Physical training is the systematic repetition of physical exercises, and it can be described in terms of its outcome (anatomical, physiological, biochemical, and functional adaptations) or its process, that is, the training load (TL) (the product of volume and intensity of training) (30). Although physical fitness tests are commonly used to assess training outcome, the training process is often described as the external load prescribed by coach (e.g., 4 × 1000 m running at 4 min·km−1 or 8 × 30-m dash at maximum velocity). However, the stimulus for training induced adaptation is the relative physiological stress imposed on the athletes (internal TL) and not the external TL (30). Therefore, to monitor and control the training process, it is important to have a valid measure of internal TL (16). This is particularly relevant in team sports where the planned external load is often similar for each team member because of the extensive use of group exercises such as small-sided games in team training sessions. For example, it was recently reported that soccer players with higher V̇O2max tend to exercise at a lower percentage of V̇O2max during small-game exercises (19). These previous results suggest that use of group training exercises, such as small-sided games, may not provide sufficient stimulus for physiological adaptation in the fitter athletes within a team (19). In addition to fitness level, other factors such as injury, illness, weather conditions, match-scheduling problems, and athlete psychological status can influence the internal TL. Consequently, when combined, these factors suggest that monitoring and controlling athletes’ internal TL is very important to ensure that each athlete receives adequate training stimulus.
There have been several attempts to quantify TL in endurance sports (5,13–16,25). It is well known that HR demonstrates an almost linear relationship with V̇O2 over a wide range of steady-state submaximal exercise intensities (2). This close relationship between HR and V̇O2 measures makes HR monitoring suitable for quantifying exercise intensity during most exercise sessions. Because HR seems to be one of the best objective way to quantify aerobic training intensity (1,17), many methods to quantify the internal TL are based on HR monitoring (5,25). In sports like soccer, HR is mainly used to determine the exercise intensity (3,18,19). To our knowledge, there are no studies using HR to monitor the overall internal TL of soccer sessions. However, it is common practice for some top professional soccer teams to systematically monitor TL using HR methods. Apart from few top level soccer teams, the routine use of HR-based method is not always feasible due to problems such as the required technical expertise, the time-consuming process of collecting HR data of all team players every training session, and the cost of numerous HR telemetric systems. An additional problem with using HR methods for quantification of internal TL in team sports such as soccer is that HR transmitter belts are not permitted during official competitive matches. This is an important limitation because the internal training load induced by a match may represent a relative high percentage of the weekly training load.
An alternative strategy to quantify internal TL was proposed by Foster et al. (13–16). This simple method (session-RPE) quantifies internal TL multiplying the whole training session rating of perceived exertion (RPE) using the category ratio scale (CR10-scale) (6) by its duration. This product represents in a single number the magnitude of internal TL in arbitrary units (AU). Previous research examining the validity of this method of measuring internal TL has shown session-RPE to be related to the percent of HR reserve (HRR) during 30 min of steady-state running and to the time spent at different intensities corresponding to HR at lactate thresholds (2.5 and 4.0 mmol·L−1) during 30 min of continuous and interval running (16). Other research has also shown the session-RPE to be significantly correlated to HR-based method of quantifying internal TL proposed by Edwards (12) in endurance athletes (13). The individual correlations between the session-RPE and Edwards’ HR method ranged from 0.75 to 0.90 (13). Although the session-RPE method was initially proposed for monitoring internal TL in endurance athletes, this method has recently been applied to basketball (9,15), where training is characterized by both aerobic and anaerobic exercises (4).
To date, there are no published studies validating this practical, simple, and inexpensive method to quantify internal TL in team sports. Therefore, the aim of this study was to verify whether the Fosters’ RPE-based approach can be considered a good indicator of internal TL in soccer players, using various HR-based methods as criteria.
Nineteen young soccer players (mean ± SD: age 17.6 ± 0.7 yr, weight 70.2 ± 4.7 kg, height 178.5 ± 4.8 cm) from the same soccer club were involved in the study. All participants were fully informed of the aims and the procedures of the study receiving both verbal and written explanation. All athletes gave a written consent according to the American College of Sports Medicine guidelines. This study was approved by the Ethics Committee of the local organization.
