Comparative Analysis of Jumping and Landing Velocity of the Young Elite Spik- ers of the Iranian National Volleyball Team While Performing Block Jump

* Corresponding Author: Razieh Yousefian Molla, PhD. Address: Department of Sports Biomechanics, Faculty of Physical Education and Sports Science, Islamic Azad University of Central Tehran Branch, Tehran, Iran. Tel: +98 (912) 2022730 E-mail: raziehyousefian@yahoo.com 1. Department of Sports Biomechanics, Faculty of Physical Education and Sports Science, Islamic Azad University of Central Tehran Branch, Tehran, Iran. Ali Fatahi1 , *Razieh Yousefian Molla1 , Mitra Ameli1


Introduction
pike skill (powerful spike/quick spike) and blocking are the most important predictors of success in volleyball competitions, and, therefore, it is important to study the factors affecting these skills [1,2]. Based on studies, factors such as mean, minimum, maximum, and S resultant velocities of jumping and landing skills play a significant role in the proper implementation and effectiveness of blocking skills and various types of spikes, followed by scoring points [3].
When performing blocking skills by defensive players against the opponent's spikers, these players must have better access to the ball at the top of the net or, in other words, to jump high. It requires special attention to the velocity factor when performing both quick and powerful spikes.

A B S T R A C T
Thus, changes in velocity can lead to differences in players' ability in the blocking position to schedule a maximum jump height and successfully perform this skill against the opponent's spike. So far, no comprehensive research has been conducted to analyze maximum, minimum, mean, and resultant velocities in performing defensive skills by powerful and quick spikers, while players and coaches in this field desperately need this information. This study aimed to compare the jumping and landing velocity of young elite spikers of the Iranian National Volleyball Team when performing block jump skills.

Materials and Methods
A total of 21 elite young male volleyball players of the National Volleyball Team of the Islamic Republic of Iran, including 11 quick spikers (with Mean±SD age of 19±0.77 years, height was 192.18±4.26 cm, and weight was 78.82±2.44 kg) and 10 powerful spikers (with Mean±SD age of 19±0.81 years, the height of 198.60±1.77 cm, and weight of 75.70±3.12 kg) participated in this study as subjects. After calibrating the force plate system (1000 Hz Kistler made in Switzerland), the two groups' subjects separately performed three vertical jumps of maximum blocking with one-minute rest intervals between each performance.
Force data in the X, Y, and Z directions were obtained directly from the force plate's output. Using the trapezoid method, the area under the force-time diagram was calculated as an impulse in each time interval. Based on the equation of motion size and impulse [4], from dividing the mean, maximum and minimum impulse data by the subject mass in each direction, the rate of change of mean, maximum and minimum velocities in that direction was obtained [5]. The minimum, maximum, and mean velocities in each direction (X, Y, and Z) and their results were considered separately for each group's next calculations.
In this study, SPSS V. 21 software was used. Descriptive statistics were used to calculate Mean±SD, and the Shapiro-Wilk test was used to examine the normality of data distribution. An independent t-test was used to analyze and compare the variables of maximum, minimum, mean, and resultant velocities in powerful and quick spikers while performing blocking skills (P≤0.05).

Results
The results of the Shapiro-Wilk test showed that the data distribution was normal. To compare the maximum, minimum, mean, and resultant velocities and examine these variables in the two groups, an independent t-test was used, the results of which can be seen in Table 1.
As can be deduced from the results of Table 1, only the mean velocity on the X and Z planes and the resultant mean velocity between the two groups was significant. The other velocity variables in the two groups of spikers were not significantly related (P˂0.05).

Discussion and Conclusion
No similar research has been conducted on comparative analysis of the maximum, minimum, mean, and resultant velocities among quick and powerful spikers during blocking. Some previous studies have examined velocity in spike skill when jumping or the effect of velocity variable on spikers' jump characteristics. For example, in a similar survey, Ficklin et al. (2014) compared two different types of spike techniques, including "traditional spike" and "swing spike while blocking". Their research results in terms of velocity on the x-axis were consistent with this research, but velocity on the y-axis was inconsistent [17]. Perhaps one of the reasons for the inconsistency was that in this study, lateral displacement in the two groups of powerful and quick spikers occurred to the same extent when blocking. As a result, no significant change was observed in the two groups.
Since the starting technique of jumping skill is different in powerful and quick spikers when blocking. Also, quick spikers have less vertical displacement than power spikers and correct less flexion in their knees before jumping, so the velocity of jump height in them is less. This study's results can help coaches, volleyball players, and professionals pay more attention to the quick execution of the blocking technique by powerful and quick spikers in different directions during the match and designing a training program.

Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed of the purpose of the research and its implementation stages. They were also assured about the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors.

Authors' contributions
All authors equally contributed to preparing this article.