Physical Characteristics of Elite Youth Female Soccer... : The Journal of Strength & Conditioning Research (2024)

Introduction

In recent years female soccer has grown rapidly, with an exponential increase in the number of opportunities to play professionally and also increased youth participation worldwide (³³). To support this growth in female soccer, the Football Association in England have created elite Regional Talent Centers (RTCs) for the identification and development of talented youth female soccer players, similar to the processes in the men's game (e.g., English Player Performance Plan (²⁹)). The RTCs operate within youth age categories (i.e., Under 10 [U10], U12, U14, and U16), whereby girls are selected to train and compete within an academy environment. The aim is to develop youth female players technically, tactically, psychologically, and physically to prepare them for the elite senior game (⁶).

Currently, there is limited research describing the physical characteristics of youth female soccer players; thus, comparative data are limited for strength and conditioning (S&C) coaches working with youth players to subsequently profile players against players beyond those within their club (^10,34). To the authors' knowledge, only 3 studies have explored the physical characteristics of youth female soccer players in England (^10,33,36). However, the studies by Taylor et al. (³³) and Wright et al. (³⁶) and were conducted before the restructuring of the girls' soccer academies in England and therefore do not reflect the current age group structuring, making comparisons difficult for current practitioners in the field. Furthermore, these data were based on 1 academy with a small sample size (U13; n = 10 and U15; n = 9 (³³) n = 14 players (³⁶)), and 1 study only reported data by chronological age (³³). Emmonds et al. (¹⁰) did consider the influence of both age and maturation on the strength characteristics of youth female soccer players; however, little is known about the influence of maturation on other physical qualities, such as lower-body power, change of direction and aerobic capacity, which are all important for soccer performance (⁶).

Research in male youth soccer (^26,31) has demonstrated the influence of maturation status on physical performance of youth players, suggesting that it may be more appropriate to consider youth athletes by maturity status instead of traditional chronological annual-age groupings (¹⁷). If S&C coaches working with elite youth female soccer players are to use physical testing data to make informed decisions about the “athleticism” of players and to inform training program design, it is important that S&C coaches are aware of the impact maturation may have on the development of specific physical qualities. Although there has been a large body of research exploring the influence of maturation on the physical development of youth male soccer players (^{4,18,19,26,31}), extrapolating male youth data and applying it to females may be erroneous given the different physiological and morphological changes that occur in males and females during maturation (³). Therefore, there is a need for further research specific to female youth soccer players that considers the influence of maturity status on physical qualities. This will allow S&C coaches and other practitioners working with youth female players to make more informed decisions about a player's physical performance in relation to her stage of development. Therefore, the purpose of the study was to evaluate the influence of maturity status on the physical characteristics of elite youth female soccer players. Such findings will help S&C coaches better understand the influence of maturation on the development of physical characteristics in young female athletes. Such findings can be then used to inform the design of individualized S&C program that are relevant to the individual stages of biological development rather than that based on the chronological age.

Methods

Experimental Approach to the Problem

A cross-sectional study design was conducted to investigate the influence of maturity status on the physical characteristics of elite youth female soccer players. All subjects undertook assessments of anthropometry and completed a physical testing battery at the start of the 2016–2017 season (i.e., September, at the end of preseason). Testing was conducted a minimum of 48 hours after competitive match play or training at each respective RTC. A standardized warm-up, including jogging and dynamic movements for 10-minutes followed by jumps and sprints of progressive intensity for 5 minutes, were undertaken before testing. This was then followed by full instruction and demonstrations of the assessments. The lead researcher undertook all testing.

Anthropometry assessments included stature, sitting height, and body mass. The testing battery included assessments of strength (isometric midthigh pull [IMTP]) on a portable force plate, lower-body power (countermovement jump [CMJ]), aerobic capacity (Yo-Yo intermittent recovery test level 1 [YYIRL1]), speed (10 and 30 m), and change of direction (CoD 505 test left and right). The YYIRL1 was not conducted at the U10 age category, as this was not in current practice at the RTCs.

Subjects

One hundred fifty-seven female soccer players (U10, n = 30; U12, n = 38, U14, n = 43, U16, n = 46) were recruited from 3 elite Tier 1 female soccer RTCs in England. Age categories were defined by chronological age on the September 1, 2016, which established their status for competition. All subjects were free from injury at the time of the study. U10 and U12 age categories trained twice per week (2 × 90 minutes pitch-based sessions and 1 × 60-minute S&C session, which included gym and field-based sessions) and U14 and U16 age categories trained 3 times per week (3 × 90 minutes pitch-based sessions and 2 × 60-minute S&C sessions), with each age group having on average 20 matches over a 35-week season.

