1. Introduction
The sport of hurling is an intermittent field sport that incorporates a variety of different explosive actions such as jumping to compete for possession in both offensive and defensive situations, performing sprint accelerations and cutting motions to change direction quickly to evade the opposition (see
Figure 1) [
1,
2]. These aforementioned explosive actions can be crucial as they often occur close to the ball, potentially determining the outcome of key events during match-play [
3]. Jump, sprint and change in direction performance are related to reactive strength performance [
4]. The drop jump (DJ) exercise in a strength and conditioning program may improve reactive strength performance, and to measure reactive strength performance, a DJ exercise test is usually employed [
5,
6,
7].
The DJ is a plyometric exercise that utilizes the stretch-shortening cycle (SSC), which is activated via a preloading countermovement [
8]. The DJ requires a subject to drop off a pre-selected height and, on landing on the ground, perform a maximal vertical jump while attempting to minimize ground contact time (GCT) [
8]. In addition to its role in athlete assessment, the DJ is a popular plyometric training exercise and has been used to enhance a variety of performance indicators, both acutely and chronically, in addition to muscle force, power, strength and rate of force development [
9,
10,
11,
12,
13,
14,
15,
16]. Given the benefits of the DJ in terms of performance training and athlete monitoring and as a performance test, the reliability of performance variables in hurling players is of upmost importance.
Previous research on the DJ has emphasized the reliability of variables such as reactive strength index (RSI), jump height (JH) and GCT. Previous research in professional basketball players showed that when performing DJs from 0.20 m to 0.50 m drop heights, the coefficient percentages (CV%) for the RSI and JH were ~2.0–4.5% and 2.5–3.5%, respectively [
17]. Similar findings were reported for RSI (ICC = 0.93, CV% = 8.47%), GCT (ICC = 0.89, CV% = 8.93%) and JH (ICC = 0.84, CV% = 8.96) on international level rugby union players from a 0.40 m drop height [
18]. RSI (ICC = 0.99), GCT (ICC = 0.98) and JH (ICC = 0.99) have also been shown to be reliable measures from a standardized drop height of 0.30 m in track and field athletes [
8]. When considering kinetic variables for the DJ, mean force (MF: ICC = 0.93; CV% = 4.5%) and peak force (PF: ICC = 0.86; CV% = 8.4%) met appropriate standards of reliability, although time to peak force (TTPF) was deemed unreliable (ICC = 0.77; CV% = 9.1%) from a 0.30 m drop height [
19]. Unreliable results have been previously found (ICC = 0.67 and CV% = 0.66) for mean power in a DJ from 0.20 m and 0.40 m drop heights [
20]. This study concluded that mean power was unreliable (ICC = 0.67, r = 0.66) when using the MyJump2 smartphone application. In addition to the contrasting findings between studies regarding reliable DJ performance, kinetic and kinematic measures, there is a dearth of studies of hurling players. Establishing the reliability of performance, kinetic and kinematic variables in the DJ exercise may contribute to the body of knowledge of the mechanisms underpinning different intervention protocols.
Therefore, the aim of this study was to estimate the intra-day and inter-day reliability and usefulness of performance (JH, GCT and RSI), kinetic (force, power, eccentric rate of force development [E-RFD] and leg stiffness [LS]) and kinematic (velocity) variables in hurling players. Based on the key references in the field, the authors hypothesize that all kinetic and kinematic DJ variables were reliable, both for intra-day and inter-day [
17,
18].
4. Discussion
The intra-day results of this study found the JH and RSI performance variables, as well as the kinetic and kinematic variables of PV, PP (absolute and relative) and E-RFD (absolute and relative), to be reliable from all drop heights as the ICC and CV% values achieved the required criteria of >0.70 and <15%, respectively (
Table 2 and
Table 3). However, the GCT performance variable was found to be reliable from drop heights of 0.20 m and 0.30 m only. Similarly, the PF (absolute and relative) and kinetic and kinematic variables were estimated to be reliable from 0.20 m, 0.30 m and 0.60 m drop heights only.
The JH and RSI findings are in agreement with previous research, where they were found to be reliable across drop heights ranging from 0.20 m to 0.50 m in athletic populations [
8,
17,
31]. Conversely, the GCT finding is conflicting with the previous literature, as GCT has also been found to be reliable across the same range of drop heights [
8,
17,
32]. However, in this study, the GCT variable only achieved the required criteria to be considered reliable from drop heights of 0.20 m and 0.30 m. The authors suggest that the lack of reliability in the GCT variable from 0.40 m, 0.50 m and 0.60 m drop heights may be due to the subjects’ training experience. Although all subjects in this study had a minimum of 1 year of plyometric training experience, they may have been unfamiliar with the DJ exercise, leading to the low ICC reliability of GCT from the highest three drop heights. Another possible explanation for this could be the increase in stretch load in the higher drop heights as greater eccentric demand occurs as a result of the higher drop heights, which could potentially cause the subjects to become overloaded, thus leading to the GCT and LS variables becoming unreliable from 0.40 m to 0.60 m and 0.40 m to 0.50 m drop heights, respectively [
33]. The lack of reliability of the PF (absolute and relative) and LS variables from the 0.40 m and 0.50 m drop heights may be caused by differences in jump strategy between trials. Significant differences have been shown in RSI, GCT, JH and take-off time when using two different jump strategies [
34]. Hence, it is possible that an altered jump strategy, along with the unfamiliarity of the subjects with the DJ exercise, may lead to low reliability in PF and LS from specific drop heights [
32]. An altered jump strategy may also explain the significant difference observed between trials for the E-RFD (absolute and relative) variables from a 0.30 m drop height. These variables should be interpreted with caution due to low reliability levels.
