The monitoring of training loads is a practical method to evaluate physical adaptation and recovery status during a training period (Bourdon et al., 2017). Methodologies to monitor training loads can be divided into internal and external measures (Impellizzeri et al., 2019). Internal measures of load can be evaluated via self-report measures [i.e., rating of perceived exertion (RPE)] or objective measures of physiological response [i.e., heart rate (HR) and blood lactate concentration] (Clemente et al., 2019). Conversely, external load measures that assist in measuring locomotion profiles via microelectromechanical systems (e.g., global navigation satellite system, inertial measurement unit, or local positioning system) can be used to evaluate physical impact and physiological strains during training sessions and competitions (Halson, 2014).

Futsal is a high-intensity intermittent and strenuous indoor sports (Barbero-Alvarez et al., 2008) requiring both aerobic and anaerobic energy systems to maintain vigorous physical and mental conditions (Naser et al., 2017). Futsal competitions are incredibly high in intensity and often cause intensive physiological and psychological strains to the players. It has been reported that high-speed running (18.1–25 km.h^–1) and sprinting (>25 km.h^–1) account for 22.6% of the total distance covered (TD) during a competitive futsal match. Additionally, field players have average HR responses between 170 and 190 beats.min^–1 in Liga Nacional de Futbol Sala (around 83% of maximal HR during the match time), representing a high cardiovascular load on players (Barbero-Alvarez et al., 2008). This cardiovascular load results from rapid changes in various activities every 8–9 s (e.g., high-speed running, sprints, change of direction, and lateral and backward activity) during matches (Álvarez et al., 2009).

Much like during match scenarios, monitoring the demands of futsal training is also important to understand the accumulation of the training load imposed on an individual player (Clemente et al., 2020). Among the possible methods of monitoring internal load, the RPE is a simple and practical tool to quantify since it is a valid, low-cost, and reliable method in measuring exercise intensity (Haddad et al., 2017). The RPE is based on psychological perception in response to training and/or exercise stimuli. An 11-points scale version from 0 (no exertion) to 10 (maximal effort), Borg CR10 scale (RPE_CR10), is developed to evaluate physiological and psychological perceptions in response to exercise strain in laboratory and field settings (Foster et al., 2001). Subsequently, the RPE_CR10 in respect to the exercise or training duration was used to quantify internal training load in sports training (Foster et al., 2001). This method has been extensively used in team sports, such as soccer (Impellizzeri et al., 2004; Clemente et al., 2019) and futsal players (Chen et al., 2020; Clemente et al., 2020; Stochi de Oliveira and Borin, 2021) due to convenience and user-friendliness.

The respiratory (RPE_respiration) and muscular (RPE_muscle) perceived exertions are variants of RPE scales. The RPE_respiration and RPE_muscle primarily focus on the breathing efforts and leg fatigue during exercise, respectively (Pianosi et al., 2014, 2015). The benefit of using both the RPE_respiration and RPE_muscle scales is that they identify specific characteristics of psychophysiological responses in cardiopulmonary and leg muscle performance. The advantage of using different RPE scales to quantify training load is related to the sensitivity of the measurement. In terms of Dalhousie pictorial scales, seven different illustrations for dyspnea (chest tightness, throat closure, breathing effort, etc.) and perceived leg exertion (light leg, heavy leg, soft leg, etc.) are used to represent different physiological strains in responses to different exercise loads. The validity to use Dalhousie scales to rate respiratory and leg muscle exertions during exercise activity has been reported in pediatric populations, compared to RPE_CR10 measure.

In terms of specific training sessions, physical demands may activate different sensory feedbacks and physiological constraints in target body systems during the performance. McLaren et al. (2016) previously reported that the sensitivity of RPE_respiration was different from RPE_muscle during incremental cycling and treadmill running in university soccer players. Furthermore, Wright et al. (2020) recently demonstrated that young female soccer players experienced stronger perceived exertions in breathing and muscular engagement during fitness and resistance training, respectively. However, no difference in respiratory and muscular perception during soccer training sessions and matches has been observed. Wright et al.’s (2020) study implied the different perceptions of sensory sources to specific training types in this population. However, the application of these measures in futsal training lacks sufficient investigation and requires further elucidation.

