Soccer is an intermittent sport with a wide variation of movement patterns. Within these varying loads from high to lower intensities, an average of 70% of maximal oxygen uptake, blood lactate values between 2-10 mmol x [L.sup.-1], a mean intensity close to the anaerobic threshold and distances of 10-12 km are covered during a match (Bradley et al., 2010; Di Salvo et al., 2009; Krustrup et al., 2006; Mohr et al., 2003; Stolen et al., 2005). These values show that the main energy demand is covered by the aerobic energy system. Although, high-intensity periods represent a small portion of the total distance covered, they are believed to be very important for the result of a soccer match (Bradley et al., 2010; Di Salvo et al., 2009). Soccer players have to perform repeated bouts of maximal exercise and need to recover quickly in between. Therefore, a high aerobic capacity is needed (Meckel et al., 2009).
There is evidence that the implementation of highintensity training (HIT) during preseason conditioning represents a possibility to enhance aerobic capacity (Dupont et al., 2004; Helgerud et al., 2001; Hoff et al., 2002; McMillan et al., 2005; Sperlich et al., 2011; Sporis et al., 2008). In most of these studies, HIT was performed over 6-10 weeks with 2-3 training session per week (Dupont et al., 2004; Helgerud et al., 2001; Hoff et al., 2002; McMillan et al., 2005), but still little is known about the integration of HIT either in daily training sessions or in special focused training blocks.
As the preseason is quite short (6-8 weeks) and conditional abilities, technical and tactical elements need to be integrated, the traditional model of training periodization seems to be unsuitable for elite soccer training and block periodization, including HIT, might be the method of choice (Issurin, 2008). Furthermore, Baar (2006) and Nader (2006) showed that concurrent development of endurance and strength has negative effects on the development of each of the conditional abilities. By using these shock microcycle blocks with highly concentrated specialized workloads, previous studies already showed that it is possible to improve endurance performance/highintensity running performance (Breil et al., 2010; Christensen et al., 2011; Garcia-Pallares et al., 2010; Mallo., 2011; Wahl et al., 2013). However, only three studies in soccer exist (Christensen et al., 2011; Mallo., 2011; Stoggl et al., 2010), with lacking information on soccer specific performance and especially on the sustainability of performance increases after a high-intensity training block. Therefore, the aim of the present study was to investigate the magnitude of effects and sustainability of a high-intensity shock microcycle according to block periodization in the preseason training of male semiprofessional soccer players.
12 healthy male soccer players (mean [+ or -] SD: 26.1 [+ or -] 4.5 years; 1.80 [+ or -] 0.05 m and 78.8 [+ or -] 6.5 kg) from a team of the sixth German league participated in the study. All players had more than 10 years of training experiences. Before the training intervention athletes had a break lasting 3 weeks where every player trained individually.
During the first half of the season weekly training amount was ~6-8 h plus one game per week. All subjects were informed about the aim of the investigation and gave their written consent to participate in the study. The study protocol was performed in accordance with the declaration of Helsinki and the Ethical Committee of the university.
A 13-day HIT shock microcycle, including interval running, dribbling exercises and small-sided games was conducted (Figure 1). Before (pre), 6 days (6d) after training (in order to assure sufficient recovery) and 25 days (25d) after training (in order to test the sustainability) soccer specific tests reflecting the characteristics of high-intensity match periods were carried out (Figure 1). Between the last HIT session and the 6d post diagnostic only technical drills were carried out. Between the 6d diagnostic and the 25d diagnostic normal training without any HIT session was performed. Normal training consisted of 4 training sessions per week (90 min each), with the main focus on technical/tactical drills and game specific variations. The investigation was conducted during the winter preparatory period. All tests were applied outside on a grass field. Athletes were instructed to avoid heavy exercises 24 h before testing.
During the shock microcycle players performed 12 additional high-intensity interval sessions with two sessions performed on day 6 and 13 (Figure 1). The HIT sessions were performed as interval running, on a dribbling track or as small-sided games (SSG) all consisting of 4 x 4 min bouts separated by 3 min active recovery. Although, the effectiveness of heart rate (HR) for controlling or adjusting the intensity of a HIT session may be limited for several reasons (Buchheit et al. 2013), we chose the HR to assess the training sessions online mainly due to practicability, especially for the control of SSG and the dribbling track. HR was only measured during the first training session. The other sessions were only controlled online (Acentas, Hogertshausen, Germany). If the intended heart rate of 90-95% of the individual maximal heart rate (HRmax) was too low during the online assessment of the intervals, athletes were advised to increase the intensity. To ensure the intended training intensity, the dribbling track was designed according to Hoff et al. (2002) and SSG were modified based on the findings of Hill-Haas et al. (2011). A standardized continuous 10-min warm-up was performed before all HIT...