## 4 Discussion  
The aim of this study was to assess jump height, covered jumping distance, and number of jumps performed at certain positions by volleyball players competing at the 2014 FIVB Volleyball Men's World Championship in Poland. The analysis showed a high degree of reliability for jump height during serve and attack, jumping flying distance covered during an attack, and number of block jumps. The strongest relationship was seen between jump components, which predominantly depend only on a volleyball player performing a specific action (e.g., jump serve or attack jump). Serving skill was the most unique activity in volleyball. SJH was similar for receivers (R1 and R2) and opposites (O), as well as for middles (M1 and M2) and setters (S). Moreover, it differed significantly between these groups of players. Such results prove that the roles and distribution of offensive and defensive forces are different for individual players. In top-level sport teams, outside hitters (A, R1, and R2) take responsibility for efficacy of jump power serve, which aims to score a point. \"Defensive\" players most often perform a jump float serve (soft), which decreases the risk of making an error related to force exerted from a player during the serve. The game action during which a setter performs a jump float (soft) bases tactics on game complex II (block, defense, setting, and counterattack), and on the block-defense system in particular for accurate and inaccurate serving reception [18; 19]. Even though jump float and jump power serve different purposes in jump technique, no statistically significant differences were observed in the number of jumps or jump flying distance. No statistically significant differences were observed in AJH, which indicates that the action was performed at the maximum possible height level, similar for all elite volleyball players. This is proven by a strong relationship between the actions (0.87 for ICC and 0.87 for CA) and an insignificant variation (0.11 with CV) (Table 1). It can be concluded that elite volleyball players (regardless their playing position) reach similar attack jump heights that are close to the maximum height (96.5% for AJ). Statistically significant differences were observed for JC. The middles (M1 and M2) and the opposites (O) performed the largest numbers of attack jumps. Setters most often set the ball for players on these positions to create a comfortable position for them to score a point. Middle blockers, due to fast pace of setting the ball, have to perform a jump in almost every offensive attack (regardless distribution of the ball). On average, middles (\(33\pm 10\) jumps on average) as well as opposites (\(31\pm 10\) jumps) performed 10 jumps more during a game than receivers (\(22\pm 10\) jumps on average). A number of jumps was also observed by [PERSON] [3]. They assessed over 200 volleyball players of U21 National Teams. The authors partly confirmed the results of the current study, highlighting that the greatest number of attack jumps were performed by middle blockers. Significant differences were seen in the components of block jumps between the middles and the other players. Volleyball players jumped, on average, up to 88.8% of a single maximum block jump. Middles performed twice as many block jumps (average \(44\pm 12\) jumps per match) than other players (O \(=26\pm 9\); R \(=24\pm 7\); S \(=22\pm 7\)), while the heights of their jumps were considerably lower (Figures 1 and 2). The main reasons for this disproportion is the role of the middles during a game, such as their somatic features and opponents' capabilities. To perform emergent actions at a comparable height, middle blockers do not need to perform jumps that are as high as outside-hitters (outside-teammates). Significant differences in block jump were observed between the receivers (R1) and opposites. This may have resulted from volleyball blocking tactics, which can become a repetitive activity at this level of play.