Decoding dynamic postural control: diagnostic insights from Y-balance test and force plate measures in chronic ankle instability

Abstract

Background

Chronic ankle instability (CAI) impairs dynamic balance and anticipatory postural control, yet the diagnostic utility of clinical versus laboratory-based measures remains unclear.

Objectives

To compare dynamic balance and anticipatory postural adjustments (APAs) during gait initiation (GI) between athletes with CAI and healthy controls, and to evaluate the diagnostic accuracy of the Y-Balance Test (YBT) and force plate–derived center-of-pressure (COP) parameters.

Methods

Sixty-two athletes with CAI and 31 healthy controls completed the YBT in three directions—anterior, posteromedial (PM), and posterolateral (PL)—as well as force plate assessments of COP displacement and velocity during the anticipatory phase of GI. Receiver operating characteristic (ROC) analyses were used to evaluate the discriminatory power of each measure.

Results

Both higher and lower CAI participants showed significantly reduced PM and PL reach performance compared with healthy controls (p < 0.001; ηp² = 0.250–0.531), while only the higher-instability group showed a deficit in anterior reach (p = 0.002). ML COP displacement and velocity were also lower in both CAI groups (p ≤ 0.033; ηp² = 0.073–0.554). No significant differences were found between the CAI subgroups, and AP COP measures were non-significant, with low power. ROC analyses showed that PL reach had the highest diagnostic accuracy (area under the curve [AUC] = 0.957; sensitivity = 0.905; specificity = 0.900), followed by PM reach (AUC = 0.857). ML COP velocity also demonstrated good diagnostic ability (AUC = 0.783). Neither YBT nor COP measures effectively distinguished higher- from lower-instability CAI.

Conclusions

Posterior directions of the YBT, particularly the PL reach, and ML COP velocity are the most reliable indicators for identifying balance deficits associated with CAI. Clinically, the PL YBT provides a practical, high-accuracy screening tool, while COP measures offer complementary mechanistic insights. Integrating both clinical and laboratory assessments may enhance evaluation, although routine field screening can rely primarily on the YBT.

Trial registration

IRCT20210604051488N1 registered on 2021-06-28.

Background

Chronic ankle instability (CAI) is a prevalent consequence of lateral ankle sprain, characterized by recurrent episodes of the ankle “giving way,” impaired proprioception, and deficits in neuromuscular control that compromise dynamic stability [1, 2]. Persistent alterations in postural control among individuals with CAI not only affect athletic performance but also increase the risk of re-injury and long-term functional decline [3]. Therefore, reliable assessment tools capable of identifying postural control deficits and differentiating the severity of instability are of considerable clinical and research importance.

The Y-Balance Test (YBT) has been widely used as a practical and reliable clinical measure of dynamic balance, with shorter reach distances indicating impaired dynamic postural control [4]. This test requires controlled multi-directional reaching while maintaining single-leg stance stability, thereby integrating proprioceptive feedback, strength, and motor control [4]. Among the three directions assessed—anterior, posteromedial (PM), and posterolateral (PL)—the latter two are particularly sensitive to functional limitations associated with CAI, reflecting the complexity of postural adjustments required to stabilize the ankle joint during multi-planar tasks [5, 6].

Despite its widespread clinical use, evidence on the discriminative validity of the YBT in at-risk populations—particularly individuals with CAI—remains limited. In this population, the YBT is commonly used to assess dynamic balance deficits and to help identify those at greater risk of ankle sprain or re-injury. Previous studies in athletes have shown that an anterior-reach asymmetry greater than 4 cm on the YBT is associated with increased odds of non-contact injury (sensitivity ~ 59%, specificity ~ 72%) [7]. Similarly, Schwiertz et al. reported that YBT reach distances in youth athletes could discriminate trained versus untrained cohorts, achieving area under the curve (AUC) values of 0.74–0.81 [8]. Several studies have also highlighted the predictive value of the YBT for ankle sprains in athletic populations. For instance, normalized PL reach values below 80% of leg length have been linked to greater susceptibility to lateral ankle sprains [9], and male collegiate athletes with poorer anterior reach performance were at higher risk of sustaining ankle sprain injuries [10]. Consistent with these findings, athletes with CAI exhibit significantly reduced anterior reach compared with controls, with a larger proportion classified as high-risk based on established anterior reach asymmetry cut-offs, accompanied by a strong effect size [11]. Nonetheless, cutoff values specifically validated for individuals with CAI remain insufficiently established.

Beyond clinical assessments, laboratory-based force plate analyses offer an advanced understanding of balance control. They quantify center of pressure (COP) displacements during both static and dynamic tasks [12]. These measures are also sensitive to training-induced changes in postural stability, further supporting their value as objective laboratory markers [13]. Among these, the anticipatory phase of gait initiation (GI) provides a unique and ecologically valid framework to evaluate postural control [14]. GI represents a complex functional task that depends on dynamic postural control to ensure a smooth transition from a stable double-leg stance to a single-leg stance [15]. Efficient GI requires precise coordination of balance and movement, placing continuous demands on motor planning and postural regulation.

