blood pressures were noted in Handcycling H1 (147 mm Hg and 85 mm Hg, respectively). In Athletics T52, the average SBP was also elevated (138 mm Hg); however, the average diastolic blood pressure remained within normal limits (79 mm Hg). The maximum SBP was 178 mm Hg and the maximum diastolic blood pressure was 112 mm Hg (in 2 different athletes). No athletes tested had submitted documentation regarding a medical history of pre-existing hypertension or frequent, unintentional episodes of AD.
Blood pressure measurements
One case of particular interest involved an athlete with an initial blood pressure reading of 176/97 and heart rate of 87. Although this athlete was allowed to compete according to the protocol, a second test later in the same day, just prior to a second event, resulted in a blood pressure measurement of 104/58 and heart rate of 102. Of note, at the time when BP measurements were made, no other clinical signs of symptoms of AD such as diaphoresis, flushing or piloerection were evident. This was true of this particular athlete as well as all other athletes tested.
Athlete compliance
All athletes were compliant with testing. In 68 of the 78 tests (87%), the athlete requested that a representative accompany him/her for testing. This representative was a team manager, coach, physical therapist or a physician. When given the opportunity to provide comments or concerns regarding the test, only two individuals made comments. One athlete expressed that he felt that testing was a positive step towards ensuring that athletes are not boosting prior to competitions. One coach expressed concern that testing interfered with the athlete's warm-up and preparation for the competition.
Discussion
The IPC considers the intentional induction of AD to be a dangerous, prohibited method of enhancing sports performance. The IPC Operational Management Plan for testing for the presence of a hazardous dysreflexic state has been implemented at three major international Paralympic competitions. There have been no positive tests. If the true prevalence of boosting is indeed low, then the current testing programme and educational measures may be functioning as an effective deterrence system. Alternatively, the lack of positive tests may be due to limitations in the testing protocol itself.
Limitations of the current testing protocol
To improve the Operational Management Plan for boosting, it is important to understand the limitations of the protocol as it currently exists in this early phase of implementation. The most significant limitation of this testing protocol is the chosen proxy marker for AD. Most of the clinical signs and symptoms of AD are difficult or impossible to measure objectively, including flushing, diaphoresis, nasal congestion, piloerection, anxiety and headache.1 Additionally, the detection of many of the specific methods that athletes use to induce AD would require unacceptably obtrusive inspections of the athletes at the competition venues. For these reasons, blood pressure was selected as an appropriate, objective proxy marker for the presence of AD.
Although dysreflexia is known to cause an acute hypertensive episode, the severity of this response can vary widely. Thus, setting a BP threshold of 180 mm Hg as a ‘positive test’ is a best attempt at choosing a level in which dysreflexia is likely to be present if an athlete was found to be that hypertensive. This threshold was purposefully chosen in order to avoid false-positive test results. Indeed, athletes may be mildly hypertensive prior to the start of a race for a number of reasons, including the physiological effects of warm up, prerace anxiety and excitement. This may certainly occur prior to what may be the most important race of an athlete's paralympic career. It would not be appropriate to disqualify an athlete who had a legitimate reason to have transient hypertension, and a threshold value of 180 mm Hg effectively avoids this unacceptable situation. Additionally, it could be argued from a medical viewpoint that it would not be safe for an athlete with high-level SCI to compete with an SBP >180 mm Hg, even if he or she was not experiencing AD.