When comparing the TE to the SWC (0.2), performance variables were found to be ‘marginal’ to ‘okay’ at detecting a small change in JH, GCT and RSI from all drop heights. However, all variables were deemed ‘good’ at detecting a moderate change (SWC [0.5]) in performance from the same five drop heights. This finding suggests that the DJ test may not be an appropriate daily monitoring tool for this population due to its inability to detect SWC. However, the usefulness of a performance test may depend on the familiarity of the subjects with the testing protocol, and the TE may be reduced as a result [
35]. This could result in making the DJ test more sensitive to detecting the SWC and, hence, making it a more appropriate athlete monitoring tool due to its more consistent results in performance test trials [
35].
The Inter-day results show that all performance variables (JH, GCT and RSI) were estimated to be reliable from all five drop heights, as they achieved the desired reliability criteria of ICC > 0.70 and CV% < 15% (
Table 5). Similarly, all kinetic and kinematic variables were estimated to be reliable based on the same criteria, except for relative PF from 0.40 m and 0.60 m drop heights, E-RFD (absolute and relative) from 0.30 m and 0.40 m drop heights and LS from a 0.60 m drop height (
Table 6).
The results of the performance variables are in agreement with the previous literature, where JH, GCT and RSI were deemed reliable inter-day from a range of drop heights (from 0.30 m to 0.60 m) in hurling players [
32]. Similarly, high levels of reliability have been reported in elite-level rugby players for all performance variables from a standardized 0.40 m drop height inter-day [
18]. Therefore, DJ performance variables are reliable and can be used by practitioners for performance testing or training reasons. Similarly, the kinetic and kinematic variables of PV, absolute PF and PP (absolute and relative) also met the reliability criteria set from all five drop heights. Relative PF (0.20 m, 0.30 m and 0.50 m), E-RFD (absolute and relative) (0.20 m, 0.50 m and 0.60 m) and LS (0.20 m, 0.30 m, 0.40 m and 0.50 m) also met the reliability criteria for these specific drop heights. The lack of reliability of these kinetic and kinematic variables from specified drop heights may be due to athlete motivation. Athlete motivation may have been altered by either individual or team performance levels over the course of the in-season period. This may have had an effect on the intra-day reliability of these variables due to the ~14–18-day time period between the first and last testing sessions. Altered levels of motivation throughout this period may have influenced the subjects’ performance during a testing session, thus influencing the inter-day reliability of certain DJ variables. Hence, these variables should be interpreted with caution for the specific drop heights due to low reliability levels.
The usefulness of inter-day reliability statistics follows similar trends to intra-day, where most variables are unable to detect the SWC, hence making them not useful as weekly monitoring tools. The GCT variable was rated as ‘good’ at detecting the SWC. However, the TE for GCT from all five drop heights was zero. This was because the mean square error (MSE) used to calculate the TE from the one-way repeated measures ANOVA was reported to three decimal places, thus giving a value of zero for MSE, which consequently provided a TE of zero. Hence, the usefulness of GCT at detecting the SWC inter-day is still unknown.
A limitation of this study was that it took place during the players’ in-season period. Although subjects were asked to abstain from vigorous exercise in the 48 h prior to testing sessions, it is unknown if the subjects adhered to this instruction. It is also possible that accumulation of fatigue occurred due to in-season training demands and match scheduling, which could decrease jump performance up to 72 h later, thus potentially influencing the results of this study [
36]. Future research could aim to estimate the reliability of performance, kinetic and kinematic variables during countermovement and DJ performance in hurling players in a well-rested state from their previous training session or competition. This could be conducted by measuring psychobiological markers or performance outcomes, such as CMJ height, isometric force or RFD.
All variables met reliability criteria intra-day except for GCT (0.40 m, 0.50 m and 0.60 m), PF (absolute and relative; 0.40 m and 0.50 m) and LS (0.40 m and 0.50 m) from those specific drop heights. In terms of usefulness, all intra-day variables were rated ‘marginal’ to ‘okay’ at detecting a small change and ‘good’ at detecting a moderate change. Similarly, all variables achieved the required criteria inter-day except for the relative PF (0.40 m and 0.60 m), E-RFD (absolute and relative; 0.30 m and 0.40 m) and LS (0.60 m) from these specific drop heights. In terms of usefulness, all inter-day variables were rated ‘marginal’ to ‘okay’ at detecting a small change, except for GCT, which was rated ‘good’. The relative peak force and leg stiffness variables rated were ‘marginal’ at detecting a moderate change. JH, GCT, PV and peak power (absolute) were rated ‘good’ at detecting a moderate change, and RSI, peak force (absolute), peak power (relative) and E-RFD (absolute and relative) were rated ‘marginal’ to ‘good’ at detecting a moderate change. In conclusion, practitioners have multiple reliable performance, kinetic and kinematic DJ measures for performance testing and training purposes. These variables could also provide the mechanisms underpinning SSC changes in relation to jumping and sprinting in club-level hurling players.