Much information has been obtained about the physical capacity of elite female soccer players (3,6,16,19,20,21). These studies have shown that the aerobic power, sprinting ability, and intermittent exercise performance vary significantly between levels of competition (20) and playing position (3,6,16,19,21) but also between players at the same playing position and playing standard (6). Measurements of HR performed during elite female games indicate that match play is physically demanding for most outfield players (20), although these studies did not determine the players' HRmax. Recent studies performed on elite male soccer players have shown that the amounts of high-intensity running and backward running decrease markedly during competitive games (12) and that the distance covered by high-intensity running in matches is closely related to the training status (8). However, little information is available about the activity profile of elite female soccer players during match play, and it has not yet been investigated whether the maximum aerobic power and intermittent exercise performance are important determinants of the physical match performance in elite female soccer. In addition, it is still unknown whether the aerobic loading during match play is related to training status.
A number of field and laboratory tests are currently used in elite soccer in order to evaluate training status of the players, to predict match performance, and to determine the effect of training. The field tests have the advantage that all players in a team can be tested frequently, easily, and rapidly at low cost. Recent studies have shown that performance in the Yo-Yo intermittent recovery test correlates well with the physical match performance of male elite soccer players (8) and top-class referees (7). Therefore, it would be of interest to investigate whether the Yo-Yo intermittent recovery test result provides a good indication of the soccer-specific fitness of elite female players.
Thus, the aim of the present study was to investigate whether differences in the physical capacity of elite female soccer players influence the activity profile and physical loading during competitive games. Another purpose was to evaluate the application of the Yo-Yo intermittent recovery (IR) test in elite female soccer.
MATERIALS AND METHODS
Fourteen elite female soccer players from the best Danish league took part in the study. Their age, height, weight, and fat percentage were 24 yr (range: 19–31), 58.5 kg (49.0–70.7), 1.67 m (1.56–1.80), and 14.6% (9.3–21.9), respectively. The playing positions were defenders (N = 5; 24 ± 4 (±SD) yr, 60.7 ± 6.3 kg, 1.68 ± 0.07 cm, and 15.4 ± 3.7%), midfielders (N = 5; 23 ± 5 yr, 56.0 ± 5.9 kg, 1.65 ± 0.04 m, and 12.5 ± 2.2%), and attackers (N = 4; 25 ± 4 yr, 58.7 ± 3.8 kg, 1.66 ± 0.04 m, and 16.1 ± 2.4%). Two of the defenders were central defenders, and the remaining three defenders were fullbacks. The players had been members of a soccer club for 14 (9–23) yr. All players had more than 3 yr of experience in the best Danish league and were regular first team members. The players were fully informed of all experimental procedures before giving their written informed consent to participate. The study was approved by the ethics committee of Copenhagen and Frederiksberg communities.
The 14 players were video recorded once during a competitive game in the best Danish league. The four matches included in the study were played within a 3-wk period in the middle of the competitive season. None of the players were injured during the observed game. Each player was filmed close up during the entire match by a VHS movie camera (NV-M50, Panasonic, Germany) positioned at the side of the pitch, at the level of the midfield line, at a height of about 15 m and at a distance of 30–40 m from the touchline. The videotapes were later replayed on a monitor for computerized coding of the activity pattern. Each locomotor activity was categorized as standing (0 km·h−1), walking (6 km·h−1), jogging (8 km·h−1), low-speed running (12 km·h−1), moderate-speed running (15 km·h−1), high-speed running (18 km·h−1), sprinting (25 km·h−1, or backward running (10 km·h−1). The above activities were later divided into four locomotor categories: 1) standing; 2) walking; 3) low-intensity running, encompassing jogging, low-speed running, and backward running; and 4) high-intensity running, consisting of moderate-speed running, high-speed running, and sprinting. The locomotor categories were chosen in accordance with Bangsbo et al. (2), except for the sprinting speed, which was determined after detailed studies of the videotapes. Thus, the average sprinting speed was calculated from measurements of the time for the players to cover known distances (i.e., between premarkers in the grass and the center circle) by sprinting. The frequency and average duration of each locomotor activity were recorded in 5-min periods throughout the game. The distance covered for each locomotor activity was determined in 5-min intervals as the product of the total time and mean speed for that activity. The total distance covered during a match was calculated as the sum of the distances covered during each type of activity. In the study by Krustrup and Bangsbo (7), it was observed that the coefficients of variation for test-retest analysis were 1, 2, 5, 3, and 3%, respectively, for total distance covered, walking, low-intensity running, high-intensity running, and backward running. Before each player analysis, the player's locomotive style was studied and several validation-tests were performed according to the predetermined locomotive categories. All match recordings included in the present study were analyzed by the same experienced observer.
