VANLANDEWIJCK, YVES C.1; VERELLEN, JOERI1; BECKMAN, EMMA2; CONNICK, MARK2; TWEEDY, SEAN M.2
In 2009, the International Paralympic Committee (IPC) endorsed extensive revisions of the IPC Athletics Classification System, which will be implemented after the 2012 London Paralympic Games (10). The stated purpose of the revised system is consistent with the IPC position stand on classification in Paralympic sport—to promote participation in sport by people with disabilities by minimizing the effect of impairment on the outcome of competition (10,11). To achieve this purpose, each class within the classification system should comprise athletes who have impairments that cause a comparable degree of activity limitation.
In wheelchair track athletics, class T54 comprises athletes with “… normal arm muscle power, and with a range of trunk muscle power extending from partial trunk control to normal trunk control … Equivalent activity limitation to person with complete cord injury between cord level T8–S4” (10). To determine whether the T54 class achieves its intended purpose—to minimize the effect of impairment on the outcome of competition—requires evaluation of the effect that reduced trunk muscle power has on wheelchair racing performance: if reduced trunk muscle power has minimal effect, then the class will achieve its purpose; if it has a large effect, athletes with partial trunk impairment will be systematically disadvantaged. In the latter instance, a case could be mounted for splitting the T54 class into two—one for athletes with unimpaired trunk and the other for athletes with partial trunk impairment.
Relatively few studies have evaluated the role of the trunk in wheelchair racing, and of those, only studies conducted since the Barcelona Paralympic Games (1992) are relevant—before that time, the sitting position was much more upright, and the biomechanics of the pushing technique was not comparable with the modern technique (12).
Since 1992, six studies have evaluated the trunk kinematics of wheelchair racing (2–5,9,13). Two principal parameters are reported on: trunk position and trunk range of movement. Although the studies vary in terms of push speed, slope, and marker position, the trunk position reported has been fairly consistent, ranging from 7° to 38° to the horizontal (i.e., forward leaning with 0° = horizontal and 90° = vertical). Junior athletes assumed a more upright trunk position of 25°–54° (4), and Wang et al. (13) also reported a more vertical position (44°–55°), although reasons for this are unclear because of missing data on trunk angle calculations. The reported amplitude of trunk movement within a propulsion cycle is small regardless of trunk position, ranging from 3° to 21° (4,13). Although these studies on the trunk kinematics of wheelchair racing provide useful background information, they are not of direct relevance to determining the effect of trunk strength impairment on wheelchair racing performance. Trunk muscle activity during wheelchair propulsion has been evaluated previously but only during propulsion of daily use wheelchairs (6,14). To the authors’ knowledge, studies evaluating the effect of impaired trunk strength on wheelchair racing performance have not been conducted.
The principal aim of this study was to evaluate the strength of association between impaired trunk strength and racing wheelchair propulsion to determine whether athletes who have impaired trunk muscle strength should compete in class T54, together with athletes who do have fully functioning trunk muscles. The hypothesis was that athletes with full trunk strength (FT) would be able to accelerate their wheelchair from standstill significantly better compared with athletes with impaired trunk muscle strength.
Before the commencement of testing, all participants provided written informed consent, which included an assurance that results from this study would not affect the athlete’s current classification. This study was accepted by the Medical Research Ethics Committee of the University of Queensland, Australia.
Participants were 10 male and 3 female international wheelchair track athletes (six Australian, four Swedish, two Canadian, and one from the United Arab Emirates). The athletes took part in the testing as part of a recovery camp that followed the Oz Day 10-km race in Sydney in January 2010. Mean ± SD age and self-reported time since injury were 25.6 ± 6.6 and 20.4 ± 8.3 yr, respectively. Diagnoses were spina bifida (n = 5), spinal cord injury (n = 4), transverse myelitis (n = 1), arthrogryposis (n = 1), myelodysplasia (n = 1), and amputation (n = 1). The level at which athletes reported being motor complete ranged from T7 to L5: two athletes were unsure, and one was an amputee. Eleven athletes were class T54 (“… normal arm muscle power, and with a range of trunk muscle power extending from partial trunk control to normal trunk control … Equivalent activity limitation to person with complete cord injury between cord level T8–S4”), and two were class T53 (“… normal arm muscle power [but] with no abdominal or lower spinal muscle activity … Equivalent activity limitation to person with complete cord injury between cord level T1–7”) (1).
