From the Field - Case Study | Curvilinear running demands for a fullback in Rugby League.

ABSTRACT

A superior ability to run and sprint in rugby league may be influential to the final result of a match. A rugby league player may be required during a game to perform sprints in a curvilinear fashion to evade opponents. This is known as curvilinear running (CR). No research on CR has yet to be conducted in rugby league. Therefore, the purpose of this case study was to investigate the CR demands of fullbacks in rugby league. 4 GPSports HPU-SPI (15Hz) files were analysed from 2 fullbacks and the raw data was exported. A total of 54 runs were identified and used in the analysis. Bin intervals of 5o were used in the study. Run angles of 5-10° (n = 17) with angles of up to 40° were run during a game. No runs were purely linear in nature with all runs performed at greater than 0°. The mean velocity during each run was 6.36m/s with a maximum velocity of 9.35m/s. The maximum acceleration was 3.30m/s/s. In summary, this investigation demonstrated the highly non-linear nature of running for a fullback in rugby league. Henceforth, curvilinear running should form part of the speed training prescription to prepare a fullback for the demands of a game.

INTRODUCTION

In rugby league, a critical element of performance is being able to run and sprint better than the opposition to maximise the chances of scoring points by crossing over the opposition endzone for what is known as a “try.” For a fullback, these running demands are accentuated with high involvement in support play as well as having to collect a ball and run after an opposition set has been completed. Instances of curvilinear running (CR) may occur when a fullback needs to evade defenders to create an overlap or a linebreak.

There is yet to be any research on CR demands in rugby league specifically. However, from other studies on CR conducted in football (3,4,12,14), it can be suggested that these runs occur at high frequency during gameplay of field- based team-sports and are very rarely completely linear in nature.

Research into CR has previously centred on track running. Several kinematic observations have been made from these studies which can be applied to the sport of rugby league. The underlying kinematics of CR include centripetal force (14), centripetal acceleration (20) and ground reaction forces (GRF) in the right and left lower extremities (8,17,19).

There are inherent changes to running kinematics on specific joint angles to compensate for running on a curve. It has been shown that the inside leg is subjected to greater angles of hip adduction, hip internal rotation and ankle inversion with greater involvement from the semi-tendinosis hamstrings and adductors (7,11) The outside leg is subject to greater angles of hip abduction and hip external rotation with greater contribution of gluteus maximus and biceps femoris hamstrings (11).

This affects left and right lower extremity GRF during CR activity. In small radii, it has been demonstrated that the outside leg produced 100-200N of force greater than the inside leg in track runners (7). However, it has been shown that on large radii, the opposite is true with the inside leg generating more force than the outside leg (8,16). Therefore, the asymmetrical nature of CR is a potential cause for insufficient maintenance of centripetal acceleration and maximal velocity as it becomes more difficult to produce GRF to overcome the athlete’s bodyweight in order to continue acceleration (7,8). Additionally, this may have implications on injury risk as one limb must generate higher forces to compensate for the body lean for the run. Therefore, based on the evidence presented, an improved ability to perform CR may be beneficial to performance in match defining situations whilst simultaneously building resilience to sprinting.

The purpose of this study is to investigate the curvilinear running demands for a fullback in rugby league using angular analysis.

METHODS

Participants

The GPS files of two male fullbacks playing for an under 20’s team competing in the New South Wales Rugby League Jersey Flegg Competition were chosen for this investigation.

Study Design

This case study was based on the paper titled “Running the Curve: Preliminary Investigation into Curved Sprinting during football match play” (12). Games during the pre-season (n = 4) were analysed and runs (n = 54) were returned from the analysis. The fullbacks wore GPSports SPI-HPU units (15Hz) in a pocket embedded in the jersey which was positioned between the scapulae. The raw data sets obtained from the GPS units included: latitude, longitude, maximum velocity, instantaneous velocity, instantaneous acceleration, X Coordinates and Y Coordinates. The process that was used to investigate CR demands in rugby league for a fullback was as follows:

Measurements

The two variables of interest were bearing change (Δo) and angle of run (o). Bearing Change was calculated by the current bearing in which the athlete was moving minus the previous bearing 0.07 seconds before (6). This was derived from Latitude and Longitude Coordinates. Angle of Run was calculated by using the angle between the tangent from the run derived by the initial trajectory of run from the first 3 data points and the chord of the start and end point of the run (12).

Data Tidying/Formulas

The Raw Data was exported from TEAM AMS software used with GPSports SPI-HPU units. Data was tidied and saved as a .csv file containing: Cumulative Time, Latitude, Longitude, Filtered Velocity (m/s), Acceleration (m/s/s), Distance (m), X Coordinates, Y Coordinates, Start/End Point of Run. Latitude and Longitude was filtered and smoothed using a 3-point moving average. The bearing of each timepoint within the run was calculated from latitude and longitudinal GPS coordinates using the following formula where φ1,λ1 is the start point, φ2,λ2 the end point and Δλ is the difference in longitude.

Θ = atan2(sin Δλ ⋅ cos φ2 , cos φ1 ⋅ sin φ2 – sin φ1 ⋅ cos φ2 ⋅ cos Δλ)

A problem with calculating bearing changes arose when a player was running at 10o and then changed to a bearing of 350o. Without a workaround, the result from the formula would be a -340o change in bearing. The actual bearing change that was desired was a “20o change” to the left. Therefore, to account for this, when the result was between -180o and 0, the original bearing change formula was used. When the result was less than -180o , the original formula +360o was used. All other results used original formula. This allowed for negative and positive bearing changes, which had implications for specification in direction of bearing change (left or right).