U10 and U12 age categories trained twice per week (2 × 90 minutes pitch-based sessions and 1 × 30-minute S&C session) and U14 and U16 age categories trained 3 times per week (3 × 90 minutes pitch-based sessions and 2 × 60-minute S&C session), with each age group having on average 20 matches over a 35-week season. The maturation groups were determined by the predicted years from peak height velocity (PHV) derived from anthropometric assessments (²⁷). Before participating in the study, institutional ethics approval was granted from the Research Ethics Committee at Leeds Beckett University. Written informed consent was obtained from both the subjects under 18 and their respective parents or guardians. Characteristics are expressed in table 1.

Procedures

Anthropometric Measurements and Maturity Status

Standing height, sitting height, and leg length were determined using previous methods descried by Malina and Koziel (²¹). For the assessment of standing height, subjects were stood in an erect posture with weight evenly distributed between both feet, heels together, arms hanging relaxed at the sides, and the head in the Frankfurt horizontal plane. Standing height was measured to the nearest 0.1 cm. Sitting height was also measured to the nearest 0.1 cm with the distance from a flat sitting surface (40-cm high) to the top of the head taken as the measurement. Subjects sat in standard erect posture with the head in the Frankfurt horizontal plane; knees were together and directed straight ahead. Subjects were dressed in shorts and t-shirt with trainers removed for the assessment of body mass. Body mass was measured to the neared 0.1 kg.

Maturity was estimated from anthropometric measurements using the protocol proposed by Mirwald et al. (²⁷) equation (equation 1) in which stature, sitting height, leg length, chronological age, and the interaction between these variables are used to predict the number of years from PHV (YPHV, maturity offset). Although some studies have questioned the use of this method (^20,22), this method was chosen because of the noninvasive nature of the assessment and the satisfactory levels of measurement accuracy (²⁷). The equation has been reported to be a reliable (R² = 0.91; standard error of estimate = 0.50), noninvasive, practical solution for the measure of biological maturity for matching adolescent athletes (²⁷) and has previously used for the assessment of maturation in youth female soccer in previous research (³⁶). Years from PHV were calculated for each subject by subtracting the age at PHV from chronological age.

Each subject was categorized into 1 of 6 maturity-offset groups (i.e., −2.5 YPHV [≤2.0], −1.5 YPHV [−1.99 to −1.0], −0.5 YPHV [−0.99 to 0.0], 0.5 YPHV [0.01 to 1.0], 1.5 YPHV [1.01 to 2.0], and 2.5 YPHV [≥2.01]). These categories were consistent with previous categories used in the literature to define maturity status (²³).

Strength

The IMTP was performed on a commercially available portable force platform (ACP; AMTI, Watertown, MA, USA) with a sampling rate of 1,000 Hz, which is consistent with previous methodologies (⁹). Subjects performed the IMTP on a customized pull rack, using a self-selected position similar to that of the second pull of a power clean, with a flat trunk position and their shoulders in line with the bar (¹²). The self-selected midthigh position was preferred because differences in knee and hip joint angles during the IMTP have previously been shown to have no influence on kinetic variables (⁵). Subjects were given 2 practice trials before testing commencing. Subjects were instructed to pull as “fast and hard” as possible and received loud, verbal encouragement (⁹). Each subject completed 2 trials lasting 5 seconds, with 5-minute rest between each trial. The highest peak force (PF) achieved over the 2 trials was considered the subject’s “best trial.” Peak force was identified as the maximum force value obtained during the best trial of the IMTP. Peak force intraclass correlation and coefficient of variation (CV) PF were r = 0.93 and CV = 3.6%. In addition to highest PF, relative PF was calculated using the ratio scaling method (i.e., PF/body mass) (¹⁴).

Lower-Body Power

Lower-body power was assessed using a CMJ in an indoor gym facility that provided a consistent stable flooring to minimize the influence of external factors (e.g., weather, foot-surface interaction) and were allowed 2-minute recovery between jumps. The CMJs were performed according to previously described methods (³⁰) using a portable photoelectric cell system (Optojump; Microgate, Bolzano, Italy). This equipment has been reported as both reliable and valid (CV = 6%; standard error of estimate = 1%) for vertical jump assessment compared with a biomechanical force plate (¹¹). Jump height was calculated using the cell system software (Optojump Next v1.7.9; Microgate). Subjects completed 3 submaximal CMJ efforts before testing. The CMJ started from an upright position. When given a verbal command, the subjects made the downward countermovement to their preferred depth and then jumped as high as possible. Subjects were required to keep their legs straight during the airborne phase of the jump. The highest jump was selected for analysis. Intraclass correlation and CV for CMJ were r = 0.96 and CV = 4.5%, respectively.

