Between-session reliability of a single CMJ per testing day

Between-session reliability was high for the CMJ (ICC = 0.986; d = 0.153; SEM 0.003; SDD = 0.01, 2.4%) (Figure 6.1), with trivial and non-significant differences (p > 0.05) noted between days.

Figure 6.1: CMJ height for testing days one and two

6.5

Discussion

As was hypothesised, a single CMJ can be considered to be a reliable jump modality for assessing between-sessions performances. Results from this study followed a similar pattern to that reported in Chapter 4 and 5, with a single CMJ measured via OptoJump appearing as highly reliable (r = 0.906) between sessions. Despite Chapter 4 showing differences existed between session one and session two, when assessing CMJ performance, this particular investigation shows a contrary view: that the performance of a single CMJ (measuring jump height) is a reliable measure for assessing post-match levels of readiness.

Support for the reliability of OptoJump has previously been reported by Glatthorn et al. (2011), who noted very high ICCs for both OptoJump CMJ height and CMJ height measured on a force plate, despite systematic differences occurring between instruments (-1.06 cm; p < 0.001). Glatthorn et al. (2011) noted excellent test-retest reliability of the OptoJump CMJ height measurement (ICC 0.989; CV 2.2%). However, the jump protocol implemented between this experimental study and that of Glatthorn et al. (2011) differs, whereby, for example, the subjects used were not considered to be elite within the research by Glatthorn et al. (2011), meaning that, as a result, clear comparisons should be not be made. The misalignment of the photoelectrical cells associated with the OptoJump could also have explained some of the differences observed, such as the concern that a subject’s foot might have broken the electrical cell, but not yet landed on the force plate. Similarly, when assessing reliability of jump

measures, Jensen et al. (2011) presented high reliability (ICC 0.966; CV 5.1%) for OptoJump

using the flight time calculation reported earlier (Chapter 5.3.2.3). When considering that the 0 5 10 15 20 25 30 35 40 45 50 Day 1 Day 2 Ju m p H ei gh t ( cm ) Testing Days

research by Jensen et al. (2011) involved similar methodology and perhaps most importantly, rugby union players, comparisons between these results are warranted.

More recent research specifically assessing CMJ performance within rugby players by Gathercole, Sporer, and Stellingwerff (2015) is also of interest. Gathercole, Sporer, and

Stellingwerff (2015) noted that flight time and peak displacement corresponded with increased training load. One might therefore assume that the results by Gathercole, Sporer, and

Stellingwerff (2015) indicate that jump height decreased alongside increases in training volume, yet the opposite is true. It is interesting for practitioners to note that there was an absence of change in the jump height variable itself. Results from the research by Gathercole, Sporer, and Stellingwerff (2015), assessing the response of CMJ performance to increases in training load in elite female rugby players, cannot be compared to the absolute CMJ values presented in this study, as both the gender of the subjects and fatigue created differ. Readers are also advised to note that the assessment of jump height used within the research by Gathercole, Sporer, and Stellingwerff (2015) incorporated a different testing protocol and that the assessed data was taken from within training periods and not from testing post-match play, as was the case in this study.

As reported in Chapter 5, when assessing two CMJ performances using the OptoJump, a SDD of 1.7% was noted to be of interest to practitioners working in the elite rugby environment. Data from this study also adds to the knowledge base, with, for example, the value of 2.4% change for a single CMJ being smaller than the value reported by Glatthorn et al. (2011). Glatthorn et al. (2011) noted high ICC values (mean 0.986), low coefficient of variation (2.5%) and low random errors averaged (2.87 cm), therefore supporting CMJ as reliable for detecting changes in longitudinal assessments. When trying to identify NMF using CMJ, jump height, assessed from flight time using the force plate, has been considered the most precise and reliable, yet as previously explained in Chapter 5, for most team-based scenarios a force plate is not readily available. When considering the financial outlay associated with force plate use, the likelihood of rugby clubs being able to implement such technology is unlikely; therefore the OptoJump

would be the next best alternative. This study adds to the knowledge base by identifying a difference of greater than 1 cm (2.4%) in jump height signifying meaningful change between sessions, while recommending the assessment of a single CMJ performance.

6.5.1 Limitations of this study

One potential limitation of this research surrounds the relatively small sample size (n=12). Despite the sample size being larger than those used in the investigations reported in Chapters 4 and 5, an even larger sample size would perhaps have presented greater depth and breadth to the knowledge base of a sole CMJ. Alongside sample size, another potential limitation of this study surrounds the self-selected CMJ protocol of the players. Players were noted potentially to have adopted differing CMJ techniques, mainly whereby depth of CMJ on the downward phase varied and width of stance was individually selected. These individually selected jump

techniques could, therefore, have altered results. It could, however, be argued that the variability in CMJ technique improved the ecological validity of this study, as not all players would normally move in the same manner especially during the competition phase of an elite rugby union playing season. Fatigue and physical mobility issues due to the physical nature of the games and the training that the players undertake, are factors that can commonly influence movement and technique.

6.6

Practical Applications

From this research, one could conclude that a learning effect does not exist between testing days for performance of a single CMJ. As a consequence, contrary to the views of past research a single CMJ measured via OptoJump appears to be a highly reliable measure of assessing between-sessions performances. Future practice recommends a meaningful change of 2.4% (1 cm) as being of note for practitioners working in the elite rugby environments, when assessing CMJ performance between sessions. This study therefore adds to the knowledge base existing for between-session assessments of CMJ performance in elite rugby union players, supporting the OptoJump as the next best alternative for assessing a single CMJ performance when force plates are not available.