Master of Science (MSc)
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Faculty of Science
Dr. Diane Gregory
Introduction: Rugby scrumming is a means of restarting play following a minor rule infringement that can occur up to 28 times per game. The scrum poses a significant injury risk, with more days missed due to injury per event than any other rugby activity. Rugby players also have a significantly higher rate of spine injury than the general population, perhaps due to flexion in combination with high compression forces, which has been cited as the main mechanism of injury. Flexion of the spine has also been associated with poor hip mobility and quadriceps fatigue in other athletic tasks and may influence rugby scrumming similarly. Therefore, the purpose of this study was to determine if spine flexion, force output and muscle activation are influenced by 1) fatigue and 2) hip mobility in individual machine rugby scrumming.
Methods: Sixteen participants with at least 4 years of rugby scrumming experience were recruited to complete the study. In the initial scrumming block, the participants completed five 5-second scrumming trials with 1-2 minutes rest in between each trial. They then performed a wall sit to fatigue and performed five more 5-second scrumming trials, this time with only 5 seconds of recovery in between each trial. The angle of each spinal region (Lumbar, Thoracic, Cervical), the muscle activation (quadriceps, lumbar and thoracic erector spinae and abdominal muscles) and the force output were all measured throughout each trial. A one-way repeated measures ANOVA was conducted (the factor being pre or post wall sit fatigue) to determine the influence of wall sit fatigue. A related samples t-test was conducted between the first and fifth trial of each block to determine the effect of repetitive scrumming, and correlations were conducted between different measures of hip mobility and the main output measures to determine the relationship with hip mobility.
Results: Wall sit fatigue led to a decrease in Thoracic Erector Spinae (left: p = 0.0003, right: p < 0.0001) and External Oblique (left: p = 0.0009, right: p < 0.0001) activation and an increase in average (p < 0.0001) and max (p < 0.0001) cervical flexion during the contact phase (contact with the scrum machine shoulder pads).
Prior to the wall sit, repetitive scrumming led to a decrease in activation of the Thoracic Erector Spinae (left: p = 0.0109, right: p = 0.0005) and left quadriceps (VM: p = 0.0271, VL: p = 0.0473) during the contact phase.
Following the wall sit, repetitive scrumming led to a decrease in Thoracic Erector Spinae Activation (left: p = 0.0462, right: p = 0.0095), and an increase in quadriceps activation (left: VM: p = 0.0367, VL: p = 0.0419; right: VM: p = 0.0238, VL: p = 0.0213). Further, repetitive scrumming led to an increase in thoracic average (p = 0.0224) and maximum (p = 0.0058) flexion angle and an increase in cervical average (p = 0.0142) and maximum (p = 0.0048) flexion angle. It also led to an increase in Lumbar spine angle deviation (p = 0.0088) and force output deviation (p = 0.0404).
Increased hip flexion range of motion was moderately related to increased impact peak force output (r = 0.55; p = 0.0290). Increased wall sit time was moderately related to increased impact peak (r = 0.52; p = 0.0376) and sustained push force (r = 0.54; p = 0.0376) for the trial prior to the wall sit.
Discussion and Conclusion: It appears that fatigue, whether induced by the wall sit or by repetitive scrumming, tends to lead to a decrease in activation of the Thoracic Erector Spinae and an increase in cervical flexion. This may be due to a variety of mechanisms: a greater extension of the lower limb leading to more compensatory flexion up the kinetic chain, direct fatigue of the Thoracic Erector Spinae, central fatigue acting on the Thoracic Erector Spinae, decreased co-contraction of the trunk as a result of fatigue, and/or disuse of the cervical region during machine scrumming. This relationship needs to be explored using more rugby specific fatigue protocols, as well as in live scrumming. Surprisingly, force output was influenced very little by fatigue, indicating that the individuals were able to use compensatory mechanisms to mitigate fatigue. There also existed very little relationship between hip mobility and spine angle, likely attributable to the lack of hip range of motion used during scrumming. There did appear to be a relationship between hip flexion range of motion and impact force, which may be attributable to a greater available distance of hip acceleration with greater hip mobility, leading to a greater impact. Wall sit time was also positively related to impact peak and sustained push which may indicate a benefit to using this test as a predictive measure to determine force output capacity in an isometric movement such as the rugby scrum.
Clayton, Joshua D., "The Influence of Hip Mobility and Fatigue on Spinal Flexion and Muscle Activation in Rugby Scrum Performance" (2016). Theses and Dissertations (Comprehensive). 1876.