Football is a sport that requires physical condition, strength, explosive power, and running ability, which are determining factors in achieving performance. Lower limb strength is the most important factor in supporting the ability to run, change direction, accelerate, and decelerate (Ruscello et al., 2018). During training and matches, football players use their lower body parts to kick the ball, run, and sometimes tackle opponents. This will undoubtedly affect the asymmetry in strength production and the biomechanical structure of the movement, as previous studies have reported.

Numerous studies have reported that most injuries in football players occur in the lower extremities, particularly the hamstrings, ACLs, and knees (Aiello et al., 2023; Alyami et al., 2023; Ferdinand et al., 2025; Huygaerts et al., 2020; Loose et al., 2019). Lower extremity strength imbalances increase the risk of injury (Benson et al., 2024; Hanief et al., 2025), particularly in female athletes. One of the best parameters for assessing injury risk is measuring differences in lower-extremity muscle strength. Valid measurements include an isokinetic dynamometer, which measures the strength of the knee extensor and flexor muscles during eccentric and concentric contractions (Kalata et al., 2020).

Strength asymmetry refers to the bilateral or ipsilateral ratio of the maximum strength values of both legs relative to a reference point. Bilateral muscle strength asymmetry in the range of 10-15% increases the risk of injury, while a hamstring/quadriceps (H/Q) strength ratio of less than 60% is associated with lower extremity injuries (Akarsu Taşman et al., 2025).

An imbalance in lower extremity strength can directly affect athletic performance and plays a significant role in preventing long-term injuries. Imbalance in strength can be influenced by ligament laxity in the knee joint, anatomical structures, and gender-related factors such as hormonal status and the menstrual cycle.

Physiological differences between male and female athletes, such as body weight, muscle mass, and body fat levels, can influence maximal quadriceps and hamstring strength, as well as interbody strength imbalances (Coto Martín et al., 2025; Kalata et al., 2020). While reports indicate that female athletes have lower force output than male athletes, further research is needed to determine whether gender influences lower-body strength imbalances.

Previous research has reported conflicting results on sex-related differences in strength asymmetry (Daneshjoo et al., 2013; Kalata et al., 2020). While some studies suggest greater asymmetry in female athletes, others report no significant differences, indicating a lack of consensus. Moreover, limited studies have simultaneously examined both bilateral and ipsilateral strength asymmetry using isokinetic assessment in male and female football players, particularly in amateur populations. Therefore, a clear research gap remains regarding the extent to which sex influences different types of lower limb strength asymmetry. Addressing this gap is important for developing evidence-based training and injury prevention strategies tailored to athlete characteristics.

This study provides a novel contribution by examining both bilateral and ipsilateral lower limb strength asymmetry in male and female football players using isokinetic assessment. Given the inconsistent findings in previous research and the limited evidence in amateur populations, this study is important to clarify sex-related differences in strength imbalance. The findings have practical implications for developing targeted training programs, improving performance, and reducing injury risk.

This study aims to examine the differences in strength imbalance between the left and right legs in the lower extremities of male and female football players. The results of the measurement and recording of participant characteristics are shown in Table 1.

* REPT: Right Extension Peak Torque; RFPT: Right Flexion Peak Torque; LEPT: Left Extension Peak Torque; LFPT: Left Flexion Peak Torque; Nm: ewton meter; Q/Q: Quadriceps/Quadriceps; H/H: Hamstring/Hamstring; H/Q: Hamstring/Quadriceps.

*p<0.05; Q/Q: Quadriceps/Quadriceps; H/H: Hamstring/Hamstring

The findings revealed significant bilateral differences, defined as strength imbalances between the two limbs, with female athletes closer to ideal values. Furthermore, there were no gender differences in ipsilateral strength imbalances, with male athletes closer to ideal values.

This outcome highlights differences among individuals and reinforces the importance of assessing each athlete individually to identify their specific weaknesses. In football players, Iga et al. (2009) found that players seldom use both legs equally, as a tendency to favor one leg over the other is linked to the brain's hemispheric dominance on the opposite side. This fact can affect specific preferences, leading to varying degrees of morphological and pathophysiological maladaptation in athletes, with one side of the body favored over the other.

