Lower Functional Bilateral Deficit Is Associated with Superior Multidirectional Performance in Soccer Players

Abstract

Bilateral deficit (BLD) is traditionally defined as the reduced capacity to produce force during simultaneous bilateral contractions compared with the summed output of unilateral actions. However, in applied sport settings, BLD is frequently estimated from countermovement jump (CMJ) performance, representing a functional rather than a direct mechanical measure of force production. Therefore, the aim of this study was to examine the association between a CMJ-derived functional BLD index and multidirectional performance in soccer players. Forty male university soccer players (age: 23 ± 1 years) performed unilateral and bilateral CMJ. The BLD index was calculated from jump height values obtained during these assessments. Participants subsequently completed the 505 change-of-direction (CoD) test, which was analyzed using two-dimensional video-based motion analysis. Participants were classified according to BLD magnitude into low, moderate, and high BLD groups. Group differences were assessed using Kruskal–Wallis tests with Bonferroni-adjusted post hoc comparisons. Additionally, Spearman correlation analyses were performed using BLD as a continuous variable. Significant between-group differences were observed across all temporal phases of the 505 test (p < 0.001), with players exhibiting lower BLD values demonstrating superior acceleration, deceleration, reacceleration, and overall CoD performance. Significant negative correlations were also observed between BLD and reaction time, acceleration, deceleration, reacceleration, CoD time, and CoD deficit (r_s = −0.42 to −0.69; p < 0.001). No significant associations were found for stride length, acceleration ability, or inter-limb asymmetry. These findings suggest that lower magnitudes of a CMJ-derived functional BLD index are associated with superior multidirectional performance in soccer players. However, given that BLD was estimated from jump performance, the results should be interpreted as associations with a functional neuromuscular performance index rather than as direct evidence of bilateral force production capacity.

1. Introduction

Soccer is an intermittent high-intensity sport characterized by explosive actions such as accelerations, decelerations, jumps, and change of direction (CoD), all of which are considered key determinants of competitive performance [1,2]. From a biomechanical perspective, the ability to accelerate and rapidly change direction largely depends on the effective production and application of force relative to body mass, as well as the capacity to generate, absorb, and redirect momentum within very short time intervals [3].

Change-of-direction performance is influenced by multiple neuromuscular factors, including force production capacity, eccentric strength, and intermuscular coordination [4]. In addition, inter-limb asymmetries have been associated with both physical performance and injury risk, although their interpretation depends on asymmetry magnitude and the athlete’s strength level [5,6]. In this context, it is important to distinguish inter-limb asymmetries, understood as functional differences between limbs, from other neuromuscular indicators emerging during bilateral tasks.

Among these indicators, the bilateral deficit (BLD) has received increasing attention in the scientific literature. BLD is defined as the reduced capacity to produce force during simultaneous bilateral contractions compared with the summed output of unilateral actions performed independently [7]. From a neuromechanical perspective, this phenomenon has been attributed to neural inhibition mechanisms, limitations in motor unit recruitment, and restrictions in inter-limb coordination during bilateral actions [7,8]. However, current evidence suggests that BLD is a multifactorial phenomenon influenced by both neuromuscular factors and the specific characteristics of the evaluated task [9].

Despite its theoretical relevance, the assessment of BLD presents important limitations in applied sport settings. Although its reference measurement is based on direct mechanical variables such as force, impulse, or torque, in practice, it is frequently estimated using performance-based variables derived from countermovement jump (CMJ) performance, particularly jump height calculated from flight time [4,10]. However, this approach does not represent a direct mechanical measure of force production but rather an outcome influenced by biomechanical and neuromuscular factors such as movement strategy, intermuscular coordination, and stretch–shortening cycle characteristics [11]. Therefore, CMJ-derived BLD should be interpreted as a functional neuromuscular performance index rather than a direct representation of the classical bilateral deficit phenomenon.

