Soccer is an intense multidirectional and intermittent field sport that demands technical ability, tactical awareness, and an exceptionally high level of physical conditioning to compete at the international level. The ability to objectively assess the physical performance has become a vital component for player development systems, player monitoring, and youth player identification at the national and international levels (8,33,39,45). As evidenced in the literature, elite players or selected players routinely achieve higher performance outcomes on sprint, strength, and jump tests as compared with subelite (10,20,38,42) or nonselected players (20,24), further demonstrating the importance of physical characteristics for soccer. Although a single performance measure cannot be isolated to determine the quality of a player (33,39,45) or their potential impact, an understanding of the strength and speed characteristics needed to be successful in the game can provide an insight into individual players’ potential for long-term success (29).
The popularity of women's soccer has witnessed rapid growth in the past 10 years as evidenced by 29 million female players worldwide and a 32% increase in participation (15). Previous research has identified that on an average pitch (105 m × 68 m), senior elite female players will cover upward of 10.4 km of field space, and perform over 1,300 different activities, with a change in the type or level of activity every 4 seconds (18,19,28,35). Although low-intensity activities including standing, walking, and jogging have been identified as the predominant movement patterns (upward of 90% of the 90-minute game), the distance covered, and the rate of fatigue during the high-intensity and sprinting activity are the main determinants between higher and lower standards of play (1,25,35,36). Elite female players complete 1.68 ± 0.09 km at high intensity and 0.46 ± 0.02-km sprinting throughout a game, which is 28 and 24% greater than for the nonelite level, respectively (29). Furthermore, it is the explosive actions such as sprints, jumps, tackling, duels, kicking, and changing pace, which will determine the outcome of a match and add to the overall demands on the athlete (38). These explosive actions are dependent on strength and speed characteristics of the individual player and should be developed from a young age.
There is a disproportional gap in the literature with the number of studies that characterize the physical performance characteristics of male players, which far outnumbers the studies on female players (24,37,43,44). Previous investigations designed to assess the physiological or speed characteristics of soccer players have typically included athletes that span a small age range, with comparisons with subelite athletes. Despite the paucity of research, it has been demonstrated that high-intensity tasks lasting several seconds tend to plateau during early to midteenage years for female athletes, where conversely gains in speed, agility, and aerobic capacity show a continual improvement into adulthood (30–32,41,44). In contrast, Mujika et al. (37) reported that senior level female soccer players (first division, 20–26 years) performed better in countermovement jump and agility tests than junior players did (second division, 16–19 years) suggesting a continual improvement in shorter duration anaerobic performance beyond teenage years for highly trained female soccer players. Current research highlights the measures of high-intensity ballistic assessment, but it fails to report on the measures of strength gains throughout this critical period of development.
To our knowledge, no study has determined the physiological characteristics of female soccer players at the international level across a wide age range with specific attention paid to the identification of player suitability for the international game. Data extending across the Fédération Internationale de Football Association (FIFA) age brackets (U17 and U20) could have a profound impact for national team coaches and sports scientists who use performance measures to evaluate players within their national organizations through providing expected values for comparative chronological ages. Thus, the primary purpose of this study was to determine the physiological characteristics of FIFA eligible elite female soccer players ranging from 14 to 36 years of age on sprinting kinetics and kinematics, isokinetic strength, unilateral jumping, and maximal aerobic velocity, in an effort to distinguish age-related and playing-time related differences. A secondary aim was to establish normative data for this cohort of athletes.
Experimental Approach to the Problem
To study the discriminative physiological characteristics of elite female soccer players across all FIFA age brackets (U17, U20, and Senior), we performed a battery of laboratory and field-based assessments during the final training period leading into an FIFA sanctioned international competition. In addition, we compared the findings of those players selected (starters) with those not selected (nonstarters) at the featured international competition of each age bracket to determine both age related and playing time–related differences.
The subjects included 51 elite female soccer players (19 ± 4.1 years), all who represented New Zealand within the 2012 calendar year of competition, including but not limited to the U17 FIFA Women's World Cup in Azerjaban, the U20 FIFA Women's World Cup in Japan and the FIFA Women's Football tournament at the London Olympics. At the time of testing, FIFA official rankings had the New Zealand Senior team ranked 23rd out of 125 (14) senior-level international teams, where the U20 team was ranked 14th out of 30 (14) among FIFA U20 age group teams, and the U17 team was ranked 20th out of 27 (16) FIFA U17 age group teams. Testing was carried out during the training build up to the featured event for each training group, and all the players were tested within 4 weeks of each other. On average, the U17 group (N = 18, 14–17 years) held 4 training sessions per week, where the U20 group (N = 18, 17–19 years) and Senior group (N = 15, 19–36 years) held 6 training sessions per week. All the players also played a minimum of 1 game per week throughout the period of testing. The players had a minimum of 1, 2, and 3 years of experience training within the national program at the U17, U20, and Senior levels, respectively. All the players were free from any injury that would prevent maximal effort during performance testing. All the procedures described in this study were approved by the Auckland University of Technology Ethics Committee. All the subjects, and guardians where appropriate, were fully informed of all experimental procedures before giving their informed consent to participate.