Preparticipation clinical evaluations confirmed that all athletes had unimpaired upper limb strength and range of movement. Trunk strength was also assessed clinically to determine whether it was full (unimpaired) or not full (impaired). The athlete sat upright on a flat seat surface without supports, and trunk strength was assessed full if 1) the athlete could lower the trunk to the knees and return to upright with no assistance from the arms; 2) the athlete could lower the trunk to 45°, hold isometrically, and maintain that position against moderate downward force and strong upward force; and 3) there was no clinically significant scoliosis or kyphosis (Cobb angle ≥ 60°). According to these criteria, six athletes had unimpaired trunk strength, and seven had impaired trunk strength.
Sitting height was evaluated with the athlete out of their wheelchair, sitting in a chair with a flat seat surface and rigid 90° backrest, which was used to assist the athlete to achieve an erect posture. The measurement was taken from the seat surface to the top of head. Mean ± SD sitting height for the group was 85.6 ± 5.3 cm. While the athlete was out of his/her wheelchair, the wheelchair was weighed on a wheelchair-accessible platform, and the combined mass of the athlete and chair was measured when the athlete returned to his/her chair. The mass of the athlete was obtained by subtracting the weight of the chair from the combined total. Mean ± SD body mass was 58.1 ± 9.6 kg.
Four measures were taken: wheelchair and wheelchair–athlete interface measures, isometric arm and trunk strength, wheelchair acceleration on track (criterion activity), and wheelchair acceleration on the ergometer (rolling resistance = four times track rolling resistance).
Wheelchair and wheelchair–athlete interface characteristics.sometric arm and trunk strength.
Racing wheelchair measures comprised length and mass, as well as diameters of the rear wheel, front wheel, and push rim. Characteristics of the athlete–wheelchair interface were measured with the athlete sitting in his/her racing chair. Position was classified as either kneeling or sitting, and if the athlete used a strap across the lumbosacral region when in his/her customary racing position, this was noted. Elbow angle was evaluated with the athlete sitting with the trunk as close to 90° to the horizontal as possible with the spine extended and hands on top of push rims. The enclosed angle between the forearm and the humerus of the dominant arm was measured. Acromion to rear tire distance was evaluated with the trunk in the fully flexed position and measured as the horizontal distance between the acromion and the front of the rear tire at the level of the rear axle (positive values are in front of the tire; negative values are behind—Fig. 1, right panel). Thigh angle was evaluated in the sitting position and measured as the angle of the femur relative to the vertical.
Isometric arm and trunk strength.
Maximum isometric arm strength and trunk strength were measured using a custom-built rig (Fig. 2). An S-type load cell (Scale Components, Slacks Creek, Australia) rated to 394 kg (1000 lb) was mounted between two aluminum plates (250 × 196 × 12 mm) and attached to the rig in front of the seat. The load cell was calibrated with a known mass before each testing session and was connected to a MuscleLab unit (Ergotest, Langesund, Norway), which converted the load cell signal and stored the data on the hard disk of a portable computer. The mounted load cell was adjustable vertically and horizontally to account for individual differences in trunk height and breadth. The seat position was adjustable in the fore–aft direction to account for individual differences in arm length. These features permitted the load cell to be positioned relative to the participant in anatomically standardized positions for each strength test. The pelvis and thighs were secured to the chair with nonelastic Velcro strapping during all tests to prevent hip flexion therefore ensuring that the muscle action was isometric. A four-point harness (Monza Autotecnica RS406, Thomastown, Australia) secured each participant’s trunk to the chair during the supported tests. Maximal voluntary contractions were performed randomly to evaluate 1) arm isometric strength and 2) trunk isometric strength. A relative trunk/arm ratio was calculated after testing.