Aerobic Capacity

Aerobic capacity was assessed using the YYIR1. The YYIR1 was selected because it has been reported as a valid and reliable test (r = 0.98; CV = 4.9%) for the assessment of soccer-specific fitness (¹⁵). The test consisted of repeated 20-m shuttle runs at progressively increasing speeds dictated by an audio bleep emitted from a CD player. Between each shuttle, a recovery period of 10 seconds is allowed involving walking around a marker placed 5 m behind the finishing line. Failure to achieve the shuttle run in time on 2 occasions resulted in the termination of the test. The final level was achieved, and total running distances were recorded.

Change of Direction Time

Change of direction was assessed using the 505 test, whereby the subjects were positioned 15 m from a turning point. Timing gates (Brower Timing Systems; IR Emit, Draper, UT, USA) were placed 10 m from the start point and 5 m from the turn point. Subjects accelerated from the start through the timing gates, turning 180° at the 15-m mark and sprinted back through the timing gates. Subjects completed 3 alternate attempts on each foot (i.e., right and left leg), separated by a 2–3 minutes of rest period. Only attempts whereby the subjects' foot crossed the 15-m mark were included. Times were recorded to the nearest 0.01 seconds with the quickest of the 3 attempts used as the final score. Data are presented as dominant (D) or nondominant (ND) foot based on preferred kicking foot. Intraclass correlation and CV for the 505 test were r = 0.995 and CV = 2.2%, respectively.

Sprint Time

Sprint times were assessed over 10 m and 30 m using timing gates (Brower Timing Systems, IR Emit). Subjects started 0.5 m behind the initial timing gate and were instructed to set off in their own time and run maximally past the 30-m timing gate. Each subject had 3 attempts, separated by a 3-minute rest period. Times were recorded to the nearest 0.01 seconds with the quickest of the 3 attempts used for the sprint score. Intraclass correlation and CVs for 10- and 30-m sprint time were r = 0.76 and CV = 4.8%, and r = 0.78 and CV = 3.9%, respectively.

Statistical Analyses

Data are presented as mean ± SDs by maturity status. All data were log transformed to reduce bias as a result of nonuniformity error. Magnitude-based inferences were used to assess for practical significance between consecutive maturity groups for each variable (¹³). The threshold for a difference to be considered practically important (the smallest worthwhile difference [SWD]) was set at 0.2× between subject SD for the comparison groups, based on Cohen's d effect size (ES) principle. The probability that the magnitude of difference was greater than the SWD was rated as <0.5%, almost certainly not; 0.5–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99.5%, very likely; and >99.5%, almost certainly (¹⁶). Effect size was rated as trivial (<0.2), small (0.2 < 0.6), moderate (0.6 < 1.2), large (1.2 < 2.0), or very large (2.0 < 4.0) (¹⁴). Where the 90% CI crossed both the upper and lower boundaries of the SWD (ES ± 0.2), the magnitude of difference was described as unclear (¹³).

Results

The anthropometric and physical characteristics of elite youth female soccer players by maturity status are presented in Table 1, and the standardized differences for anthropometric and physical characteristics between consecutive maturation groups are shown in Table 2. Stature and sitting height were very likely to most likely greater in more mature players. Likely to most likely differences in leg length were observed in consecutive maturity groups until 0.5 YPHV, with only possibly differences observed between consecutive maturity groups after PHV. Likely to most likely differences in body mass were observed between consecutive maturity groups, with more mature players being heavier than less mature players.

Peak force was likely to most likely greater for more mature players. However, differences in relative PF between consecutive maturity groups were possibly small to possibly trivial, except for between maturity groups −0.5 YPHV and 0.5 YPHV, where a likely difference was observed. There were likely differences in CMJ between consecutive maturity groups −2.5 to −0.5 YPHV, but only possibly small differences between groups −0.5 and 0.5 YPHV were observed. Most likely differences in CMJ height were observed in maturity groups 1.5 YPHV and 2.5 YPHV. Both 10- and 30-m sprint times were lower in more mature players, with possibly to most likely differences observed between consecutive maturity groups until 1.5 YPHV. Differences in 10- and 30-m sprint times between the maturity groups 1.5 YPHV and 2.5 YPHV were possibly trivial to possibly small, respectively.