All the subjects were required to avoid strenuous exercise 24 hours before testing, and therefore, testing was scheduled immediately after their prescribed rest day. All the subjects performed a standardized warm-up consisting of 10–15 minutes that included general exercises such as jogging, shuffling, sprinting, multidirectional movements, and dynamic stretching. Performance was assessed in a single session with the tests completed in the following order: anthropometrical assessment, sprint kinetics and kinematics unilateral jumping, and isokinetic strength. Maximal aerobic velocity was further assessed during a group training session within the 4-week testing period.
Isokinetic Strength Assessment
Maximal torque generated by the knee extensors, knee flexors, hip flexors, and hip extensors was obtained on an isokinetic dynamometer (Cybex Norm; Phoenix Healthcare, Nottingham, United Kingdom). The motor axis was visually aligned with the axis of the knee joint for knee flexion and extension, and at the axis of the hip joint for hip flexion and extension. The subject was either seated (truck reclined to 15° from the vertical plane) or lying horizontal and stabilized so that only the knee or leg was moving with a single degree of freedom. The subject's dominant leg was assessed, as determined as the preferred kicking leg. All the subjects performed tests in the following order: concentric knee flexion and extension, eccentric knee flexion and extension, and concentric hip flexion and extension at the set experimental maximal velocity of 60°·s−1, chosen to allow safe and reliable measurement of concentric and eccentric strength. After a standardized warm-up consisting of 2 rounds of 3–5 submaximal (∼50 and ∼80%) contractions, 5 maximal repetitions were performed for the concentric measures, whereas 3 maximal repetitions were performed for the eccentric measures. All the subjects were encouraged to put forth maximal effort during all the tests performed. For all trials, peak torque (PT, newton meter) was recorded and averaged for the 5 repetitions during concentric knee and hip flexion and extension, and the 3 repetitions during eccentric knee flexion and extension.
Sprinting Kinetics and Kinematics
Sprint kinetics and kinematics were assessed on a nonmotorized treadmill (NMT; Woodway; Force 3.0, Waukesha, WI, USA). The NMT sprint performance variables were measured after a warm-up on the NMT consisting of 3 minutes of continuous jogging interspersed with 3 sprints at gradually increasing speeds (∼60% of their perceived maximal speed). During the maximum effort sprints that followed, the subjects ran over the NMT with 4 embedded vertical load cells mounted under the running surface and were connected to a mounted horizontal load cell, which measures horizontal force via a nonelastic tether and harness that was attached around their waists. The horizontal load cell was attached to a metal vertical strut with a sliding gauge, which locked into place to avoid any movement during testing. The sliding gauge allowed the horizontal load cell to be adjusted vertically in accordance with the height of each subject so that the tether was at an angle greater than the horizontal for each participant while standing, so as to maintain the horizontal position of the tether during the forward lean adopted when sprinting on the NMT. The subjects were instructed to sprint maximally from a standing split stance start (left leg forward) on the researcher's instruction and to maintain their effort for >6 seconds. This procedure was repeated twice more interspersed with at least 2 minutes of passive recovery. Mechanical data were sampled at 200 Hz during the sprint period allowing instantaneous collection of vertical forces (F v), horizontal forces (F h), and power (Pmax). Peak values of force and power were averaged over 10 steps at constant maximum velocity. Contact times (Ct) and flight times (Ft) were also recorded and averaged over the 10 steps. The Ct was determined from the time of force applied to the treadmill exceeding 0 N and returned to 0 N, whereas Ft was determined from the time between the end of the ground contact period of 1 foot to the beginning of the ground contact period for the opposite foot. Stride frequency was determined from the following formula: 1/(Ct + Ft), whereas stride length was determined from the following formula: running velocity/step frequency. The interclass correlation coefficient (ICC) and coefficient of variation (CV) for sprinting kinetics and kinematics, and isokinetic variables are presented in Table 1.
