Open-access Bilateral asymmetries and the performance profile in isokinetic knee assessment in young soccer players according to peak height velocity

Assimetrias bilaterais e perfil de desempenho na avaliação isocinética do joelho em jovens jogadores de futebol de acordo com o pico de velocidade de crescimento

Asimetrías bilaterales y perfil de rendimiento en la evaluación isocinética de la rodilla en jóvenes jugadores de fútbol según el pico de velocidad de crecimiento

ABSTRACT

The aim of the study was to compare isokinetic performance (PT) of knee extensors (ext) and flexors (flex), H:Q ratio, and PT asymmetry in young soccer players based on somatic maturation. 80 soccer players (aged 12-17) were sampled, classified as pre-PHV (n=19) and post-PHV (n=61). Isokinetic dynamometry assessed knee extensors/flexors at 60º/s. Pre-PHV had lower body mass/height than post-PHV (P < 0.001). Pre-PHV exhibited lower PT for dominant and non-dominant leg in ext and flex (P < 0.001). No differences in asymmetry (ext: 31.6% vs 31.1%; flex: 31.6% vs 34.4%) and H:Q ratio (dominant: 42.1% vs 32.8%; non-dominant: 26.3% vs 39.3%) between PHV groups, where asymmetry was considered >15%, and cutoff points for each limb H:Q were set at ≥60%. Study showed increased isokinetic performance according to PHV, but no effect on asymmetries and H:Q ratio.

Keywords:  Youth training; Somatic maturity; Injury prevention; Muscular strength

RESUMO

Comparar o desempenho isocinético (PT) dos extensores (ext.) e flexores (flex.) do joelho, a relação I:Q e a assimetria do PT em jogadores jovens de futebol de acordo com a maturação somática. 80 jogadores de futebol (entre 12 e 17 anos), pré-PVC (n=19) e pós-PVC (n=61), foram avaliados no dinamômetro isocinético nos ext/flex do joelho (60º/s). O grupo pré-PVC apresentou menor massa corporal/altura do que o grupo pós-PVC (P < 0,001), além de menor PT na perna dom e não-dom na ext. e flex. (P < 0,001). Não houve diferenças na assimetria (ext: 31,6% x 31,1%; flex: 31,6% x 34,4%) e relação I:Q (dom: 42,1% x 32,8%; não-dom: 26,3% x 39,3%) entre os grupos, onde assimetria foi considerada >15%, e os pontos de corte para a relação H:Q em cada membro foram definidos como ≥60%. O desempenho isocinético aumentou com PVC, mas sem efeito sobre as assimetrias e relação I:Q.

Palavras-chave:  Treinamento de jovens; Maturidade somática; Prevenção de lesões; Força muscular

RESUMEN

Comparar el rendimiento isocinético (PT) de los extensores (ext.) y flexores (flex.) de la rodilla, y la relación I:Q en jóvenes jugadores de fútbol según la maduración somática. Evaluamos a 80 jugadores (entre 12 y 17 años), pre-PVC (n=19) y post-PVC (n=61), en el dinamómetro isocinético (ext/flex de la rodilla a 60º/s). El grupo pre-PVC tuvo menor masa corporal/altura (P < 0,001) y PT en la pierna dom y no-dom en ext. y flex. (P < 0,001). No hubo diferencias en la asimetría (ext: 31,6% vs 31,1%; flex: 31,6% vs 34,4%) y relación I:Q (dom: 42,1% vs 32,8%; no-dom: 26,3% vs 39,3%) entre los grupos, donde la asimetría se consideró >15%, y los puntos de corte para la relación H:Q en cada miembro ≥60%. El rendimiento isocinético aumentó con el PVC, pero sin impacto en las asimetrías y la relación I:Q.

Palabras clave:  Entrenamiento juvenil; Madurez somática; Prevención de lesiones; Fuerza muscular

INTRODUCTION

The assessment of strength and muscle imbalances in the lower limbs through isokinetic assessments in soccer players are common and are of fundamental importance for training, performance, and injury prevention (Oberg et al., 1986; Croisier et al., 2008; Parulytė et al., 2011; Eniseler et al., 2012; Evangelidis et al., 2015; Baroni et al., 2020), especially in young players (Bonetti et al., 2017). Therefore, the production of peak torque (PT) at 60º/s and variables derived from PT, such as the conventional ratio between hamstrings and quadriceps (H:Q ratio), which is defined as the ratio between the peak torque of the hamstring and the quadriceps and is generally measured during a concentric contraction, and the asymmetry of PT between dominant and non-dominant limbs, are among the most common and main parameters to be considered in an isokinetic evaluation of soccer players' knee (Grygorowicz et al., 2017; Baroni et al., 2020). Thus, the literature has shown that when evaluated at 60º/s, soccer players often reach percentage values ​​in the range between 60 to 65% in H:Q ratio (Baroni et al., 2020) and that bilateral asymmetries above 15% can increase the risk of injuries (Croisier et al., 2008) in this population.