Change of direction time was less in more mature players with possibly to very likely differences observed between maturity groups, except for between maturity groups −0.5 YPHV and 0.5 YPHV, where differences between groups were possibly trivial. Likely small differences in distance covered on the YYIRL1 were observed between maturity groups 0.5 YPHV and 0.5 YPHV. All other differences between consecutive maturity groups were possibly trivial.

Discussion

The aim of this study was to investigate the influence of maturation on the physical characteristics of youth female soccer players. Findings demonstrate that speed, CoD, lower-body power, and aerobic fitness were improved in more mature players. However, the development of physical characteristics was nonlinear between consecutive maturation groups. Strength and conditioning coaches need to consider the maturity status of youth female soccer players when evaluating physical testing data and consider the nonlinear development of physical qualities. Such data can also be used as comparative data by S&C coaches working in youth female soccer when assessing the performance of their own players.

Differences in leg length between consecutive maturity groups were greatest between groups −2.5 and −0.5 YPHV. These findings are consistent with normal somatic growth, whereby the peak leg length growth occurs just before PHV (¹⁷). The development of anthropometric characteristics with advancing maturity likely accounts for a number of observed changes in physical characteristics between consecutive maturity groups (¹⁹) and therefore highlighting the importance of regularly assessing maturity status (approximately every 3 months (¹⁷)).

Peak force was greater in more mature female soccer players, which may be attributed to biological changes associated with advanced maturity, including increased body mass (³). Furthermore, given that more mature players were typically older, the greater PF may also be explained by an increased exposure to a structured S&C program within the academies at the older age groups (i.e., structured gym-based resistance training at the U14 and U16 age categories twice per week for 60 minutes). However, when made relative to body mass, relative PF did not increase linearly between consecutive maturity groups, highlighting the importance of considering relative vs. absolute measures of PF. Greatest relative PF was observed in the maturity group −0.5 YPHV (0.99–0.0 YPHV), which may be related to hormonal and morphological changes (i.e., increase in muscle mass) reported to occur around PHV (³). These findings are consistent with previous longitudinal (⁷) and cross-sectional (²) strength assessment research of nonelite female athletes. A likely moderate difference in relative PF was observed between maturity groups −0.5 YPHV and 0.5 YPHV, with lower relative PF in the more mature players (0.5 YPHV). In line with this finding, there were also unclear changes in lower-body power (CMJ height) between these respective maturity groups. Together these findings suggest that female soccer players may experience a reduction in relative PF and consequently lower-body power at 0.5 YPHV (0.01–1.0 YPHV). This may be explained by a potential increase in fat mass associated with peak weight velocity that occurs in females 3.5–10.5 months after PHV (³), which has a nonfunctional role for athletic performance. However, a limitation of this study was that it was not possible to obtain body composition data for players, and therefore, the reason for the observed differences is speculative and requires further research. Nonetheless, S&C coaches should be aware of this possible reduction in relative PF after PHV in youth female soccer players, given the known relationship between strength and athletic performance (³²) and the relationship between low relative strength and increased risk of injury in children (⁸). Findings support the need for youth female soccer players to regularly undertake structured strength training as part of their training program, particularly after PHV.

Both 10- and 30-m sprint times were less in more mature players, indicating faster sprint times with advanced maturity. However, findings demonstrate that acceleration ability (10 m) and maximum speed (30 m) are unique physical qualities, which do not develop at the same rate between consecutive maturity groups. Greatest differences in 30-m sprint time between consecutive maturity groups was observed between −2.5 and 0.5 YPHV, with very likely to likely differences observed. This may be explained by the very likely to most likely differences in leg length between these respective groups, which has been reported in male youth athletes to account for improvements in stride length and thus sprint time (²³). In contrast, 10-m sprint time may be influenced more by relative strength, improved running mechanics, and neuromuscular control (²⁴). Players −0.5 YPHV had faster 10-m sprint times than players 0.5 YPHV. As previously discussed, this was in line with a likely moderate difference in relative PF between these consecutive maturity groups, which may have had a negative influence on force production capabilities (²⁵). Again, this supports the need for youth female soccer players to regularly undertake strength training as part of their weekly training schedule because strength development in addition to the development of correct movement patterns and neuromuscular control underpins the development of other physical qualities (³¹).