Although the avoidance of false-positive results is of utmost importance, the current testing protocol may also utilise a blood pressure threshold that is too high to detect athletes who are purposefully inducing AD. During testing in London, one athlete was noted to have an initial blood pressure reading of 176/97 and heart rate of 87. Given the current SBP threshold of a 180 mm Hg, this athlete was not disqualified. A second test later in the same day, just prior to a second event, resulted in a blood pressure measurement of 104/58 and a heart rate of 102. This dramatic decrease in blood pressure within a relatively short period of time raised concerns for AD at the time of the first test. An additional matter of concern was the presence of a lower heart rate at the time of the first test when the athlete was hypertensive, a finding consistent with AD. Furthermore, the athlete's race time was 5% faster in the first race when he/she was hypertensive, as compared with the second race when he/she was not hypertensive. The combination of these findings was suspicious for either the unintentional or intentional induction of AD just prior to the first race. Indeed, this single test suggests that boosting still may occur, and provides an example for the limitations of the current testing protocol. An alternative physiological explanation for the change in cardiovascular parameters may have been if the athlete was dehydrated at the time of the second test.
A second limitation of the current testing protocol is the choice of sports that have been targeted for testing. Thus far, testing has focused on athletes from the paralympic classifications Athletics T51/T52/T53 and Handcycling H1, with a focus on the class of Athletics T52. It is important to note that concern for boosting has also been raised in wheelchair rugby. However, health professionals with knowledge of this sport propose that athletes may unintentionally develop AD due to the nature of the sport itself. There are two elements of wheelchair rugby that may lead to the unintentional induction of AD. First, the frequent and sometimes violent collisions that occur during competition may elicit a dysreflexic response. Second, the practice of tightening straps across the athlete's legs in order to secure the athlete into his/her wheelchair may provide a noxious stimulus to an extent that induces dysreflexia. Over-tightening leg straps in both wheelchair rugby and in wheelchair racing can be seen as either an unintentional or quasi-intentional means of inducing AD prior to competition. It would be very challenging, if not impossible, for officials to distinguish the intentional induction of AD (ie, boosting) from the unintentional induction of AD that is caused by the rigorous nature of the sport itself.
A third limitation of the current protocol is the low number of women tested (6 out of 56 athletes). This gender disparity is likely due to the low number of female competitors within the classifications of paralympic sport involving athletes with high-level SCI. Given this disparity, the current testing results are difficult to generalise more broadly towards the larger population of female athletes with SCI.
It is additionally recognised that with the current protocol, the timing of the testing prior to competition may still leave time for an athlete to boost after testing has been completed. Although this remains a concern, in a practical sense, it becomes increasingly difficult for an athlete to self-inflict a noxious stimulus within 45 min prior to the start of a race. The reasons for this are twofold. First, urinary bladder distension to the degree in which boosting is effective typically requires at least 30 min to accumulate. Second, given that an athlete enters an area near the first call tent (in Athletics) or within the call tent (in Handcycling) just prior to the race, there are often many officials and other athletes in the immediate area, thus making tampering or inflicting a noxious stimulus increasingly difficult.
A final limitation of the current testing protocol is the use of automated sphygmomanometers for blood pressure measurements. Automated cuffs were chosen because they feature digital displays that can be easily read, thus minimising the concerns about the subjectivity of reporting results that can be seen with manual sphygmomanometers due to both observer and methodological errors. However, in skilled hands, the accuracy of a manual sphygmomanometer is considered superior to that of an automated machine.
Considerations for future modifications of the operational management plan
These initial experiences raise several considerations for future directions towards the continued development of a deterrence programme for the intentional induction of AD that is fair, efficient and effective. First and foremost, the IPC will continue to broaden its educational efforts regarding the ethical and health-related risks inherent to competing in a dysreflexic state. In addition to providing safety information to athletes, coaches, medical personnel and officials at the time of major competitions, this will also be made available on the IPC website, via mailing and via educational sessions outside of games-time.20 The IPC will notify all National Paralympic Committees regarding how to access this web-based safety information. Ultimately, it remains a primary goal of the IPC to provide educational materials that raise awareness around the inherent dangers of AD and create a competition environment in which the intentional induction of AD is universally recognised as a dangerous, prohibited method.