Certainly the behavior of these parameters is closely related to the increase in force production in each muscle group (Grygorowicz et al., 2017; Baroni et al., 2020), however, considering that biological maturation causes changes in all systems, tissues and body functions (Malina et al., 2009) the production of strength in the lower limbs can be influenced by both the increase in chronological age and the progress of biological maturation (Philippaerts et al., 2006; Deprez et al., 2015; Malina et al., 2015; Duarte et al., 2018), making it difficult to interpret the main parameters obtained in an isokinetic evaluation of teenage soccer players.

Some studies in this population have explored the influences (Croix et al., 2003; Croix, 2007; Duarte et al., 2018) and relationships (Gillen et al., 2020) between biological maturation, body size, force production (PT) of knee extensors and flexors, and their association with H:Q ratio and limb asymmetry in soccer players. However, this area remains underexplored, particularly concerning athletes at different moments of peak height velocity, impacting athletic performance due to varying rates of individual body development during maturation (Philippaerts et al., 2006). Therefore, the aim of the study was to compare the performance in the isokinetic evaluation of knee extensors and flexors in the parameters PT, H:Q ratio and PT asymmetry between lower limbs of young soccer players and the association according to peak height velocity in order to understand the profile of young soccer players according to somatic maturation status.

Thus, it is believed that young athletes at different growth rates present differences in PT production, H:Q ratio and PT asymmetry between the lower limbs, caused by the advancement of biological maturation in different body systems.

METHODOLOGY

Sample

The sample included 80 young soccer players (aged 12-17 years; 14.6 ± 1.1 years; 59.2 ± 10.9 kg; 172.8 ± 11.0 cm) from three youth teams in Londrina, Brazil. Training consisted of 4-5 weekly sessions lasting 120-180 minutes, covering similar soccer training, strength, and conditioning sessions. Players and guardians were informed, gave consent following ethical guidelines (National Health Council 466/2012; local ethics committee approval Proc. 2.650.232/2018). Inclusion criteria: I) Enrolled in soccer teams; II) Not in pre-evaluation period; III) Injury-free during evaluation. Excluded: injured, dismissed, dropouts or who do not present valid data. 17 players were excluded.

Experimental design

In this cross-sectional study, data collection occurred in September-October 2018, just before the team's pre-season. It happened over two days (D1 and D2). On D1, anthropometric measurements were conducted, and on D2, Isokinetic Evaluation was performed. Athletes recovered for 48 hours before data collection, with a 21-hour gap between tests, totaling 69 hours of rest before the isokinetic assessment. All players were familiar with the procedures.

Anthropometry

Body mass was measured using a Seca digital scale with 0.1 kg accuracy, and height with a portable Harpenden stadiometer accurate to 0.01 cm, following the criteria by Gordon et al. (1988). Chronological age was calculated as the difference between the birth date and evaluation date.

Somatic maturity

Somatic maturity was estimated using Moore et al. (2015) model, which calculates the years from the peak height velocity (PHV) using a predictive equation involving chronological age (CA) and height (H) data, therefore, the equation used in the study was: -7999994 + (0.0036124 × (CA × H) (R2= 0.896; SEE = 0.542). Somatic maturity status was classified as pre-PHV for those with negative values and post-PHV for those with positive values based on the estimated PHV in years.

Isokinetic evaluation

Prior to data collection, subjects did a 5-min warm-up: 50-60 rpm pedaling on Monark 814E ergometer cycle, dynamic 30-sec stretches for quadriceps, hamstrings and hip adductors. Knee muscles assessed with Biodex System 4 dynamometer (Biodex Medical Systems, Shirley, NY, USA). The athletes were chosen randomly, performing three sets of five repetitions at 60°.s-1 for each leg, 60-sec rest between sets. Athletes identified the dominant limb, segment-specific weighing was performed using a knee flexion angle of 30°. For familiarization, three practice repetitions were employed. Participants were verbally encouraged to exert maximum strength. Concentric peak torque (PT) data for knee extension/flexion, representing the maximum peak torque extracted from all repetitions, was recorded for conventional H:Q ratio analysis (Oberg et al., 1986). Asymmetry was considered >15% (Bonetti et al., 2017), and the cutoff points for each limb H:Q ≥60% (Baroni et al., 2020) were adopted.