The available evidence regarding the relationship between BLD and sports performance remains inconsistent. Some studies have reported that greater BLD values are associated with superior performance in unilateral tasks such as CoD [4], whereas others have found no significant associations between BLD and sprint or CoD performance [9]. In contrast, investigations based on mechanical variables have demonstrated that lower BLD values are associated with greater force and impulse production, both considered key determinants of explosive actions [12]. Furthermore, recent studies in sports characterized by a predominance of unilateral actions have reported positive associations between BLD and CoD performance, although with moderate magnitudes and task-dependent effects [13].

In line with these findings, recent reviews have highlighted that the relationship between BLD and performance is highly dependent on the task, the variable used for its calculation, and the specific demands of the sport [14]. In soccer, the available evidence remains limited and inconclusive, emphasizing the need for studies examining this phenomenon using more contextualized and sport-specific approaches.

Additionally, unilateral training interventions have been shown to improve CoD performance while modifying BLD magnitude, suggesting that this index demonstrates considerable plasticity and may reflect specific neuromuscular adaptations to training [15]. In sports such as soccer, where unilateral actions predominate, performance may be more closely associated with the ability to efficiently apply force under dynamic unilateral conditions rather than with maximal bilateral force production.

Therefore, it is necessary to further explore the interpretation of BLD not as a direct marker of force production capacity but as a functional indicator influenced by coordination and task-specific factors. To date, no study has simultaneously analyzed how different magnitudes of a CMJ-derived functional BLD index are associated with acceleration, CoD performance, and inter-limb asymmetry in soccer players.

Accordingly, the aim of the present study was to analyze the association between a functional BLD index derived from CMJ height and multidirectional performance in soccer players. It was hypothesized that greater values of this functional index would be associated with lower multidirectional performance, reflecting reduced efficiency in force application and coordination during high-intensity actions.

2. Materials and Methods

2.1. Participants

Forty male university soccer players (age: 23 ± 1 years; height: 178.6 ± 0.6 cm; body mass: 78.5 ± 7.9 kg; body fat: 10.8 ± 1.0%) voluntarily participated in this study. Body fat percentage was assessed using bioelectrical impedance analysis (BIA) with a Tanita BC-418 MA body composition analyzer (Tanita Corp., Tokyo, Japan). All participants competed at the national level and participated in five weekly training sessions, including technical, tactical, and physical components. To be included, participants were required to be free from lower-limb injuries within the six months preceding data collection. Additionally, they were instructed to avoid strenuous physical activity for at least 24 h before testing.

Sample size was estimated a priori using G*Power software (version 3.1, Heinrich Heine University, Düsseldorf, Germany) based on a one-way ANOVA with three independent groups. Assuming a moderate effect size (f = 0.30), a significance level of α = 0.05, and a statistical power (1 − β) of 0.80, the minimum required sample size was calculated to be 36 participants [16]. All procedures were conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of the Universitat de València (UV-INV_ETICA-1995574). Written informed consent was obtained from all participants prior to participation.

2.2. Study Protocol

All assessments were completed during a single experimental session conducted under controlled environmental conditions. Upon arrival, participants first underwent anthropometric measurements, including body mass and height assessment. Subsequently, standardized instructions regarding the testing procedures were provided by the investigators.

Before data collection, participants completed a standardized warm-up lasting approximately 10 min, consisting of low-intensity running, dynamic mobility exercises, and progressive sport-specific actions, including accelerations and submaximal jumps. Following the warm-up, participants performed the bilateral CMJ, unilateral CMJ with the dominant limb, unilateral CMJ with the non-dominant limb, and subsequently the 505 CoD test in a fixed sequence. This order was selected to standardize testing conditions and minimize the potential influence of fatigue induced by the CoD task on jump performance. A recovery period of 2–3 min was provided between attempts to reduce fatigue-related effects.

For each test, two valid trials were completed, and the best performance was retained for subsequent analysis. Trials considered technically incorrect were discarded and repeated. For the CMJ assessments, trials were considered technically incorrect when participants removed their hands from their hips, touched the ground with the contralateral limb during unilateral jumps, lost balance during take-off or landing, or failed to follow the standardized jumping instructions. During the 505 CoD test, trials were repeated if participants failed to place the turning foot on the turning line, did not complete the 180° turn as instructed, slipped, or lost balance during the execution of the test. Standardized verbal encouragement was provided throughout all assessments to ensure maximal effort. The temporal sequence of the experimental protocol is illustrated in Figure 1.