The load cell plate was positioned at shoulder height in front of the sternum (Figs. 2A and B). To evaluate isometric arm strength, the participant was fully supported by the backrest, both elbows were in 60° of flexion in the sagittal plane, and the palms of the hands were on either side of the center of the load cell plate, which was positioned at the same height as the rotation center of the glenohumeral joints. The humeri were internally rotated, and the forearms were pronated such that the fingers of each hand faced the center of the load cell (Fig. 2A). The elbow angle was selected to optimize the isometric force that can be exerted by this muscular chain (8). To help maintain correct arm position and to ensure that participants distributed force evenly through the center of the load cell, the heels of hands were placed on the lateral edges of the plate with the fingers overlapping. Elbow joint angle was established using a 360° goniometer (SunShine Diagnostics and Measuring Instruments, New Taipei City, Taiwan). To measure isometric trunk strength, participants were seated without backrest support by placing the trunk in 30° of trunk flexion (Fig. 2B) determined by real-time video feedback (Dartfish ProSuite v.4.0, Fribourg, Switzerland). The video camera (Sony DCR-HC19E PAL, Tokyo, Japan) was positioned at 90° to and 1.5 m from the strength rig at the height of the participant’s hip. Participants were instructed to maintain the trunk position while pushing. Both hands were placed on the load cell, the upper extremities in identical position as described for Figure 2A. Elbow joint angle was established using a 360° goniometer (SunShine Diagnostics and Measuring Instruments).
Participants were instructed to “obtain peak force with a slow, steady maximal effort” reaching peak force after 2 s, which was held for a further 3 s. Participants performed two submaximal practice trials and were given visual feedback of the force–time curve using the MuscleLab software. This facilitated adjustments in rate of force development during subsequent trials to ensure a slow buildup of force. Participants then produced three maximum voluntary contractions with 30-s rest between trials. Strong verbal encouragement was given to promote maximum voluntary contraction.
Wheelchair acceleration test—track.
Acceleration on the track was evaluated with the athlete in his/her own racing wheelchair, permitting individualized positioning and strapping as applied in competition. A Cheetah LMT (AMR Sports, Queensland, Australia) was used to measure linear displacement of the wheelchair. The Cheetah LMT unit consists of 100 m of a fishing line (Berkley FireLine, Spirit Lake, IA) wrapped around a spool (10-cm circumference) that had a hole positioned every 1 cm. The circumference of the spool passed in front of a photoelectric device. The unit was secured to a table surface 40 cm high, and the line was attached to the rear of the wheelchair at the same height. As the wheelchair was accelerated, the fishing line was pulled from the spool, causing it to spin in front of the photoelectric device. Each hole in the spool circumference permitted light to reach the device, sending a pulse to a connected onboard microprocessor. The onboard microprocessor measures the time taken between each pulse (every 1-cm increment). The Cheetah LMT analog signal was analog-to-digital converted and stored as time per centimeter on the hard disk of a portable computer. From these data, the acceleration and deceleration of the wheelchair–athlete system could be calculated at any time point.
Before each testing session, cones were placed at the 15-m point on both sides of lane 5 of the 100-m straight of an outdoor 400-m running track. After warming up, participants performed three maximal 15-m sprint starts, with the best of the three used for analysis. After each 15-m sprint, the athlete stayed in his/her propulsion position, placed hands on the steering device at the front end of the wheelchair (ensuring relatively constant frontal surface area), and coasted down to 90 m. Average deceleration during coast down was −0.173 ± 0.015 m·s−2. This information was subsequently used to calibrate the ergometer.
To evaluate wheelchair acceleration with increased rolling resistance (four times track rolling resistance), propulsion tests were performed in the athletes’ own competitive wheelchair, which was mounted on a custom-designed roller ergometer. Athletes were allowed positioning and strapping as in competition. The axle of the rear wheels was positioned straight above the ergometer roller, which had a radius of 0.09 m. A hydraulic bridge brought a custom-designed chock in place to secure the wheelchair in the fore–aft direction without interfering with the other dimensions including the backward tipping movement of the wheelchair. Backward tipping of the chair is essential in simulating realistic wheeling conditions and will appear when too much backward momentum is generated through force application on the hand rims or backward shifting of the athlete’s center of gravity.
Realistic wheelchair–user mass inertial forces were simulated using a flywheel (Fig. 1). The flywheel was connected to the roller through a system of chains and cogwheels to adjust for differences in weight of the participants. Adjustments were made by changing the transfer ratio between the roller and the flywheel, calculated using the following formula:
Equation (Uncited)Image Tools
with R2, R3, and R4 as the radius of the roller, the roller cogwheel, and the flywheel cogwheel, respectively. M(body + chair) represents the mass of the wheelchair and user, whereas Iflywheel represents the inertia of the flywheel.
Velocity of the wheelchair was continuously monitored using a photoelectric cell counting the number of revolutions of the flywheel with an accuracy of 1/60 per flywheel revolution. Markers were placed on the axle of the rear wheel, the rear wheel felly, the greater trochanter, and the seventh cervical vertebra (C7). Trunk kinematics in the sagittal plane was captured at 300 Hz using a CASIO EX-F1 camera (CASIO, DVC, Drenthe, The Netherlands).