Statistical analysis

The sample description is presented as mean and confidence interval. Data normality was tested using the Kolmogorov-Smirnov test. The effects of the “maturity group” factor on body size, maturity, peak torque variables, and H:Q ratio were conducted through the Multivariate General Linear Model (GLM). Bonferroni post-hoc analysis was adopted to identify differences between maturity groups, and partial effect size (PES) was used to determine effect size. The Chi-square test with likelihood ratio correction was employed to observe possible associations between H:Q ratio and asymmetry in dominant and non-dominant lower limbs with maturity status groups. Analyses were performed using the SPSS statistical package (v. 25, IBM SPSS, Chicago, IL), and a significance level of 5% was adopted.

RESULTS

Table 1 presents the description of the sample in the variables of body size, somatic maturity, peak torque and H:Q ratio in the lower limbs by maturity status groups in young soccer players. Athletes from post-PHV status were older and had greater body size than youth from pre-PHV status (P<0.001). When compared to the post-PHV groups, the pre-PHV group showed lower performance in peak torque in dominant leg extension (ED) (-38.6%), Dominant leg flexion (FD) (-35.5%), END (-45.4) and non-dominant leg extension (FND) (-43.6%) (P<0.001). In the H:Q ratio in the dominant and non-dominant member no differences were found.

Table 1
Description of the sample by group of somatic maturity status of young soccer players in mean and confidence interval.

Table 2 shows the association of lower limb asymmetry in PHV groups. There were no associations between asymmetry and H:Q ratio with maturity status groups (P<0.05). It is worth noting, around 62% of the young athletes, regardless of their PHV classification, presented symmetry in the knee flexion and extension tests. In addition, more than 76% of the youths in the pre-PHV group had asymmetry H:Q ratio values in the non-dominant limbs.

Table 2
Association of asymmetry and H:Q ratio of dominant and non-dominant limbs in knee flexion and extension in different maturity status groups.

DISCUSSION

The study aimed to compare isokinetic muscle strength in knee extensors and flexors among young soccer players, considering the parameters PT, H:Q ratio, and asymmetry between members, and their associations with maturity status. Findings suggest no links between maturity status and H:Q ratio (60-65%) in both limbs, nor with extension and flexion asymmetries. Peak torque and body size were notably lower for the pre-PHV group.

The study's results show that post-PHV athletes exhibit higher muscle strength, in line with prior research (Holm et al., 2005, Forbes et al. 2009). In a five-year longitudinal study, Duarte et al. (2018) assessed isokinetic force (PT) at 180º/s of knee extensors and flexors in youth aged 11 to 16 years. These authors observed in two multilevel predictive models, involving chronological age, height and fat-free mass, that each year of chronological age progress led to an increase in PT of about 5.4 Nm in knee extension and 2.7 Nm in knee flexion. Holm et al. (2005) also noted age group differences in PT production, even when controlling for body weight during adolescence. Thus, the highest PT among young post-PHV soccer players aligns with their greater body mass, height, and age, all linked to enhanced lower limb strength (Philippaerts et al., 2006; Forbes et al., 2009; Deprez et al., 2015; Duarte et al., 2018).

The post-PHV group's non-dominant H:Q ratio are below and furthest from the 60-65% acceptable range for professional soccer players (Baroni et al., 2020). This deviation can be attributed to the combined effect of increased training impact from youth to professional levels and the positive influence of chronological age progress on isokinetic performance (Croix et al., 2003; Duarte et al., 2018). For instance, a study comparing under 17 players to professionals found that isokinetic variables for knee muscle flexion and extension could effectively differentiate professional players. This underscores the importance of consistent strength training and ongoing player monitoring (Mazuquin et al., 2015).