Countermovement Jump

The CMJ assessment consisted of two trials performed under three conditions: Bilateral CMJ, unilateral CMJ with the dominant limb, and unilateral CMJ with the non-dominant limb. The highest jump height obtained in each condition was retained for subsequent analysis. A 60 s recovery interval was provided between attempts [17]. Jump performance was assessed using a contact platform system (Chronojump, Barcelona, Spain) used in combination with Boscosystem software (version 1.6.2), previously validated for the evaluation of vertical jump performance [18]. Intraclass correlation coefficients were not available for all CMJ conditions and therefore could not be reported.

During the bilateral CMJ, participants adopted an upright standing position with their gaze directed forward and their hands placed on their hips to minimize the influence of arm swing. Participants then performed a rapid downward movement through hip and knee flexion to a self-selected depth, immediately followed by an explosive extension of the lower limbs until take-off while maintaining full body extension during the flight phase.

For the unilateral CMJ, the same procedure was performed independently with the dominant and non-dominant limbs. The contralateral limb remained flexed and off the ground throughout the movement. Participants were instructed to maintain trunk stability and minimize lateral compensations to ensure that force production was generated exclusively by the tested limb.

Jump height was estimated from flight time (FT) according to the equation proposed by Bosco et al. [19]:

$${\mathrm{h}=(\mathrm{FT}}^2\times \mathrm{g})/8$$

where h represents jump height, FT corresponds to flight time, and g represents gravitational acceleration.

Take-off velocity was calculated from jump height according to the projectile motion equation:

$$\mathrm{v}=\mathrm{g}\ast \mathrm{F}\mathrm{T}/2$$

where v represents take-off velocity, g represents gravitational acceleration, and FT corresponds to flight time. Relative power output (W·kg⁻¹) was calculated automatically by the Boscosystem software using the flight-time-derived jump performance variables obtained during each CMJ trial.

Bilateral Deficit Calculation and Classification

The BLD index was calculated according to the equation proposed by Howard and Enoka [20]:

$$\mathrm{B}\mathrm{L}\mathrm{D}\%=\left({\displaystyle \frac{\mathrm{B}\mathrm{i}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{l} \mathrm{C}\mathrm{M}\mathrm{J}}{\mathrm{C}\mathrm{M}\mathrm{J} \mathrm{D}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t}+\mathrm{C}\mathrm{M}\mathrm{J} \text{Non–dominant}}}\ast 100\right)-100$$

Negative values indicated the presence of bilateral deficit, whereas positive values reflected bilateral facilitation (BLF).

According to the literature, athletes may be classified based on the presence of a bilateral deficit or bilateral facilitation [10]. However, all participants in the present study exhibited negative BLD values. Therefore, participants were categorized according to deficit magnitude into low BLD (0 to −9.9%; n = 10), moderate BLD (−10 to −19.9%; n = 18), and high BLD (−20 to −30%; n = 12). Although previous studies have proposed classification systems based on tertiles [21], the present categorization was adopted to facilitate the identification of distinct neuromuscular performance profiles within the analyzed sample. Consequently, these cut-off values should be interpreted as sample-specific and exploratory in nature rather than as universal thresholds.

Importantly, BLD was estimated from CMJ height derived from flight time, which does not represent a direct mechanical measure of bilateral force production. Since jump height is influenced by multiple biomechanical and neuromuscular factors, including intermuscular coordination, movement strategy, and stretch–shortening cycle characteristics, the obtained values should be interpreted as a functional index of neuromuscular performance rather than a direct measure of bilateral force deficit. To complement the categorical analysis and reduce the likelihood that the findings were dependent on group classification, Spearman’s rank correlation coefficients were calculated between BLD values treated as a continuous variable and the different 505 performance variables.