Physiological Measurements during Match Play
HR was recorded in 5-s intervals during the same competitive games using a Polar Vantage NV HR monitor (Polar Electro Oy, Kempele, Finland). The chest monitor and wrist receiver, weighing ∼100 g, was placed on the player approximately 45 min before kickoff.
Within 3 wk of the competitive game, a player carried out a laboratory treadmill test and the Yo-Yo IR test.
Laboratory treadmill testing.
The laboratory treadmill running test was performed to determine an individual HR-V̇O2 relationship, HRmax, maximal pulmonary oxygen uptake (V̇O2max), and performance in an exhaustive incremental test. The protocol consisted of treadmill running at speeds of 9, 11, and 13 km·h−1 in 6-min bouts separated by 2-min rest periods, followed by the exhaustive incremental test. The latter test started at a running speed of 13 km·h−1 for 2 min and continued at 15 km·h−1 for 30 s with a stepwise 1 km·h−1 speed increment every 30 s until exhaustion. Time to exhaustion was recorded as the test result. Oxygen uptake was measured during the last 2 min of each submaximal running speed and during the incremental test by a MedGraphics CPX/D online system (St. Paul, MN). HR was recorded in 5-s intervals during the entire protocol by a Polar Vantage NV HR monitor (Polar, Kempele, Finland). A blood sample was taken immediately after each exercise bout and hemolyzed within 10 s in an ice-cold Triton X-100 buffer solution and analyzed for lactate using a YSI 2300 lactate analyzer (Yellow Spring Instruments, OH.) (5). Individual V̇O2max and HRmax were determined as the peak values reached in a 15- and 5-s period, respectively, during the last part of the incremental test.
The Yo-Yo IR level 1 test.
The test was performed outdoors on artificial grass as described by Krustrup et al. (8). Briefly, the test consisted of repeated 2 × 20-m runs at a progressively increased speed controlled by audio bleeps from a tape recorder. Between each running bout, the players had a 10-s rest period, during which 2 × 5 m of jogging was performed. When a player had failed twice to reach the finish line in time, the distance covered was recorded as the test result. The test was performed after a 10-min warm-up period. The duration of the test was 8–20 min. All the players had been familiarized with the test procedure previously.
Mean data and ranges are presented. Differences between 15-min periods within the match were evaluated using repeated-measures ANOVA. When a significant interaction was detected, data were subsequently analyzed using a Tukey post hoc test. Differences between the first and second half were determined using the Student paired t-test. Differences between the different team positions were tested with ANOVA. Correlation coefficients were determined and tested for significance using a Pearson's product-moment test. A significance level of 0.05 was chosen.
The total number of activity changes was 1459 (range: 1336–1529), corresponding with an activity change every 4 s on average (Fig. 1). The number of high-intensity runs performed during the game was 125 (72–159) with an average duration of 2.3 s (2.0–2.4) (Fig. 1). The number of sprints was 26 (9–43) (Fig. 1). Standing, walking, and running at low intensity accounted for 16% (10–23), 44% (38–53), and 34% (26–43) of the total match time, respectively. In the remaining 4.8% (2.8–6.1) of the total time, the players performed high-intensity running. The total distance covered during a match was 10.3 km (9.7–11.3). The amounts of low- and high-intensity running were 9.0 km (8.4–9.8) and 1.31 km (0.71–1.70), respectively. Sprinting accounted for 0.16 km (0.05–0.28). The distance covered at a high intensity decreased (P < 0.05) by 30% (0.27 to 0.19 km) and 34% (0.24 to 0.16 km) from the first to the last 15-min period of the first and second halves, respectively. The total number of headers and tackles was 8 (3–19) and 14 (7–21), respectively.
HR during match play.
Mean HR during a match was 167 beats per minute (bpm) (range: 152–186), which corresponded to 87% (81–93) of HRmax being 193 bpm (175–212) (Fig. 2). Peak HR reached during the game was 186 bpm (171–205), corresponding with 97% (96–100) of HRmax. Neither the mean nor the peak HR was different between the 15-min periods during the game (Fig. 2). Based on individual HR relationships from the treadmill test, average V̇O2 during match play was estimated to be 2.2 L·min−1 (1.8–2.9) or 77% (69–84) of V̇O2max. Peak V̇O2 during match play was estimated to be 96% (92–99).