This finding is in contrast to the findings of youth male soccer players who have been reported to display an improvement in sprint times after PHV (²³). These differences may be the result of the different physiological changes that occur in males and females with the onset of maturation, with males experiencing a greater increase in lean muscle mass, which results in improved expression of both concentric strength and power (¹⁶). As previously discussed, an increase in body mass and fat mass in females after PHV may possibly explain why 10-m sprint times increased in the maturity group 0.5 YPHV. Therefore, coaches working with youth female soccer players who are −0.5 to 0.5 YPHV need to be aware that players may experience increase in sprint times during this period of development and consider this when evaluating the physical performance of players.

Change of direction times were less in more mature players, indicating more efficient CoD ability; however, greatest differences between consecutive maturation groups were observed −2.5 to −1.0 YPHV (likely to very likely). Improvements in CoD time in players at this stage of maturation may be explained by improvements in neuromuscular control and coordination (⁸). Players at the U10 and U12 age groups included in this study were regularly taking part in a structured S&C session each week, in addition to 2 field-based soccer training sessions. Given that previous research has shown that CoD time can be improved in less mature players using neuromuscular training (⁸), potentially, this may have also facilitated improvements in neuromuscular control beyond the natural development of this physical attribute at this stage of development. Differences in CoD time between maturity groups circa-PHV (−0.5 to 0.5 YPHV) were unclear. Given that relative strength has previously been reported to be strongly correlated with CoD time in female athletes, the lower relative PF observed in females 0.5 YPHV in this study may explain why differences in CoD time at these maturity groups were unclear.

Distance covered on the YYIRL1 was greater for more mature players. Likely differences were observed between maturity groups −0.5 to 0.5 YPHV. This is consistent with findings for 8- to 16-year-old untrained youth females, where aerobic fitness was observed to be greatest around circa-PHV and decrease after PHV (²⁸). Previous research has reported that growth-related changes to the central and peripheral cardiovascular system, including increases in stroke volume and cardiac output, and changes in muscular function and metabolic capability occur around the onset of PHV (³). This may explain the likely differences in aerobic capacity observed between maturity groups −0.5 to 0.5 YPHV. Furthermore, research has shown that percentage body fat is an important factor in the variation of aerobic fitness of youth females (^1,28). Therefore, unclear differences in distance covered on the YYIRL1 test between maturity groups after PHV in this study may be explained by a possible increase in fat mass at this stage of development. As such, it is important that S&C coaches working with this cohort actively look to develop the aerobic capacity of youth female players after PHV.

Although this study provides S&C coaches with a better understanding of the influence of maturity status on the physical development of youth female athletes, it must be noted that this study is not without its limitations. First, the estimation of maturation from somatic measures and predictive equations rather than using a measure of biological maturation likely results in some degree of error (^20,22), which coaches must consider when interpreting the data. Analysis of such data is further complicated by the different categories used in the literature to define maturity status. Given that players may not have all entered the RTC at the same age, a second limitation of the study was that it was not possible to obtain information on the training age of the players, which may influence physical performance. Therefore, future research should also look to consider the training age of players in addition to other variables not evaluated in this study, which may impact on physical performance (i.e., menstrual cycle, training loads). Finally, this study adopted a cross-sectional design, thus future studies should look to employ longitudinal designs to infer development trajectories as opposed to differences by maturation status.

Practical Applications

It is recommended that S&C coaches regularly monitor anthropometric variables to detect periods of rapid growth and maturation, which may impact upon the physical characteristics of youth female soccer players. Strength and conditioning coaches need to be aware that relative PF may decrease after PHV, which may impact upon player’s lower-body power, 10-m acceleration, and CoD time. Therefore, coaches should consider a player’s stage of biological age when evaluating physical testing scores or designing S&C programs in addition to other factors, such as training age. Given the importance of strength for athletic performance (³²), it is recommended that S&C coaches should look to improve neuromuscular strength and fundamental movement skills in players before PHV. These qualities can be developed by working on correct running mechanics, multiplaner jumping and landing tasks, and sprinting as part of fun and engaging pitch-based warm-ups (³⁵). Players circa-PHV may experience decreases in relative PF; therefore, it is important that S&C coaches focus on the development of strength during this period of development. However, it is important that coaches are aware that players at this stage may also experience a reduction in coordination; therefore, the focus of resistance-based exercises must first be on technique (³⁵), which can be developed as part of structured gym-based training sessions and continuing to develop running mechanics and jumping technique as part of pitch-based training sessions. Post PHV, players may benefit from individualized gym- and pitch-based conditioning programs. Furthermore, findings of this study suggest that the coaches should look to actively develop the aerobic system in players after PHV. Manipulation of small-sided games combined with short-duration, intermittent, high-intensity, running drills may provide an efficient training stimulus while concurrently developing technical and tactical skills within the same session.

References