## 4 Discussion  
The most important finding of this study is that the small change in technical execution of a CMJ test was associated with subsequent JH in professional volleyball players when the measurements were performed using the same device. Introducing more rigorous testing procedures in S2 and changing execution of the CMJ during the flight phase led to a statistically significant decrease in JH. Also, individual analysis of players' jumps indicated that a standard deviation in JH data drastically decreased by approximately 30%. It highlights the importance of carrying out the measurements in the same rigorous manner with special attention to execution of the test. To our knowledge, this kind of investigation has not been carried out so far. Monitoring fluctuations in weekly CMJ measurements during the competitive season was described in division I basketball players [40] but not within professional athletes. A large amount of data was taken into the statistical analysis (540 jumps in S1 and 468 in S2; 1008 jumps in total) so the results are not based on three or four measurements that took place during the competitive season [38,39]. The results prove that the testing procedures need to be carried out with particular caution to assess CMJ height appropriately. As the practitioners are often not able to afford 'gold standard' devices and they are forced to use cheaper alternatives during in-field testing, they should be aware of the limitations of the device implemented into measurements [50]. [PERSON] et al. [28] highlighted some key recommendations regarding testing procedures to improve its reliability in sports science. They placed the emphasis on describing testing considerations regarding various subjects such as equipment, environmental conditions, tester's and athlete's familiarization with the test and equipment. Researchers underline the importance of executing the test in the same manner [27,28], and despite having instructions as to how the test should be performed, a discrepancy within execution of the test by different investigators can be observed [26]. Our work confirmed that the proper execution of the test is critical in obtaining reliable results, especially when no 'gold standard' device is introduced into measurements. Our measurements that took place in S1 could be considered suboptimal as we allowed more freedom in the movement to imitate athlete's technical action (as in volleyball block), and it is in opposition to current recommendations [26]. These measurements were examined in the same manner consistently, but the addition of one technical cue in S2 changed the dynamics of the measurements. The approximately 4% decrease in JH and decrease in standard deviation by approximately 30% is a discrepancy that would probably not be expected while using the same testing device. Contact mats are considered as an appropriate tool to test vertical JH for slow stretch-shortening cycle as in CMJ [34,51], and the particular mat used in this study was also suggested to be appropriate [41, 42, 43]. However, some authors indicate that contact mats are appropriate to test JH in college-aged \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Mean Individual Result** & **Standard Deviation of** & **Mean Individual Result** & **Standard Deviation of** \\  & **of Subjective** & **Individual Result of** & **of Subjective** & **Individual Result of** \\  & **Monitoring in S1** & **Subjective Monitoring in S1** & **Monitoring in S2** & **Subjective Monitoring in S2** \\ \hline
1 & 6.32 & 0.61 & 6.71 & 0.51 \\
2 & 6.82 & 0.58 & 6.6 & 0.57 \\
3 & 7.11 & 0.68 & 7.07 & 0.68 \\
4 & 6.58 & 0.57 & 6.63 & 0.63 \\
5 & 6.59 & 0.74 & 6.8 & 0.76 \\
6 & 6.44 & 0.69 & 6.65 & 0.6 \\
7 & 6.63 & 0.74 & 6.7 & 0.69 \\
8 & 6.56 & 0.42 & 6.52 & 0.44 \\
9 & 6.38 & 0.49 & 6.56 & 0.64 \\ x & 6.60 & 0.61 & 6.69 & 0.61 \\ SD & 0.24 & 0.11 & 0.16 & 0.10 \\  & \(p\) & & 0.173 & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Individual results of players’ subjective monitoring.