From a biomechanical perspective, GI involves anticipatory postural adjustments (APAs) that prepare the body for forward movement. These adjustments are characterized by activation of the tibialis anterior and inhibition of the gastrocnemius, producing a backward and lateral shift of the COP toward the stepping limb. This coordinated action facilitates forward and contralateral displacement of the center of mass (COM), enabling step execution [16].

In individuals with CAI, these neuromechanical processes—comprising the integration of neural commands, coordinated muscle activation, and controlled COP shifts necessary for APAs during GI—are frequently disrupted. Evidence indicates that APAs are altered at multiple levels, including central nervous system (CNS) activity, muscle activation patterns, and the biomechanics of the task. For example, Beyraghi et al. reported that individuals with CAI exhibited diminished preparatory brain activity preceding GI, assessed via EEG, which was accompanied by altered COP trajectories [17]. Similarly, Yousefi et al. found that individuals with CAI exhibited a longer reaction time phase, shorter APA duration, and earlier soleus activation in the injured limb [18]. Furthermore, force plate analyses in CAI populations have demonstrated altered COP trajectories during GI, characterized by reduced displacement and velocity in both anticipatory and locomotor phases, reflecting inefficient APAs and limited COM excursion [17–20]. These impairments may compromise stability during transitional movements and increase the risk of subsequent injury.

Evaluating the diagnostic utility and defining cutoff thresholds of these measures enables clinicians and researchers to identify individuals at elevated risk for developing CAI. Receiver-operating characteristic (ROC) analysis quantifies the accuracy of diagnostic tests in discriminating between distinct patient states, thereby providing evidence for optimal cutoff values that balance sensitivity and specificity [21].

However, to our knowledge, no studies have investigated the discriminative validity of force plate–derived COP parameters during the anticipatory phase of GI in individuals with CAI, nor directly compared these laboratory-based measures with field-based tests such as the YBT. Dynamic COP measures during this phase provide insights beyond traditional static balance assessments by capturing the feedforward postural control required for functional movements. As a result, they offer complementary information on functional stability and may have greater potential for discriminating individuals with CAI. Nonetheless, cutoff values specific to this population and task remain to be established.

Accordingly, the present study aimed to evaluate the sensitivity and specificity of the YBT and force plate–derived COP parameters during the anticipatory phase of GI in (1) distinguishing individuals with CAI from healthy controls, and (2) differentiating between relatively higher and lower degrees of instability. It was hypothesized that COP measures obtained during GI would exhibit greater discriminative validity for identifying CAI, whereas the PM and PL directions of the YBT would provide complementary, objective indicators of impaired postural control.

Method

Study design

This observational study employed a cross-sectional design and was conducted in accordance with the STROBE guidelines.

Participants

A total of 62 athletes with CAI and 31 healthy controls were recruited from sports physiotherapy clinics, sports federations, and the university community via posters and social media advertisements (Table 1). All participants provided written informed consent, and the study protocol was approved by the Ethics Committee of Tehran University of Medical Sciences (IR.TUMS.FNM.REC.1401.146).

Table 1.

Demographic characteristics

Parameters	Healthy (N = 31)		Lower-instability CAI (N = 31)		Higher-instability CAI (N = 31)		P	Post-hoc comparison
	Mean	SD	Mean	SD	Mean	SD
Age (years)	28.00	3.32	25.06	6.72	27.87	6.17	0.07	-
Height (cm)	177.97	7.68	176.50	6.90	174.19	10.87	0.24	-
Weight (kg)	78.69	11.38	72.53	12.25	74.39	14.39	0.16	-
CAIT (0–30)	30.00	0.00	19.88	2.62	10.97	3.29	< 0.001*	Healthy > Low > High
Physical activity (hours/week)	9.14	2.28	9.67	4.38	8.97	4.27	0.75	-
Females (N (%))	12 (39%)		10 (32%)		11 (35%)		0.87	-

Bolded letters and asterisks indicate statistically significant differences between groups (p < 0.05)

CAIT Cumberland Ankle Instability Tool, CAI  Chronic Ankle Instability

Inclusion criteria for CAI group were (1) Age 18–40 years; (2) Self-reported history of a first-ever significant ankle sprain occurring more than 12 months prior to enrollment (to ensure sufficient history for development of chronic ankle instability) [2]; (3) No ankle sprain within the 3 months prior to enrollment (to exclude acute or sub-acute injuries) [2]; (4) At least two episodes of “giving way,” recurrent sprains, or sensations of ankle instability within the past 6 months [2]; (5) Cumberland Ankle Instability Tool (CAIT) score ≤ 24 (scores range from 0 to 30, with lower scores indicating greater instability) [2]; (6) Recreationally active, participating in sports such as volleyball, basketball, soccer, or handball at least three times per week for a minimum of 30 min per session [22]; (7) No history of lower extremity fractures or surgery; (8) No known psychological or neurological disorders; and (9) No current use of prescription or self-medication.

