Improvements in athletic performance can be achieved by appropriate periodization of training and recovery (22). The monitoring of training load is important in providing objective information on how athletes respond to training programs. In addition, if training load is not monitored and recovery is insufficient, then athletes may be at risk of overreaching or overtraining along with a concomitant decrease in performance (24). In soccer, it is of practical interest for coaches to measure and control the intensity of training programs to quantify improvements in fitness and especially in aerobic capacity. Strong aerobic capacity is important for soccer performance because ∼90% of the energy contribution during a soccer match play is provided by the aerobic metabolism (2). Analyses of match play have shown that the average work intensity in games is around 80-90% of maximal heart rate (HRmax) (2,25) and 47-75% of maximal oxygen uptake (V̇O2max) (38). Improvements in aerobic capacity in soccer players can enhance match performance in areas such as the total distance covered, number of sprint efforts and ball actions, and maintain a similar technical performance despite exercising at a significantly higher intensity (25) and reducing the fatigue-induced decline in short-passing ability (30).
An accurate and direct measurement of exercise intensity in aerobic training sessions is to analyze V̇O2 using portable breath-by-breath metabolic gas systems (6,8). However, metabolic gas systems are often expensive and not available to every soccer club. In addition, it is practically impossible to measure the oxygen uptake of all players from the squad simultaneously by using the metabolic gas system. Moreover, the use of such devices will modify the natural style and activity of players when training by limiting player to player contact especially in certain game situations such as tackling and heading actions. Therefore, direct measurements of the internal intensity in soccer match play or training are difficult to obtain, despite the fact that external intensity indicated by measures of distance coverage and running speed has been heavily investigated using computerized video motion analyses (13,14).
Indirect measures such as HR (17,31) and rate of perceived exertion (RPE) (10,29) are alternative means for estimating internal intensity and exercise load in soccer players. Previous studies have reported strong correlations between measures of HR and V̇O2 ranging from 0.84 to 0.99 among adult soccer players (17,26). Additionally, it has been reported that the HR-V̇O2 relationship in adult soccer players when tested on the treadmill is not different from that obtained in on-field testing (17). This experimental evidence suggests that it is logical and more practical to use HR measures to estimate players' V̇O2 during on-field situations (17). However, previous studies in this area were performed using adult soccer players (17,26,33) and to the best of our knowledge, the HR-V̇O2 relationship in young soccer players has not been investigated. The HR-V̇O2 relationship in young soccer players should therefore be determined to interpret metabolic information by using the HR measures obtained in the field.
Although a strong association exists between V̇O2 and HR, there are psychological factors that may not be taken into account by simple HR measures (43) in addition to some characteristics of physical activity profiles in soccer that are not well reflected by HR, for example, sprinting and explosive actions (35). Subjective RPE is used as a measurement of integrated afferent information during exercise (43) and is well associated with physiological measurements such as HR, ventilation, respiratory rate, and V̇O2 (40). Recently, RPE has also been used as a global indicator to monitor intensity in exercise training session among athletes (20,21) and adolescent soccer players (10,29). However, no information on the association between RPE and V̇O2 in younger soccer players is available in the literature.
We hypothesized that more accurate estimation of V̇O2 in young soccer players can be achieved by employing HR and RPE simultaneously as compared to each of these measures alone. Moreover, it is practical for coaching staff to measure objective HR and subjective RPE during field training to estimate exercise intensity (V̇O2). However, no previous study has provided an equation to accurately estimate V̇O2 from HR and RPE among young soccer players. Therefore, the main purpose of this study was to assess the accuracy of estimation of V̇O2 in elite youth soccer players by using measures of HR and RPE.
Experimental Approach to the Problem
There is no information on the association between RPE and V̇O2 in younger soccer players available in the literature, and the HR-V̇O2 relationship in young soccer players should be determined to interpret metabolic information by using the HR measures obtained in the field. In this study, all players participated in the first V̇O2max test in a sport science laboratory in which V̇O2, HR, and RPE were simultaneously recorded. The present exercise protocol has previously been used to determine V̇O2max in young soccer players (8,9). To estimate V̇O2 from HR and RPE, regression equations were determined from the above parameters. Two weeks after the first test, all players performed the same V̇O2max test to validate the regression equations. The differences between the estimated values in the first test and the actual values in the second test were compared.