505 Change-of-Direction Test and Asymmetry Assessment

The 505 CoD test was performed using the traditional spatial configuration, consisting of a 10 m acceleration approach, a further 5 m section to the turning line, execution of a 180° turn, and a 5 m return phase. Although the spatial structure of the test was consistent with the established 505 protocol, the present study implemented an adapted video-based analysis approach that enabled the segmentation of the movement into acceleration, deceleration, turning, and reacceleration phases. This procedure was adopted to provide phase-specific information regarding multidirectional performance while preserving the fundamental characteristics of the original test [4,22].

All trials were recorded using a smartphone camera (iPhone 11; Apple Inc., Cupertino, CA, USA) operating at 60 fps and 1080p resolution. The camera was positioned perpendicular to the plane of motion to ensure a consistent sagittal view throughout testing. Video recordings were subsequently analyzed frame-by-frame using Kinovea^® software (version 0.9.5), which has demonstrated acceptable validity and reliability for the temporal analysis of sprinting and CoD actions [23]. Given the recording frequency of 60 fps, the temporal resolution of the analysis was approximately 0.017 s per frame. Although this resolution may not be sufficient for precise sprint timing, it was considered adequate for the consistent identification of the key movement events delimiting each phase of the test.

The purpose of the video analysis was not to determine absolute sprint times with millisecond precision, but rather to consistently identify the mechanical events defining each phase of the movement under identical recording conditions for all participants. This approach enabled relative comparisons between groups while assuming that any measurement error remained systematic across observations. The reliability of event identification was not formally assessed through intra-rater or inter-rater analyses. However, all recordings were analyzed by the same experienced evaluator using standardized operational criteria.

To standardize the analysis, operational definitions were established for each event comprising the test. Reaction time was defined as the interval between the visual stimulus and the first observable displacement of the center of mass, which was considered the onset of movement. Although reaction time was quantified as an additional variable, it was analyzed independently and was not included in the calculation of the traditional 505 completion time. The acceleration phase extended from movement initiation until the center of mass reached the 10 m mark. The onset of deceleration was identified as the first foot contact with the ground after this point. The CoD phase commenced with foot contact on the 15 m line and ended with the first ground contact following completion of the 180° turn, which marked the beginning of the reacceleration phase. Reacceleration continued until participants crossed the 5 m line during the return phase, which was considered the end of the test (Figure 2).

Briefly, velocity was calculated from the time required to traverse predefined spatial segments during the 505 test. Specifically, the time elapsed between segment entry and exit was determined through frame-by-frame video analysis, and velocity was calculated as the ratio between segment length and traversal time (v = d/t). Subsequently, acceleration-related variables were derived from changes in velocity over time. Acceleration and reacceleration were calculated as the change in velocity divided by the corresponding time interval (a = Δv/Δt), whereas deceleration was quantified using the same approach during phases characterized by a reduction in velocity. This procedure enabled the quantification of the acceleration, deceleration, and reacceleration phases associated with CoD performance.

A phase-specific CoD deficit was calculated as follows:

$$\mathrm{C}\mathrm{o}\mathrm{D} \mathrm{D}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}=\left(\right(\mathrm{D}\mathrm{e}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e}+\mathrm{R}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e})-\mathrm{A}\mathrm{c}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e})$$

This variable represents the additional temporal cost associated with braking, changing direction, and reaccelerating during the 505 test. Lower values reflect greater efficiency during direction-change maneuvers, whereas higher values indicate a greater performance cost associated with deceleration and subsequent speed restoration [24].

Acceleration ability was calculated as follows:

$$\mathrm{A}\mathrm{c}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{A}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}=(\mathrm{R}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e}-\mathrm{D}\mathrm{e}\mathrm{c}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{T}\mathrm{i}\mathrm{m}\mathrm{e})$$

This variable was intended to provide an index of the athlete’s capacity to regain speed following the braking phase. Lower values indicate a shorter reacceleration period relative to the preceding deceleration phase and therefore greater efficiency in restoring momentum after the change of direction.