The control group consisted of healthy participants with no history of ankle sprain, no gait or balance impairments within the six months prior to the study, and no known conditions affecting balance. Including a healthy control group allowed for meaningful comparisons with individuals presenting CAI. Controls were matched to the CAI groups based on age (± 5 years), BMI (± 2 units), and sex distribution, ensuring comparable demographic characteristics across groups and minimizing potential confounding effects.

Sample size

The sample size for the diagnostic study was calculated using MedCalc software (version 23.4.0). Based on an anticipated area under the curve (AUC) of 0.7, a null hypothesis value of 0.5, a negative-to-positive subject ratio of 2:1, a statistical power of 0.8, and an alpha level of 0.05, a total of 93 participants were required. A post hoc power analysis was conducted for the ANOVA comparisons based on the observed effect sizes (partial eta squared, ηp²). As shown in Table 2, the majority of parameters achieved adequate statistical power (≥ 0.80), confirming that the sample size was sufficient to detect group differences. Two parameters—AP COP displacement and AP COP velocity—showed lower power (0.48 and 0.26, respectively), indicating that results for these measures should be interpreted with caution.

Table 2.

One-way ANOVA results for YBT scores and force plate–derived COP parameters across healthy, lower-instability CAI, and higher-instability CAI groups

Parameters	Healthy (N = 31)		Lower-instability CAI (N = 31)		Higher-instability CAI (N = 31)		F	P	ηp²	Power	Post-hoc comparison
	Mean	SD	Mean	SD	Mean	SD
YBT (Anterior) [%]	110.82	25.13	98.31	19.96	90.76	22.21	6.487	0.002*	0.126	0.91	Healthy > High; Low ≈ Healthy, High ≈ Low
YBT (posteromedial) [%]	123.05	10.75	105.71	13.80	102.49	13.15	22.740	< 0.001*	0.336	1.00	Healthy > Low & High; Low ≈ High
YBT (posterolateral) [%]	125.54	13.27	98.67	14.09	93.62	11.03	56.371	< 0.001*	0.556	1.00	Healthy > Low & High; Low ≈ High
COP Displacement [ML] (cm)	-2.36	0.72	-1.80	0.77	-1.34	0.97	11.477	< 0.001*	0.205	0.99	Healthy > Low & High; Low ≈ High
COP Displacement [AP] (cm)	-2.04	0.93	-1.95	1.10	-1.54	0.87	2.329	0.103	0.050	0.48	-
COP Velocity [ML](cm/s)	9.79	3.84	6.61	3.18	5.33	3.03	14.086	< 0.001*	0.240	1.00	Healthy > Low & High; Low ≈ High
COP Velocity [AP](cm/s)	7.82	4.03	7.58	4.54	6.35	3.56	1.178	0.313	0.026	0.26	-

Negative sign in AP direction indicates the backward displacement of COP and in ML direction means displacement towards the step-initiating limb

Bolded values and asterisks (*) represent statistically significant differences between groups (p < 0.05)

YBT Y-Balance Test, COP Center of Pressure, CAI Chronic Ankle Instability, AP Anteroposterior, ML Mediolateral

Tests

The order of test administration for GI and the YBT was randomized for each participant. Participants were familiarized with the tasks prior to testing to minimize learning effects, and a 5-minute rest was provided between tests to reduce fatigue.

Gait initiation

Participants stood barefoot in a relaxed posture on a force platform (Bertec Corporation, Columbus, OH, USA), where ground reaction forces (GRFs) and COP trajectories were recorded. They maintained eyes open, with feet positioned at a 10° outward angle and heels separated by 6 cm, ensuring even distribution of body weight between both feet. To maintain consistent foot placement across trials, the outline of each foot was traced onto a sheet of paper, which was then secured to the force platform for the duration of the testing session. Participants performed two practice trials prior to data collection to become familiar with the task. Athletes with CAI were instructed to initiate forward stepping with their affected limb, as this limb reflects the pathological side and was the primary focus of the neuromechanical assessment. Healthy control participants initiated stepping with their dominant limb, which serves as the functional reference in individuals without ankle pathology and aligns with established methodological practices in CAI research. All participants initiated stepping in response to a ‘go’ stimulus [23]. Participants with CAI were instructed to initiate forward stepping with their affected limb, as this limb reflects the pathological side and was the primary focus of neuromechanical assessment. Healthy control participants initiated stepping with their dominant limb, which serves as the functional reference in individuals without ankle pathology and aligns with established methodological practices in CAI research. To allow natural movement patterns, participants were not given any instructions regarding step length or walking speed and were allowed to initiate the step at their preferred, comfortable pace, ensuring that habitual GI strategies were captured without imposing artificial constraints. Each GI task was repeated five times, with 30-second rest intervals between trials to minimize fatigue effects. The mean of the five trials was analyzed to enhance reliability [14].