Athletes were allowed a 5-min familiarization with the wheelchair ergometer and warmed up without any supplementary resistance applied to the roller. Thereafter, two sprint start tests were performed with increased resistance on the roller. Resistance was added through a belt over the flywheel and a pulley system to reach coasting down conditions of −0.600 m·s−2, which is approximately four times the resistance during level propulsion stroking. Coasting down trials were conducted before the sprint start tests to confirm the standardized simulation of resistance.
On the track and the ergometer, distances covered after 1, 2, and 3 s were determined from the output of the respective measurement devices. Video images from the ergometer performances were loaded into Dartfish ProSuite (Version 4.0; Dartfish) to determine trunk excursion during acceleration. Trunk position on the ergometer is described in terms of the angle between the horizontal and a line from C7 to the greater trochanter (the trunk rotation axis) (Fig. 1, left panel). Trunk angles were calculated at three hand positions during the propulsion phase: hand contact (HC), hand at 90° or 3 o’clock on the push rim, and hand release (HR). HC and HR were identified visually and verified through the ergometer acceleration curve. The angular excursion of the hand on the push rim is called the push angle and is defined as the angle between HC and HR. The timing variables push time (time the hands are in contact with the push rims), recovery time (the time the hands are not in contact with push rims), and cycle time (total of push time + recovery time) were also measured.
For the strength measures, data were exported from MuscleLab into Excel (version 2007; Microsoft, Redmond, WA) and processed using Scilab (Scilab Consortium, INRIA, Le Chesnay, France). For each trial, the isometric force used for further analysis was calculated as the mean force during the 2-s period with the least variability. To ensure a plateau was achieved, a trial was deemed acceptable if the calculated force was a minimum of 95% of the peak force registered during the trial. The best trial was then used for statistical analysis.
Statistical analyses were conducted in SPSS (version 17.0; SPSS, Inc., Chicago, IL). Relative trunk strength (the ratio of isometric trunk strength/isometric arm strength) was calculated for each athlete, and the relative trunk strength of FT and NFT were compared using a Mann–Whitney U test. To compare distance pushed at 1, 2, and 3 s on the ergometer and on the track, female athletes without full trunk strength (NFT♀) were removed from the sample, and male athletes with full trunk strength (FT♂) and males without full trunk strength (NFT♂) were compared using a Mann–Whitney U test. In each case, significance level was set at P < 0.05. Spearman rank correlations were used in evaluating the association between strength measures and performance data.
Table 1 presents mean athlete, wheelchair, and wheelchair–athlete interface data for the total group as well as the three main subsets—NFT♀, NFT♂, and FT♂. No female athletes with full trunk strength participated in the study. Wheelchair configuration was very consistent both within and across groups—rear wheel size was between 0.66 and 0.71 cm (mean ± SD = 0.70 ± 0.2 cm), hand rim diameter was between 0.32 and 0.41 cm (mean ± SD = 0.37 ± 0.2 cm), and wheelchair mass was between 8.0 and 10.0 kg (mean ± SD = 9.33 ± 0.69 kg). Greater variability was evident in terms of the wheelchair–athlete interface, in particular, the elbow angle in the upright position (hands on top of the hand rims) and acromion to rear tire (Fig. 1, right panel). The range of thigh angles to the horizontal was very consistent both across and within groups.
In Table 2, arm strength (force generated with full backrest), trunk strength (force generated without backrest), and relative trunk strength (ratio of trunk strength/arm strength) are presented for three groups: NFT♀, FT♂, and NFT♂. Results confirm the outcome of the clinical division: NFT♂ and NFT♀ have relative trunk strength that is equal (M = 0.29), and a Mann–Whitney U test of difference indicated that this ratio in participants without full trunk strength (men and women combined) was significantly lower than the ratio of those with full trunk strength (M = 0.42) (P = 0.02).
Table 2 also presents distance covered at 1, 2, and 3 s on both the ergometer and the track. The distance covered at 1, 2, and 3 s was greater on the track for each of the groups than it was on the ergometer, clearly indicating that the resistance on the ergometer was higher than it was on track. A Mann–Whitney U test of difference indicated that despite the significant difference in relative trunk strength between FT♂ and NFT♂, there was no significant difference between any of the performance variables for these groups. Spearman correlation coefficients between strength measures and performance measures were small and not statistically significant. Note that the correlations between trunk strength and performance on the ergometer (the high-resistance condition) were higher than for the track (realistic conditions).