Commonly used performance tests were performed before and after 7 wk of training to evaluate the subjects training progress. An incremental treadmill run to exhaustion was completed using the protocol of Helgerud et al. (18), where the treadmill running velocity was increased by 1 km·h−1 every 5 min, at an inclination of 3%. Once capillary blood lactate concentrations [La−] were elevated above 4 mmol·L−1, the treadmill speed was increased 1 km·h−1 every 30 s until exhaustion. V̇O2max was measured using a breath-by-breath automated gas analysis system (VMAX29, SensorMedics, Yorba Linda, CA). Flow, volume, and gas concentrations were calibrated before each test using routine procedures. The highest HR measured during the test was used as maximum reference value. At the end of each step and 3 min after exhaustion, capillary blood samples (25 μL) were collected from the ear lobe and immediately analyzed using an electroenzymatic technique (YSI 1500 Sport, Yellow Springs Instruments, Yellow Springs, OH). Before each test the analyzer was calibrated following the instructions of the manufacturer using standard lactate solutions of 0, 5, 15, and 30 mmol·L−1. The following lactate thresholds were calculated from blood [La−] measures taken during the incremental test:
1) Lactate threshold (LT), the intensity that elicited a 1.5 mmol·L−1 increase in [La−] above baseline values (50–60% of V̇O2max) (18).
2) Onset of blood lactate accumulation (OBLA), the intensity corresponding to a fixed [La−] value of 4 mmol·L−1 (28).
Field data collection.
Training data were collected during the first 7 wk of the competitive season (from Sep-tember to November). The training program was planned by the coach of the team. The researchers did not alter the original training program. Except for the second week of the study, all players trained four times (Monday, Tuesday, Wednesday, Thursday) and participated in an official match each week (Saturday). In the second week, only three training sessions were completed. The heaviest aerobic training was usually completed during the Monday sessions. During the Tuesday sessions, the first 30 min of training were generally dedicated to speed development consisting mainly of sprint and plyometric training exercises. Running interval training (4 × 1000 m) was completed only two times during the 7 wk of the study. Most of the physical conditioning training was performed using small group exercises. Small-sided games with individual technical and tactical objectives were also extensively performed.
HR was recorded every 5 s during each training session using HR monitor with individually coded HR transmitters to avoid interference (VantageNV, Polar Electro, Kempele, Finland). The mean HR recorded during the briefing before each training session was used as rest HR. To reduce HR recording error during training, all athletes were regularly asked to check that their HR monitors were functioning of properly (at least every 10 min). The investigators were immediately available to solve these problems such as erroneous HR values or technical/transmission problems. After every training session, HR data were downloaded on a portable PC using the specific software and subsequently exported and analyzed using the Excel software program (Microsoft Corporation, U.S.).
Internal training load indices determination.
The session-RPE was determined by multiply the training duration (minutes) by session RPE as described by Foster et al. (16). Each athlete’s session-RPE was collected about 30 min after each training session to ensure that the perceived effort was referred to the whole session rather than the most recent exercise intensity. In this study, the Italian translation of the CR10-scale modified by Foster et al. (16) was used. This scale was modified in order to better reflect the American idiomatic English (Table 1). It is unlikely that these minor changes affect the reliability and the validity of the original Borg’s CR10-scale. All athletes had been familiarized with this scale for rating perceived exertion before the commencement of the study.
Various HR-based TL were used as the criterion measure of internal TL. The HR-based method proposed by Edwards (12) was used by Foster et al. to validate the use of RPE-TL to monitor endurance training (13). This HR-based method was also used as criterion measure of TL in a study examining the session-RPE method during nonsteady state and prolonged exercise (15). For these reasons, we calculate Edwards’ TL from training sessions HR data recorded and collected during the 7 wk of training. The Edwards’ method determines internal load by measuring the product of the accumulated training duration (minutes) of 5 HR zones by a coefficient relative to each zone (50–60% of HRmax = 1, 60–70% of HRmax = 2, 70–80% of HRmax = 3, 80–90% of HRmax = 4, 90–100% of HRmax = 5) and then summating the results.