Additionally, the Operational Management Plan will likely be adjusted so that testing occurs as close to the start time of each race as is possible. In this way, athletes will have less opportunity to induce AD after testing is completed, yet before the start of a race. In a games-time setting, moving the testing closer to the start time must be balanced against the requirement that testing should not interfere with the athlete's normal physical and psychological preparation for the race. Additionally, testing must be done far enough before the start of the race so that there is time to recheck an athlete's blood pressure several minutes later if the first value is above the threshold limit.
Regarding the testing protocol itself, further research is needed to improve the current methods of testing and to provide evidence supporting its efficacy. Priority should be given to the identification of objective measures for AD that may be more appropriate than the current use of blood pressure thresholds. These may include the measurement of catecholamine levels in blood or urine (once standard baseline levels have been established for this particular athletic population) or the use of other physiological parameters such as the sympathetic skin response. Additionally, modification of the testing protocol may include the concept of a ‘blood pressure passport’, analogous to the ‘athlete blood passport’ that has been instituted by several international sports federations and antidoping organisations. Blood pressure passports may allow for the development of athlete-specific thresholds for the presence of AD rather than the current use of a standard threshold that is applied uniformly to all athletes. Although a promising concept, this model might not be feasible in practice. Unlike blood parameters such as haemoglobin, which tend to be fairly stable over many years within a given athlete, cardiovascular parameters such as blood pressure and heart rate are highly variable, even from hour to hour.
Finally, as resources allow, testing will be expanded to include an increased number of relevant events within the sport of athletics. Guided by prior prevalence data,11 this will likely include testing of the T52 800 m race. Although the expansion of testing to include the sport of wheelchair rugby is of interest, additional research is needed towards the development of a practical, objective method to distinguish between the unintentional versus intentional induction of AD prior to making this change.
Conclusion
The IPC strives to promote safe and fair competition, and considers boosting to be a prohibited method of enhancing sports performance that can have potentially catastrophic health consequences for the athlete. A programme to educate the athletes and others about the dangers of boosting and to detect the presence of the intentional induction of AD in paralympic athletes has been implemented by the IPC at three major games. Thus far, there have been no positive tests. Knowledge and experience gained during this initial testing period will be used to further refine the testing protocol. Testing will be continued, and the education of athletes, team physicians, trainers, coaches and officials will remain a cornerstone of the IPC's strategy to further deter this practice.
What are the new findings?
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The International Paralympic Committee's (IPC) policy on autonomic dysreflexia (AD) and boosting is presented in the literature for the first time.
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There were no positive tests for the presence of the intentional induction of AD, otherwise known as boosting, at the prior three major paralympic competitions.
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Given the findings of one athlete with borderline results, boosting may still occur.
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The limitations of the IPC's current testing protocol are discussed.
How might it impact on clinical practice in the near future?
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Team physicians, trainers and coaches must work together to educate athletes regarding the dangers of autonomic dysreflexia (AD) and deter its practice.
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The International Paralympic Committee will continue to improve the testing and deterrence programme for boosting.
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For athletes with high spinal cord injury, team physicians must be aware to monitor for the signs and symptoms of either intentional or unintentional AD.
Acknowledgments
The authors with to extend their gratitude to Dr Oriol Martinez, Dr Norma Angelica Patino Marques, and Dr Wayne Derman (Members of the IPC Medical Committee) for their assistance with the implementation and development of this protocol. Additionally, they wish to thank members of the medical services of the 2008 Beijing Paralympic Games, 2011 Guadalajara Parapan American Games, and 2012 London Paralympic Games for their assistance with logistical and administrative support throughout the testing period.
References
Supplementary materials
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Files in this Data Supplement:
- Data supplement 1 - Online appendices
Footnotes
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Contributors All of the authors contributed in a substantial manner to the planning and carrying out of the testing, literature review and/or manuscript preparation.
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Competing interests None.
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Provenance and peer review Commissioned; externally peer reviewed.
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▸ References to this paper are available online at http://bjsm.bmj.com