Volleyball places high requirements on a player's speed, agility, upper-body and lower-body muscular power, and maximal aerobic power (Gabbett, 2008; Sattler et al., 2015). Therefore, coaches and professionals involved in volleyball are interested in the potential effectiveness of different training regimes and improvement of those conditioning capacities are known to be important determinants of success (Pereira et al., 2015). One of such training regimes is plyometric training. Plyometric training uses the physiological phenomenon of a stretch-shortening cycle in order to enhance the ability of the neuromuscular system to produce maximal force in the shortest possible time (Markovic and Mikulic, 2010). Due to the characteristics of the game, which involve repeated jumping, frequent sprinting and changes in directions, this training regime is a particularly popular method for fitness development in volleyball players (Kim and Park, 2016; Pereira et al., 2015; Trajkovic et al., 2016).
Previous studies have investigated the effects of plyometric training on conditioning capacities in volleyball (Lehnert et al., 2017; Marques et al., 2008; Sheppard et al., 2008; Voelzke et al., 2012). Voelzke et al. (2012) evaluated the effectiveness of resistance training with additional plyometric exercises (n = 8) and electromyostimulation plus plyometric exercise (n = 9). In general, their results showed significant improvement in jumping performance as a result of both modalities (improvement of approximately 5%), whereas the latter additionally promoted speed and agility performance in male volleyball players (Voelzke et al., 2012). Sheppard et al. (2008) investigated the concurrent effects of training using accentuated eccentric load during jumping (n = 8) vs. nonloaded training (n = 8) in high-performance volleyball players (mixed gender groups). The results indicated that more intensive plyometric training (with additional loads) yielded superior jumping performance (improvement of 11% in displacement capacity) in comparison to regular jumping training with the player's own body mass. In a study on 10 elite female volleyball players, Marques et al, 2008 reported changes in strength and power performance as a result of a 12-week program performed during the in-season (10 regular plus 2 additional sessions consisting of combined resistance- and plyometric-exercises), and reported improvements in muscular strength (13% and 18% for squat and bench-press, respectively), ball-throwing- (13%), and countermovement jump (4%). In two studies done on female junior volleyball players, the authors reported significant improvements as a result of 5-week and 6-week plyometric training on generic- and specific-jumping performances (Kristicevic and Krakan, 2016; Trajkovic et al., 2016). Similarly, Pereira et al. (2015) confirmed significant improvements in jumping-and throwing-capacities (between 5.3% and 20.1%) among 14-year old female volleyball players after 8-week plyometric training. Very recently, Polish authors reported training-induced changes in different physical performances in 12 female junior volleyball players (
Skill-based conditioning games (training-games or small-sided games) are a popular method for improving the skill and fitness levels of players from different team sports, including volleyball (Corvino et al., 2014; Gabbett and Mulvey, 2008; Schelling and Torres, 2016). The basic idea behind skill-based conditioning is the fact that the greatest improvements in fitness and performance occur when the training stimulus simulates the physiological and technical demands of competition (Gabbett, 2008). In volleyball, it is reasonable to expect that skill-based conditioning can improve those capacities that are regularly improved through plyometric training, such as jumping and throwing. Indeed, volleyball skill-based conditioning includes different plyometric exercises (jumping, spiking, etc.), which are also included in plyometric training in different forms. Therefore, skill-based conditioning games have already been studied as potentially effective not only for improving technical skills but also for increasing the conditioning capacities of volleyball players (Gabbett et al., 2006; Gabbett, 2008). In short, skill-based conditioning improved sprinting capacities over 5 and 10 meters but did not contribute to better results in vertical jump, spike jump, or overhead medicine ball throw in junior volleyball players (Gabbett et al., 2006). In additional investigation, it has been suggested that a combination of skill-based-conditioning (i.e. training oriented toward improvement of conditioning capacities), and skill-based-instructional training (oriented toward development of specific volleyball skills), is likely to confer the greatest improvements in conditioning parameters and skill in junior elite volleyball players (Gabbett, 2008). To the best of our knowledge, no study has simultaneously examined the effects of plyometric- and skill-based conditioning on possible improvements in volleyball player physical capacities.
From the previous literature overview, it is evident that there is a lack of studies on the effectiveness of plyometric training on fitness indices in high-level female volleyball players. Additionally, information on the differential effects between plyometric- and skill-based-conditioning in female volleyball players is particularly lacking. Therefore, the aim of this study was to evaluate the concurrent effects of plyometric- and volleyball-skill-based training on changes in sprinting-, jumping- and throwing-capacities in high-level female volleyball players. Increased knowledge about these training modalities will allow a better understanding of the concurrent effects of these two popular training methods in volleyball. The initial hypothesis of this study was that the plyometric-training will induce more positive changes than skill-based conditioning, in studied conditioning qualities.
In this randomized controlled study, the sample of participants originally consisted of 50 high-level female volleyball players from Kosovo, members of teams participating at the highest competitive level (i.e., first division players) (21.9 [+ or -] 2.0 years of age; 1.76 [+ or -] 0.06 m; 61.2 [+ or -] 7.1 kg). Total sample was divided into plyometric- (n = 25) and skill-based group (n = 25). All participants were older than 18 years, and had played volleyball for at least 8 years prior to the study. Plyometric- and skill-based conditioning were performed as an addition to the regular technical and tactical volleyball training (see later for training details). Prior to the study, the participants were informed about the possible risks and benefits of the study, and their participation in the study was voluntary. The study was approved by the corresponding author's Institutional Ethical Board, and all participating players provided written consent for the study participation. However, in this study we included only those participants who participated in at least 80% of training sessions. Therefore, a final sample included 41 participants (21.8 [+ or -] 2.1 years of age; 1.76 [+ or -] 0.06 m; 60.8 [+ or -] 7.0 kg; 21 and 20 participants in plyometric- and skill-based-group, respectively).
The plyometric- and skill-based conditioning protocols were performed twice per week during the 12-week period at the beginning of the season. A single session for both programs lasted up to 60 minutes (10-15 min of standardized warm-up, 25-40 min of skill-based or plyometric conditioning depending on program, and 10-15 min of cool-down and stretching).
Plyometric training in general included lower-body plyometric exercises (jumping exercises), and upper body plyometric exercises (throwing exercises). Lower body plyometrics included (from low- to high-demanding exercises): leg hops, vertical jumps, tuck jumps, lateral/diagonal jumps, broad jumps, obstacle jumps, different types of box jumps (step-ups, box shuffles, etc.), and drop-jumps. In general, jumps were performed as (i) two-leg jumps with two leg landings (low intensity), (ii) two leg jumps with one leg landings (medium and high intensity, depending on exercise), and (iii) one-leg jumps with alternate leg landings, or one-leg jumps with same leg lending (high intensity). High intensity jumps were introduced to training from 6th to 8th week, and regularly applied from 9th to 12th week. From 9th to 12th week of training, some players performed loaded (weighted) jumps, with loads of maximally 5% of player's body mass, depending on one's fitness level and motor proficiency. Upper body plyometric exercises included: explosive push-ups, jumping spider (combination of explosive push-ups and jump), clapping push-ups, and different forms of exercises with medicine ball (i.e. throws, passes). Throws were performed in different directions (upward, horizontal, downward, etc.) with 1-kg medicine ball (one-arm throws for medium intensity, and two-arm throws for low intensity), and 3-kg medicine ball (two-arm throws for medium and high intensity depending on exercises. When it was possible...