The physical demands of soccer players changed over the last 10 years due to modern game philosophies and tactics. Especially, the distances covered in the higher intensities and the number of explosive actions such as accelerations, turns, and jumps have increased (Di Salvo et al., 2010; Mohr et al., 2003). Therefore, a player's sprint ability and the dynamic and explosiveness of his movements are some of the most crucial criteria in today's talent scouting.
The results of our previous meta-analysis of EMS methods reveal that EMS training can be an effective alternative to the traditional resistance training and/or power training for developing maximal strength, speed strength, sprinting and jumping performance in elite athletes (Filipovic et al., 2012). Several studies with elite athletes revealed positive effects of EMS on performance (Babault et al., 2007; Billot et al., 2010; Brocherie et al., 2005; Maffiuletti et al., 2000; 2002; Pichon et al., 1995; Willoughby and Simpson, 1996). Despite an already high level of strength of elite athletes, several studies were able to verify high gains of 30% in the maximal strength of the lower body muscles (Babault et al., 2007; Kots and Chwilon, 1971; Willoughby and Simpson, 1996; 1998). Some studies were able to directly transfer the strength gain into an improved vertical jumping (Babault et al., 2007; Herrero et al., 2006; Kots and Chwilon, 1971; Maffiuletti et al., 2000; 2002; Pichon et al., 1995) and sprinting performance of up to -4,8% (Herrero et al., 2006; Kots and Chwilon, 1971) within 3-6 weeks (cf. Ferrando et al., 1998; Filipovic et al., 2012). Other studies showed no effects on sprinting performance (Babault et al., 2007; Billot et al., 2010). Previous investigations revealed that the combination of EMS training with specific jump training (Herrero et al., 2006; Martinez-Lopez et al., 2012; Maffiuletti et al., 2002) or high-performance training have positive effects on the strength transfer and thus enhancing motor abilities such as jumping or sprinting performance in elite athletes (Babault et al., 2007; Maffiuletti et al., 2000; Malatesta et al., 2003; Pichon et al., 1995).
All of these studies applied EMS only to defined muscles of the lower body with single electrodes. With the new generation of EMS-devices several muscle groups can be trained simultaneously through an electrode belt- and vest system (e.g., miha bodytec, Augsburg, Germany). In comparison to the local EMS method (see above) there is only little research about applying WBEMS methods with trained athletes (Kreuzer et al., 2006; Speicher et al., 2009) and almost no data about implementing WB-EMS in the training routine of elite team athletes for systematically enhancing sport performance.
The documentation of the CK-level is a widely used parameter in high-performance sports to control training intensity and recovery. Intense physical exercise stresses muscle tissue, which elevates the level of the CK in the blood serum. Accordingly, the serum CK-level is used as an indicator for muscle damage. Extreme muscle stress and subsequent damage or several intense stimuli on consecutive days can cause a summation of CK values. In contrast to voluntary exercise, EMS artificially activates muscle contraction without resistance load. Studies investigating the stimulation intensity and the responds of creatine kinase to EMS have shown that the electrical stimulus can produce higher muscular stress and consequently a higher degree of muscle fiber damage than voluntary stimulus (Boeckh-Behrens et al., 2006; Jubeau et al., 2008; Kreuzer et al., 2006; Steinacker, 1999). Jubeau et al. (2008) assume that this could be due to the different recruitment of motor units during EMS compared with voluntary contractions.