The gradient of the tangent and chord was then calculated using:

m = (y2 - y1) / (x2 - x1).

From here, the y intercept was calculated using the formula:

y - mx = c

where y is the y coordinate of the first point of the sprint and m is the gradient as calculated. Finally, the angle of each

run was calculated using this formula where m1 and m2 are the two gradients previously calculated.

θ = tan-1

(|(m1 - m2) / (1 + m1×m2)|)

This was then converted from radians into degrees to make this more interpretable:

θ = θ × 180/π

Three plots were generated using the ggplot2 and ggridge package on Rstudio and exported to .pdf format (Figure 1).

To be eligible for the plot, the velocity of the run had to be greater than or equal to 5.5m/s and a run duration of greater than 1 second. The ggridge package allowed for analysis of the distribution of values using Joy Plots which was represented by the Bearing Visualisation plot.

Interpretation of Plot

As shown in Figure 2, the plots showed a graphical representation of the run using X and Y coordinates. The blue line represented the chord from start to end point of the run. The red arrow represented the initial trajectory of the run. The intensity of speed and acceleration was represented by the gradient line. In this case, the athlete was running at a velocity between 6m/s and 9.5m/s. The player was also accelerating and decelerating at various points during the run.

The player was shown to be moving in the right direction with a positive bearing change of between 10-15o.

RESULTS

Table 1 - Running demands of a fullback in rugby league.

The runs were categorised into bins of 5o intervals up to 40o. Most of the runs occurred within the 0-5o (n=17) and 5-10o (n=17) but could be greater than 40o in angle (n=2). Within a run, changes from 12o to 24o in bearing were shown. Mean speeds for the runs was 6.36m/s with a maximum of 9.35m/s. Maximum accelerations of up to 3.30m/s/s were observed during CR in gameplay. Run distances ranged from 10.6m to 42.2m.

DISCUSSION

This case study has demonstrated that the fullback performs a higher frequency of curvilinear runs (n=37) compared to linear runs (n=17) during a rugby league game. This is the first case study to investigate curvilinear running demands in rugby league. Previous research conducted in football showed many runs that were performed in gameplay were also between 0-5o (12). The angle in which players run at may also be highly influenced by the various technical and tactical demands of rugby league. The findings from this case study may also have implications for training and testing of rugby league athletes to determine curved running sprint ability. Presently, most of the sprint testing used in rugby league is linear in nature (9).

As shown in Table 1, a fullback may need to perform a curvilinear run that is up to 40o in angle. This may be important for speed training by designing drills that replicate these “worst case” scenarios. This also reinforces the need to consider individualisation as a key principle to athletic development especially amongst elite athletes where capacity for adaptations is limited (18). Considerations around the various positional demands that a rugby league athlete may encounter should be made. For example, outside backs would be subjected to greater amounts of CR performed at higher running speeds compared to middle forwards who often perform short distance high intensity accelerations for hit-ups (13). Athletes should train specifically for the demands of the sport to maximise transfer from training to competition (18). The significance of using high intensity contextual training to develop technical and tactical skills has been highlighted in the literature (1). Although the impacts of running with the ball is less pronounced in elite rugby athletes (2), running with the ball is still an important skill to train so that the athlete is exposed to actions they may encounter during gameplay. Finally, chronic sprinting has been shown to mitigate the risk of hamstring injuries (10).

Future investigation should focus on the energy cost of CR. This “cost” should be based on degree of bearing change and should be allometrically scaled based on body weight. This may be beneficial for practitioners to gain a more holistic understanding of external workloads in rugby league. Currently, the main limitation to using current workload metrics is that it assumes that all work being performed is linear which does not hold true for team sports (5,15). Centripetal force calculations may also be derived from GPS coordinates (14). Additionally, future research should also investigate curvilinear running demands of positions other than the fullback.

PRACTICAL APPLICATIONS

Overall, this case study may provide physical performance staff with a supplementary tool for monitoring running workload. CR is one of the most demanding movements in team-based sports because direction changes are performed concurrently to speeds which are close to or above high-speed running thresholds. It also demonstrates that fullback running demands are mostly non-linear in nature with high frequency of evasion and support play associated with this position. Thus, CR should be a considered as piece of the complex programming puzzle in team-sport. Below is an example of how CR can be applied in programming. It has been shown that male semi-professional football players are able to complete a 17m curved run with a 9.15m radius in approximately 2.5 seconds (15) in a standardised test. As there is yet to be any evidence performed on rugby league players, it is suggested that male outside backs would run a 20m curved run on a 9.15m radius in approximately 3 seconds at maximal intensity. For female outside backs, it would be suggested that they would be able to run a 20m curved run on a 9.15m radius in approximately 3.5 seconds at maximal intensity. In the field, it may be difficult to precisely measure specific angles for the prescription of drills such as arc runs. However, by manipulating the radius of the arc, an approximation of angles can be determined. As shown by Figure 3, for the same given distance, using smaller radii would require the athlete to run a “smaller angle” whereas using larger radii would require the athlete to run a “larger angle.” Due to the intensive nature of arc running, it is recommended that athletes are exposed to linear sprinting drills prior to the introduction of curvilinear running.

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