Volleyball is characterized as an intermittent sport, with frequent high-intensity actions, involving explosive bursts, short body displacements and numerous jumps (Sheppard et al., 2007). One of the main physical abilities required in this modality is lower limb muscle power, expressed by the numerous jumps performed during the games, which are important both for the attacking and blocking actions (Sheppard et al., 2007; 2008; 2009). Some studies have reported positive changes in the ability of performing vertical jumps during a competitive season (Hakkinen, 1993), as well as during the years of training in the transition from the junior to the senior category (Sheppard et al., 2012). Besides this, a greater ability to perform serving and blocking actions has been suggested as a discriminating factor between the more successful teams, who win championships, and the less successful ones (Sheppard et al., 2009). It is noteworthy that in modern volleyball, most players serve using jumps in order to have higher vertical amplitude in contact with the ball, thereby increasing the power of the action. Therefore, a well-planned pre-competitive period is essential to ensure that the athletes enter the competitive period with an optimal performance in this and other abilities.
Volleyball, like other team sports, presents a calendar with a short pre-competitive period and a long competitive period. Thus, during the pre-competitive period, intensification of the training load is a frequently used strategy aimed at preparing the athletes to face the demands of a long competitive period and adapt rapidly (Coutts et al., 2007a; Coutts and Reaburn, 2008). Such a strategy requires careful control of training loads, since the application of excessive loads and insufficient recovery may induce negative adaptations to training, leading the athlete to non-functional overreaching or overtraining (Borresen and Lambert, 2009; Foster et al., 2001; Kentta and Hassmen, 1998). Appropriate load control, in turn, is based on accurate quantification and frequent monitoring of performance and psychophysiological changes resulting from the balance between the stress and recovery (Borresen and Lambert, 2009; Coutts et al., 2007d; Coutts and Reaburn, 2008).
In this sense, the session rating of perceived exertion (session-RPE) method (Foster, 1998; Foster et al., 2001) is presented as a simple low cost strategy, validated to quantify the internal training load in several sports (Foster, 1998; Foster et al., 2001; Coutts and Reaburn, 2008; Coutts et al., 2007b; 2007d; Impellizzeri et al., 2004; Milanez et al., 2011), including volleyball (Bara Filho et al., 2013), and has been shown to be consistent with the manipulation of external load.
Among the numerous jumps performed in volleyball games (Sheppard et al., 2007), a substantial percentage are performed with a countermovement, implying a rapid eccentric phase leveraging the concentric phase of the jump. Thus, the countermovement vertical jump test (CMJ) is frequently used to evaluate performance in this modality (Sheppard et al., 2008; 2009; 2012). Fatigue caused by exhaustive exercises, especially those involving stretch-shortening cycle, implies a loss in neuromuscular function (Horita et al., 1999; Komi, 2000). The reduced performance in the CMJ, which is related to the magnitude of muscle damage and neuromuscular fatigue, may persist for several days (Komi, 2000; Horita et al., 1999). A study by Delextrat et al. (2012) reported a decrease in the CMJ height on the third day of a regular training week in basketball players. In rugby athletes, Coutts et al. (2007a) showed a reduction in CMJ performance after a pre-competitive period with intensified training loads. Johnston et al. (2013) also identified impaired muscular function during the performance in the CMJ in response to an intense period of rugby fixtures. Thus, the CMJ appears to be a sensitive index of the fatigue occasioned by the intensification of training load, especially in sports like volleyball, in which the stretch-shortening cycle is repeated hundreds of times a week. However, the sensitivity of CMJ to the fatigue caused by the intensification of training has not yet been tested in volleyball.
