Assessing stability and post-collision stabilisation in elite level Rugby Union athletes

Abstract

Rugby union is a football code incorporating running, jumping, changes of direction and multidirectional high velocity collisions. The demands of the sport make postural stability an area of importance for players, coaches, and medical practitioners. Despite this importance, methods, and practice in the quantification of various elements of postural stability are not well developed or standardised. A rugby union team is commonly split into two position units, forwards, and backs, each with distinct anthropometric, physiological and match action characteristics which present high levels of variance in stability as currently measured in practice. The first study aimed to evaluate current measures of stability used within elite rugby union and assess the value of those measures in a research context. It found that the measure and test protocol utilised, the modified Balance Error Scoring System (mBESS), was appropriate only in assessing individual variances, as scores were influenced by height, mass, positional unit and grouping in each plane of movement, in bi and unipedal conditions. This study concluded that more ecologically valid forms of stability should be investigated. The second study aimed to identify variables which are appropriate for player comparisons and wider research purposes in assessment of both static and dynamic stability. Time to stabilisation (TTS) was demonstrated not to be significantly affected by anthropometric or positional factors and was therefore recommended for use in rugby stability research. Static sway area, assessed using a unipedal Stork test, was identified as a measure of static stability unaffected by playing position. The principal threat to maintaining stability in rugby union match play is the tackle, a match event in which opposition players attempt to bring a player in possession of the ball to the ground to halt their forward progress or remove the ball from their possession. The impact forces involved in this action are increasingly well understood, however the effect of those forces on stability of the ball carrier, and the response to those forces, are not. A novel device was constructed to replicate such impacts in a laboratory setting. The third study aimed to establish the reliability and validity of this device, referred to as the impact machine. The impact machine demonstrated within-session coefficient of variance of <5% in accurately delivering specified forces of between 881 and 4410 N, successfully replicating forces recorded in previous field and laboratory studies. Between-session reliability achieved an inter class coefficient of R = 0.886. Following the establishment of an appropriate test protocol, the fourth study aimed to identify kinetic variables which may influence static, dynamic and impact-response stability in male rugby union athletes. Counter movement jump (CMJ), drop jump (DJ) and isometric mid-thigh pull (MTP) variables were measured and placed into a stepwise multiple linear regression model for each aspect of stability. No relationships were found between CMJ height, maximal rate of force development, DJ height, DJ contact time or peak MTP force and static, dynamic or impact response stability. This suggests different systems influence different aspects of stability and that the physical capacity measures tested, although correlated, are not related to stability. Further work is required to verify these novel results. The stability measures selected removed positional differences in static and dynamic stability assessments but failed to do so in impact-response stability. This is likely due to different anthropometric and force production characteristics between forwards and backs. These findings suggest that stability and stabilisation should be measured in a task and player specific manner, and that TTS is a suitable variable for use in comparison of players and tasks in an elite male rugby population. Future work should look to identify effective stabilising strategies for each type of collision and response, and the physical capacities which may influence them.