Inter-Limb Asymmetry Calculation

Inter-limb asymmetry was determined using the CoD performance time when turning on the dominant and non-dominant limbs. Bilateral asymmetry was subsequently calculated according to the following equation [25,26]:

$$\mathrm{A}\mathrm{s}\mathrm{y}\mathrm{m}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{y} \mathrm{I}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{x}=\left({\displaystyle \frac{\mathrm{D}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t} \mathrm{L}\mathrm{i}\mathrm{m}\mathrm{b}-\mathrm{N}\mathrm{o}\mathrm{n}-\mathrm{D}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t} \mathrm{L}\mathrm{i}\mathrm{m}\mathrm{b}}{0.5\ast \left(\mathrm{D}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t} \mathrm{L}\mathrm{i}\mathrm{m}\mathrm{b}+\mathrm{N}\mathrm{o}\mathrm{n}-\mathrm{D}\mathrm{o}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{n}\mathrm{t} \mathrm{L}\mathrm{i}\mathrm{m}\mathrm{b}\right)}}\right)\ast 100$$

Absolute values were used to quantify the magnitude of asymmetry, irrespective of its direction. Asymmetry was expressed as a percentage, where 0% represented perfect symmetry between limbs, whereas higher values indicated greater inter-limb asymmetry.

2.3. Statistical Analysis

Statistical analyses were performed using SPSS software (version 25.0; IBM Corp., Armonk, NY, USA). Data normality was assessed using the Shapiro–Wilk test, together with visual inspection of histograms and skewness and kurtosis values. Since normality assumptions were not satisfied, data were descriptively reported as median and interquartile range (IQR; Q1–Q3) [27]. Although non-parametric tests were ultimately applied, the a priori sample size estimation based on a one-way ANOVA was retained as a conservative approximation to ensure an adequate sample size for detecting moderate between-group effects.

Differences among groups according to bilateral deficit magnitude (low, moderate, and high BLD) were analyzed using the Kruskal–Wallis test for independent samples. When significant overall differences were identified, pairwise comparisons were performed using Bonferroni-adjusted post hoc analyses.

Effect size for overall comparisons was estimated using epsilon squared (ε²), whereas the pairwise effect size was calculated using the r coefficient derived from the z statistic. Additionally, Spearman’s rank correlation coefficient (rho) was used to examine the associations between functional BLD and multidirectional performance variables. Correlation magnitudes were interpreted as trivial (<0.10), small (0.10–0.29), moderate (0.30–0.49), large (0.50–0.69), very large (0.70–0.89), and nearly perfect (≥0.90) [28]. Statistical significance was established at p < 0.05.

3. Results

All data exhibited a non-normal distribution (p < 0.05). Descriptive statistics for bilateral and unilateral CMJ performance are presented in

Table 1. Descriptive statistics of countermovement jump performance in unilateral and bilateral conditions (n = 40).

Variable	Jump Height (cm) (Mdn [IQR])	Take Off Velocity (m·s−1) (Mdn [IQR])	Power Relative (W·kg−1) (Mdn [IQR])	SEM	CV (%)
CMJ Dominant limb (cm)	20.45 [3.01]	2.00 [0.15]	34.93 [5.12]	0.49	8.73
CMJ Non-dominant limb (cm)	20.81 [3.22]	2.02 [0.16]	35.21 [5.19]	0.52	9.49
CMJ Bilateral (cm)	38.43 [5.24]	2.75 [0.19]	48.84 [6.10]	0.85	4.65

Mdn: median; IQR: interquartile range; SEM = standard error of the mean; CV = coefficient of variation.

.

The coefficient of variation (CV) was higher in unilateral jumps (8.7–9.5%) compared to the bilateral condition (4.65%), indicating greater relative variability when force production was performed unilaterally. Additionally, the standard error of the mean (SEM) and the narrow confidence intervals indicated an adequate level of measurement precision. Overall, although bilateral CMJ demonstrated greater absolute performance (38.43 cm), unilateral CMJ showed higher variability, which is a relevant factor for interpreting the BLD index.