The study examined limb asymmetry as an important variable. Existing literature has often highlighted that an imbalance between dominant and non-dominant limbs can increase the risk of injury, especially when the asymmetry is 15% or greater (Knapik et al., 1991; Croisier et al., 2008). When considering the descriptive data on the frequency of asymmetry between limbs no differences were observed between pre- and post-PHV, in spite of this, it is observed that we can see that more than 30% present asymmetry in both extension and flexion.

This is the first study comparing, between PHV groups, the asymmetry parameters in the knee flexion and extension of dominant and non-dominant limbs. A similar study, but it did not take into account the PHV groups, by Bonetti et al. (2017) assessed knee extensor and flexor performance in 32 young soccer players, averaging 17 years of age, using concentric isokinetic evaluation (60º/s) and the results showed approximately 10% non-dominant limb flexor asymmetry. The authors attributed those differences to soccer's demands, such as supporting weight and resisting torque from the dominant limb during actions like kicking (Rahnama et al., 2005; Cheung et al., 2012). Although no association between the H:Q ratio and the PHV group was observed, the lower percentages of H:Q ratio in the non-dominant limb found in the PHV group may be related to the athletes' maturational stage. These changes lead to imbalances in motor control and muscle size-force generation disproportions (Deprez et al., 2015; Malina et al., 2015), which can affect performance in concentric isokinetic evaluations, a dynamic force production task.

Although results demonstrate different behavior of PT variables between PHV groups, there was no association between PHV and H:Q ratio (60%) in both limbs, as well as asymmetries in extension and flexion. In other words, maturity status alone does not seem to affect the likelihood of young athletes having limb asymmetry or acceptable H:Q ratio values. Other factors, such as systematic training and the nature of training performed by young athletes, may be relevant in explaining these findings.(Deprez et al., 2015; Duarte et al., 2018).

These results contradicted our initial hypothesis, which suggested that post-PHV soccer players might exhibit greater limb asymmetry and lower H:Q ratio due to variations in the growth and development of muscle and bone tissue (Malina et al., 2009), potentially impacting lower limb strength and physical performance differences among advanced and late-maturity individuals (Sherar et al., 2005; Philippaerts et al., 2006; Malina et al., 2009, 2015; Carvalho et al., 2017).

However, this study has limitations worth noting. Due to its cross-sectional design, it is impossible to establish cause-and-effect relationships, in addition, failure to control variables such as chronological age and height could influence the final result. While previous research indicates that PHV influences increased strength production in young soccer players (Sherar et al., 2005; Philippaerts et al., 2006; Malina et al., 2009, 2015; Carvalho et al., 2017), variables which are related to the performance in isokinetic evaluations of young soccer players (Croix et al., 2003; Duarte et al., 2018). Additionally, although we had control over the fact that teams had 4-5 training sessions per week with similar training components such as strength training and conditioning, we did not have control over the specific methodologies used in each strength training session, as each club has its own approach. Therefore, future studies should consider longitudinal designs, diverse sports modalities, and factors like player position, practice time, and training types, while also considering bone age as a more suitable indicator of biological maturation. For subsequent investigations, it is suggested to explore the relationship between imbalances in muscle strength production during athletes' developmental phase and their evolution throughout their careers. It is essential to examine athletes at different stages of their sporting careers, especially as they age, where the accumulation of training loads and the aging process can increase injury risks, as addressed in a previous study with volleyball players (Berriel et al., 2022)

The results of this study can be utilized for future comparisons about muscle strength parameters and imbalances in soccer players based on their maturational status. It provides valuable insights for soccer technical committees and coaching staff and serves as a benchmark for future studies involving soccer players at different maturity stages. Emphasizing the importance of addressing strength and muscle imbalances during athlete training, the data obtained here inform injury prevention and strength training protocols for young soccer players. It can also be a reference for comparing similar studies in young athletes from different soccer teams.

CONCLUSION

The study showed an increase in lower limb isokinetic muscle performance in young players who have already gone through the PHV. The percentage of players with asymmetries between legs and with H:Q ratio out of the recommended were not affected by PHV. The fact that asymmetries between limbs and H:Q ratio do not show differences between PHV, which is directly related to the risk of injury, emphasizes the responsibility of the coaching staff in seeking the muscular balance of athletes regardless of their current maturational status.

ACKNOWLEDGEMENTS

The authors would like to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).

  • FUNDING
    None.

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Publication Dates

  • Publication in this collection
    06 Dec 2024
  • Date of issue
    2024

History

  • Received
    08 Sept 2023
  • Accepted
    16 Oct 2024
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