The isoinertial training method, used to improve hypertrophy (Tesch et al., 2004; Norrbrand et al., 2008), neuromuscular functions (Norrbrand et al., 2010), power (Gual et al., 2016) and sprint time (Gonzalo-Skok et al., 2016) owes its efficacy to an accommodated resistance and optimal individualized eccentric overload (Tesch et al., 2017). The resistance intervention is based on the application of eccentric overloads generated by an isoinertial device, which uses the inertia of rotating flywheels during the athlete's movements. The flywheel technology, which simulates the mechanism of a toy yo-yo (Tesch et al., 2017), allows unlimited linear resistance loads during concentric and eccentric muscle actions, with the possibility of regulating the resistance overloads in each repetition. This device provides resistance force (in the eccentric phase) proportional to that generated by the athlete's concentric effort. The inertial force is generated by the rotating cone-shaped flywheel, which allows the athlete to move freely within the three spatial dimensions (Suarez-Arrones et al., 2018). The training load of this device can be regulated by increasing the speed of movement or by adding flywheel weights (Nunez et al., 2018). Due to the absence of frictional force, the energy of the concentric and eccentric phases is identical, allowing a great eccentric effort, with a very low metabolic cost (Caruso and Hernandez, 2002).
It is well known that the energy needed to perform eccentric actions is about one fifth of that required for concentric actions of the same cycle (Tesh et al, 2017). Several studies have pointed out that training protocols in which the eccentric phase of movement is overloaded, produce greater strength improvements than those in which the load is constant during both the concentric and eccentric phases (Doan et al., 2002). Prolonged eccentric exposure can enhance sport performance and prevent injuries (Martinez-Aranda and Fernadez-Gonzales, 2017). Cormie et al. (2010) reported that eccentric training improves concentric force and velocity, enhancing the storage and utilization of elastic energy. Several studies that have analyzed the effectiveness of the isoinertial method, have shown greater strength and power production, expressed in different joint angles (Dolezal et al., 2000). Eccentric strength is particularly needed in sports requiring Change of Direction (COD), where the athlete must decelerate and stabilize the body in the shortest possible time to then re-accelerate in a new direction (Chaabene et al., 2018). Since the phases of a soccer game require three-dimensional deceleration and acceleration actions with COD, (Fiorilli et al., 2017) the use of a cone-shaped device and transmission pulleys could provide additional benefit, allowing multidirectional movements in multiple planes (Chiu and Salem, 2006). Moreover, the accentuated eccentric phase enhances the neural adaptation that improves coordination and shooting precision (Norrbrand et al., 2010).
In recent decades a body of evidence has been developed that supports youth resistance training (di Cagno et al., 2013) as a fundamental means for physical development (Behm et al, 2008). Resistance training among youths is able to elicit improvements in overall motor performance and thus reduce the frequency of injuries (Zwolsky et al, 2017). Traditionally, plyometric training is used to overload the eccentric phase movement, allowing changes in the pattern of neural activation during the stretch-shortening cycle (SSC) and positively influencing the force and the start of the concentric phase (Chimera et al., 2004). Plyometric training is a safe and feasible method for physical conditioning in young athletes, improving neuromuscular function and soccer performance (Negra et al, 2017; Bedoya et al., 2015). Rubley et al. (2011) reported an improvement in kicking distance in prepubertal and pubertal soccer athletes after 10 weeks with twice-weekly sessions of plyometric training based on jumps, hops, skips, footwork and sprint drills. Nevertheless, plyometric training is based on the use of gravitational overloads, whereas inertial eccentric overload training provides a source of linear resistance from the spinning cone (Norrbrand et al., 2010). The eccentric overload provided by the isoinertial device may be applied directly to specific technical elements, such as COD and shooting movements, allowing the athlete to transfer the external variable overload effects to the real team sport performance. Moreover, isoinertial training provides unknown and unpredictable loads that stimulate different and continuous neuromuscular adaptations during each repetition (Van Hooren et al., 2017). A strong correlation has been found between isoinertial training and athletic performance with unknown loads (Hernandez-Davo et al., 2017). This effect could lead to superior benefits than traditional plyometric training intervention.