3.1. Association Between BLD and 505 Test Performance

Acceleration performance variables according to BLD level showed statistically significant between-group differences across all measures analyzed, with small-to-moderate effect sizes (r = 0.21–0.58) (Figure 3).

Significant differences were observed between groups for reaction time. Post hoc analyses revealed that the low BLD group exhibited significantly shorter reaction times than the high BLD group (Z = −2.53, p = 0.027; r = 0.55). No significant differences were found between the low and moderate BLD groups (p = 0.055; r = 0.43) or between the moderate and high BLD groups (p = 0.062; r = 0.34).

With respect to acceleration performance, the low BLD group demonstrated significantly greater values than both the moderate (Z = 2.48, p = 0.013, r = 0.48) and high BLD groups (Z = 3.89, p < 0.001, r = 0.77), whereas no significant differences were observed between the moderate and high BLD groups.

The greatest between-group differences were observed during the deceleration phase. Specifically, the low BLD group achieved significantly greater values than the moderate (Z = 4.02, p < 0.001, r = 0.78) and high BLD groups (Z = 4.31, p < 0.001, r = 0.84), with no significant differences identified between the moderate and high BLD groups.

A similar pattern was observed during the re-acceleration phase, where performance progressively decreased as BLD increased. Significant differences were identified between the low and moderate BLD groups (Z = 3.83, p < 0.001, r = 0.74) and between the low and high BLD groups (Z = 4.34, p < 0.001, r = 0.80). Furthermore, the moderate BLD group exhibited significantly greater values than the high BLD group (Z = 2.39, p = 0.017, ES = 0.44).

3.2. Change-of-Direction Performance

Significant differences were observed in CoD performance (Figure 4). The low BLD group exhibited significantly shorter CoD times than both the moderate BLD group (Z = −2.89, p < 0.01, r = 0.74) and the high BLD group (Z = −5.35, p < 0.001, r = 0.82). Moreover, the moderate BLD group also demonstrated significantly shorter CoD times than the high BLD group (Z = −3.10, p < 0.01, r = 0.84).

A similar pattern was observed for CoD deficit. The low BLD group displayed significantly lower CoD deficit values than both the moderate BLD group (Z = −3.31, p < 0.01, r = 0.76) and the high BLD group (Z = −5.42, p < 0.001, r = 0.69). In addition, the moderate BLD group exhibited a significantly lower CoD deficit than the high BLD group (Z = −2.73, p < 0.05, r = 0.65).

No significant differences were observed between groups for stride length (p = 0.661) or acceleration ability (p = 0.350).

3.3. Inter-Limb Asymmetry

Inter-limb asymmetry during the CoD tasks also showed significant differences between groups. As illustrated in Figure 5, the low BLD group exhibited greater asymmetry values (Mdn = 11.01%) compared to the moderate BLD group (Mdn = 0.58%; z = 3.99, p < 0.001, r = 0.73).

Similarly, the high BLD group (Mdn = 7.36%) also differed significantly from the moderate BLD group (z = −3.43, p < 0.01, r = 0.81). No significant differences were observed between the low and high BLD groups (z = 0.69, p = 0.49, r = 0.20).

To complement the group-based analyses, Spearman correlation coefficients were calculated using BLD as a continuous variable (Figure 6). Significant negative correlations were observed between BLD and reaction time (p = 0.001, r_s = −0.420), acceleration time (p < 0.001, r_s = −0.520), deceleration time (p < 0.001, r_s = −0.605), reacceleration time (p < 0.001, r_s = −0.644), CoD time (p < 0.001, r_s = −0.687), and CoD deficit (p < 0.001, r_s = −0.639). No significant correlations were observed for stride length (p = 0.245, r_s = −0.158), acceleration ability (p = 0.246, r_s = 0.158), or inter-limb asymmetry (p = 0.736, r_s = 0.045).