Futsal training camps include team preparation, player selection, building and developing team tactics and formations, physical and mental preparation, and team squad readiness. In general, domestic training camps (DTC) provide benefits for testing squad members in individual and team performance, periodizing physical and mental preparation, and tactical strategies prior to tournaments (Clemente et al., 2020). Conversely, overseas training camps (OTC) have advantages in creating a simulative environment of official tournaments and thus familiarizing players with the usual intensity of psychophysiological responses during competitions (Lu et al., 2019; Chen et al., 2020). However, a comparative study to examine training loads between DTC and OTC has not been reported elsewhere.

In light of the above, this study compares the different training measures during DTC and OTC in under 20 (U-20) male futsal players, and examines relationships between the RPEs and traing impulse (TRIMP)/TD. It was hypothesized that there would be significant differences in measured variables between DTC and OTC. The secondary hypothesis was that the relationship between perceived exertions and TRIMP/TD would be established during DTC and OTC.

This study is the first to quantify training loads using RPE_respiration and RPE_muscle during DTC and OTC in male futsal players. The primary findings revealed that DTC demonstrated larger TD and TRIMP than that of OTC. Whereas RPE_CR10 was significantly larger during OTC than that of DTC, no significant differences in RPE_respiration and RPE_muscle were observed between DTC and OTC. The second finding in this study was that RPE_respiration and RPE_muscle demonstrated a positive linear association to RPE_CR10. Thus, either RPE_respiration or RPE_muscle can be used to quantify internal load during DTC and OTC in futsal players. Additionally, individual variability of measuring variables varied from camp to camp. Three different RPE scales showed similar dispersion and tendency of individual and group values across the DTC. Finally, TD and TRIMP are independent markers to training loads quantified by RPE scales during short-term futsal training camps.

We found that mean values of TD and TRIMP were significantly larger during DTC than that of OTC. It seems that there is a large demand of physical engagement during futsal DTC. Conversely, the RPE_respiration and RPE_muscle demonstrated no difference between the two different types of training camps. These findings highlighted the discrepancy of subjective and objective assessments of training loads in futsal DTC and OTC. Conversely, the different activity profiles that experienced between DTC (generally focused on fitness development, player selection, and technical evaluation) and OTC (usually incorporates more friendly matches and fewer training sessions to prepare for competition) could be a factor to explain these findings (Chen et al., 2020). Another possible explanation for these findings could be greater intersubject variability of RPE measures. Nevertheless, it is challenging to compare our observations with other populations, levels of players, or sports due to limited studies reporting internal or external loads during OTC.

It is interesting to note that the RPE_CR10 was significantly larger during OTC than that of DTC. Although larger values of RPE_respiration and RPE_muscle were observed during OTC, the results did not approach statistical significance. Indeed, the RPE_CR10 is a self-report tool to reflect the overall engagement of psychophyiolgoical efforts during the training or match. The higher values of RPE_CR10 and sRPE_CR10 reported during OTC might be related to a high frequency of friendly matches. Futsal is a high-intensity intermittent sports (Spyrou et al., 2020) with characteristics of quick decision-making on top of highly demanding technical and tactical performance (Corrêa et al., 2016). It seems that the players experienced higher perceptive loads rather than physiological strains during OTC as evidenced by TRIMP and TD metrics. However, the psychological stress and mental effort were not evaluated in this study. Thus, the overall contribution of psychological aspects on the intensity of RPE level is unknown.

We observed a positive association between RPE_respiration and RPE_muscle scales during both DTC and OTC. These findings are supported with previous studies that observed longitudinal changes in RPE_respiration and RPE_muscle scales in young professional adult soccer players during weekly training sessions and in-season matches (Los Arcos et al., 2014, 2016, 2017). The RPE_muscle and RPE_respiration scales showed similar features when quantifying the training load during weekly training sessions of a competitive period in professional soccer players (Los Arcos et al., 2017). Furthermore, different RPE scales permit researchers to obtain similar patterns of internal load during short-term specific training sessions. McLaren et al. (2020) recently demonstrated the usefulness of RPE_respiration and RPE_muscle scales in detecting improvements in high-intensity running profiles during a 2-week repeated sprint training intervention in semiprofessional soccer players. In this study, the positive association identified in all RPE scales indicates the potential usefulness of pictorial RPE measures in quantifying internal load in futsal training.