V̇O2max was 2.89 L·min−1 (range: 2.58–3.29) or 49.4 mL O2·min−1·kg−1 body weight (43.4–56.8). Performance in the incremental treadmill test (ITT) and Yo-Yo IR test was 4.49 min (3.38–5.17) and 1379 m (600–1960), respectively. The V̇O2 during treadmill running at speeds of 9, 11, and 13 km·h−1 was 1.72 (1.42–1.97), 2.13 (1.89–2.44), and 2.43 (2.12–2.84) L·min−1, respectively, corresponding to 59% (52–72), 74% (67–83), 84% (78–92) of V̇O2max. The corresponding HR was 133, 152, and 168 bpm, respectively, or 69% (60–82), 79% (71–90), and 87% (81–97) of HRmax. Blood lactate at rest and at running speeds of 9, 11, and 13 km·h−1 was 0.8 (0.5–1.4), 1.0 (0.7–1.5), 1.5 (1.0–2.6), and 2.9 (1.4–5.1) mmol·L−1, respectively. Peak blood lactate reached after ITT was 8.0 mmol·L−1 (5.8–10.3).
Training status in relation to match activities and HR.
The correlation coefficients for the interindividual relationships between the various test results and physical match performance are summarized in Table 1. Briefly, total match distance did not correlate with V̇O2max or ITT performance but correlated (P < 0.05) with the running speed at 2 mM and the Yo-Yo IR test result (Fig. 3, Table 1). Positive correlations (P < 0.05) were observed between the amount of high-intensity running and all four test results with r values from 0.76 to 0.83 (Fig. 4, Table 1). The sum of high-intensity running performed in the last 15-min periods of the two halves correlated (P < 0.05) with the Yo-Yo IR test result (r = 0.83) as well as ITT performance, running speed at 2 mM lactate and V̇O2max with r values from 0.55 to 0.74 (Fig. 5, Table 1). No relationship was observed between %HRmax during match play and any of the test results (r = −0.01 to 0.38) (Fig. 2, Table 1).
The most important finding of the present study is that the physical match performance of elite female soccer players varies in close association with differences in the physical capacity of the players. Moreover, the present study revealed that the Yo-Yo IR test is a good predictor of elite female soccer players' ability to perform high-intensity running throughout competitive matches.
The 14 elite female soccer players investigated in the present study covered a total distance of ∼10.3 km and had on average ∼1400 activity changes during a game. These values are similar to or slightly lower than values reported for elite male soccer (1,2,12,13,14). On the other hand, the average distance covered by high-intensity running of 1.3 km corresponds with less than two thirds of that in elite male soccer (1.9–2.4 km) (2,12). Interestingly, small between-player variations were observed in the total number of activity changes (1336–1529, SD = 4.9% of mean) and the distance covered at low intensity (8.4–9.8 km, 4.8%), whereas large variations were found in the distance covered by high-intensity running (0.71–1.70 km, 20.8%). The latter finding was mainly due to large variations in the number of high-intensity running bouts, as the mean duration of the intense running bouts were similar among the players (Fig. 1). It was furthermore observed that the amount of high-intensity running decreased markedly within each half and that 13 of 14 players did the least high-intensity running in the last 15-min period of the first or second half. The finding of a large decline in high-intensity running at the end of the game is in accordance with a recent study on elite male soccer by Mohr et al. (12) and can be taken as a sign of fatigue. Together, the time-motion analysis revealed that elite female soccer consists of multiple brief intense exercises separated by low-intensity activities and that one of the main factors differentiating between good and poor physical performance is the amount of high-intensity running.
HR measurements were used to provide information about the aerobic energy turnover during the match play. The mean HR ranged from 152 to 186 bpm with an average value of 167 bpm, which is similar to or slightly higher than what has been observed in other studies investigating elite female (15) and elite male soccer (1,2,4,15,17). The peak HR reached during games was observed to vary as much as from 171 to 205 bpm. In the present study, each player's HRmax was determined and the average and peak HR observed during games could therefore be expressed in percentage of HRmax. The average and peak HR corresponded with 87% and 97% of HRmax, respectively, with rather small variations in average (81–93%) and peak values (96–100%) (Fig. 2). These results indicate that essentially all outfield players have a high aerobic loading throughout a game with periods of near-maximal values. It was, moreover, observed that the HR in the percentage of HRmax did not correlate with any of the fitness assessments (Fig. 2), suggesting that the aerobic loading was not dependent on the player's physical capacity. This finding shows that the aerobic energy system is highly tasked also for players with superior physical capacity, indicating that the activity profile and energy use during match play is adjusted based on the player's physical ability.