Unilateral Leg Power
The assessment of unilateral leg power for both the horizontal countermovement jump (HCJ) and lateral countermovement jump (LCJ) was adapted from Meylan et al. (34). For the HCJ, the subjects began by standing on the designated testing leg with the foot at the starting line and with hands on the hips. Each subject was instructed to sink to a self-selected depth as quickly as possible before jumping as far forward as possible and landing on 2 feet. For the LCJ, each subject stood on the designated testing leg with the foot at the starting line and hands on the hip. Each subject was instructed to sink to a self-selected depth as quickly as possible and then jumped as far laterally to the inside and landed on 2 feet. Three trials were performed on the dominant leg, as determined by the leg used to kick the ball, interspersed with 2 minutes of passive rest between both the HCJ and the LCJ.
Maximal Aerobic Velocity
Maximal running speed was determined by use of the 30:15 intermittent fitness test (IFT) (30:15IRT) (5). The 30:15IFT protocol consists of 30-second shuttle runs, which are interspersed with 15-second passive recovery periods. Running velocity was initiated at 2.2 m·s−1 (8 km·h−1) and speed continued to increase 0.14 m·s−1 (0.5 km·h−1) for every 30-second shuttle run completed. The subjects were required to run back and forth over a 40-m track at the given pace, governed by a prerecorded beep. The velocity attained during the final stage completed was determined as the subject's maximal aerobic velocity or V IFT. The V[Combining Dot Above]O2max can be further estimated from the V IFT according to the following formula: V[Combining Dot Above]O2max30-15IFT (milliliters per minute per kilogram) = 28.3 − 2.15G − 0.741A − 0.0357W + 0.0586A × V IFT + 1.03V IFT. Here G stands for Gender, female = 2, male = 1; A is for age; and W stands for weight (4).
A subgroup of 10 players performed a second testing session within 7 days of initial testing to determine the test-retest reliability of the sprint and isokinetic variables. Interclass correlation coefficient (ICC) and CV were calculated for each variable. A 1-way analysis of variance was used to compare the physiological and anthropometrical characteristics between each age group (U17, U20, and Senior). When required, comparisons of the group means were performed using Fisher's least significant difference (21) post hoc analysis to determine pairwise differences. Data are presented as mean ± SD throughout. Statistical significance was accepted at p ≤ 0.05. The groups were then subsequently divided to determine differences, via an independent T-test between starters and nonstarters. Because of the relatively small sample size, differences between the starters and nonstarters, and age group groups were further analyzed using effect size (ES) statistics. The ES of <0.2, <0.6, <1.2, and >1.2 were considered trivial, small, moderate, and large, respectively (26).
The test-retest reliability data are shown in Table 1. Table 2 presents a physical profile of all U17, U20, and Senior elite female soccer players. There was a significant difference (p < 0.05) for body mass between all the groups, and therefore, isokinetic strength measures and sprint kinetics have been represented in both absolute and relative values. Results for isokinetic strength show relative differences for concentric hip extension and flexion, and eccentric knee flexion; the U17 group displayed a lower relative PT for concentric hip extension (2.37 ± 0.68 N·m−1, p < 0.05), concentric hip flexion (0.92 ± 0.31 N·m−1, p < 0.05), and eccentric knee flexion (1.82 ± 0.43 N·m−1, p < 0.05) as compared with both the U20 and Senior groups. Relative horizontal force was significantly greater for the U17 group (4.14 ± 0.62 N·kg−1, p < 0.05) compared with that for all the other groups. Differences were observed in sprint kinematics for the U17 group compared with that for all other groups for all variables, including greater contact time (202.87 ± 20.30 milliseconds, p < 0.05), greater flight time (87.07 ± 16.90 milliseconds, p < 0.05), and lower step frequency (3.49 ± 0.33 Hz, p < 0.05), but an increased step length (1.52 ± 0.30 m, p < 0.05) compared with all the other groups. Differences in leg power were also observed between all the groups, the Senior group jumped a greater distance in both lateral (154.60 ± 15.70 cm, p < 0.05) and horizontal (162.80 ± 12.60 cm, p < 0.05) jumps compared with the U17 and U20 groups. The U20 group was significantly slower than both the U17 and Senior groups for maximum sprinting velocity (4.89 ± 0.26 m·s−1, p < 0.05). The Senior group represented significantly higher scores (19.20 ± 1.2 km·h−1, p < 0.05) and (50.29 ± 2.89 ml·kg−1·min−1, p < 0.05) for V IFT and V[Combining Dot Above]O2max(30-15IFT), respectively, compared with both the U17 and U20 groups.