Raw signals were low-pass filtered using a 6th-order Butterworth filter with zero-phase shift and a 10 Hz cut-off frequency. The APA phase of GI was defined as the interval from COP displacement onset to the most posterolateral COP position beneath the step-initiating limb. COP displacement onset was determined as the point at which the vertical GRF exceeded the mean plus 2 SD of the first 500 ms of quiet stance [9, 10] (Fig. 1).

Fig. 1.

The schematic representation of the Center of Pressure (COP) trajectory during gait initiation

Displacement and velocity of the COP in the anteroposterior (AP) and mediolateral (ML) directions were subsequently computed using the following equations, where n represents the number of data points, FsF is the sampling frequency (500 Hz), and X and Y denote COP positions in the ML and AP directions, respectively [16, 23].

Y-balance test

Athletes with CAI stood on their injured limb (stance leg), whereas healthy participants stood on their dominant limb, reaching with the contralateral limb (moving leg) in three directions: anterior, PM, and PL. The PM and PL directions were oriented perpendicular to each other (90°) and formed 135° angles with the anterior direction. Participants were instructed to reach maximally in each direction while maintaining postural stability on the stance leg. Participants performed two familiarization trials per direction before testing to minimize learning effects. Each participant performed three recorded trials per direction, with 30-second rest intervals between trials and 1-minute rest between different reach directions. Reach distance was measured from the center of the stance foot to the farthest point reached and subsequently normalized to lower limb length—measured from the anterior superior iliac spine to the medial malleolus—by dividing the reach distance by limb length and multiplying by 100, yielding a percentage of limb length. Each participant performed three trials per direction, and the mean value across trials was used for analysis. Trials were discarded and repeated if the participant failed to maintain a unilateral stance on the stance leg (e.g., the reach foot touched the floor), failed to keep the entire plantar surface of the stance foot in contact with the ground (e.g., lifting the heel), or failed to return the reach foot to the starting position in a controlled manner (e.g., loss of balance) [24–26].

Severity of perceived ankle instability

The CAIT was used to assess the severity of perceived ankle instability. The questionnaire comprises nine items, with a maximum score of 30, where higher scores indicate greater ankle stability. In the present study, participants with a perfect score of 30 were classified as healthy controls, whereas those with a CAIT score ≤ 24 were considered to have CAI. For individuals with CAI, scores ≥ 16 were classified as lower instability and scores < 16 as higher instability. The Persian version of the CAIT has demonstrated good internal consistency (Cronbach’s α = 0.78 for the right ankle and 0.79 for the left ankle) and excellent reliability (ICC (2,1) = 0.88; 95% CI: 0.86–0.90) in athletes [27].

Statistical analysis

All statistical analyses were performed using SPSS version 23 (IBM Corp., Armonk, NY, USA). Data were first screened for normality using the Shapiro–Wilk test, and all variables were confirmed to be normally distributed. Homogeneity of variances was verified using Levene’s test. Additionally, residuals from all ANOVA models were saved and tested for normality using the Shapiro–Wilk test, confirming that residuals were normally distributed for all dependent variables. Normality was also visually inspected using Q–Q plots, which showed that residuals followed a normal distribution.

Continuous demographic variables (e.g., age, height, weight, CAIT score, weekly physical activity) were compared across the three groups (healthy, lower-instability CAI, higher-instability CAI) using one-way ANOVA, while categorical variables (e.g., sex) were compared using the chi-square test.

One-way ANOVA was conducted to compare YBT scores and force plate–derived COP parameters among the three groups. When a significant main effect was observed, Bonferroni-adjusted post hoc comparisons were performed to identify pairwise differences. Effect sizes for ANOVA were reported as ηp² and interpreted as small (0.01–0.06), medium (0.06–0.14), and large (≥ 0.14).

In addition, separate ROC curve analyses were conducted to evaluate the discriminative ability of the YBT and COP parameters in (1) distinguishing individuals with CAI from healthy controls, and (2) differentiating between lower and higher instability levels within the CAI group. Sensitivity, specificity, and AUC values were calculated, and the optimal cutoff points were determined using the Youden Index. Higher sensitivity minimizes the likelihood of missing individuals who truly have the condition (reducing false negatives), whereas greater specificity decreases the probability of incorrectly classifying healthy individuals as affected (reducing false positives). An AUC value between 0.7 and 0.8 is considered acceptable, values between 0.8 and 0.9 indicate excellent discriminative ability, and those above 0.9 reflect outstanding accuracy in differentiating between groups [28]. The Youden Index [J = (sensitivity + specificity) – 1] was used to identify the cutoff value that maximized the overall diagnostic performance of each parameter [29].

All statistical tests were two-tailed, and a significance level of p < 0.05 was adopted for all analyses.