The IPC Position Stand on Classification in Paralympic Sport states that development of evidence-based systems of classification requires studies that quantify the relative strength of association between measures of impairment—e.g., impaired strength or impaired range of movement—and performance in Paralympic sport (11). Conceptually, this study is important because it is the first to address the research need identified in the position stand, empirically evaluating a key premise of wheelchair racing classification.
The premise investigated is that athletes who do not have fully functioning trunk muscles should compete in the same class as athletes who do have fully functioning trunk muscles—T54. Specifically, this study evaluates the strength of association between trunk strength and wheelchair acceleration from standstill, the part of wheelchair racing in which abdominal strength is likely to have the greatest role. To accentuate this role, we also investigated how the relationship between abdominal strength and acceleration was affected by increased pushing resistance (four times track rolling resistance) on a purpose-built ergometer.
In our sample, diminished abdominal strength had no effect on the athletes’ capacity to accelerate the wheelchair from standstill. No significant differences were observed between FT♂ and NFT♂ in distance covered after 1, 2, and 3 s (Table 2). Furthermore, correlations between isometric trunk strength and wheelchair track acceleration were low (0.27–0.32) and nonsignificant, accounting for only 7%–10% of the variance in performance. There are no conditions in track athletics under which the trunk is likely to have a bigger role than in acceleration from standstill. Therefore, for track athletics, our results indicate that although there is a range of trunk impairment within the class, the effect of this impairment of performance is minimal. Conclusively, the hypothesis that FT would be able to accelerate their wheelchair from standstill significantly better compared with NFT has to be rejected. These results provide evidence that the current practice of grouping athletes with some trunk function in the same class as FT is valid. There are two important caveats to this finding.
The first caveat is that results from this study do not permit inferences about the current practice of placing athletes who have no trunk function into a separate class (T53) from those who have partial or full trunk function (T54). This is because our clinical assessment was only designed to identify people who did not have full trunk function—it did not allow us to subdivide those without full trunk function into those with partial trunk function and those with none. Having said this, the male athlete who covered the least distance on both the ergometer and the track at 1, 2, and 3 s was a T53 athlete with a self-reported lesion level of T7 (motor complete) and a trunk strength score of zero, indicating that further research should be undertaken to investigate whether and at what point severely reduced abdominal strength affects acceleration.
The second caveat is that although the correlation between trunk strength and acceleration under both track (low resistance) and ergometer (high resistance) conditions was not statistically significant, the correlation under high-resistance conditions (r = 0.41–0.54) was almost double the correlation under low-resistance conditions (r = 0.27–0.32), accounting for 18%–28% of the variance in performance. This makes sense because the isometric role of the trunk would be greater when the resistance was greater. This result indicates that it is possible that in road racing, particularly those races where athletes have to do a lot of hill climbing, the effect of trunk impairment on performance may be greater than it is on track. This issue requires further investigation.
The novel methods for measuring arm strength and trunk strength that are described in this article are an important methodological advancement and are highly relevant for the development of measures of strength impairment that will be required for developing evidence-based systems of classification. In contrast to manual muscle testing (the method of strength impairment assessment currently used in most Paralympic sports), the measures we used were instrumented and produced an objective, continuous measure that was ratio rather than ordinal. When assessing strength impairment for the purposes of classification, it is vital that athletes who have positively influenced their impairment scores through training are not competitively disadvantaged by being placed into a less impaired class. One important means of guarding against this possibility is to use modalities of impairment measurement that are not sports specific (2). The isometric testing mode with a slow buildup to maximal voluntary contraction was selected because such contractions are known to have a low association with dynamic performance (1,7) but will still adequately capture the effects of neurological compromise. A measure with these characteristics is critical because it is the effects of neurological compromise that should be the principal determinant of an athlete’s class; the measure should be as resistant to the effect of effective athletic training as possible.
With the back fully supported (Fig. 2A), the prime movers in this upper limb muscular chain are the same as those principally used for wheelchair propulsion—anterior deltoid, pectoralis major, and triceps brachialis (2). The selected arm position—90° shoulder abduction, 45° horizontal shoulder flexion, and 120° elbow extension—optimizes the isometric force that can be exerted by this muscular chain (8). Therefore, the isometric measure of strength used in the current study will provide a single composite measure of upper limb strength that reflects the person’s ability to apply force to the push rim (7,8).
