Tennis is an intermittent sport involving high-intensity efforts interspersed with periods of low-intensity activity, during which active recovery (between points) and passive periods (between changeover breaks in play) take place (Fernandez-Fernandez et al., 2009; Kovacs, 2007). Although the technical and tactical skills are considered the most predominant factors in tennis performance (Smekal et al., 2001), players also need a mixture of fitness qualities such as speed, agility, and power combined with a well-developed aerobic fitness in order to achieve high levels of performance (Fernandez et al., 2006; 2009). During competitive matches, mean heart rate (HR) values range between 70 and 80% of maximum ([HR.sub.max]), and peak values around 90 to 100% of [HR.sub.max]. Average oxygen uptake (V[O.sub.2]) values correspond to approximately 50 to 60% of maximum oxygen consumption (V[O.sub.2max]), with values above 80% of V[O.sub.2max] during intensive rallies (Kovacs, 2007). Rating of perceived exertion (RPE) has been reported as ranging from 5 to 7 au (arbitrary units on a scale of 1 to 10) (Coutts et al., 2010; Gomes et al., 2013), and 10 to 16 (on the Borg 20-point scale) (Mendez-Villanueva et al., 2010). Thus, it seems that the ability to maintain a high technical efficiency during those phases of high-intensity intermittent exercise (which can result in fatigue) is an important feature of successful tennis players (Mendez-Villanueva et al., 2007).
Elite tennis players travel and compete year round and have a demanding calendar. This can result in athletes focusing on competition and thus compromising training, leading to suboptimal recovery, conditioning, and overall preparation (Duffield et al., 2013; Fernandez-Fernandez et al., 2009). Because of the ever-increasing demands imposed on players, there is a progressive reduction of total training time devoted to preparation, with preseasons normally reduced to 5 to 7 weeks duration. At the highest levels of the game, this preseason is even reduced further with the increase in high-paid exhibition matches or non-sanctioned team events/tournaments. During the preseason, most tennis players do prioritise fitness training during the first couple of weeks, while the maintenance of technical and tactical skills also seems to be a key factor. Thus, coaches are increasingly relying on an integrated approach to conditioning and skill-based work, often resulting in the programming of game-specific, on-court exercises that include both technical and tactical assignments as part of sport-specific conditioning (Buchheit et al., 2009). High-intensity training (HIT) (i.e., work and rest intervals ranging from 15 s to 4 min; 90-100% velocity at the level of V[O.sub.2max]; HR values ~90% of [HR.sub.max]; work-to-rest ratios of 1:1 to 4:1) (Laursen and Jenkins, 2002) that incorporates skills and movements specific to the sport has been reported to result in physiological responses that mirrored aspects of both average and maximal match-play and can be used as a training method aiming to improve tennis-specific fitness levels (Fernandez-Fernandez et al., 2012; Reid et al., 2008). Previous research has shown that the implementation of HIT protocols during preseason conditioning (i.e., 2-3 training sessions per week for 6-10 weeks) leads to enhanced sport-specific performance (Dupont et al., 2004; Fernandez-Fernandez et al., 2012; Sperlich et al., 2011). However, little is known about the integration of HIT in daily training sessions or in short periods of concentrated training.
As the tennis preseason is probably the shortest of all the major sports, the training schedule and how to organise the main physical abilities in order to achieve optimal training outcome and performance remain unclear. Block periodisation--also described as 'a training cycle of highly concentrated specialized workloads' or shock microcycle (Issurin, 2010), including HIT, in which training periods are divided into shorter periods (1-4 weeks) with the main focus of improving a few specific abilities (i.e., V[O.sub.2max])--might be an alternative (Garcia-Pallares and Izquierdo, 2011; Issurin, 2008). While the potential benefits of this periodisation model have been theorised (Issurin, 2010), only a few studies have shown its relative effectiveness, mainly in endurance athletes (Breil et al., 2010; Garcia-Pallares and Izquierdo, 2011; Ronnestad et al., 2014). Results have shown that the use of shock microcycles leads to performance improvements in different sports (e.g., rowing, soccer, ski) (Breil et al., 2010; Christensen et al., 2011; Garcia-Pallares et al., 2010; Wahl et al., 2013). However, there is a lack of information about the use of these shock training microcycles in intermittent sports such as tennis. Therefore, the purpose of the present study was to investigate the effects of HIT addition to the normal training content in several physical performance indicators during the preseason training of high-level male tennis players.
Twelve healthy male tennis players (mean [+ or -] SD: age 21.9 [+ or -] 2.0 years; height 1.82 [+ or -] 0.22 m and weight 76.4 [+ or -] 5.9 kg) with a ranking between positions 500 and 800 (668.1 [+ or -] 105.1) in the Association of Tennis Professionals (ATP) volunteered to participate in this study. Players trained 17 [+ or -] 2.5 h x [wk.sup.-1] and had a training background of 12 [+ or -] 2 years. All the players were free of cardiovascular and pulmonary disease and were not taking any medications. Prior to any participation, the experimental procedures and potential risks were explained fully to the players, and all provided written informed consent. The study was approved by the local ethics committee and conformed to the Declaration of Helsinki.
A 17-day HIT shock microcycle, including running exercises based on the 30:15 intermittent fitness test (30:15ITF) and on-court specific exercises organised in 13 training sessions (~30 min each), was conducted (Figure 1). Before any baseline testing, all participants attended two familiarisation sessions to introduce the testing and training procedures and to ensure that any learning effect was minimal for the baseline measures. Fitness tests (30:15IFT, 20 m sprint, countermovement jump [CMJ], repeated sprint ability [RSA]) were conducted before (pre-test) and 5 days after the intervention (posttest). Between the last HIT session and the post-test, only on-court training combined with moderate intensity strength training and injury prevention (e.g., core training, shoulder strengthening, and flexibility) sessions were performed. Normal training consisted of 5 training sessions per week (60-90 min each), with the main focus on technical/tactical drills and game-specific situations (e.g., sessions were designed by coaches to address the specific priorities of each athlete). Therefore, players were submitted to an overall training volume of ~22 h during the shock microcycle. The investigation was conducted during the European winter preparatory period (November-December). All tests were conducted on an indoor synthetic court. To reduce the interference of uncontrolled variables, all subjects were instructed to maintain their habitual lifestyle and normal dietary intake before and during the study. The subjects were told not to exercise on the day before a test and to consume their last (caffeine-free) meal at least 2 h before the scheduled test time.
30-15 Intermittent Fitness Test: Supramaximal intermittent performance with changes of direction was assessed using the 30-15IFT (Buchheit, 2008), which consisted of 30-s shuttle runs interspersed with 15-s passive recovery periods. The athletes had to run back and forth between two lines set 40 m apart at a pace dictated by an auditory signal. The speed was set at 8 km x [h.sup.-1] for the first 30-s run and was increased by 0.5 km x [h.sup.-1] every 45-s stage thereafter. The speed during the last completed stage was noted as velocity obtained in the intermittent fitness test ([V.sub.IFT]). The reliability of [V.sub.IFT] has been shown to be good (intra-class correlation coefficient [ICC] = 0.96; typical error [TE] 0.33 km x [h.sup.-1]) (Buchheit et al., 2008). HR was monitored and recorded at 5-s intervals during the test (Polar S610, Kempele, Finland), and maximum HR ([HR.sub.max]) was determined as the highest 5-s mean value.
Speed Test: Running speed was...