4. Discussion

The main finding of the present study was that lower magnitudes of BLD, derived from CMJ performance, were associated with superior performance across all temporal phases of the 505 test. Specifically, players with lower BLD values demonstrated shorter reaction, acceleration, deceleration, reacceleration, and overall CoD times, as well as lower CoD deficit values. Importantly, these findings were supported not only by the group-based comparisons but also by the correlation analysis performed using BLD as a continuous variable, which revealed significant associations between BLD magnitude and key CoD-related measures. Collectively, these results suggest that lower BLD magnitudes are associated with more effective multidirectional performance in soccer players.

From a biomechanical perspective, these findings may be related to the movement demands associated with acceleration and CoD actions. Acceleration performance primarily depends on the rapid application of net horizontal force relative to body mass, particularly during the initial sprint steps [28,29]. In contrast, CoD performance involves a complex mechanical sequence composed of deceleration, center-of-mass reorientation, and subsequent reacceleration [3,30]. Within this framework, the present findings suggest that players with lower BLD may exhibit movement characteristics associated with more effective execution of rapid directional transitions.

The strongest correlations observed in the present study were found for CoD time (r_s = −0.687), CoD deficit (r_s = −0.639), and reacceleration time (r_s = −0.644), followed by deceleration time (r_s = −0.605) and acceleration time (r_s = −0.520). These findings suggest that the association between BLD and performance may be particularly relevant during the most demanding phases of CoD performance. The large between-group differences observed during deceleration and reacceleration further support this interpretation. Deceleration has been described as one of the most demanding phases of CoD performance from a neuromuscular perspective [3]. Similarly, reacceleration requires the rapid restoration of movement velocity following directional reorientation [3]. Although kinetic variables were not directly measured in the present study, the observed differences suggest that lower BLD values may be associated with more effective movement execution during demanding multidirectional actions.

From a neuromuscular perspective, lower BLD magnitudes may reflect more efficient intermuscular coordination and reduced bilateral inhibition during explosive actions. Previous studies have demonstrated that lower BLD values are associated with greater peak force and mechanical impulse production [7,12]. Although these variables were not directly assessed in the present study, such findings may provide a potential explanation for the superior multidirectional performance observed in players with lower BLD values. Therefore, the present findings suggest that multidirectional performance may be influenced not only by physical capacities but also by the efficiency with which movement is coordinated during soccer-specific actions.

However, the literature remains inconsistent regarding the relationship between BLD and linear sprint or CoD performance. Some studies have reported no significant associations between BLD and sprint or CoD performance in soccer players [9], whereas others have suggested that greater BLD magnitudes may be associated with superior CoD performance [4,10]. These discrepancies may be explained by methodological differences in BLD quantification and by the specific nature of the tasks evaluated. In the present study, BLD was estimated from CMJ height, meaning that the obtained index represents a functional indicator of neuromuscular performance rather than a direct measure of bilateral force production. Unlike approaches based on direct mechanical variables such as force, torque, or impulse, jump height integrates coordinative, technical, and movement-efficiency components [11,31,32]. Therefore, CMJ-derived BLD should be interpreted as a task- and context-dependent phenomenon rather than a universal marker of neuromuscular performance.

An additional contribution of the present study was the inclusion of a correlation analysis using BLD as a continuous variable. The significant associations observed across the entire sample indicate that the relationship between BLD and CoD performance is not solely dependent on the categorization of athletes into low, moderate, and high BLD groups. Instead, the findings suggest that BLD was associated with multidirectional performance along a continuum, further supporting its relevance as a neuromuscular determinant of CoD ability in soccer players.

Another interesting finding was the relationship between BLD and inter-limb asymmetry. Group comparisons revealed significant differences in asymmetry between BLD categories. However, no significant correlation was observed between BLD magnitude and asymmetry when BLD was analyzed as a continuous variable (r_s = 0.045, p = 0.736). This discrepancy suggests that the relationship between BLD and asymmetry may not be linear and that both variables likely represent distinct neuromuscular constructs.