When the backrest is removed (Fig. 2B), the abdominal muscles become part of the active muscular chain. The force applied to the load cell will be equal to the combined mass of the trunk and arms in the 30° forward leaning position, plus the maximum isometric force that can be exerted by the weakest link in the chain—in this case, the abdominals. Our results indicate that this procedure provides a valid measure of abdominal strength: the groups that were clinically assessed as having trunk strength that was not full had lower absolute strength and lower relative trunk strength than the group that was clinically assessed as having full trunk strength. Moreover, the relative trunk strength of FT♂ is comparable with that of a sample of 60 healthy, active, nondisabled male participants (0.46) who were recently evaluated by our group using the strength testing protocols described in this article. Future studies will be able to use this method of upper limb strength assessment to investigate the effect of impaired arm strength on wheelchair propulsion for various sports—wheelchair basketball, wheelchair rugby, wheelchair racing, and wheelchair tennis.
Wheelchair racing is a dynamic sport in which equipment and pushing techniques are constantly evolving. For this reason, meaningful interpretation of biomechanical analyses of wheelchair racing requires accurate descriptions of the characteristics of both the wheelchairs used and the wheelchair–athlete interface. The data reported in this study provide comprehensive information for meaningful interpretation and, in this regard, are an exemplar for future studies. Two wheelchair–athlete interface characteristics with a possible effect on force generation need further investigation, i.e., strapping and positioning. Table 1 indicates that six of seven NFT used lumbosacral strapping compared with only two of six of the FT. One possible effect of this strapping would be to enhance trunk stability and increase force generation on the hand rims. However, this is speculative because no studies ever addressed the effect of strapping. Variability in positioning is mainly the women sitting in a lower/backward position compared with the rear wheel axle. In this sample, this is not related to body height. Especially, the backward-oriented location of the center of gravity will affect the capacity to accelerate from standstill because the force generated on the hand rims will create a backward tilting reaction torque.
Several studies have used ergometers that fixed the front wheel to the ergometer (2,5,9,13). However, one of the design features of the ergometer that enhanced the validity of our study was that the front of the wheelchair was not attached to the ergometer, allowing tipping of the chair. This is important because in track racing conditions, when an athlete applies a propulsive force to the push rim, a backward reactive rotational moment will cause the front wheel to lift and the wheelchair to tip backward. Athletes counteract this moment by shifting the center of gravity forward and downward with increased trunk flexion. This position allows athletes to maximize the amount of force they can apply to the push rim without tipping over; however, the vast majority of elite-level athletes are able to apply considerably more force to the push rim than they are able to counteract through positioning. Wheelchair ergometers obliging the wheelchairs to be attached to the front will not allow tipping, meaning that athletes can apply maximum force to the push rim, altering the kinetics and, potentially, the kinematics of the movement compared with the movement they would perform when racing.
This study is conceptually important because it is the first to make a significant contribution toward the development of evidence-based classification methods in Paralympic sport. In our sample, diminished abdominal strength had no effect on the athletes’ capacity to accelerate the wheelchair from standstill. Therefore, for track athletics, our results indicate that although there is a range of trunk impairment within the class, the effect of this impairment of performance is minimal. These results provide evidence that the current practice of grouping athletes with some trunk function in the same class as FT is valid. The study does not address the question of whether classes T53 and T54 should be combined, and the results cannot be interpreted to provide any indication. It is possible that in road racing, particularly those races where athletes have to do a lot of hill climbing, the effect of trunk impairment on performance may be greater than it is on track, and this issue requires further investigation.
This research was supported by the Australian Research Council (grant LP0882187), the IPC, the Australian Sports Commission, and the Australian Paralympic Committee. S.M.T.’s work was supported by the Motor Accident Insurance Commission, Australia.
The authors thank the following people for their expertise and hard work in collection of these data: Dr. Vicky Tolfrey, Professor Thomas Janssen, Dr. John Bourke, Professor Brendan Burkett, Dr. Laurie Malone, Mr. John Lenton, Ms. Ciata Cooper, Ms. Anna Mickenbecker, Ms. Jemima Spathis, Ms. Sissy Taufika, Mr. Elliot Dolan-Evans, and Mr. Sven Blomqvist.
The authors have no conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.