Therefore, the aim of the present study was to assess the effects of flywheel inertial training on explosive and reactive strength, sprint ability, COD performance and soccer shooting precision, when compared to traditional plyometric training of the same duration and volume. To the best of our knowledge, no previous study has analyzed the effects of an inertial eccentric-overload training program on soccer shooting precision in young soccer players. It was hypothesized that athletes would benefit by adhering to a specific training program based on uncertain eccentric overloads used in similar conditions to those in which they compete, stimulating neuromuscular improvement and co-ordination.
Thirty-four junior male highly trained soccer players volunteered to participate in the present study and were randomly assigned to the FEO (n = 18, aged 13.21 [+ or -] 1.21, weight 51.25 [+ or -] 6.71 Kg, height 1.65 [+ or -] 0.10 m, BMI 19.16 [+ or -] 2.22 Kg/[m.sup.2]) and the PT (n = 16, aged 13.36 [+ or -] 0.80, weight 52.10 [+ or -] 5.23 Kg, height 1.68 [+ or -] 0.07m, BMI 19.45 [+ or -]2.06 Kg/[m.sup.2]). All players belonged to the same club and had at least 3 to 4 years of experience. Their regular exercise practice included 4 field-based training sessions lasting approximately 120 minutes and included warm-up, plyometric training, technical and tactical activities, small side games and one competitive match. All the players were new to structured eccentric overload training. To be eligible for the study, players were required to meet the following criteria: to be joint or bone injury free at the moment of recruitment and to not make use of drugs or other substances that could influence the correct execution of the tests proposed by this study. Information about the study purpose was given to all participants and their parents before obtaining their written consent. The study was designed and conducted in accordance with the Declaration of Helsinki and approved by the local bioethical committee.
The present study used a controlled randomized repeated-measure research design to assess the effects of six weeks of soccer training with the implementation of two inertial eccentric-overload training sessions per week, on young male soccer players. The effects on explosive and reactive strength, linear sprint, agility, and COD and shooting precision improvement were evaluated. The randomization in the FEO and the PT, was performed as follows: a progressive number was assigned to each of the 34 enrolled and eligible participants. Successively, a random number list (from 1 to 34 with no repeated numbers) was generated using online software (https://www.random.org/sequences/, Dublin, Ireland). The list of participants was rearranged according to the random number list; the participants were then allocated to the different groups in blocks of two participants per group following the order FEO and PT. Baseline homogeneity of the two groups was assessed after randomization (relative to all the primary and secondary outcomes).
The sample size (34 total participants) was calculated a priori with G*Power 126.96.36.199 (G*Power software, Dusseldorf, Germany). The computation of the total sample size was calculated using an a priori method in order to have an [alpha] error probability = 0.05 and a Power = 0.95 with Pillai' V = 0. Pre and post intervention, participants of both groups underwent a three -day testing session, to assess explosive and reactive strength, sprint ability, agility, COD ability, and soccer shooting precision (Figure 1).
After the familiarization session, all participants took part in the testing session at the same time of the day in the same order: on the first day a lower-limb strength test was completed; on the second day there were COD, agility and sprint tests; and on the third day, the soccer shooting precision tests were performed. Participants were requested to avoid strenuous activities for at least 48 hours before each test.
The ground contact time and flight time of all jump tests were measured using Optojump (Microgate, Bolzano, Italy). This is an optical acquisition system, developed to measure flight time and ground contact time to a precision of 1ms. It has an excellent reliability, ranging from 0.982 to 0.989 (Glatthorn, 2011).
Squat Jump (SJ): In the squat jump test, participants were required to assume a static squat position with 90[degress] knee flexion, and to perform a purely concentric action with the instruction to jump as high as possible (SJ_h). The knee angle was monitored with a Medigauge electronic digital goniometer and adjusted prior to each jump. The testers observed the subjects in order to identify any visual signs of countermovement. When countermovement was observed, the subjects were asked to repeat the attempt after 45 s of rest. Participants in the test were required to have their hands placed on their hips during the...