Individual training adaptations in response to chronic exposure to physical exercise are related to athletes' individual physical fitness level and the magnitude (i.e., intensity and duration) of the prescribed training load (TL) (Impellizzeri et al., 2005). As a result, the quantification of the individual responses to a given external TL is important to optimize physical fitness performance. However, in a team sport such as soccer, the extensive use of group exercises where inter-player TL differences vary depending on the type of ball-drill performed (Los Arcos et al., 2014) make the prescription of training difficult in relation to individual characteristics (Impellizzeri et al., 2005). For instance, those players with the highest V[O.sub.2max] showed the lowest percentage of V[O.sub.2max] during the small group play (Hoff et al., 2002). Therefore, the remarkable differences in TL accumulation between soccer players (Akubat et al., 2012; Impellizzeri et al., 2005; Manzi et al., 2013) and the difficulty to ensure an appropriate level of training stimulus for each soccer player, when a team-based training approach is undertaken, suggest how important the quantification of the individual's TL is (Alexiou and Coutts, 2008) and the relevance of this information that makes possible the identification of the players with insufficient or excessive TL and individual training strategies.
The quantification of TL is generally based on both external (e.g., distance, power output, number of repetitions) and internal (e.g., oxygen uptake, heart rate, blood lactate, rate of perceived exertion) indicators of effort intensity (Buchheit, 2014). In order to know the stimulus for exercise-induced adaptations, Foster et al. (2001) proposed the session-rating of perceived exertion (sRPE) as a practical tool for evaluating internal TL in endurance and team sport athletes (Foster et al., 1996, 2001, 1995; Foster, 1998). This method requires from the players to subjectively rate the intensity of the entire training session using a modified version of category ratio scale (CR-10 scale) developed by Borg et al. (1998), the Foster's 0-10 scale (Foster et al., 2001). This intensity value is then multiplied by the total duration (minutes) of the training session to create a single measure of internal TL (sRPE-TL) in arbitrary units (AUs) (Foster et al., 2001). Previous investigations have shown sRPE-TL to compare favourably with more sophisticated methods of quantifying TL such as heart rate (HR) in endurance (Foster et al., 2001), team sport (Alexiou and Coutts, 2008; Casamichana et al., 2013; Foster et al., 2001; Impellizzeri et al., 2004; Scott et al., 2013), and resistance-trained athletes (Day et al., 2004). Specifically for soccer training, several studies have described substantial positive correlations between sRPE-TL and HR-based TL (Alexiou and Coutts, 2008; Casamichana et al., 2013; Impellizzeri et al., 2004; Scott et al., 2013). Thus, the sRPE-TL appears to be a good indicator of the global internal load for soccer training (Impellizzeri et al., 2004). Furthermore, RPE method is easy, versatile and cheap (Borg, 1982), it does not require technical expertise and the TL is measured quickly without chance for technical errors (Alexiou and Coutts, 2008).
Despite the potential limitations of HR measures to quantify TL in team sports (Borresen and Lambert, 2009), previous training studies have generally reported substantial and positive associations between the changes in HRbased TL and physical fitness parameters in soccer players (Akubat et al., 2012; Castagna et al., 2011; 2013; Manzi et al., 2013). For example, after 6 to 8 weeks of pre-season training, large to very-large correlations were reported between training time spent at high-intensity (> HR at 4 mmol x [l.sup.-1]) and changes in running speed at 2 and 4 mmol x [l.sup.-1] of blood lactate (Castagna et al., 2011; 2013), maximal oxygen uptake (V[O.sub.2max]) (Castagna et al., 2013) and performance in the Yo-Yo IR1 (Castagna et al., 2013). Moreover, after 8 weeks of pre-season training, Manzi et al. (2013) reported large to very-large correlations between HR-derived individualised training impulse (iTRIMP) TL and changes in the running speed at 4 mmol x [l.sup.-1] of blood lactate and performance in the Yo-Yo IR1. Similarly, Akubat et al. (2012) reported a large association between the HR-derived iTRIMP and changes in the running speed at 2 mmol x [l.sup.-1] of blood lactate in young soccer players after six week of in-season training. Interestingly, in this last study authors did not find any substantial association between changes in several aerobic fitness variables (i.e., running speed at 2 mmol x [l.sup.-1] and 4 mmol x [l.sup.-1] of blood lactate) and overall sRPE-TL (Akubat et al., 2012). Thus, despite the reported validity and advantages of the RPE derived TL (Alexiou and Coutts, 2008; Impellizzeri et al., 2004), it is still unclear how useful this measure is for monitoring changes in physical fitness performance in soccer players.