Results

Participant characteristics

The demographic characteristics of participants did not differ significantly between groups, except for the CAIT score (Table 1).

Group differences

Descriptive statistics and ANOVA results for YBT scores and force plate–derived COP parameters are presented in Table 2.

For the YBT, ANOVA revealed significant group effects for all three reach directions. In the anterior reach, F(1, 93) = 6.49, p = 0.002, ηp² = 0.126. Post-hoc Bonferroni comparisons showed that the higher-instability CAI group had significantly shorter reach distances than healthy controls (mean difference = 20.06, p = 0.002, 95% CI: 6.26–33.86, ηp² = 0.309), whereas differences between the higher- and lower-instability CAI (mean difference = 12.51, p = 0.098, 95% CI: −1.56–26.57, ηp² = 0.148) and between lower-instability CAI and healthy participants (mean difference = 7.55, p = 0.628, 95% CI: −7.02–22.13, ηp² = 0.060) were not significant.

For the PM reach, F(1, 93) = 22.74, p < 0.001, ηp² = 0.336. Healthy participants showed significantly greater reach distances than both higher-instability CAI (mean difference = 17.34, p < 0.001, 95% CI: 9.64–25.04, ηp² = 0.250) and lower-instability CAI groups (mean difference = 20.56, p < 0.001, 95% CI: 12.43–28.70, ηp² = 0.320), while no significant difference was observed between the two CAI groups (mean difference = 3.22, p = 0.958, 95% CI: −4.63–11.07, ηp² = 0.011).

For the PL reach, F(1, 93) = 56.37, p < 0.001, ηp² = 0.556. Healthy participants had significantly greater reach distances than both higher-instability CAI (mean difference = 31.92, p < 0.001, 95% CI: 24.19–39.65, ηp² = 0.531) and lower-instability CAI participants (mean difference = 26.87, p < 0.001, 95% CI: 18.71–35.04, ηp² = 0.445), while the difference between the two CAI groups was not significant (mean difference = − 5.05, p = 0.364, 95% CI: −12.93–2.83, ηp² = 0.028). These results indicate that YBT reach distances are substantially reduced in CAI participants compared with healthy controls, particularly in the PM and PL directions.

For ML COP parameters during the anticipatory phase of GI, ANOVA showed significant group effects for displacement, F(1, 93) = 11.48, p < 0.001, ηp² = 0.205, and velocity, F(1, 93) = 14.09, p < 0.001, ηp² = 0.240. Post-hoc comparisons indicated that higher-instability CAI participants had significantly lower ML COP displacement than healthy controls (mean difference = 1.02, p < 0.001, 95% CI: 0.50–1.54, ηp² = 0.206), and the lower-instability group also showed reduced displacement compared with healthy participants (mean difference = 0.56, p = 0.033, 95% CI: 0.03–1.08, ηp² = 0.073). The two CAI groups did not differ significantly (mean difference = 0.46, p = 0.090, 95% CI: −0.05–0.97, ηp² = 0.050). Similarly, ML COP velocity was significantly lower in higher-instability CAI compared with healthy participants (mean difference = − 4.46, p < 0.001, 95% CI: −6.56 to − 2.36, ηp² = 0.554) and in lower-instability CAI compared with healthy controls (mean difference = − 3.18, p = 0.001, 95% CI: −5.29 to − 1.06, ηp² = 0.387). The difference between CAI subgroups was not significant (mean difference = − 1.28, p = 0.399, 95% CI: −3.35 to 0.78, ηp² = 0.093).

For AP COP displacement and AP COP velocity, ANOVA revealed no significant group effects (p > 0.05). However, these results should be interpreted with caution due to the low statistical power (0.48 and 0.26, respectively).

Diagnostic accuracy for identifying CAI versus healthy controls

Table 3 presents the diagnostic accuracy of the YBT parameters for distinguishing athletes with CAI from healthy controls. The PL reach demonstrated the highest diagnostic performance (AUC = 0.957, p < 0.001; cutoff = 109.995%, sensitivity = 0.905, specificity = 0.900, Youden index = 0.805), followed by the PM reach (AUC = 0.857, p < 0.001; cutoff = 110.425%, sensitivity = 0.651, specificity = 0.933, Youden index = 0.584). The anterior reach showed a lower but still significant discriminative ability (AUC = 0.637, p = 0.033; cutoff = 74.455%, sensitivity = 0.984, specificity = 0.367, Youden index = 0.351). Overall, the PL direction exhibited excellent sensitivity (0.905) and specificity (0.900), with the highest Youden index (0.805), indicating superior diagnostic performance among the YBT directions.

Table 3.