The results of linear regressions show no relationships between RPE scales and TD/TRIMP. This finding indicated independent markers of training load monitoring between the subjective and objective tools. Subjective assessment of training load can be used to understand the variation in an individual’s perception of daily changes in psychological and physiological status during sports training. Conversely, objective assessment of training load provides a quantitative measure of physiological responses and exercise performance to understand the accumulation of training and match loads. Collectively, our findings suggested no relationship between RPE scales and TRIMP/TD variables during futsal DTC and OTC. A combination of subjective and objective assessments to evaluate training loads in futsal training is recommended.

It should be noted that a within-subject approach was used during this study period (Costa et al., 2019, 2021; Figueiredo et al., 2021). Compared to TD and TRIMP, our results demonstrated that three different RPE scales showed similar dispersion and tendencies between individual and group values across the DTC. However, such observations may not exist during the OTC (see Figure 2). It is possibly related to the difference in training tasks among the OTC. In our study samples, the OTC consisted of several friendly matches to test the team’s competitive level. Costa et al. (2021) found that a training camp consisting of training sessions and friendly matches demonstrated a large variability of sRPE_CR10 in female football players. Interestingly, there was a long interval between DTC 5 and DTC 6 (83 days) in our study. Despite the similar training contents with DTC 2, 3, and 4, a large dispersion of individual training load was observed in DTC 6. This finding was related to the initial fitness level before the training camp and competition status in the participants’ home teams (2-week break during the Chinese New Year, and no competition schedule). During the periodic training camps, coaches and strength and conditioning practitioners should consider the large individual difference in fitness level when the players return for training camps.

The first limitation of this study is that respiratory and muscular RPE used pictorial rating to quantify the perception of sensory feedback, compared to arbitrary units used in RPE_CR10. This study players had extensive experience using the pictorial RPE_respiration, RPE_muscle, and sRPE_CR10 measurements. Despite similar reports among different RPE scales found in our study, there still exists the potential that other populations, such as players with less training experience or those who are not familiar with this method, may show differing results. Second, the different perceived efforts might be related to the players’ initial fitness capacity. This study did not report the association between fitness capacity and individual perception of training stresses (Azcárate et al., 2020). A potential bias of the individual peak HR that changed across the timeline of training camps could be the third limitation of this study. The peak HR determined during the Yo-Yo intermittent recovery level 1 test in the first OTC may not be equivalent to actual HR_max in each training camp engagement. Finally, the task difference during DTC and OTC may result in large intraindividual variability in this study. The main focus of friendly matches during OTC could lead to a fluctuation of daily training loads and recovery status to more/less playing time per individual. Future studies need to measure these individual changes during the training camps.

The present findings revealed significant positive correlations among RPE_CR10 and pictorial RPE_respiration and RPE_muscle during the futsal DTC and OTC. Our study reported that pictorial RPE measures and RPE_CR10 have similar outcomes as monitoring tools during futsal training. Coaches and sports practitioners are encouraged to use one of the RPE measures concurrently with time spent in HR zones and TD measures during futsal training camps.

In conclusion, TD and TRIMP of short-term DTC were larger than that of short-term OTC. However, the RPE_CR10 is higher during OTC than that of DTC. No differences in perceived measures of muscular and respiratory RPE were identified. Additionally, three different RPE measures showed similar individual dispersion and group tendency across the DTC. The interpretation of perceived efforts via respiratory, muscular, and Borg CR10 quantifications provides a valuable resource in monitoring internal load in futsal DTC and OTC. No relationships among the perceived measures, TRIMP, and TD were observed in this study highlighting that it is essential to implement multiple tools when recording training loads in futsal players. Using a combination of subjective and objective measures to monitor training loads during short-term futsal training camps is warranted.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by Human Ethics Committee of the University of Taipei. Written informed consent to participate in this study was provided by the participants’ legal guardian/next of kin.

Y-XL contributed to the study conceptualization, project administration, investigation, methodology, and writing (including reviewing and editing) of the manuscript. FMC and PB contributed to the study conceptualization and writing (including reviewing and editing) of the manuscript. ZC-M and C-HC contributed to the study data analysis and writing (including reviewing and editing) of the manuscript. S-CC contributed to the statistical analysis and writing (including reviewing and editing) of the manuscript. C-DK and Y-SC contributed to the study conceptualization, methodology, supervision, and writing (including reviewing and editing) of the manuscript. All authors contributed to the article and approved the submitted version.

C-DK is a consultant for Leadtek Research Inc. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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