The observed differences in bilateral strength asymmetry between male and female athletes may be explained by several underlying mechanisms. One possible explanation relates to differences in neuromuscular control and limb dominance, as repeated unilateral actions in football (e.g., kicking and cutting) may lead to asymmetric muscle development (Bishop et al., 2018; Dos’Santos et al., 2021). Male athletes, who generally produce greater absolute force, may also be more susceptible to developing strength imbalances due to higher mechanical loading during training and competition (Maloney, 2019; Read et al., 2019).

Additionally, physiological differences such as muscle mass distribution, tendon stiffness, and hormonal influences may contribute to variations in strength asymmetry between sexes (Brownstein et al., 2021; Tøien et al., 2023). Female athletes, despite having lower absolute strength, may exhibit more balanced inter-limb coordination and movement control, which could explain why their asymmetry values are closer to the recommended threshold (Bishop et al., 2018; Dos’Santos et al., 2021).

The absence of significant differences in ipsilateral asymmetry suggests that the hamstring-to-quadriceps (H/Q) ratio is relatively consistent between sexes. This finding aligns with previous studies indicating that neuromuscular coordination between agonist and antagonist muscles tends to be preserved regardless of sex, particularly in trained populations (Baroni et al., 2020; Hanief et al., 2025; Tengku Kamalden et al., 2021). This may indicate that although inter-limb asymmetry differs, intra-limb muscle balance is maintained similarly across groups.

The results of this study also occurred in several other sports such as volleyball and tennis. In volleyball, players frequently perform multiple vertical jumps starting from the same leg during play. Landing occurs in a significantly unbalanced position, leading to a substantial and uneven engagement of the lower limb muscles (Taylor et al., 2019). Tennis involves a sequence of active movements, frequent changes in direction, and quick speeding up or slowing down. This can lead to differences in how the lower limbs function in both young and adult tennis players (Fernandez-Fernandez et al., 2010).

In 2015, Bailey and his team conducted a study on differences in isometric strength and countermovement jump performance between genders (Bailey et al., 2015). Their findings indicated that women exhibited greater asymmetry than men. Bell and others noted that female athletes showed greater strength imbalances than male athletes (Bell et al., 2014). Our research results show that the level of strength imbalance between the two sides of the body of female soccer players is closer to the figure considered ideal than that of male players.

The results of this study have significant implications across various areas, including training, injury prevention, and athlete performance. The findings on differences in power imbalances between the left and right arms in male and female athletes indicate that training programs should be tailored to each individual's gender and strength level. Female athletes who exhibit modest differences in leg strength still require ongoing monitoring to maintain muscle strength balance, whereas male athletes require additional attention to reduce leg imbalances.

The absence of differences in ipsilateral asymmetry between men and women indicates that the hamstring-to-quadriceps strength ratio is similar, suggesting that knee-strengthening and stabilization training methods can be applied equally to both male and female athletes. Furthermore, the results of this study emphasize the importance of individual assessment of strength imbalances, as specific leg dominance and sport-specific movement habits can lead to different nervous and muscular system adaptations among athletes.

Practically, these findings serve as a basis for developing training programs aimed at preventing injuries, restoring body function after injury, and improving athlete performance, particularly through isokinetic testing, particularly in sports that require repeated single movements such as soccer.

This study demonstrated differences in bilateral strength asymmetry between male and female football players, with female athletes tending to have values closer to the ideal range. However, no significant differences were found in ipsilateral strength asymmetry between the sexes. These findings suggest that inter-limb strength balance and hamstring-to-quadriceps ratios should be evaluated separately, as leg dominance and sport-specific demands may influence asymmetry patterns.

From a practical perspective, the results highlight the importance of isokinetic assessment in designing individualized training programs aimed at reducing injury risk and improving performance.

However, these findings should be interpreted with caution, as the relatively small sample size and focus on amateur athletes may limit the generalizability of the results to other populations. Future research should include larger and more diverse samples, as well as additional variables such as playing position, limb dominance, and training load, to provide a more comprehensive understanding of strength asymmetry in football players.

We would like to thank all participants involved in this study, especially the coaches and parents. We also extend our gratitude to Batangas State University for its financial support.

The data supporting this study's findings are available on request from the corresponding author. The data are not publicly available because they contain information that could compromise the privacy of research participants

This research received funding from Batangas State University under the fundamental research scheme.

The author hereby declares that this research is free from conflicts of interest with any party.