are types of vertical jumps, and constitute the most useable tests for the assessment of the neuromuscular coordination of athletes (Claudino et al., 2017). In addition, CMJ is used for the assessment of the fitness level of injured athletes after rehabilitation periods, and for detecting their fitness to return to play (Clan- ton, Matheny, Jarvis, & Jeronimus, 2012; Henderson, Barnes, & Portas, 2010; Olsson et al., 2013). Force- time (F-T) curve variables of CMJ are utilised to assess neuromuscular and biomechanical features related to lower extremity dynamics (Claudino et al., 2017; Kol- lias et al., 2001). Additionally, vertical jump tests are performed in accordance to various aims, such as the assessment of the developed force and power in elite ath- letes who participate regularly in competitions (Cormie, McGuigan, & Newton, 2010). McGinnis et al. (2016) have used CMJ to assess the effect of fatigue on per- formance. Olsson et al. (2013) showed that CMJ and other types of jumps could be used for the assessment and evaluation of athletes’ performances, after Achilles tendon tears. Previous research studies have shown that CMJ has an appropriate validity for assessing explosive Introduction Vertical jump is an integral part of preparation exercises in most modern sports, and require high velocity and power. Many research studies have focused on athletes’ vertical jumps, and especially on the attained height (Runge, Rittweger, Russo, Schiessl, & Felsenberg, 2004; Russo et al., 2003; Salles, Baltzopoulos, & Rittweger, 2011). Vertical jump is also used for assessing the explosive power of lower extremities, forecasting, and monitoring athletes. Force and power are known as rela- tive factors of sport performance (Carlock et al., 2004; Slinde, Suber, Surer, Edwén, & Svantesson, 2008). Most of the previously published research studies assessed lower extremity power by countermovement jumps (CMJ) (Kollias, Hatzitaki, Papaiakovou, & Giat- sis, 2001). CMJ, both free-handed and akimbo-style, * Address for correspondence: Javad Sarvestan, Department of Health and Sports Medicine, Faculty of Physical Education and Sport Sciences, University of Tehran, 15th St., North Karegar St., 1417614418 Tehran, Iran. E-mail: javadsarvestan@ut.ac.ir Relationships between force-time curve variables and jump height during countermovement jumps in young elite volleyball players Javad Sarvestan 1, *, Mahdi Cheraghi 2 , Masoud Sebyani 1 , Elham Shirzad 1 , and Zdeněk Svoboda 3 1 Faculty of Physical Education and Sport Sciences, Tehran University, Tehran, Iran; 2 Sport Biomechanics Center, National Olympic and Paralympic Academy of Iran, Tehran, Iran; and 3 Faculty of Physical Culture, Palacký University Olomouc, Olomouc, Czech Republic Copyright: © 2018 J. Sarvestan et al. This is an open access article licensed under the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Background: The importance of vertical jumps has been extensively recognised in sports. Furthermore, the main indicator of success in vertical jumps is the attained height, however, there is lack of knowledge on how the eccentric and concentric force-time curve variables affect the jump height. Objective: The objective of this study was to assess the relationships between eccentric and concentric variables of the force-time curve and jump height in youth vol- leyball players. Methods: Twelve elite volleyball players (male, Iranian national youth volleyball players, 17 ± 0.7 years) have participated in this study. The correlations between the force-time variables, including the peak force, relative peak force, peak power, average power, relative peak power, peak velocity, and modified reactive strength index, of the eccentric and concentric phases and jump height have been assessed based on Pearson’s correlation coefficient. Results: Results showed that the average power (r = .70, p = .034), relative peak power (r = .75, p = .029) and peak velocity (r = .98, p = .004) of the concentric phase and modified reactive strength index (r = .83, p = .014) significantly correlated with the jump height. Conclusions: Relative peak power and average power of the concentric phase are shown to affect jump height in young volleyball players, whereby the vertical jump is their integral part. Keywords: stretch-shortening cycle, CMJ, eccentric, concentric Acta Gymnica, vol. 48, no. 1, 2018, 9–14 doi: 10.5507/ag.2018.003