Diagnostic accuracy of YBT and COP parameters in distinguishing healthy athletes from those with CAI

Parameters	AUC	P Value	95% Confidence Interval		Cutoff Score	Sensitivity	Specificity	Youden
			Lower Bound	Upper Bound
YBT (Anterior) [%]	0.637	0.033*	0.514	0.760	74.455	0.984	0.367	0.351
YBT (posteromedial) [%]	0.857	< 0.001*	0.782	0.931	110.425	0.651	0.933	0.584
YBT (posterolateral) [%]	0.957	< 0.001*	0.920	0.994	109.995	0.905	0.900	0.805
COP Displacement [ML] (cm)	0.751	< 0.001*	0.649	0.853	-1.950	0.667	0.793	0.460
COP Displacement [AP] (cm)	0.600	0.126	0.477	0.722	-1.687	0.571	0.621	0.192
COP Velocity [ML](cm/s)	0.783	< 0.001*	0.687	0.880	5.630	0.556	0.897	0.452
COP Velocity [AP](cm/s)	0.557	0.378	0.432	0.683	6.525	0.556	0.655	0.211

Bolded values and asterisks (*) represent statistically significant differences between groups (p < 0.05)

YBT  Y-Balance Test, COP Center of Pressure, CAI Chronic Ankle Instability, AP Anteroposterior, ML Mediolateral

Regarding COP parameters, significant discrimination between groups was observed for ML COP displacement (AUC = 0.751, p < 0.001; cutoff = − 1.950 cm, sensitivity = 0.667, specificity = 0.793, Youden index = 0.460) and ML COP velocity (AUC = 0.783, p < 0.001; cutoff = 5.630 cm/s, sensitivity = 0.556, specificity = 0.897, Youden index = 0.452). In contrast, AP COP displacement (AUC = 0.600, p = 0.126; cutoff = − 1.687 cm, sensitivity = 0.571, specificity = 0.621, Youden index = 0.192) and AP COP velocity (AUC = 0.557, p = 0.378; cutoff = 6.525 cm/s, sensitivity = 0.556, specificity = 0.655, Youden index = 0.211) did not reach statistical significance.

Overall, the PL reach direction and ML COP velocity exhibited the greatest discriminative ability, suggesting their potential as the most reliable indicators for identifying CAI-related balance deficits.

Diagnostic accuracy for differentiating severity of CAI

Table 4 presents the diagnostic performance of YBT and COP parameters for differentiating higher- from lower-instability CAI. None of the examined parameters reached statistical significance (all p > 0.05). The results were as follows: anterior YBT (AUC = 0.632, p = 0.073), PM-YBT (AUC = 0.567, p = 0.361), PL-YBT (AUC = 0.594, p = 0.201), ML COP displacement (AUC = 0.615, p = 0.115), AP COP displacement (AUC = 0.590, p = 0.221), ML COP velocity (AUC = 0.610, p = 0.134), and AP COP velocity (AUC = 0.565, p = 0.379). Among these parameters, the anterior YBT reach and ML COP displacement showed relatively higher, though still limited, discrimination capacity. Moreover, the sensitivity (first value) and specificity (second value) were as follows: anterior YBT (0.750, 0.629), PM-YBT (0.607, 0.600), PL-YBT (0.321, 0.943), ML COP displacement (0.516, 0.844), AP COP displacement (0.548, 0.656), ML COP velocity (0.677, 0.531), and AP COP velocity (0.581, 0.594). Overall, these findings indicate that while YBT and COP measures can distinguish CAI from healthy individuals, their ability to differentiate varying severities of instability within the CAI group remains limited.

Table 4.

Diagnostic accuracy of YBT and COP parameters in differentiating higher-instability CAI from lower-instability CAI

Parameters	AUC	P Value	95% Confidence Interval		Cutoff Score	Sensitivity	Specificity	Youden
			Lower Bound	Upper Bound
YBT (Anterior) [%]	0.632	0.073	0.494	0.771	101.640	0.750	0.629	0.379
YBT (posteromedial) [%]	0.567	0.361	0.425	0.710	103.520	0.607	0.600	0.207
YBT (posterolateral) [%]	0.594	0.201	0.449	0.739	105.300	0.321	0.943	0.264
COP Displacement [ML] (cm)	0.615	0.115	0.473	0.757	-1.965	0.516	0.844	0.360
COP Displacement [AP] (cm)	0.590	0.221	0.448	0.732	-1.660	0.548	0.656	0.205
COP Velocity [ML](cm/s)	0.610	0.134	0.470	0.749	5.090	0.677	0.531	0.209
COP Velocity [AP](cm/s)	0.565	0.379	0.421	0.708	5.695	0.581	0.594	0.174

Bolded values and asterisks (*) represent statistically significant differences between groups (p < 0.05)

YBT Y-Balance Test, COP Center of Pressure, CAI Chronic Ankle Instability, AP Anteroposterior, ML Mediolateral

Discussion

This study provides new insights into how CAI impacts dynamic balance and anticipatory postural control during GI. Athletes with CAI, regardless of severity, exhibit challenges in both maintaining dynamic balance and generating effective anticipatory adjustments. Moreover, the PL reach and ML COP velocity emerged as the most sensitive indicators of these deficits. They provide complementary perspectives on impaired dynamic stability. The PL reach reflects a field-based, practical measure, while ML COP velocity provides precise, laboratory-based quantification. However, neither measure reliably distinguished between higher- and lower-instability CAI, indicating that while they effectively differentiate CAI from healthy individuals, they may not capture more subtle variations in severity. This highlights the need for more refined, task-specific assessments to better classify CAI subgroups.

