Postural Control Measures of Neuromuscular Fatigue in Rugby Union
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Managing player load, fatigue and recovery in rugby union is deemed important given the potential impact on performance and injury risk (Cross et al., 2016; Quarrie et al., 2017). Whilst countermovement jump (CMJ) tests are the most common test of neuromuscular fatigue (NMF) in applied sport settings (Taylor et al., 2012), there remain some limitations to the real-world implementation of such testing for regular NMF monitoring purposes (Starling & Lambert, 2018). In particular, the maximal contractile nature of such tests results in reluctance of athletes to perform them in periods of post-match fatigue (Carling et al., 2018). As such, some have proposed tests of proprioception and postural control as novel tests of NMF, which provide the main advantage of being minimally physically demanding and not requiring maximal motivation from athletes (Austruy, 2016; Clarke et al., 2015). Therefore, the aim of this research was to assess the use of postural control measures of single-leg balance and landing on a force plate for monitoring neuromuscular fatigue in professional rugby union players. Initially, the inter-trial and inter-test reliability of postural control measures were determined to understand the inherent variability of these tests (study 1). Then, the acute post-training changes related to specific internal and external load measures (study 2) as well as changes across different accumulated weekly load profiles (study 3) were determined in order to understand responsiveness of the tests. Finally, the use of single-leg balance and landing tests 36h post-match at the beginning of a weekly micro-cycle when monitoring tools are most applicable was investigated (study 4). To do the above, thirty-five male professional rugby union players were recruited to participate in this investigation. Measures of postural control included single-leg balance and landing tests performed on a force plate (9260AA6, Kistler Instruments, Winterthur, Switzerland) sampling at 1000 Hz. Participants performed balance tests by completing two 20 s trials on each leg with hands on hips and eyes closed. Landing tests were performed by jumping off two legs from a starting point 1 m away and landing on one leg in the centre of the force plate. Participants performed three trials on each leg and were instructed to jump as high as possible whilst sticking and holding the landing. To investigate reliability, postural control tests were performed in the morning on the first day of training following 48 h rest on two consecutive training weeks with similar prior loads (study 1). To investigate the responsiveness to acute rugby union training loads, postural control tests were performed prior to training and again immediately following a typical training day consisting of three training sessions (gym, skills, and rugby) (study 2). To investigate the responsiveness to different accumulated internal and external load profiles, postural control tests were performed in the morning on the first day of the week across three distinct load profiles representing normal, spiked, and higher accumulated loads (study 3). And to investigate the responsiveness of postural control tests for monitoring fatigue post-match, postural control tests were performed in the morning on the first day of training following a bye week (rested) and again 7 days later on the first day of training 36h post-match (study 4). The key findings within this research include: 𝗦𝘁𝘂𝗱𝘆 𝟭 • Single-leg balance sway velocity demonstrates acceptable reliability, and superior reliability than sway velocity in the anterior-posterior and medio-lateral directions. • Single-leg landing peak force and impulse demonstrate acceptable reliability, whilst time to stabilization should be used with caution due to the high variability and poor reliability between dominant and non-dominant legs. 𝗦𝘁𝘂𝗱𝘆 𝟮 • Whilst countermovement jump eccentric rate of force development demonstrated the largest impairment following a typical rugby union training day, single-leg balance sway velocity on the non-dominant leg showed similar responsiveness to countermovement jump measures of jump height and concentric impulse. • Bodyload, session rating of perceived exertion training load, and Banister’s training impulse appear to be the main contributing factors to countermovement jump and balance impairment following typical rugby union training days. 𝗦𝘁𝘂𝗱𝘆 𝟯 • Single-leg balance sway velocity was not significantly different between normal, spike, and higher three-week accumulated load profiles. • Accumulated load profiles demonstrated a significant interaction in which single-leg landing impulse decreased over time under spike load conditions and increased over time under higher load conditions. 𝗦𝘁𝘂𝗱𝘆 𝟰 • When monitoring postural control 36h post-match at the beginning of a weekly micro-cycle, most measures were within ranges observed without a prior match load (i.e. rested), however possibly small single-leg landing impulse impairments may remain. • In higher match load groups, single-leg balance sway velocity may also demonstrate impairment 36h post-match. Whilst the current investigation demonstrates potential responsiveness of single-leg balance sway velocity and single-leg landing impulse measures to acute and accumulated loads, the low signal to noise ratio of these tests may challenge their effectiveness. However, as demonstrated by the responsiveness of landing impulse and sway velocity on the dominant leg 36h post-match using specific thresholds for meaningful change derived from reliability results, there remains some potential for identifying individuals experiencing larger residual postural control disturbance resulting from higher match loads. It is likely that such athletes may be averse to maximal performance tests at the beginning of the weekly micro-cycle, and balance and landing tests could provide objective rationale for the appropriate modification of training and recovery. As a practical suggestion for implementation, it may be appropriate to use postural control tests early in the weekly micro-cycle to inform readiness to train, and CMJ tests later in the micro-cycle to inform readiness to perform.
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