Comparisons between the starters and nonstarters of the 3 age groups are represented in Tables 3–5. Starters for the U17 age group (Table 3) displayed greater isokinetic leg strength, with a significant difference shown in absolute concentric knee flexion (94.40 ± 12.70 N·m−1, p < 0.05) and both absolute (164 ± 39.30 N·m−1, p < 0.05) and relative (2.81 ± 0.57 N·m−1, p < 0.05) eccentric knee extension. The U17 Starters also produced greater VIFT (18.90 ± 0.40 km·h−1, p < 0.05) and relative vertical force (23.66 ± 1.92 N·kg−1, p < 0.05) while running than the nonstarters did. As shown in Table 4, U20 starters had a greater absolute (177 ± 49.30 N·m−1, p < 0.05) and relative (2.93 ± 0.61 N·m−1, p < 0.05) isokinetic hip extension strength, with a significantly greater absolute (1,348 ± 223 N·m−1, p < 0.05) and relative (22.46 ± 1.60 N·m−1, p < 0.05) vertical force production while running. There was also a pronounced increase in V IFT (19.4 ± 0.70 km−1·h−1) and derived V[Combining Dot Above]O2max(30-15IFT) (49.08 ± 1.68 ml·kg−1·min−1, p < 0.05) compared with nonstarters. Senior starters (Table 5) had a greater peak V IFT (20.1 ± 0.70 km·h−1, p < 0.05) maximal sprinting velocity (5.20 ± 0.40 m·s−1, p < 0.05) and a higher derived V[Combining Dot Above]O2max(30-15IFT) (51.59 ± 2.07 ml·kg−1·min−1, p < 0.05) as compared with nonstarters. When comparing across the age span, all the starters had a significantly higher V IFT, and consequently a greater derived V[Combining Dot Above]O2max(30-15IFT), as compared with nonstarters.
To our knowledge, this is the first study to report the physiological characteristics of female soccer players at the international level across a wide age range (14–36 years) with specific attention paid to starters vs. nonstarters, providing an insight into the physical qualities that are important for playing success. This study also provides normative data for elite players competing at the international level across a wide age range. The results of this study support previous findings in that gains in speed, strength, leg power, and aerobic capacity show continual improvement into adulthood (30–32,41,44). Furthermore, the current findings support the notion that physiological gains are greater during the early to midteenage years.
Playing frequency within the national training pool was significantly higher for the U20 and Senior groups than for the U17 groups, but with no effect for the starters vs. nonstarters, as all the players train under the same set schedule. Experience within the national pool showed a significant difference between groups, but only a large ES was shown for the U17 starters over nonstarters. Moderate ESs were shown for height within the U17 starters compared with nonstarters, and this suggests that the players who are physically more mature, regardless of chronological age, are more often selected as starters. This reflects the inclusion of player identification and development models, which have been adapted within the New Zealand Football framework (24); younger players are being developed earlier and subsequently earning starting positions at the international game earlier.
A focus for this study was evaluating player's isokinetic hip and knee flexor and extensor (5) strength using dynamometry across the groups. As a significant difference in body mass was detected between groups, isokinetic strength measures were represented in absolute and relative to body mass. The findings for PT across all the age groups in all conditions are in agreement with those of previous authors in that PT increases with age throughout maturation (3,13,24,40). Although the relative PT increase in concentric knee flexion and extension was stable between groups, relative PT for both hip flexion and extension was significantly stronger for the U20 group compared with that for both the U17 and Senior groups of players. This supports the findings of Forbes et al. (17) that PT increases are not necessary linear with age, muscle group or mode of contraction, and that each display a different pattern. Le Gall et al. (29) found no difference in knee flexion or extension strength in male soccer players aged 14–17 years. Therefore, we speculate that the difference observed in this study and that of Forbes et al. (17) is specific to elite athletes, more specifically soccer players and in particular the impact of elite training pathways. Rochcongar et al. (40) also reported specific concentric knee extensor strength gains at the U20 equivalent in their sample of 166 elite junior soccer players, where an increase in absolute PT of 35% was shown after the mean age of 15 (our U17 age bracket). Our data are consistent with these overall gains in strength between the ages of 15 and 20 years. Further to this, hip flexion and extension were significantly weaker in the under 17 population compared with all other groups. Why this occurs is outside the realm of this study; however, for clinicians, trainers, and coaches, the above findings could prove insightful for targeted strength training, with the U17 or late pubertal athletes highlighted as an area where a marked improvement may be generated. Hence, there may be a window of opportunity where younger athletes can make strength gains (2,32), potentially resulting in improved performance.