Another HR-based method of determining internal TL in the present study was the training impulse (TRIMP), described by Banister (5). Training impulse was determined using the following formula:
in which TD is the effective training session duration expressed in min and HRR is determined with the following equation:
Recently, Lucia et al. (21) proposed another approach to determine internal TL in endurance athletes (Lucia’s TRIMP). TL is calculated using this method by multiplying the time spent in three different HR zones (zone 1: below the ventilatory threshold; zone 2: between the ventilatory threshold and the respiratory compensation point; zone 3: above the respiratory compensation point) by a coefficient (k) relative to each zone (k = 1 for zone 1, k = 2 for zone 2, and k = 3 for zone 3) and then summating the results. This method is similar to that of Edwards (12). The main difference between Edward’s and Lucia’s method is that the HR zones defined by Lucia et al. (21) are based on individual parameters obtained in laboratory, whereas Edward’s method uses standardized predefined zones. In the present study, LT were used instead of ventilatory thresholds. A similar approach was used by Foster et al. (16), who reported significant relationships between session-RPE and relative time spent in three different zones defined by HR at 2.5 and 4 mmol·L−1 LT. For weeks 1–4, the LT of the first laboratory test was used, whereas for weeks 5–7, the tests results performed at the end of the training period investigated were taken as reference.
The relationships between session-RPE and the various HR-based TL were analyzed using Pearson’s product moment correlation. Mean weekly session-RPE was analyzed using a one-way ANOVA, followed by Scheffé’s post hoc test. Statistical significance was set at P < 0.05. For the statistical analysis, the software package STATISTICA (Version 6.0, StatSoft, Tulsa, OK) was used.
Maximum oxygen uptake of this group of young soccer players was not statistically different before and after 7 wk of training (56.8 ± 3.9 mL·kg−1·min−1 vs 57.1 ± 4.0 mL·kg−1·min−1). Similarly, HRmax (187.6 ± 6.7 beats·min−1 vs 189.6 ± 5.7 beats·min−1) and maximal aerobic speed reached in the treadmill incremental test (16.7 ± 1.1 km·h−1 vs 17.0 ± 1.1 km·h−1) were unchanged after training. The HR at LT in the first and second laboratory tests was 162.0 ± 11.9 beats·min−1 and 163 ± 7.9 beats·min−1, corresponding to 85.5 ± 5.3 and 86.9 ± 3.8% of HRmax, respectively. The HR at OBLA in the first and second laboratory test was 171.5 ± 8.3 beats·min−1 and 171.2 ± 7.1 beats·min−1, corresponding to 90.5 ± 3.4 and 91.3 ± 3.4% of HRmax, respectively. These absolute and relative HR were not significantly different between the two testing sessions.
The various HR-based TL and session-RPE were collected from 476 training sessions. Individual correlations were determined on a minimum of 17 to a maximum of 27 training sessions data. Correlations between session-RPE and HR-based TL were all significant (P < 0.01 to P < 0.001). Individual correlations are presented in Table 2. Figure 1 shows that session-RPE and Edwards’ TL described similarly the team TL during the 7 wk of training, confirmed also by the significant correlation between team session-RPE and team Edwards’ TL (Fig. 2).
The mean weekly internal TL (weekly periodization) described using session-RPE is shown in Figure 3. The mean session-RPE of Monday, Tuesday, Wednesday, and Thursday were 634 ± 116 AU, 550 ± 67 AU, 453 ± 83 AU, and 343 ± 65 AU, respectively (N = 19). For descriptive purposes and to obtain a more representative value of match RPE (625 ± 60 AU), only data of players that played more than 80 min were used (N = 12). Peak internal TL was reached the first day of the training week (after a day of total recovery) (Fig. 1). Further analysis of the individual training weeks within this study showed that there was some variability in the placement of peak internal TL sessions within the week. For example, during week 2 and 7 peak daily sessions were completed on the second day of training. However, most of the sessions with higher internal TL were completed at least 3 d before match. This was deliberately planned by the coach to allow for adequate recovery before competitive matches.