Although little is known about the underlying mechanisms, some authors speculate neural adaptations such as a preferential activation of the large motor units of the type-II fibers with EMS as the main factor for the increase in strength (Hortobagyi et al., 1999; Maffiuletti et al., 2000; 2002; Pichon et al., 1995; Willoughby and Simpson, 1996). Hortobagyi and Maffiuletti (2011) concluded that EMS-programs up to six weeks may induce alterations in muscle metabolism. However, the authors stated that the increase in MVC (maximum voluntary contraction) is not a result of overt muscle hypertrophy and more due changes in some elements of the nervous system. Studies which applied EMS for time periods longer than six weeks suggested that hypertrophy might occur in the late phase of such programs (cf. Hortobagyi and Maffiuletti, 2011; Gondin et al., 2005, 2011; Ruther et al., 1995). Anabolic hormones such as the human growth hormone (hgH) play a dominant role in the regulation of protein metabolism and can be an indicator for muscle growth and hypertrophy. Jubeau et al. (2008) investigated the acute effect of EMS on hgH. They observed a significant increase in hgH for the EMS-group compared to traditional strength training. However, the authors did not investigate the changes in muscle mass. IGF-1 is also an anabolic hormone that is part of a signaling network that is involved in exercise-induced remodeling processes in the muscle (Goldspink, 2005; Hameed et al., 2003). In adaptation to resistance training IGF-1 increases the protein synthesis in the muscle cells (Butterfield et al., 1997; Ferrando et al., 1998). Further, IGF-1 activates satellite cells to proliferate and differentiate and thus could modulate skeletal muscles (Adams and Haddad, 1996). Resistance exercise is a powerful stimulus for the endocrine system. Studies have shown that the hormonal response to resistance exercise depend on several factors including number of sets, repetitions, training intensity and volume and rest intervals (Crewther et al., 2006). Research studies dealing with the acute response to IGF-1 have shown that strength exercise can elevate circulating IGF-1 and free IGF-1 (Kraemer and Ratamess, 2005; Rahimi et al., 2010). In contrast, other studies have shown no changes in acute IGF-1 after resistance exercise (Kraemer and Ratamess, 2005). No studies have yet investigated the acute metabolic responses to WB-EMS in elite athletes.
To our knowledge this is the first field-practice study that systematically implements a WB-EMS program in training routine of elite soccer players over 14 weeks to increase performance and test the WB-EMS method on practicability in professional soccer. Little is known about the underlying mechanisms of EMS, e.g. no studies have yet investigated the acute metabolic responses to WBEMS in elite athletes.
For these reasons the aim of this study was to implement a dynamic whole-body EMS program in the in season training routine of elite soccer players on the basis of our previous studies to investigate the effects on maximal strength, sprinting and jumping performance, and kicking capacity. A further objective of this study was to investigate the effects on hormonal (insulin-like-growth-factor-1) and enzymatic (creatine kinase) parameters in order to explain possible adaptation mechanisms such as hypertrophy.
Twenty-two professional male soccer players, competing in the 4th division of the German Soccer Federation (DFB), voluntarily participated in this study. To our knowledge this is the first study to implement WB-EMS in the training routine of elite soccer players. In accordance with the principles of players' preference intervention, only players who did not have a strong group preference were randomized into either the WB-EMS group (EG) or the Jump-Training group (TG). To cope with possible dropouts in the WB-EMS group the players were assigned into a larger intervention group (EG) and a smaller control-group (TG): WB-EMS group (EG, n = 12; age 24.9 [+ or -] 3.6 years; height 1.84 [+ or -] 0.05 m; mass 80.6 [+ or -] 9.2 kg), control-group (TG, n = 10; age 26.4 [+ or -] 3.2 years; height 1.82 [+ or -] 0.07 m; mass 78.3 [+ or -] 9.3 kg). All players were professional soccer players and performed 6-7 training sessions per week and competed once a week in the championships. The standard training sessions lasting 7090min including technical skill activities, offensive and defensive tactics, athletic components with various intensities, small sided game plays and 20-30min of continuous play. The playtime in championship- or friendly matches of the test persons were recorded during the study period (Table 1). The players were asked to maintain their usual food intake und hydration. During the study no additional strength training for the lower body was allowed. The study was conducted in the second half of the season from January until May. All players had at least five years of experience in systematic strength training. During the first half of the season strength training sessions were part of their daily soccer training routine minimum once a week. None of the players had trained with EMS before. All players were informed about the procedures and risks of the study and written informed consent was obtained. All experimental procedures performed were approved by the Ethic Committee of Human Research of the German Sport University Cologne.
The study was designed as a randomized training study including a jump training group with simultaneous EMS (EG) and a jump training group without EMS (TG) as control group in order to investigate the effects of the WB-EMS stimulus on maximal strength and performance and exclude the possible effects of the squat jumps. The study was conducted during the second half of the competitive season. The training interventions were conducted twice per week in addition to 6-7 soccer training sessions and a match on the weekend. Performance was assessed before (baseline) after 7 weeks (wk7), and after...