Many athletes target an increase in strength in order to improve sporting performance. Specific to Rugby Union, an increase in strength has shown to distinguish between levels of competition, and as such, measuring strength is required for the implementation and monitoring of effective training programs. Traditionally, one-repetition maximum (1-RM) testing has been implemented as it is a direct measure of dynamic strength that is commonly incorporated into training programs. However, 1-RM testing is not very practical in-season and research suggests it is less safe and produces more fatigue than alternate methods of testing. Isometric testing is an alternative to 1- RM testing that has been shown to be reliable and effective at reducing testing time and reducing fatigue, loading, and injury risk. Anecdotally, however, isometric testing such as the isometric mid-thigh pull, and isometric back squat are less feasible in season due to the injuries Rugby Union athletes obtain. An alternative isometric test is an isometric belt squat which is strictly a lower-body method of evaluating strength; however, to date, the reliability and effectiveness of the isometric belt squat is unknown. This thesis is separated into four chapters. Chapter One is a review of literature that highlights the importance of strength in sport, the changes in strength athletes sustain throughout a season, how reliability can be measured, the traditional methods of strength testing, isometric testing, and the specific methods of isometric testing commonly used in Rugby Union. The literature shows the isometric mid-thigh pull, and isometric squat are reliable methods of measuring strength that have been shown to correlate to 1-RM testing. Previous research comparing the isometric belt squat to an isometric squat show a reduction in lower-back stress and an increase in peak vertical force in the isometric belt squat, suggesting it is a potentially safer and more accurate evaluation of lower-body strength for athletes. However, there is no research into the stability reliability of the isometric belt squat. Furthermore, the literature has shown that while in competition, an increase in skill training results in a decrease in resistance training to balance training load while maintaining concurrent training. Therefore, periodised training programs are required in-season to minimise the negative effects concurrent training has on the maintenance/development of strength. As concurrent training conflicts with an increase in strength within a season and the evaluation of the fluctuations of strength levels helps guide training programs, valid and reliable strength testing is required to inform effective exercise prescription. Chapter Two investigated the reliability of the isometric belt squat. Both between- and within-session reliability were measured during a five-week investigation with each athlete completing the isometric belt squat four times. Intraclass correlation coefficients (ICC), coefficient of variation (CV), typical error (TE), smallest worthwhile change (SWC), and p-values were calculated to assess the reliability between-sessions, whereas a CV was used to quantify within-session reliability. The between-session reliability indicated the isometric belt squat was a reliable measure of strength after each athlete had completed one familiarisation period with a range of measures deemed reliable between Trials 2 and 4 (TE= 504.56 N, CV = 8.7%, ICC = 0.86, p-value = 0.021, SWC0.5 = 647.39 N). The within-session data was also considered reliable which was conducted after the familiarisation period (CV = 7.2%). Overall, the between- and within-session data indicated the isometric belt squat test is a reliable measure once each athlete has completed one familiarisation trial. Chapter Three investigated the change in strength that occurred in professional Rugby Union athletes across a competitive season. This study was conducted over a 15-week block (end of preseason to end of competition) whereby each athlete completed five trials of the isometric belt squat. Effect size (ES), probability statistics, and a percentage of change in strength were used to highlight the change in strength between the five trials. Overall, the change in strength from Trial 1 to 5 was ‘trivial’ (+0.44%, ES = 0.04, p value = 0.871). The ‘trivial’ increase in strength across the season indicates the maintenance of strength from the end of preseason to end of competition is feasible despite the interference concurrent training imposes. Overall, as concluded in Chapter Four, the results from the two studies in this thesis show that the isometric belt squat is a reliable measure of strength both between- and within-sessions once each athlete has completed one familiarisation period, and that strength can be maintained across a professional rugby union season. The limited literature surrounding the isometric belt squats reliability when applied to Rugby Union, and the change in strength across a competitive Rugby Union season is an area that needed to be addressed. This thesis described the literature surrounding isometric testing, identified the stability reliability of a isometric belt squat, and assessed strength changes across a season in professional Rugby Union athletes. This information can be used to enhance strength monitoring in professional team sports such as Rugby Union, Rugby League, Football, and Netball that have access to force plates or strain gauges.