YBT performance in CAI

Consistent with prior research, athletes with CAI exhibited substantial impairments in dynamic balance, as shown by reduced YBT reach distances across all directions [30–32]. Individuals with CAI typically present with a combination of mechanical and functional (sensorimotor) impairments [33]. Mechanical deficits include arthrokinematic restrictions, limited dorsiflexion range of motion (DFROM) in both open- and closed-chain conditions, and reduced joint stiffness [34, 35]. Functional impairments include deficits in proprioception [36], delayed peroneal reaction time [37], altered motoneuron pool excitability [38], reduced muscle strength [39], impaired neuromuscular coordination [40, 41], and diminished postural control [40]. Together, these limitations constrain dynamic postural control and contribute to shorter YBT reach distances. Importantly, prior evidence indicates that the relative contribution of these impairments differs across YBT directions [5, 6].

For instance, Terada et al. demonstrated that dorsiflexion mobility (assessed via the weight-bearing lunge test for closed-chain and a bubble inclinometer for open-chain) and self-perceived ankle stiffness significantly predicted anterior reach, whereas these variables had little influence on PM or PL performance [6]. Similarly, Gabriner et al. reported that anterior reach was primarily associated with dorsiflexion mobility and plantar cutaneous sensation (measured via algometry), while PM and PL directions were more strongly predicted by ankle muscle strength (quantified via dynamometer) and static postural control measures (time-to-boundary, TTB) [5]. Collectively, these findings indicate that anterior YBT performance is more sensitive to mechanical and sensory constraints at the ankle, whereas posterior reaches depend more heavily on strength, neuromuscular coordination, and postural control mechanisms.

In line with these observations, our study showed that only the higher-instability CAI subgroup exhibited significantly shorter anterior reach than healthy controls, whereas both CAI subgroups demonstrated pronounced deficits in the PM and PL directions, with no significant differences between them. This pattern suggests that dynamic balance impairments are most evident in the posterior directions, likely reflecting the higher neuromuscular coordination demands of multiplanar stability, while anterior reach is affected primarily in individuals with more severe mechanical limitations.

In line with this interpretation, a recent scoping review highlighted that clinicians and researchers commonly assess dynamic postural control in individuals with ankle instability using the maximum reach distance in the PM direction of the YBT [42]. Experimental studies further demonstrate that disrupting somatosensory input in healthy individuals reduces PM-YBT performance by approximately 4.6%, while more accurate torque control during eccentric dorsiflexion predicts better outcomes [43, 44]. Beyond ankle-specific factors, deficits in PM reach are also linked to reduced hip-abduction and external rotation strength, with individuals with CAI exhibiting greater trunk and pelvis rotation during reaching tasks [45, 46]. Collectively, these findings indicate that PM-YBT performance reflects not only ankle sensorimotor control but also multi-joint coordination, corroborating our observation of pronounced deficits in CAI participants.

No significant differences were observed between higher- and lower-instability CAI groups, although both were impaired compared with healthy participants. This suggests that dynamic balance deficits in CAI do not necessarily scale with self-reported instability, possibly because individuals with lower-instability CAI adopt compensatory strategies—such as increased proximal muscle activation or greater reliance on visual and vestibular inputs—that partially mitigate deficits.

COP-based postural control alterations

Athletes with CAI, especially those with greater instability, exhibit impaired anticipatory postural adjustments prior to GI compared with healthy controls. These findings are in line with previous studies [17–20]. These alterations indicate impaired feedforward postural control. Normally, the CNS relies on proprioceptive input to construct internal models of body dynamics, which allow the brain to predict movement outcomes and generate APAs that stabilize the body before movement onset [47, 48]. In CAI, recurrent ligament injuries and partial deafferentation lead to proprioceptive deficits that disrupt these internal models, resulting in less accurate anticipatory control and delayed activation of stabilizing muscles [49, 50]. Consequently, individuals with CAI may adopt more cautious and less efficient preparatory strategies, manifested as smaller and slower ML COP shifts. These findings indicate impaired predictive control rather than merely mechanical limitations. Supporting this interpretation, recurrent sprains and ligament damage in CAI may induce partial deafferentation and proprioceptive loss, which have been linked to maladaptive cortical reorganization and impaired sensorimotor processing in previous studies [17, 51, 52].

