The trampoline gymnasts need high levels of power in their lower limbs to achieve an optimal height in the initial part of a routine and dispose of more aerial time for the functional part to encompass forwards and backward somersaults and twists (Jensen et al., 2013). Resistance training under close supervision and with correct guidance has been proven effective and beneficial in developing strength and power and enhancing athletic performance in young athletes (Behringer et al., 2010; Granacher et al., 2011; Harries et al., 2012; Granacher et al., 2016; Suchomel et al., 2016; McQuilliam et al., 2020). In general, it has been suggested that lifting high-load resistances (i.e., ≥80% of 1 repetition maximum, 1RM) increases strength and power in young athletes with advanced or intermediate experience (Faigenbaum et al., 2009; Lesinski, et al., 2016; Behm et al., 2017). Lifting high loads (HL) requires contractions at an unintentional slow velocity enabling more time for cross-bridge formation and thus producing higher forces than when lifting low loads (Fenwick et al., 2017). Training with HL is preferential to increase maximal (1RM) or near-maximal strength (Folland and Williams, 2007; Lopez et al., 2021), and it has the potential to maximize recruitment of the motor unit pool in fewer repetitions (Lopez et al., 2021) and with less fatigue in the central nervous system than low loads (LL) training when both strength trainings are performed to momentary failure (Farrow et al., 2021). On the other hand, training with moderate loads (that is, 60%–79%) to LL (that is, 30%–59%) inflicts less stress on the joints and tendons than HL (Bohm et al., 2015) and have demonstrated the ability to increase power in a broad load spectrum (McBride et al., 2002) and the ability to apply force at high velocity in the most resistance exercises, particularly in jumping exercises in young athletes (Behm et al., 2017). Lifting LL with maximal exertion enhances high-velocity strength (McBride et al., 2002) through an increment in motor unit firing rate (Van Cutsem et al., 1998) and refinement in neural modulation at high speed (Behm and Sale, 1993). However, since high levels of maximal strength underpin power output (McQuilliam et al., 2020), and HL have demonstrated more remarkable neural adaptations than low loads (Jenkins et al., 2017), some coaches and sports scientists have employed a combination of HL and LL or the so-called contrast training to increase lower-limb power output and jump performance (Duthie et al., 2002; Marshall et al., 2021). For all these reasons, contrast training can be a valuable tool for improving the jumping ability of trampoline gymnastics athletes.

In this regard, contrast training has been defined as a workout that involves using exercises with loads in a wide range of the force-velocity profile within a sequence, that is, alternating HL and LL exercises set for set (Marshall et al., 2021). Several works have found that contrast configurations improved strength and power in young athletes (Franco-Marquez et al., 2015; Bauer et al., 2019; Fathi et al., 2019). It is suggested that contrast configuration could bring a performance enhancement in submaximal or LL high-velocity exercises when those are preceded by a biomechanically similar HL (>80% of dynamic or isometric MVC) in the same circuit set by set (Duthie et al., 2002). The physiological mechanisms that explain the chronic adaptations produced after a contrast training period remain to be determined. The prescription of HL as a conditioning activity in a contrast training sequence is not always possible with young athletes, either due to a lack of strength in the trunk muscles that limits the correct execution of the exercises or because their sport modality already implies many impacts on joints and tendons. The trampoline practice entails a high volume of impacts on joints, tendons, and ligaments, especially in the spine and lower limbs (Grapton et al., 2013). Somersault is a commonly used skill in trampoline and one of the movements that most injury produces (Lindner and Caine, 1990; Grapton et al., 2013); consequently, the athletes sustain lower limb ligament damage frequently. Trampoline gymnasts would benefit from using effective training strategies to improve jumping performance, but on the other hand, they do not impose high stress on the joints and ligaments. The use of lower limb LL blood flow restriction exercises (LL-BFRE) has proven to be an efficacious alternative to high-load exercises (HL-RE) when used alternately within the same week or alternating weekly (Hansen et al., 2020). The employment of LL-BFRE can provide a less stressful stimulus on joints and tendons with a similar level of neuromuscular adaptations (Luebbers et al., 2019). Some studies have reported that low-load resistance exercises (i.e., ∼30% 1 RM) with a high number of repetitions (12–30) (Pope et al., 2013) in combination with BFR (LL-BFRE) are effective at increasing muscle mass and strength across a wide range of populations (Loenneke et al., 2014; Loenneke et al., 2012b; Centner and Lauber, 2020; Pearson and Hussain, 2015). Interestingly, LL-BFRE and HL-RE resulted in similar levels of muscle water content (i.e., muscle swelling) (Freitas et al., 2017), mechanisms associated with post-activation performance enhancement (PAPE); however, LL-BFRE has been barely used in complex and contrast training sequences (Cleary and Cook, 2020; Doma et al., 2020). Cleary and Cook (2020) reported that in a complex strength session, both HL-RE and LL-BFRE failed to produce a PAP effect (Cleary and Cook, 2020). Nevertheless, they also pointed out that the absence of effect might be caused by an ineffective complex training protocol or other individual factors (Cleary and Cook, 2020).

