A biomechanical analysis of running load in professional rugby league

Professional rugby league athletes are exposed to a large amount of physical load each week. Global Positioning Systems (GPS) are commonly used to calculate external load experienced by athletes. However, external load calculated from GPS may not be representative of mechanical forces exerted on skeletal structures, such as those experienced at the foot-ground interface during running as external mechanical load. Modern wearable technology such as inertial measurement units, can be placed directly on the lower limb and may provide further insight into the external mechanical load experienced by athletes than is currently available, while complimenting the information collected from GPS units. Therefore, the purpose of this thesis was to explore the use of modern wearable technology and its application in professional rugby league.

The first research paper, Chapter Two, reviews the published literature characterising the physical demands of rugby league match-play. Often studies which attempt to characterise the physical demands of professional rugby league match-play have relatively small sample sizes, based on only one or two clubs, making generalisation of findings difficult. A total of 30 studies were included in the meta-analysis and 16 variables were quantitatively assessed. Results of the meta-analysis determined that half of the synthesised variables demonstrated significant differences between position groups, indicating that each playing position in rugby league has unique demands. These results highlight that positional specific training is required to best prepare athletes for the demands of competitive match-play and that external load experienced may be position-specific. The manuscript outlining the demands of rugby league has been published as: Glassbrook, D. J., Doyle, T. L. A., Alderson, J. A., & Fuller, J. T. (2019). The demands of professional rugby league match-play: A meta-analysis. Sports Medicine - Open, 5(1), 24.

The second research paper, Chapter Four, expands on the findings of Chapter 2, and specifically examines the most intense periods of rugby league match-play. Therefore, providing insights into an aspect of match-play which may not only contribute to match outcomes, but also to recovery times after matches due to the high intensity nature of this movement. This study also aimed to determine which physical tests correlate with peak match-play running performance. The relationship between generic tests of physical capacities, and match running performance were determined by physical tests performed in the preseason period, and peak match running results from the first three matches of the subsequent Australian National Rugby League competition, in sixteen professional rugby league players from one club. The aerobic, lower body strength and power, and upper body power capacity of each player were considered. Their physical capacities were directly compared to an analysis of the most intense periods of early season match running performance. The results of this study highlight that not all generic tests of physical qualities are related to peak match running performance, and only those with significant correlations are likely to be able to indicate how players may perform during match-play. The manuscript outlining the peak intensity demands of professional rugby league match-play is in press as: Glassbrook, D. J., Fuller, J. T., Wade, J. A., & Doyle, T. L. A. (2020). Not all physical performance tests are related to early season match running performance in professional rugby league. Journal of Strength and Conditioning Research.

In an effort to develop an instrumentation strategy that would allow for direct measurement of acceleration load the third research paper, comprising Chapter Five, examines the effect of inertial measurement unit location on lower limb segmental acceleration magnitude and acceleration trace pattern. Accelerometers are commonly placed on the tibia to measure segmental accelerations; however, the dorsal foot may be a preferred attachment location in an applied sporting setting to minimise injury risk. Sixteen recreationally active participants performed a sprint protocol on a non-motorised treadmill. Accelerometers were positioned bilaterally on the medial tibia, and bilateral dorsal foot surfaces. Unsurprisingly, resultant accelerations were statistically greater at the dorsal foot than the tibia on both sides for 50 - 60% of the gait cycle. Larger accelerations at the dorsal foot, than at the distal tibia can be explained by a greater range of movement (degrees of freedom) of the ankle joint, and by the sensor placement location relative to the hip. That is, a larger centripetal acceleration at the dorsal foot compared to the tibia due to the greater radius from the centre of rotation (hip). The results of this study highlight that dorsal foot location can be used to effectively measure accelerations when it is not feasible to place the accelerometer on the tibia, however, results between the two locations should not be compared. The manuscript outlining the differences in accelerations measured at the tibia or dorsal foot while sprinting has been published as: Glassbrook, D. J., Fuller, J. T., Alderson, J. A., & Doyle, T. L. A. (2019). Foot accelerations are larger than tibia accelerations during sprinting when measured with inertial measurement units. Journal of Sports Sciences, 38(3), 248-255.

The fourth research paper, Chapter Six, takes the results from Chapter 5, and employs wearable technology in an applied sports setting to quantify acceleration load and asymmetry at the lower limb. Current analysis methods of lower limb symmetry during match-play employ GPS technology that may not be best suited to the task. Instead, research grade accelerometers placed directly on the dorsum may provide better information about load symmetry. This study proposes a new technique to quantify acceleration load, and lower limb asymmetry during on-field team sport play using inertial measurement units. This technique analyses the IMU acceleration data in a novel way, by area under the curve (AUC) and percentage of time (%Time) spent in seven acceleration categories (negative to very high, < 0 g to > 16 g). Four professional rugby league players wore two accelerometers, one attached to each foot by the boot laces, during match simulations. Clinically significant asymmetry as measured by AUC was observed for all but one participant, and only in negative (< 0 g) and very high accelerations (> 16 g). The results of this pilot study highlight that a wearable located on the footwear is feasible to use during rugby league match-play. The proposed technique is able to quantify acceleration load and detect lower limb asymmetries during match-play at the source of impact and loading, and is therefore likely to provide information about external mechanical load that current torso methods do not capture. This manuscript has been published as: Glassbrook, D. J., Fuller, J. T., Alderson, J. A., & Doyle, T. L. A. (2020). Measurement of lower-limb asymmetry in professional rugby league: A technical note describing the use of inertial measurement units. PeerJ, 8:e9366.

The fifth and final research paper, Chapter Seven, was the culmination of this thesis as it brought together a number of data sources and new analysis techniques to quantify how acceleration load is transmitted through the body in response to an extended running protocol. Nineteen males participated in two sessions, a maximal aerobic fitness test to determine individualised treadmill speeds for the second session, and an extended running protocol. In the second session participants wore a portable metabolic system, and four accelerometers, one on each foot, the lower back, and the upper back. In both sessions, the participants ran on an instrumented treadmill, and in the second session ground reaction forces (GRF) were measured via the instrumented treadmill. The running protocol elicited statistically significant increases in all metabolic variables over time. No statistically significant changes in any peak impact accelerations from the accelerometers were observed. Statistically significant AUC results were not observed at the feet, but were observed as significant increases at both upper body locations. No statistically significant changes were observed in impact peak GRF, but were observed as significant increases in active peak GRF. The results of this study indicate that the effect of an extended running task on acceleration load are manifested in the upper body, and are effectively measured by AUC. The manuscript detailing the changes in acceleration load, in response to an extended running protocol has been submitted as: Glassbrook, D. J., Fuller, J. T., Alderson, J. A., Wills, A., & Doyle, T. L. A. (2020). Changes in acceleration load as measured by inertial measurement units manifest in the upper body after an extended running task. The Journal of Applied Biomechanics.

Collectively, the results of this thesis address an important gap in the professional rugby league literature by providing insights into the application of wearable technology for assessing the physical demands of professional rugby league match-play. This thesis demonstrated 1) GPS is capable of measuring the global and peak external running demands of professional rugby league match-play, 2) IMU placement location will significantly influence the acceleration load results during running, 3) acceleration load at the lower limb can be quantified during team sport match-play with an accelerometer, and 4) acceleration load changes, measured by an accelerometer, manifest in the upper body during a prolonged running task.