## 4 Discussion
Naturally, their playing time could slightly differ, but they generally practiced a similar amount of time (as hours of practice are restricted by the club environment) and participated in nearly the same amount of official games (30 games in S1 and 33 games in S2). Thus, it cannot be said that the smaller mean JH negatively influenced players' volleyball performance as they advanced to the play-off series in S2 and could not achieve it in S1. Also, according to Table 7, players' subjective readiness to train did not significantly differ between S1 and S2 (was even slightly higher in S2) so we can assume that their subjective readiness to measurements was similar in S1 and S2 and did not affect the results (Table 7). Another argument may concern the lack of measurements involving one repetition maximum (1 RM)of multi-joint lower body movements such as deadlifts, squats or power cleans. The latest research indicates that the measurements of this type are not commonly introduced within the high-performance environment during the competitive season [18]. The isometric mid-thigh pull (IMTP) test is the most popular choice within the practitioners (35%), but it was not performed within a population of our study due to the lack of measurement device. Despite not performing frequent IMTP test or 1 RM or 3 RM measurements of multi-joint lower body movements, the athletes regularly participated in resistance training that involved different forms of exercises such as squats, trap bar deadlifts, power cleans, etc., to promote their maximal strength and rate of force development. Therefore, we can assume that their level of maximal strength most likely remained unchanged within the competitive seasons and was not a limiting factor to CMJ measurements. However, as the athletes' training prescriptions changed weekly to optimize their performance during the competition, it should be taken into consideration that their physical condition could also slightly vary during the measurements because of these frequent changes. Apart from characteristics describing physical condition of the athletes during the measurements, a variety of other external factors could also be associated with their performance. Despite having the same recovery modalities through both competitive seasons (ice baths, sauna, pneumatic compression, visits with a qualified physiotherapist), the athletes had a freedom to choose their recovery modalities on a daily basis. As it was not taken into consideration during the measurements, it could also play a small role in players' results. Additionally, the athletes were acquainted with general guidelines of sports nutrition, but their caloric intake was not strictly controlled during the measurements, and it could influence their energy level and recovery. Also, in addition to diet, hydration may play a role in athletes' performance and can affect individual fluctuations in an athlete's body mass [54; 55]. Another factor that influences performance is sleep [56], and its restriction was found to impair athlete's maximal jump performance [57]. As the competitive season is long and exhaustive, mental fatigue could also arise and influence an athlete's mental health and motivation [8; 9]. Thus, the psychological factors could also play a role in an athlete's result during the measurements. Despite the inability to control all the external variables that could be associated with CMJ performance, it should be highlighted that apart from the proper execution of a jump, a number of other factors may correlate with results of the test. Our study has practical applications and can be transferred to high-performance environments. Despite force platforms being a 'gold standard' for CMJ measurements, the practitioners are often forced to use cheaper alternatives in in-field testing, and contact mats are often chosen for this matter. Our results provide an additional context to already existing guidelines regarding testing procedures [26; 27; 28].