Questionnaires are also shown to be valid, simple and practical strategies for monitoring the effects of training loads (Kentta and Hassmen, 1998). Among them, the Recovery and Stress Questionnaire for Athletes (RESTQSport) (Costa and Samulski, 2005; Kellmann and Kallus, 2001) is capable of simultaneously monitoring the stress and recovery in response to team sports training (Coutts and Reaburn, 2008; di Fronso et al., 2013; Noce et al., 2011), whilst also being sensitive for monitoring the effects of training loads during periods of intensification and to identify/predict overreaching (Coutts and Reaburn, 2008; Coutts et al., 2007d; Gonzalez-Boto et al., 2008; Nederhof et al., 2008). Another tool that is thought to be equally sensitive to the effects of training loads is the Total Quality Recovery Scale--TQR--(Kentta and Hassmen, 1998), used to monitor the state of the psychophysiological recovery of athletes (Brink et al., 2010; Suzuki et al., 2006).
Serum creatine kinase (CK) is a biochemical marker widely used in sports science as a diagnostic parameter for quantifying the degree of muscle damage (Brancaccio et al., 2010; Hartmann and Mester, 2000) or changes in the permeability of the muscle cell membrane (Goodman et al., 1997). Studies show a sharp rise in CK levels after prolonged high intensity exercises (Nederhof et al., 2008; Waskiewicz et al., 2012), after exercises with a large eccentric component (Snieckus et al., 2012), as well as in response to a period of intensified training (Coutts et al., 2007a; 2007b). In addition, in soccer players, an increase above the 90th percentile of CK found in the athletes' plasma, during a competition, is suggested to detect fatigue and muscle overload (Lazarim et al., 2009).
The sensitivity of the aforementioned performance, psychological and biochemical markers in monitoring the overload effects of training in athletes of various sports leads us to the hypothesis that these markers will present the same sensitivity for monitoring the effects of training load intensification during a pre-competitive period in volleyball. Nevertheless, this has not yet been investigated in previous studies and is justified based on the perceived need of coaches and trainers to monitor biological signs of excessive internal training loads. Thus, the aim of this study was to test the sensitivity of CMJ performance, RESTQ-Sport, TQR and CK to deliberate intensification of training load during the pre-competitive period on high-level volleyball players.
Experimental approach to the problem
The present study involved a mesocycle of 25 days preparation of a high-level volleyball team (adult team competing in the 1st division of Brazilian volleyball--Brazilian Men's Volleyball Super League) for a state competition, in which the team was among the 4 finalists. The beginning of the investigated period corresponded to the seventh mesocycle of the team season, with competitions at national and state level.
In order to assess the sensitivity of the markers to the intensification of training loads, a proportion of the players were submitted to deliberate intensification of training load (IT group). The training load was deliberately intensified for 11 days (first period--FP) and reduced afterwards for 14 days (second period--SP). Another group continued training with normal (i.e., habitual) training (NT group) load. This group was used as the "control". Performances in the CMJ, RESTQ-Sport, and the CK were evaluated at three different moments: on the first day of training, considered as baseline, after FP and after SP. The TQR was evaluated at the beginning and end of each microcycle.
Sixteen male athletes belonging to a high-level Brazilian volleyball team participated in the study. The team's technical staff planned the training in order to test the effect of intensification of training loads on performance indicators. Accordingly, the coach presented a list of the eight athletes who had more potential to be starters in the championship. These athletes composed the IT group. These players were 23.37 [+ or -] 2.94 years old, 88.18 [+ or -] 5.26 kg, 1.95 [+ or -] 0.06 m with a 7.80 [+ or -] 2.13% fat percentage. A group of eight players with lower chances of being starters (non-starters) was submitted to the training that is habitually planned in the pre-competitive period of the investigated team, composing the NT group. These athletes were 19.75 [+ or -] 1.48 years old, 77.52 [+ or -] 11.51 kg, 1.88 [+ or -] 0.07 m with a 9.07 [+ or -] 2.71% fat percentage. Hence, we emphasize that this study was not a randomized controlled trial, because the randomization of players into the groups would not have been in the interests of the team, and so, it would be counterproductive to its goals. Nevertheless, we consider the experimental design adequate to describe the longitudinal effects of short-term overload in volleyball, having, as a reference, a group of players training normally.
The study was approved by the local Ethics Committee...