Meanwhile, even though many studies have reported the effects of training with LL-BFRE on muscular strength and hypertrophy (Pearson and Hussain, 2015), the evidence of LL-BFRE’s influence on power or jumping performance is unclear. Some authors have reported that LL-BFRE or jumping exercises with blood flow restriction in the legs did not affect the power or jumping performance (Abe et al., 2005; Horiuchi et al., 2018). Conversely, Cook et al. (2014) found that three weeks of BFR training with moderate-load (i.e., 70% 1 RM) induced significant increases in strength and countermovement jump performance in young adult rugby players (Cook et al., 2014). In addition, it was found that an increase in height, flight time, and power of drop jump when LL-BFRE, bodyweight lunges with occluded legs, are performed 6–16 min before the drop jumps (Doma et al., 2020). In brief, the LL-BFRE seems to be efficient to warm-up previous to power exercises or be part of a resistance training sequence, as contrast training, to enhance posteriorly executed jumping exercises. Nevertheless, to the best of our knowledge, the effect of LL-BFRE as a conditioning activity on a contrast training sequence has not been investigated with preadolescent or adolescent athletes.

Therefore, the present work aims to study the effects of LL-BFRE, as a conditioning activity, into a contrast training scheme, on strength and power outcomes in high-level preadolescent trampoline gymnasts.

We found that preadolescent trampoline gymnasts increased their jump height (CMJ, SJ) and strength capabilities (IMTP) regardless of employing HL or blood flow restricted LL as conditioning activity within a contrast training strength program performed two days per week for ten weeks. These results suggest the potential usefulness of BFR with low loads as a conditioning activity in a contrast training sequence in preadolescents athletes. However, our results must be interpreted with caution since we could not have a group that only trained specific trampoline sessions and thus compare the magnitude of change between the two interventions group with a control group. Participants’ allocation was based on the rank obtained from the maximal isometric mid-thigh pull test in the pre-intervention to prevent participants’ force level from influencing the present study’s independent variable (i.e., the type of conditioning activity). Moreover, the groups did not significantly differ in any jump measures (i.e., CMJ, SJ, DJ) in the PRE, meaning that post-training differences could not be attributed to unequal group composition or pre-experimental biases.