In accordance with a number of previous studies (3,6,16,19,20,21), the elite female players in the present study had marked between-player variations in physical capacity. For example, the V̇O2max of the players varied from 43 to 57 mL·min−1·kg−1, and the Yo-Yo test performance ranged from 600 to 1960 m. When plotting the fitness assessments obtained in the present study to the amount of backward running, low-intensity running, and total distance covered, no correlations were observed for V̇O2max and treadmill test performance, whereas the Yo-Yo test result and the running speed at 2 mM lactate were weakly correlated with the amount of low-intensity running and total distance covered (Fig. 3). In contrast, it was observed that V̇O2max, treadmill test performance, running speed at 2 mM lactate, and the Yo-Yo test result all were correlated with the amount of high-intensity running (Fig. 4). From the fairly high correlation coefficients, it appears that all the applied fitness assessments provided a good indication of the distance covered at a high intensity during a game, which is one of the main factors differentiating between good and poor physical match performance. It should be emphasized, however, that players covering a similar match distance of 1.25 km of high-intensity running ranged in V̇O2max from 46 to 53 mL·min−1·kg−1, in treadmill test performance from 4.0 to 4.8 min, in running speed at 2 mM lactate from 11 to 13 km·h−1, and in Yo-Yo test performance from 1100 to 1500 m (Fig. 4). On that basis, it may be considered whether all the observed players used their entire physical capacity during the game. For example, it is well-known that defenders perform less high-intensity running during a game than midfielders and attackers (1,2,8,10,12,13), and it has been discussed whether this is due to tactical or physical limitations of the defenders (1,2,8). We observed a huge range in the amount of high-intensity running within the group of defenders (0.7–1.7 km), but the average value for high-intensity running was only slightly lower for the defenders compared to the midfielders (17%) and attackers (13%) (Fig. 4). The HR measurements revealed that all players had an average HR above 80% of peak HR and that the aerobic loading was equally high for the defenders, midfielders, and attackers (86, 88, and 88% of peak HR, respectively; Fig. 2B). Moreover, all players except one performed less high-intensity running toward the end of the first and/or second halves. Together, these observations suggest that the observed games were physically demanding for all the players and that each of the players performed an amount of high-intensity running close to her optimal level. In support of this notion, Mohr et al. (12) observed small match-to-match variations in the amount of high-intensity running during elite male soccer games (coefficient of variance = 9%).
The present study also tested whether the Yo-Yo IR test can be used to evaluate match-specific physical capacity of elite female soccer players. In accordance with what has been observed for elite male soccer players (8) and top-class referees (7), the present study showed that the Yo-Yo test performance correlated with the amount of high-intensity running performed in games. The present study also revealed a positive correlation between maximal oxygen uptake and high-intensity running during a game, whereas no such relationship was observed for elite male soccer (8). These findings indicate that the aerobic power is more important for the physical match performance in elite female than elite male soccer and may be explained by a lower anaerobic capacity in elite female players than in their male counterparts (9,18). In accordance, the peak blood lactate of 8 mM reached after exhaustive incremental running performed in the present study is seemingly lower than the values reported for elite male soccer players (>10 mM) (11). An indicator of the ability to sustain fatigue during match play, that is, the sum of the high-intensity running performed in the last 15-min periods of the two halves, was also plotted against the four fitness assessments. As can be seen in Figure 5, the Yo-Yo test was observed to correlate closely with the physical performance toward the end of the two halves. Significant correlations, with lower correlation coefficients, were also observed between the sum of high-intensity running in the last 15-min periods and the incremental treadmill test performance, running speed at 2 mM lactate, and maximal oxygen uptake (Fig. 5). Together these findings provide evidence that the performance on the Yo-Yo IR test is a good predictor of physical match performance throughout elite female soccer games. Measurements of maximal aerobic power is an equally good indicator of the total amount of high-intensity running during a game but seems to be a less sensitive tool to evaluate the ability to sustain fatigue during match play.
In summary, the present study demonstrated that the physical capacity is an important determinant of the number of intense actions performed by elite female soccer players in a game and that the aerobic loading during match play is high, independent of training status. Moreover, it was shown that the Yo-Yo IR test can be used as an indicator of the physical performance of elite female players throughout competitive matches.
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