Diagnostic accuracy of YBT and COP parameters

ROC analysis demonstrated that among the YBT directions, the PL reach exhibited excellent diagnostic accuracy (AUC = 0.957), characterized by both high sensitivity and specificity. The PM direction also showed good discriminatory ability (AUC = 0.857), whereas the anterior reach was only moderately effective (AUC = 0.637). These findings emphasize that posterior reach directions, which require greater control of multiplanar and proximal stability, are particularly sensitive and specific for identifying postural control deficits in individuals with CAI. In our study, the optimal cut-off scores were 110.0% of leg length for the PL reach, 110.4% for the PM reach, and 74.5% for the anterior reach, providing practical thresholds for clinical assessment.

Despite the widespread clinical use of the YBT, research on its discriminative validity in populations with CAI is limited. Schwiertz et al. [8] reported AUC values of 0.74–0.81 for discriminating trained versus untrained youth athletes. Compared with these findings, our study demonstrated higher accuracy—particularly for the PL reach—further supporting its use as the most reliable direction for clinical screening, while anterior and PM reaches can provide supplementary information.

Regarding laboratory-based COP measures, ML velocity and displacement demonstrated the highest diagnostic accuracy (AUC = 0.751 and 0.783, respectively), with cut-off values of − 1.95 cm for ML displacement and 5.63 cm/s for ML velocity. Anterior–posterior parameters were less effective (AUC < 0.60), suggesting that frontal-plane instability may be an important feature of CAI. Force plate–based metrics may be useful for detecting subtle neuromechanical alterations that are not consistently captured by standard functional tests.

From a clinical perspective, the YBT—particularly the PL direction—may serve as a rapid, accessible, and time-efficient screening tool, with diagnostic accuracy comparable to force plate–based assessments. Clinicians can use the cut-off scores to identify individuals with impaired dynamic balance, guide targeted rehabilitation, and monitor improvements over time. While COP analysis provides additional mechanistic insight, the YBT remains superior for clinical decision-making and field-based screening, making the combination of both approaches ideal for comprehensive evaluation of CAI-related postural control deficits.

Differentiating CAI severity

Although clear differences were observed between athletes with CAI and healthy controls, neither the YBT nor COP measures differentiated between individuals with higher and lower levels of instability. This indicates that these measures are suitable for distinguishing pathological from non-pathological conditions, but limited for grading severity within the CAI population. Participants were classified using CAIT scores; however, perceived instability may not directly reflect objective postural control performance. Individuals reporting greater instability may compensate through increased activation of proximal stabilizers, such as the hip and trunk, thereby preserving test performance. Consequently, conventional clinical and laboratory assessments may detect major deficits but appear less sensitive to subtle severity-related differences.

Limitations and future directions

This study examined the diagnostic accuracy of YBT and COP parameters, which limits inferences regarding the underlying mechanisms of postural impairments. The lack of kinematic and electromyography (EMG) data constrains insight into compensatory strategies and neuromuscular contributions to balance deficits. Future research should incorporate neural and muscular assessments to elucidate the mechanisms of anticipatory postural control in CAI and determine whether targeted interventions can improve ML COP dynamics and YBT performance, thereby enhancing both diagnostic precision and rehabilitative outcomes. Additionally, the cross-sectional design precludes causal interpretations of the observed deficits.

Conclusion and clinical implications

The PL reach of the YBT and ML COP velocity are the most informative indicators for identifying athletes with CAI. Clinically, the PL YBT provides an efficient, accessible tool for screening and monitoring postural deficits, while COP measures can complement it by offering additional insights into neuromechanical control. For routine sports rehabilitation, the YBT—particularly in the PL direction—remains the preferred method due to its practicality and diagnostic accuracy. Integrating laboratory-based assessments may further refine evaluation, but is not necessary for standard screening. However, distinguishing instability severity likely requires complementary neurophysiological or kinematic assessments to guide individualized interventions.

Abbreviations

ANOVA

Analysis of Variance

AP

Anteroposterior

APAs

Anticipatory Postural Adjustments

AUC

Area Under the Curve

CAI

Chronic Ankle Instability

CAIT

Cumberland Ankle Instability Tool

CNS

Central Nervous System

COM

Center of Mass

COP

Center–of–Pressure

EMG

Electromyography

GI

Gait Initiation

GRFs

Ground Reaction Forces

ML

Mediolateral

PM

Posteromedial

PL

Posterolateral

ROC

Receiver Operating Characteristic

YBT

Y–Balance Test

Authors’ contributions

R.KH. contributed to conceptualization, resources, methodology, supervision, and writing—review and editing. S.S.S., R.F., and Z.B. contributed to data curation, formal analysis, and writing—original draft preparation. All authors have read and approved the final version of the manuscript.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Data availability

The data that support the findings of this study are available on request from the corresponding author.

Declarations

Ethics approval and consent to participate

All procedures were conducted in accordance with the principles of the Helsinki Declaration. Ethical approval was obtained from the Ethics Committee of Tehran University of Medical Sciences (IR.TUMS.FNM.REC.1401.146). Written informed consent was obtained from all participants prior to participation in the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.