The present work is the first investigation to study the effects of contrast-type resistance exercise training on the performance of high-level early adolescent trampoline gymnasts. The increments observed CMJ (HL-RE: 12.4 ± 5.4% and LL-BFRE: 17.2 ± 8.7%) and SJ (HL-RE: 14.6 ± 9.4% and LL-BFRE: 27.6 ± 15.3%) in both groups were much higher than those reported in the literature for pubertal volleyball players when trained with plyometrics (CMJ: 3.4% and SJ: 4.1%) or combining resistance and plyometric exercises (CMJ: 6.3% and SJ: 7.1%) (Fathi et al., 2019). In the last experiment, authors implemented a control group that continued their regular volleyball training, and they reported no changes in CMJ and SJ after 16-weeks of regular training (Fathi et al., 2019). A higher volume of jumps is performed during trampoline gymnastics sessions than in volleyball training, and likely a control group would show an improvement in those jumps. However, the magnitude of improvement reached in the present study after only ten weeks of two different contrast training configurations in pre-pubertal athletes warrants more studies that might highlight the contribution of that training strategy to increasing jump capabilities in young athletes. It has been signaled that jump trainability is mediated by biological maturation (Moran et al., 2017), that conclusion is derived from a meta-analysis of plyometric controlled trials in young individuals where authors indicated that adaptative responses were higher between the mean ages of 10 and 12.99, and between 16 and 18 years, than the mean ages of 13 and 15.99 despite greater exposure (Moran et al., 2017). Surprisingly, the athletes from our study were ∽14 years old what becomes the results obtained in the present study even more relevant because we found significant improvements in athletes that were within a period of lowered response to maximize performance.

Trampoline athletes in LL-BFRE improved more SJ than CMJ (27.6 ± 15.3% vs. 17.2 ± 8.7%, respectively), contrasting to HL-RE that experimented a similar improvement in both exercises (14.6 ± 9.4% vs. 12.4 ± 5.4%, SJ and CMJ respectively). SJ and CMJ performance depends on common factors related to changes in muscle structure (Moran et al., 2017) and neural drive to muscles and on specific factors related to the ability to manage SSC (Kozinc et al., 2021). We believe that even the athletes in LL-BFRE reduced their EUR, more increment in CMJ in LL-BFRE might show that LL-BFRE did not weaken their capability to use the eccentric phase but simply increased more SJ. We also consider that one possible reason for this outcome might be the higher increment in muscle mass in the LL-BFRE group. The muscle mass increment could have contributed to the improvement of SJ and to a lesser extend in CMJ (Van Hooren and Zolotarjova, 2017; Radnor et al., 2018). Jumps with a previous countermovement require mastery stretching shortening cycle (SSC) characterized by an eccentric “stretching” action prior to a subsequent concentric “shortening” action (Nicol et al., 2006). Performance during an SSC is attributed to muscle pre-activation, the stretch reflex, and the release of stored passive-elastic energy in the muscle-tendinous tissue (Groeber et al., 2021). An augmentation in muscle mass is usually produced by an increment in contractile and no-contractile tissue. Differences in the proportion of contractile and no-contractile augmentation because of the type of conditioning activity within the contrast sequence might be the reason for the differences in the increment of performance between SJ and CMJ.

As aforementioned, trampoline athletes in LL-BFRE gained more muscle mass than those in HL-RE (6.6 ± 3.1% vs.3.6 ± 2.0%, respectively). The result of our study is consistent with the evidence from other previous studies employing BFR with young adult populations (Loenneke et al., 2012b). Furthermore, it has been observed that LL-BFR training stimulated physiological factors associated with skeletal muscle hypertrophy conversely to training without BFR with equal relative loads (20% 1RM) or even higher (50% 1RM) (Haddock et al., 2020). Since LL-BFRE performed more repetitions than HL-RE and athletes lifted at moderate effort intensity, we suggest that these circumstances might be responsible for the muscle mass changes. It has to be highlighted that the maturity process during the intervention period could have influenced the results. Food intake was not controlled during the study; however, athletes made their meals in the dining room inside the training center; thus, it is not expected that there have been considerable differences in the quality and quantity of the food intake.

On the other hand, the current study demonstrated that a contrast training program using low-loads with BFR as a conditioning activity might provide an effective and equivalent positive influence on maximal strength compared to using high-loads for preadolescents athletes. Our results agree with the evidence from other previous studies employing low load BFR with other populations (Loenneke et al., 2012b; Yamanaka et al., 2012; Luebbers et al., 2019; Hansen et al., 2020) and high school adolescent weightlifters (Mohamed et al., 2017; Luebbers et al., 2019). No side-effects (hazards or unbearable discomfort) were reported during the ten weeks of training, which implicates that LL-BFRE integrated into a contrast training sequence is effective and safe for young athletes’ strength development. Potential mechanisms of LL-BFRE training for young athletes was not within the scope of the current study, but previous research has suggested that the increased hormone level (such as plasma concentration of growth hormone) (Manini and Clark, 2009), fast-twitch fibers recruitment, and muscle cell swelling (Loenneke et al., 2012a) might be the factors for improvement of maximal strength in LL-BFRE.