It is interesting to note that for leg power, only the Senior group was significantly greater compared with the younger athletes. The phenomenon is supported by the relationship between strength and power, which dictates that an individual cannot possess a high level of power without first being relatively strong. This assertion is supported by the robust relationship that exists between maximal strength and maximal power production (11). Because the Senior athletes have greater overall concentric and eccentric muscle strength, their ability to generate power should be heightened. Starters of both the U17 and Senior group had significantly stronger values compared with those of nonstarters for both isokinetic strength and leg power (i.e., horizontal and lateral jumping distance). Those who can transition strength into leg power may be more likely to become a Senior starter.
A major finding of this article was the consistently greater maximal running velocity and derived V[Combining Dot Above]O2max(30-15IFT), across all the age groups, and between starters and nonstarters. Large ESs were shown for both measures between starters and nonstarters with the most profound difference found at the U20 level. The V IFT is related not just to a measure of maximal aerobic function but to anaerobic capacity, neuromuscular, change of direction qualities, intereffort recovery ability, and repeat sprint ability providing a broad picture of a player’s ability on the park (4). Because these qualities are the foundation for which an elite soccer player is built on (8,9), the relationship between V IFT and its derived V[Combining Dot Above]O2max(30-15IFT) is of the greatest importance. Reaching end stage V IFT elicits V[Combining Dot Above]O2max(30-15IFT) (6,7); therefore, providing a clear indication of a player’s performance capability as supported by Helgerud et al.’s (22) V[Combining Dot Above]O2max is an important variable of match performance. Because velocities attained in a match are a direct indication of the level or standard of play (5), the measure of V IFT not only provides an indication of a subject’s performance capability but it also reflects their position as a starter or nonstarter.
Because sprinting is an essential component of soccer and can distinguish between the level of play (39), we further investigated sprint kinematics, kinetics, and maximum speed across the U17, U20, and Senior groups. Although sprinting can be divided into a number of phases, the focus for this article was in attaining maximal velocity over a 6-second period (12). The sprint kinetics we measured displayed a unique pattern across all age brackets. Vertical force (F v) increased in a linear fashion across the age groups, whereas both horizontal force (F h) and power (Pmax) were the greatest for the U17 age bracket, followed by the Senior group and U20 with significantly lower outputs. When compared with the maximum sprinting velocity associated with these measures, the trend is apparent again, at the U17 group sprinting at a greater velocity than of the older players. In terms of sprint kinematics, the U17 group, although having an increased contact time and decreased step frequency with each foot strike, covered greater distance (step length) and flight time with each step forwards than did both the U20 and Senior groups. Because more time is spent on the ground, they were able to produce more peak horizontal force and peak power, while generating speed throughout the sprint compared with the older athletes. This increased contact time may be explained by the observed lower isokinetic muscle strength and muscle stiffness associated with their developmental age compared with the U20 and Senior teams (27). As a whole, the older and thus physically more mature athletes are able to generate a higher step frequency that in turn will benefit them in longer distances, as shown by the linear increase in high-intensity running capacity across the age groups. Thus, despite the increased maximum sprinting ability of the U17 players, they are smaller, weaker, and have a decreased running capacity at high intensity in comparison with the U20 and Senior teams. It should be noted that several members of the U17 group were track sprinters who also competed at the national level, and thus are exceptionally fast for their age. Three of these players were starters, whereas 1 is a nonstarter. It is unlikely that other comparative U17 groups will possess athletes with these attributes.
Although this article is a cross-sectional analysis, precaution must be taken for the interpretation of the results, because the maturational process is unique to each individual, assessed and therefore, crossover between chronological age groups may be present for physical maturation milestones. These data suggest that there is little difference in sprint kinetics and kinematics postmaturation as denoted by the findings of the U20 and Senior squads, but there may be a window of accelerated adaptation to training (2) for the increased development of these characteristics, before the completion of sexual maturation.
This study contributes to the literature by providing normative data for coaches, trainers, and clinicians working with elite and international level female soccer players across all FIFA eligible age brackets. The ability to objectively assess the physical performance of female soccer players and in turn compare those results with the normative data collected will not only improve but will also guide player development systems, monitoring, and selection at the national and international levels. The inclusion of starters and nonstarters may lead to greater specificity in training and group selection as opposed to pure chronological comparisons. Our data suggest that players should aim at implementing individualized training for the development of speed, maximal aerobic velocity, and leg strength before the completion of sexual maturation to meet the physical requirements of the senior level international game.
The authors thank the soccer players for their participation in this study, and the New Zealand Football coaching staff for their support of this research. The technical assistance of Karyss Adams was greatly appreciated. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association. This research was performed without funding.
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