Materials and methods

4.3.1 Participants

Eight elite rugby union players (age 21.0 ± 4.4 years, height 185.0 ± 8.0 cm, mass 90.0 ± 8.2 kg) from the same professional rugby club volunteered for the study. Participants were all healthy and active individuals who had no current injury issues. All subjects provided written

informed consent to participate and Salford University Research and Ethics Committee approved the study.

4.3.2 Jumps

Three types of jump were tested within this study, with two repetitions of each jump

performed on three separate days. Participants were asked to standardise activity levels and dietary intake for the 24 hours prior to testing. The jumps performed included a CMJ, a SJ and a SLDJ. Prior to testing, participants engaged in two familiarisation sessions to ensure that techniques were appropriate and standardised. Participants placed their hands on the hips, for all jumps, to eliminate contribution of arm movement (Taylor, 2012). This was also the method to which all participants were accustomed during regular testing in their sporting environment. In line with previous studies, assessing CMJ and SJ performance, protocols such as hands on the hips throughout the jump and extended legs throughout flight to prevent tucking of the knees, (which had been reported to cause inaccuracies) were implemented (Flanagan, Ebben, & Jensen, 2008; Taylor, 2012). Prior to performing the jumps, subjects were asked to perform five minutes of stationary cycling and two minutes of prescribed dynamic stretching.

4.3.2.1 Countermovement Jump (CMJ) technique

The CMJ was performed from a standing position, with the whole plantar part of the foot touching the jumping surface and the hands resting on the hips. A counter movement was conducted by the participants until the knee angle reached approximately 90°, then

immediately the participants jumped as high as they could, with their legs remaining straight upon flight, therefore preventing any tucked legs which would lead to inaccurate measurement. Upon landing the participants made contact with the testing surface with knees extended, only flexing to absorb the impact once contact had been made with the floor. Participants were encouraged to jump as high as possible, prior to each jump, with all participants receiving verbal feedback about their performance after each jump.

4.3.2.2 Squat Jump (SJ) technique

The participants had hands on their hips throughout the SJ, and when cued the participants moved from a tall standing starting position into a semi squat position, which they held for three seconds before commencing their jump. After the jump participants received verbal feedback about their performance and where encouraged to jump as high as possible, with no

countermovement jump made at the start of the jump, as identified from visual inspection of the force-time data. If countermovement occurred, participants rested for a further 60 seconds and then repeated the attempt. Previous research by La Torre et al. (2010) assessing the effect of starting knee angle during SJ showed that an increase height, peak force, and maximal velocity may occur as a result of angle amplitude. The notion of knee angle was therefore considered within this testing protocol, with an angle of greater than 90° recommended in accordance with the research by La Torre et al. (2010).

4.3.2.3 Single Leg Drop Jumps (SLDJ) technique

Participants were instructed to place hands on the hips throughout the SLDJ and instructed to “jump for height” when landing in the area directly below the raised starting box position. The box that the subjects started on for the SLDJ was 30 cm high and the box the subjects finished on was 14 cm above the floor where the lasers lay, enabling them to perform correctly this unilateral movement.

4.3.3 Procedure

Despite the testing being conducted post-match, the time-points of assessment were consistent throughout this study and the training and match protocol prior to testing commencing on each occasion was standardised. In addition, the players tested within this study were accustomed to games, as this testing was conducted during the competitive phase of the players’ season, meaning no differing effects of game fatigue could have altered results between weeks. The order of the jumps was standardised, to minimise the effect of fatigue or order during subsequent testing sessions, with one-minute rest intervals between jumps allowing restoration of the phosphagen system to ensure maximal effort. As the unilateral jump involved the most eccentric forces, SLDJ’s were performed last. SJ and CMJ were performed first and second respectively, as they were deemed less fatiguing and are bilateral in nature, providing a natural progression towards unilateral jumps. The CMJ was seen as a good progression from the SJ, hence it was the second jump to be performed. Additionally, in accordance with previous research, it was thought that the order detailed above for these jumps could potentiate each jump (Harman et al., 1990), giving maximal results. Possible potentiation within this study, however, was not considered to be of particular concern, as the jumps used were commonly performed by the elite athletes participating and despite the order being standardised, a protocol of jump technique and the activities performed in the days prior to testing was replicated throughout.

4.3.3.1 Instrument

The instrument used within this research to assess jump height was the OptoJump (Microgate, Bolzano, Italy), which was shown to have excellent reliability (Glatthorn et al., 2011), with recommendations for the use of flight time previously reported (Cormack, Newton, McGuigan, & Doyle, 2008) and used in previous research (Buchheit et al., 2008). An OptoJump is described as an optical measuring system consisting of transmitting and receiving bars containing light Emitting Diodes (LED), which communicate with each other. OptoJump systems are presently used by many elite sports teams, making it possible to measure flight and contact times with an accuracy of 1/1000 of a second. The OptoJump was placed on the gym floor, with one box either end of the OptoJump to enable performance of the SLDJ. The sampling frequency was 1000 Hz and the sensors were located 3 mm from the testing surface upon which the OptoJump

bars were placed. The OptoJump was 1 m in length with spacing between sensors being 1.041cm apart, meaning that there were 96 sensors per metre of OptoJump.

4.3.3.2 Jump Height and Flight time

Jump height was calculated via flight time using the following equation utilised by McMahon et al. (2015) and adapted from that proposed by Bosco et al. (1983).

Jump Height = (9.81 m.s-2 _{x flight time}2_{) /8}

To ensure accuracy and reliability, participants used a consistent landing technique on CMJ and SJ, where the legs and hips are extended until contact was made with the floor. Flexing of the knees or the hips delays contact with the mat and therefore distorts flight time.

4.3.4 Statistical Analyses

All statistical analysis was conducted on SPSS for windows, with an a priori alpha level set at p < 0.05. Within-session reliability was tested via intraclass correlation coefficients (ICC) (Model 3, 1). Between-session reliability was determined using both ICC and RMANOVA with

Bonferroni post-hoc analysis, or non-parametric equivalent, assessing learning effects. Partial eta squared was reported as recommended by Cohen (1988) and calculated to see if there were any meaningful differences between testing days. Despite the sample population being elite and therefore difficult to recruit for, the low sample size (n = 8) involved in this study meant that effect sizes were calculated. In accordance with the views of Buchheit (2016), the sole use of p values should be discounted against as they are sample size dependant. Effect sizes (ES) were also determined using the Cohens d method, and interpreted based upon the criteria suggested by Rhea (2004)and interpreted as follows; trivial = < 0.25, small = 0.25 - 0.5, moderate = 0.50 - 1.0 and large > 1.0. Post-hoc statistical power was calculated using G Power 3.1 (Faul et al., 2009), for a large effect size of 0.5, a total n = 8 was sufficient to deliver an actual power of 0.76. The reliability was considered acceptable if the ICC r≥0.8 (Cortina, 1993). SEM and smallest detectable differences (SDD) were calculated to provide information upon whether a change in an individual’s performance is significant, with SEM calculated using the formula: SD pooled ∗ 1−ICC and SDD calculated from the formula: ( 1.96∗ 2 ∗

SEM). Limits of agreement were represented using Bland-Altman plots within sessions for day one of testing.