The contrast training sequence is characterized by high-load resistant strength exercises followed by lighter-loads with similar movement pattern power exercises and plyometric exercises (Marshall et al., 2021) that try to cause a PAPE effect within the sets and between them. Initially, some authors suggested that the acute physiological effects linked to post-activation potentiation (PAP) (e.g., improvement of the sensitivity of the Ca²⁺ released by the sarcoplasmic reticulum due to phosphorylation of the regulatory light chain of myosin) caused by a precedent conditioning activity is the cause of improved performance in high-speed strength or power exercises during a session (Sale, 2002). The repetition of contrast training configurations can achieve a long-term improvement in the performance of activities such as jumping, throwing, or all those that depend on high values of rate of force development (Sale, 2002; Tillin and Bishop, 2009). However, other authors have exposed their concerns about PAP causing an improvement in force production after conditioning activity because the specific physiological mechanism of PAP is based on the contractile response after a conditioning activity that could or not contribute to the posterior performance enhancement (Cuenca-Fernandez et al., 2017; Boullosa et al., 2020). Conversely, it has been indicated that highly probable that other physiological effects such as changes in muscle temperature, the water content of muscles and cells, and muscle activation may be responsible, at least in part, for the improvement in voluntary strength and power (Blazevich and Babault, 2019) after a conditioning activity, the so-called PAPE (Cuenca-Fernandez et al., 2017). These physiologic mechanisms related with PAPE may have been responsible to performance increases in HL-RE and LL-BFRE contrast training sequences in our study. Cleary and Cook (2020) studied the acute effects of HL-RE and LL-BFRE in a complex training sequence in college-aged men, and they did not find an increase in the subsequent vertical jumps’ height. Conversely, our results support that HL-RE and LL-BFRE could provide a performance enhancement in jump and strength capabilities when employed chronically (i.e., ten weeks) within a contrast training sequence in pre-adolescents. If PAPE effects are responsible for improving the performance observed in contrast training with HL or LL-BFR as conditioning activity, it should be clarified in future studies. Furthermore, it has to be highlighted that LL-BFRE group trained their lower limb extension pattern at execution speeds closer to those sport-specific. As stated almost three decades ago, velocity specificity of resistance training has demonstrated that the greatest strength gains occur at or near the training velocity (Behm and Sale, 1993). Therefore, the LL-BFRE group was training at a higher velocity than HL-RE, trampoline-specific performance in longer training interventions. The results of our study should be contrasted with future designs that analyze structural and neural chronic adaptations of contrast training using LL-BFRE in young athletes’ power performance.

Moreover, our study did not find an interaction effect on reactive, explosive strength (DJ), but simple effects showed an increment in the HL-RE and no changes in LL-BFRE. As shown in Figure 2, the responses to the DJ test in HL-RE presented high variability between participants. The reason that could explain high variability in the DJ responses of the HL-RE group may be that high loads training presented specific difficulties related to the capacity to place the load on the gymnasts’ back and subsequent demands for their core strength and stability. Core weakness limits load progression and applying force efficiently when performing strength exercises like back or front squat. Nevertheless, high loads contribute to stiff muscle-tendon structures (Fenwick et al., 2017), which can explain the magnitude of response experimented by some athletes.

Low-intensity resistance training with BFR could be implemented into an integrated strength training program with a combination of power or plyometric exercise to effectively improve the strength and jump height performance in lower limbs in preadolescent athletic populations. Our study has provided valuable implications for coaches working with elite young athletes, especially when using high loads in a contrast training structure is not possible.

Implementing LL-BFRE in place of HL-RE in a contrast training structure is safe and might be equally effective in improving lower-body strength and power in preadolescent trampoline gymnasts.