Jumping ability has been considered as fundamental for successful performance in many sports (Sheppard et al., 2008). Depending on the sport, the importance of jumping ability can be affected by the direction of the jump. Considering the tactical nature of jumping activities in team sports like basketball, football (soccer) and volleyball, a vertical jump height is often considered to be critical performance outcome (Sattler et al., 2012). For example, a greater jump height achieved during a jump in basketball produces more favourable condition for shots and rebounds. In volleyball, the achievement of a greater jump height provides a clear advantage in the attack and block actions. Also, several studies have shown a positive association between jumping ability and other motor abilities (Wiskff et al., 2004; Maulder and Cronin, 2005).
The countermovement jump (CMJ) and drop jump (DJ) are reliable and valid for the evaluation of jumping performance (Arteaga et al., 2000; de Villarreal et al., 2009). At this point it is worth noting that both tasks represent different muscle action patterns (Flanagan and Comyns, 2008). The CMJ is classified as slow SSC movements and DJ as fast due to shorter contraction time and a smaller range of motion when compared to CMJ.
Plyometric exercises are widely believed to contribute to positive neuromuscular adaptations to high eccentric forces and corresponding improvements in vertical jumping ability (Markovic 2007; de Villarreal et al., 2009; Stojanovic et al., 2017). This training approach is effective due to increase fibre force and contraction velocity (Malisoux et al. 2006). The main mechanism explaining the effects of plyometric exercises is related to a specific muscle performance in the stretch-shortening cycle (SSC). This sequence of concentric (shortening) contraction preceded by the intense eccentric action (stretch) increased force and velocity compared to concentric action alone (Bobbert et al. 1996). The effect of the SSC is due to the storage and utilization of the elastic energy, the stretch reflex and tendon reflex (Bosco et al., 1982; Kawakami et al., 2002). A typical plyometric training includes jumps in place, standing jumps, multiple hops and jumps, bounds and drop jumps (Baechle and Earle, 2008). Jumps in place involve jumping and landing in the same spot. Standing jumps like vertical jumps, jumps over barriers are performed with maximal effort. Multiple hops and jumps involve repeated movements. Bounds are performed in a horizontal direction. Drop jumps consist of jumping off a box, a two-legged landing, and jump upward or to another box immediately after landing.
Assisted and resisted training methods have been adapted from sprint training (Rumpf et al., 2016) as a novel solution in the plyometric regime. Assisted plyometrics usually involve mainly countermovement jumps and drop jumps with the aid of elastic bands or tubing fixed between the body harness and point (e.g. the ceiling of a training hall) above the body. This system pulls on an individual upward (Makaruk et al., 2014; Sheppard et al., 2011). There are several types of resisted plyometric exercises that are performed under varied external conditions like water, sand and additional external loads. The most common among them are aquatic plyometrics (Robinson et al., 2004) and plyometric exercises with a weight vest (Khlifa et al., 2010), elastic bands (pulling downward) (Argus et al., 2011) and dumbbells (Markovic et al., 2011). The ground contact time (CT) during a jump is a basic parameter differentiating assisted and resisted methods. Assisted plyometrics provides shorter CT (Tufano et al., 2018), while resisted plyometrics results in longer CT (Dell'Antonio et al., 2016; Makaruk et al., 2010) as compared to the traditional plyometrics.
The conception of assisted and resisted methods in a plyometric intervention is based on two general training principles, i.e. specificity and overload (Baechle and Earle, 2008). Specificity aims to produce a high transfer of training exercises to sports performance by emphasizing similar movement patterns, muscle action and contraction velocity to those during sports competition. This idea was used by Makaruk et al. (2010), who found that drop jump training showed a positive tendency for changes in force at peak power during the CMJ test. Therefore, the authors claimed that plyometric training with a weight vest (resisted plyometrics) could be a specific and effective stimulus for athletes who require power production against large resistance, e.g. in shot put. In turn, the overload principle states that disturbance of the homeostasis of the body, including cells, tissues, and organs, is required for effective training adaptation. According to Sheppard et al. (2011), assisted plyometrics could be a novel 'overloading' stimulus for the athletes in jumping sports who have a narrow window of adaptation for jumping performance development. This type of plyometric exercises promote an improvement in jumping ability by decreasing an effective mass of a jumper and an increasing peak acceleration during jump due to unloaded condition (Sheppard et al. 2011). Following this observation, they found that a 5-week assisted jumping training allowed young elite male volleyball players to increase jump height for CMJ, while traditional jump training did not provide significant enhancement in jump height. Reducing impact landing forces is the other reason why assisted or resisted plyometric exercises are implemented into the training programs (Argus et al., 2011; Donoghue et al., 2011). Some studies have demonstrated that the aquatic environment (Robinson et al., 2004) or sand (Impellizzeri et al., 2008; Miyama and Nosaka, 2004) induced less muscle damage in comparison to a solid surface. Moreover, research revealed that aquatic- and land-based plyometric training programs provided similar gains in athletic performance (Arazi et al., 2012; Robinson et al., 2004).
Although traditional plyometric training programs have been shown to improve jumping ability in athletes who already achieved of jumping ability (de Villarreal et al., 2009; Stojanovic et al., 2017), the use of traditional plyometric training methods may be insufficient for the improvement of jump height (Argus et al. 2011). Several original studies provided evidence that incorporation of the assisted and resisted plyometric methods using non-standard devices (e.g. rubber bands) or environment (e.g., water) during plyometric training programs may pose a more effective approach to enhance jumping ability as compared to the traditional plyometrics in athletes (Argus et al. 2011; Sheppard et al., 2011) and non-athletes (Kibele et al., 2015). Conversely, other studies found greater jumping effects for traditional plyometric method relative to resisted plyometrics in recreationally trained students (McClenton et al., 2008). To our knowledge, no systematic review has been conducted to determine and compare the effects of different plyometric training methods on vertical jumping ability in adults. Clarifying the influence of plyometric training interventions on jump performance in adults appears to be important for three reasons: (i) to determine the effects of traditional, assisted and resisted plyometric training methods as compared to control group (no plyometric training), (ii) to identify if assisted and resisted plyometric methods are more effective than traditional plyometrics and (iii) to provide sport coaches with a critical evaluation of the current plyometric methods concerning sport level. The objective of this systematic review and meta-analysis was to compare the effects of traditional, assisted and resisted plyometric methods on vertical jumping ability in healthy adults.
This systematic review with meta-analysis was conducted according to the criteria of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis Statement (PRISMA). A literature search was performed using the MEDLINE (via EBSCO), SPORTDiscus (via EBSCO), Scopus and Web of Science Core Collection databases, with no restriction of dates until June 1, 2019 (with an update until February 11, 2020), for peer-reviewed articles published in the English language. The following groups of keyword combined with Boolean operators were used as search terms: ("plyometric*" OR "jump* exercise*" OR "plyometric* training" OR "jump* training" OR "ballistic* training" OR "ballistic* exercise*" OR "power training" OR "explosive training") AND ("power" OR "reactive strength index" OR "rate of force development" OR "jump* height" OR "countermovement jump" OR "drop jump" OR "depth jump" OR "vertical jump*" OR "center of body mass" OR "flight time" OR "contact time" OR "vertical velocity"). The electronic data search and screening based on titles and abstracts were conducted independently by three authors (MS, MC, and BS). The duplicate articles were rejected. The scanned articles were discussed during the meeting of all the...