A total of 46 young male soccer players from a Chinese regional Under-14 team competing at the highest level of competition for their age category participated in this study. All tests were performed during the competitive season. All players participated in training twice a week with each session lasting for ∼2 hours, in addition to the regular weekly competitive game. Their respective age, soccer experience, body mass, height, and body mass index were as follows: 13.5 ± 0.7 years, 4.2 ± 1.5 years, 50.9 ± 8.8 kg, 1.65 ± 0.1 m, and 18.5 ± 2.0 kg·m−2. Only outfield players participated in this study. The study was conducted according to the Declaration of Helsinki and the protocol was fully approved by the Clinical Research Ethics Committee before the commencement of the assessments. Before participation, written informed consent was received from all players and parents after detailed explanation about the aims, benefits, and risks involved in this investigation. All players were instructed not to perform vigorous exercise 48 hours before the day of testing. All players were familiar with the V̇O2max test protocol because they performed the same test every 2 months throughout the season.
On 2 occasions separated by 2 weeks, players ran on a 5.5% slope motorized treadmill (cos10198, h/p/cosmos, Nussdorf-Traunstein, Germany) for 4 minutes at 7 km·h−1, followed by a 1 km·h−1 increment every minute until exhaustion. In the first V̇O2max test, breath-by-breath V̇O2 and the corresponding HR were measured. The V̇O2 values obtained at the completion of each treadmill velocity increment were later divided by the V̇O2max measurement to obtain a %V̇O2max for each submaximal stage (velocity increment). The regression formula to estimate %V̇O2max and V̇O2 was based on these submaximal or incremental V̇O2 values. The second V̇O2max performed 2 weeks later used the same procedure and measures. Players performed both V̇O2max tests at the same time on each testing day to minimize circadian variation in the HR (15). The regression equation derived from the first test was used to estimate the %V̇O2max and V̇O2 of the second test and was compared to the true measured value of the second V̇O2max test.
Cardiorespiratory variables were determined using a calibrated breath-by-breath system (MetaMax 3B, Cortex, Leipzig, Germany). The following criteria were met by all players when V̇O2max was tested: (a) a plateau in V̇O2 despite an increase in treadmill speed; (b) a respiratory gas exchange ratio >1.1; and (c) blood lactate > 6 mmol·L−1. V̇O2max was determined as the average of last 30 seconds of the test, and HRmax was the highest value attained at exhaustion (7). Previous studies have shown that the coefficient of variance of this incremental treadmill V̇O2max test was <5% (39). V̇O2max values were represented relative to body mass (mL·min−1·kg−1) and scaled to body mass (mL·min−1·kg−0.75) (3).
The RPE with 10-point scale (29) was obtained at the last 5 seconds of each running speed. All players were familiar with the use of RPE. The RPE was previously implemented during soccer training for 8 weeks, which was equivalent to 16 training sessions. The players were asked to provide their respective RPE during and after each of these soccer training sessions.
The HR was determined from a portable monitor and recorded every 5 seconds (Polar, Kempele, Finland). Blood lactate was collected 3.5 minutes after the V̇O2max test, with 25 μL samples of capillary blood withdrawn from fingertip. Blood lactate concentration was subsequently measured using an enzymatic method (YSI 1500, Yellow Springs Instruments, Ohio, USA).
Data are expressed as mean ± SEM. The normality distribution of the data was checked with the Kolmogorov-Smirnov test. Pearson product-moment correlation coefficient was used to examine the relationship between parameters in the first V̇O2max test, and the test-retest reliability of the V̇O2max tests. The magnitude of the correlations was determined using the modified scale by Hopkins (28): trivial: r < 0.1; low: 0.1-0.3; moderate: 0.3-0.5; high: 0.5-0.7; very high: 0.7-0.9; nearly perfect >0.9; and perfect: 1. Stepwise multiple regression was used to estimate V̇O2 values by HR and RPE. A paired-sample t-test was used to compare the estimated values in the first test and the actual values in the second test. Significant level was defined as p ≤ 0.05.