The present study is the first to apply the Foster’s RPE-based approach (16) to quantify internal TL in soccer, and to demonstrate significant correlations between this method and other published methods based on the HR response to exercise. These correlations (ranging from 0.50 to 0.85) were slightly lower than those reported by previous investigators (r = 0.75–0.90) (13). A possible explanation for the lower correlations in the present study could be the in- creased anaerobic contribution to energy provision during soccer training. The increased anaerobic contribution may account for the increased internal TL through increased RPE. Previous research supporting this suggestion has demonstrated increased subject RPE during intermittent protocols in comparison with a steady-state exercise session matched for total work, despite no differences in V̇O2 and HR between the two exercise protocols (11). These investigators also suggested that the increased RPE during the intermittent work protocol may be due to the increased contribution of anaerobic mechanisms to energy provision (11,27). Because soccer training can be characterized by intermittent exercises relying on both aerobic and anaerobic sources for energy provision (3), the different perceived exertion with similar mean HR may explain the reduced strength of the correlations between the session-RPE and HR-based TL in comparison to those reported by previous research on endurance athletes (13).
As RPE represents the athlete’s own perception of training stress, which can include both physical and psychological stress, the session-RPE method may provide a valuable measure of internal TL. Borg’s CR10 is considered a global indicators of exercise intensity including physiological (oxygen uptake, HR, ventilation, beta endorphin, circulating glucose concentration, and glycogen depletion) and psychological factors (23). As a consequence, RPE-based quantification of TL could be considered an accurate indicator of global internal TL. Research has shown that the combination of HR and [La−] predicts RPE more accurately than either variable taken alone (7). This previous research suggests that RPE may be a more reliable measure of exercise intensity when both anaerobic and aerobic systems are appreciably activated, such as is the case during intermittent activities like soccer training and match play (3). Hence, these findings emphasize the usefulness of RPE to monitor exercise intensity due to its psychobiological nature (8).
Although RPE has been shown to accurately reflect exercise intensity, it is possible that players could perceive the same physiological stimulus differently as a consequence of their individual psychological state (24). Researchers investigating overreaching and overtraining support this suggestion, as RPE has been reported to increase during a standardized exercise test when athletes are in an increased fatigue state (29). Furthermore, during overreaching, RPE for a given HR was reported to increase (22), suggesting that RPE could be more sensitive to accumulated fatigue than HR. This characteristic of RPE may have partially determined the moderate correlations between HR-based TL and session-RPE found in some subjects of this investigation. Consequently, the use of RPE to monitor exercise intensity could be considered a valuable tool to detect excessive training-related fatigue in athletes and also potentially viable in monitoring responses to training and preventing overtraining (20,29).
Quantification of internal TL is also necessary to analyze the periodization of training (26). In team sports, appropriate periodization of internal TL during the training week is important to ensure adequate physiological stimulus is provided while allowing adequate time for recovery before competition days. Commonly, heavy training sessions are not imposed to players in the days immediately before or after competition matches, in order to avoid excessive physical strain that could impair recovery and reduce performance (10). This general approach to the weekly training structure is common among many soccer teams and other team sports where weekly competition is required (10,26).
The RPE-based method for quantifying internal TL is simple and practical. However, in order to be used reliably, it is necessary to follow correct standardized procedures including player education and familiarization with the CR10-scale, and standard timing of rating should be followed (15). In the present investigation, the players were accustomed to use the CR10-scale to classify training intensity, as this method was routinely used in both their soccer training and laboratory-based physiological testing sessions. The use of RPE during incremental tests is a good approach as it allows the athlete to readily associate RPE scores through a full range of exercise intensities. However, when laboratory tests cannot be conducted, it is possible to familiarize players with incremental field tests. The familiarity of our subjects with the use of the CR10-scale made it simple to attain valid exertion ratings after each training session. The timing of the rating is also important to minimize influence of the last effort during training on the player’s RPE of the whole training session. For this reason in this study, the last 15–20 min of each training session were dedicated to cool-down, and RPE were asked for after 30 min from the end of the session.
In summary, based on our results and the literature reviewed, Foster’s RPE-based method seems to be a good indicator of global internal TL in soccer. This method does not require expensive equipment such as telemetric HR systems and may be very useful and practical for coaches to monitor soccer players internal TL. Furthermore, the present results suggest that the RPE-based method may assist in the development of specific periodization strategies for individuals and teams. However, the moderate correlations we found do not support this method as a valid substitute of HR, as only about 50% of variance in HR was explained by session-RPE. This simple method has the potential to become a valuable tool for coaches and sport scientists to monitor internal TL, but further studies are necessary to fully validate this TL quantification strategy.
The authors would like to thank Prof. Maurizio Fanchini and the Pro Patria Football Club for their collaboration in this study. We also acknowledge the soccer players involved in this investigation.
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