To our knowledge, all previous soccer studies that have assessed the relationship between longitudinal changes in TL and physical fitness performance have only paid attention to aerobic fitness variables (Akubat et al., 2012; Castagna et al., 2011, 2013; Manzi et al., 2013). In addition to aerobic fitness, neuromuscular factors (i.e., strength, power, speed) are important performance determinants of soccer match-play physical performance (Stolen et al., 2005) and are heavily taxed during matches and training sessions (Ascensao et al., 2008; Krustrup et al., 2006; Mohr et al., 2004; Rampinini et al., 2011; Robineau et al., 2012; Thorlund et al., 2009). Despite neuromuscular factors being perceived as crucial for soccer physical performance, to the best of our knowledge, no previous study has assessed the relationship between changes in TL and neuromuscular fitness parameters in soccer. In order to evaluate these associations between RPE-derived TL and both aerobic and neuromuscular fitness components, the differentiated use of respiratory sRPE-TL (sRPEres-TL) and muscular sRPE-TL (sRPEmus-TL) (Aliverti et al., 2011; Borg et al., 2010; Green et al., 2009; Mahon et al., 1998) may offer, also in team sports (Arcos et al., 2014; Yanci et al., 2014; Weston et al., 2014), an interesting alternative to the global sRPE.
The purpose of this study was, therefore, to examine the usefulness of sRPEres-TL and sRPEmus-TL for monitoring changes in several aerobic fitness and neuromuscular performance variables during 9 weeks of soccer training in young professional players.
Nineteen young professional male soccer players (20.2 [+ or -] 1.9 years, height 1.81 [+ or -] 0.07 m, body mass 73.8 [+ or -] 7.3 kg) belonging to the same reserve team of a Spanish La Liga Club participated in this study. Two players were excluded from the final analysis due to long-term injuries (> 4 weeks) and three other players could not perform the second physical fitness assessment. Eventually, the group was reduced to 14 players (6 defenders, 5 midfielders, and 3 forwards; 20.6 [+ or -] 1.7 years, height 1.79 [+ or -] 0.06 m, body mass 73.5 [+ or -] 7.0 kg). One player did not complete the jump and sprint tests due to acute injury during the second physical fitness assessment. Players were professional and their training regimen was identical to the 1st Division team. As such, players were not involved in any other professional activity aside soccer. They competed during the 2011-2012 in the Spanish 2ndB division Championship. Players had between 0-3 years of competitive experience in this Championship and at least 10 years of soccer training experience. Goalkeepers were excluded from the study. All participants were notified of the research procedures, requirements, benefits and risks before giving informed consent. The study was conducted according to the Declaration of Helsinki, and the study was approved by the local Ethics Committee.
This study was performed during pre-season (5 weeks) and in-season competitive periods (4 weeks), from July to September. Before and after the 9 weeks, the subjects were tested to determine physical fitness performance. Players trained 5-8 times a week (31 training sessions in total) and played 1-2 friendly matches per week (8 friendly matches in total) during pre-season, while they trained 4-6 times a week (19 trainings in total) and played an official match per week (4 official matches in total) during the competitive period. Furthermore, during the competition period an additional friendly match was played. During pre-season, friendly matches were played in the middle of the week (Wednesday) and in the weekend (Saturday or Sunday). All official matches were played during weekends. The participants that played lesser minutes (substitutes) during the competition period typically performed...