In the first V̇O2max test, values for HRmax, relative and scaled V̇O2max of the players were 199 ± 1 b·min−1, 54.9 ± 0.9 mL·min−1·kg−1, and 146.7 ± 2.5 mL·min−1·kg−0.75, respectively. The %HRmax was highly correlated with V̇O2 (r = 0.67, p < 0.001) and very highly correlated with %V̇O2max (r = 0.90, p < 0.001) (Table 1). Similarly, RPE was highly correlated with V̇O2 (r = 0.53-0.55, p < 0.001) and very highly correlated with %V̇O2max (r = 0.77, p < 0.001) (Table 1). Compared to RPE, the %HRmax had a higher shared variance with V̇O2 (45 vs. 28-30%) and %V̇O2max (81 vs. 59%). Furthermore, the combined use of %HRmax and RPE to estimate %V̇O2max (R2 = 83%, p < 0.001, Table 2) was similar to the use of %HRmax (R2 = 81%) alone. Higher explained variances were observed when %HRmax and RPE were used to estimate %V̇O2max (R2 = 83%, equations 1 and 2, Table 2), but lower values were observed when estimating V̇O2 (R2 = 45% and 46%, equations 3 and 4, Table 2). To validate the 4 equations presented in Table 2, the values estimated by the equations were compared with the actual values obtained in the second V̇O2max test. Small and nonsignificant (p > 0.05) mean differences were observed between the estimated and actual values (Table 3). High test-retest reliability was observed for the V̇O2max tests (r = 0.90-0.91, Table 3).
The first purpose of this study was to examine the relationship between V̇O2 and HR and RPE, respectively, in a group of regional-level youth soccer players. This study showed that during continuous endurance exercise, the %HRmax was highly correlated with V̇O2 and very highly correlated with %V̇O2max. The applicability of the HR-V̇O2 equation obtained in laboratory setting to on-field soccer exercise has been examined by Esposito et al. (17). The authors reported that data on the HR-V̇O2 relationship obtained in the laboratory setting were not statistically different from those observed when performing a field-based soccer training circuit involving jumping, change of direction, walking, jogging, running with and without the ball, forward, backward, and lateral movements (17). Esposito et al. (17) reported that the correlation coefficients of the HR-V̇O2 relationships obtained in the laboratory and on the field were 0.98 and 0.99, respectively. In addition, Hoff et al. (27) found that the correlation coefficient of the HR-V̇O2 obtained during treadmill running was 0.84. Findings from these studies (17,27) suggest that HR is a valid indicator of aerobic demand during soccer activities. In contrast, Ogushi et al. (38) found that HR was 25% higher during match play compared to figures obtained in a laboratory protocol at the same level of V̇O2. This disparity may be caused in part by the additional requirements involved in emotional stress and complex mental processes during match play. With the above-mentioned reasons, it is suggested that HR is a valid indicator of aerobic demand in continuous endurance soccer training session (17,27). However, the limitation of using HR as a measure of internal training load is that it is a relatively poor method in match play (38) and high-intensity and short-duration exercises such as sprinting and plyometric training (12,29).
This study showed that RPE was highly correlated with V̇O2 and very highly correlated with %V̇O2max. The validity of self-regulation of exercise intensity guided by the RPE has been confirmed in treadmill running (16,18,23), cycling (5,19), arm and leg ergometry (32), and rowing ergometry (36). The RPE-based training load is recently reported as being an accurate indicator of exercise intensity during soccer training (29,35). However, in the present study, RPE was shown to be a poor predictor of V̇O2 because only 28% of V̇O2 and 59% of %V̇O2max can be explained by RPE (Table 1). This result suggests that in youth soccer players, RPE measures alone cannot be accurately employed to estimate V̇O2. Therefore, RPE should not be used to estimate V̇O2 nor %V̇O2max during continuous endurance training session in youth soccer players. In contrast, RPE can be used as a measure of internal training load during noncontinuous training sessions such as small-sided games and interval training drills (12). However, psychosocial factors can influence up to 30% of the variability in an RPE score (44), and a future study comparing match play and laboratory measures of HR and RPE measures in young soccer players is warranted.