Traditionally, inter-limb asymmetries have been associated with impaired physical performance and increased injury risk [5,6]. Nevertheless, recent evidence suggests that certain asymmetries may represent sport-specific functional adaptations rather than dysfunctional movement patterns [4]. In sports such as soccer, characterized by a high predominance of unilateral actions, the repetitive execution of specific tasks such as kicking, dominant-leg support, and preferential CoD movements may promote motor specialization and functional lateralization.

From this perspective, the coexistence of lower BLD and greater asymmetry observed in some groups may represent a sport-specific neuromuscular adaptation associated with multidirectional performance. Certain levels of asymmetry may facilitate movement strategies that are advantageous during CoD tasks, particularly during actions requiring rapid transitions between limbs and high levels of unilateral dynamic stability. This interpretation is partially consistent with previous studies showing that unilateral training can modify both BLD magnitude and inter-limb asymmetries while simultaneously improving CoD performance [15].

Biomechanically, CoD performance requires high levels of unilateral postural control during braking and reacceleration phases [3]. Consequently, soccer players may develop preferential motor patterns that enhance task-specific movement efficiency. This may help explain why greater levels of functional asymmetry were not necessarily associated with poorer performance in the present study.

Nevertheless, this interpretation partially contrasts with previous investigations reporting negative associations between inter-limb asymmetry and sprint, acceleration, or CoD performance [33,34]. These discrepancies may be attributed to methodological differences in asymmetry quantification, as well as to the specificity of the functional tasks evaluated. As highlighted by Dos’Santos et al. [3], different CoD protocols may induce distinct neuromuscular demands, thereby modifying both the magnitude and functional meaning of the observed asymmetries.

Overall, the present findings reinforce the idea that BLD and inter-limb asymmetry represent distinct neuromuscular constructs. Whereas BLD may reflect aspects of neuromuscular coordination between limbs, functional asymmetries appear to be more closely related to task-specific motor control strategies and unilateral specialization during dynamic movements. Therefore, in soccer players, certain asymmetries may coexist with superior multidirectional performance and should not necessarily be interpreted as negative from a functional standpoint.

From an applied perspective, these findings suggest that asymmetry interpretation should consider the sporting context, the specific task evaluated, and the functional profile of the athlete. In soccer, where high-intensity unilateral actions predominate, performance may be influenced more by the ability to efficiently execute sport-specific movements than by perfectly symmetrical bilateral performance. Consequently, training programs should not only focus on bilateral force development but also on movement quality, eccentric control, and performance during deceleration and reacceleration tasks.

Several limitations should be acknowledged. First, the cross-sectional design prevents causal inferences regarding the relationship between functional BLD and multidirectional performance. Second, the CMJ assessments (bilateral, dominant, and non-dominant) were performed in a fixed sequence rather than in a randomized order. Although this approach was adopted to standardize testing conditions and minimize the potential influence of fatigue from the subsequent CoD task, a potential order effect cannot be completely excluded. Third, the sample consisted exclusively of male university soccer players, limiting the generalizability of the findings to other competitive levels, age groups, and female athletes. Finally, the phase-specific variables derived from the 505 test were obtained through frame-by-frame analysis of video recordings captured at 60 fps. Although all analyses were performed by the same evaluator using standardized operational criteria, no formal intra-rater or inter-rater reliability assessment was conducted, and reliability indices (ICC, CV, or SEM) were not established for these measurements. Therefore, some degree of measurement error cannot be ruled out when interpreting the phase-specific performance variables.

5. Conclusions

Lower values of a CMJ-derived functional BLD index were associated with superior multidirectional performance in soccer players. Specifically, players with lower BLD values demonstrated better acceleration, deceleration, reacceleration, and CoD performance, resulting in shorter CoD times and lower CoD deficit values. These findings were supported by both group-based comparisons and correlation analyses.

Because BLD was estimated from CMJ height rather than direct mechanical measures of force production, the observed associations should be interpreted as reflecting neuromuscular coordination and movement efficiency rather than bilateral force production capacity. Therefore, a CMJ-derived functional BLD index may represent a practical tool for profiling multidirectional performance in soccer players.