The second purpose of this study was to estimate V̇O2 and %V̇O2max by using HR and RPE together. We found that the combined use of both %HRmax and RPE to estimate %V̇O2max (R2 = 83%) was similar to %HRmax alone (R2 = 81%) but better than using RPE alone (R2 = 59%). Furthermore, higher explained variances were observed when %HRmax and RPE were used to estimate %V̇O2max (R2 = 83%) but lower values were observed when estimating V̇O2 (R2 = 45 and 46%). This study showed that %HRmax had higher shared variance with V̇O2 (45 vs. 28-30%) and %V̇O2max (81 vs. 59%) as compared to RPE. It appears that %HRmax (instead of RPE) is a better indicator of internal training load and can accurately estimate %V̇O2max during continuous endurance training. This is in agreement with the findings of Desgorces et al. (12), which showed that HR is a more sensitive indicator of internal training load compared to blood lactate and RPE during on-field endurance training session.
In this study, a very high correlation (r = 0.76) was observed between RPE and %HRmax. This association was stronger than that previously reported in soccer-specific small-sided games (r = 0.60) in amateur adult soccer players (10). The lower explained variance observed in the previous study might be partly attributed to differences in age and fitness level (4). This discrepancy may also be because of the nature of the exercise undertaken. For example, small-sided game training sessions with an intermittent running nature at different speeds (1) and which greatly vary in intensity (31). As the anaerobic contribution in small-sided games is considerably larger than that in continuous running endurance training, this may affect the RPE-HR relationship (12). In this regard, Alexiou and Coutts (1) found that higher correlations between RPE and HR were found with less intermittent, aerobic-based training session in women soccer players. In this study, a continuous incremental protocol was employed with much less anaerobic contribution than that observed during small-sided games, which may have contributed to the difference in findings. In future studies, research could be conducted to test the validity of alternative effort perception scales such as the OMNI scale (41,42) in regressing against data on HR or V̇O2 in young soccer players.
A limitation of this study identified beforehand was the mode of exercise analyzed as only straight line running was done on a motorized treadmill. The pattern and mechanics of treadmill running are different from those observed on the pitch and may have modulated RPE. Running actions in soccer match play are more closely represented by the movements observed in shuttle running protocols that require 180° turns and abrupt accelerations and decelerations inducing significantly higher HR responses and RPE values (11). Therefore, future studies are recommended to develop and test regression equations using shuttle-type exercise protocols in a field setting combined with physiological measures from portable gas analyzers to collect data in situations that are better representative of the demands of the game.
The ability to accurately control and monitor training load to analyze variations in fitness in soccer players is an important aspect of effective coaching. There are also concerns about the negative impact of intensive participation in training and match play on player's health especially at youth levels. In this study, we found that the use of %HRmax alone to estimate percentage of maximal V̇O2 (%V̇O2max) during continuous endurance training was as accurate as using a combination of HR and RPE measures in young soccer players. By measuring the %HRmax, coaches will be able to detect youth players who have differing physiological responses (%V̇O2max) to the same continuous endurance-training session. These players can subsequently receive individualized training to enhance their aerobic fitness to meet the overall team standard (37). In addition, the high risk of injury in youth soccer players over time suggests the need for frequent evaluation and control of the physical stress placed on players in training (34). Furthermore, it seems that RPE is more useful as a measure of internal load during noncontinuous (e.g., intermittent and sprint) exercises but not in estimating %V̇O2max during continuous endurance exercise.
The authors have no conflicts of interest that are directly relevant to the content of this article.
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