Title: Inter-limb Functional Asymmetry and the Development of Pain in Collegiate Volleyball Players: A Preliminary Study
Dr.
RyotaImaiPh.D
1✉Phone+81-745-54-1601Emailryo7891@gmail.comAyaKusumoto2Emailaya.kusumoto.13@gmail.com
TakanariKubo3Emailt@med.mie-u.ac.jp
DaisukeBaiPh.D.
4Emailh1371019@kio.ac.jp 1A
Graduate School of RehabilitationOsaka Kawasaki Rehabilitation University158 Mizuma597-0104Kaizuka CityOsakaJapan 2Department of RehabilitationSaiseikai Nara Hospital643 4chome, 630-8145Hachijo, Nara city, NaraJapan
3Clinical Research Support CenterMie University HospitalTsuJapan
4Meiji Yasuda Health Development Foundation12F 3-22-7151- 0053Yoyogi, TokyoShibuyaJapan
Ryota Imai1* Ph.D, Aya Kusumoto2, Takanari Kubo3, Daisuke Bai4, Ph.D.
1 Graduate School of Rehabilitation, Osaka Kawasaki Rehabilitation University, 158 Mizuma, Kaizuka City, Osaka 597 − 0104, Japan. Email: ryo7891@gmail.com
2 Department of Rehabilitation, Saiseikai Nara Hospital, 643 4chome, Hachijo, Nara city, Nara 630–8145, Japan. Email: aya.kusumoto.13@gmail.com
3 Clinical Research Support Center, Mie University Hospital, Tsu, Japan. Email: kubo-t@med.mie-u.ac.jp
4 Meiji Yasuda Health Development Foundation, 12F 3-22-7, Yoyogi, Shibuya, Tokyo 151 − 0053 Japan. Email: h1371019@kio.ac.jp
*Corresponding author:
Dr. Ryota Imai,
Graduate School of Rehabilitation, Osaka Kawasaki Rehabilitation University, 158 Mizuma, Kaizuka City, Osaka 597 − 0104, Japan.
Tel.: +81-745-54-1601; Fax: +81-72-446-6767. Email: ryo7891@gmail.com
Keywords:
asymmetry
athletes
pain
volleyball
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Abstract
Purpose
Functional asymmetry is a potential risk factor for musculoskeletal pain in athletes, but how it affects volleyball players has not been widely investigated. We therefore aimed to explore the association between inter-limb asymmetry in performance-based tests and the development of lower extremity pain in collegiate volleyball players over a competitive season.
Methods
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In this preliminary study, 26 male collegiate volleyball players (mean age = 20.6 ± 1.0 years) underwent preseason testing, including the star excursion balance test (anterior direction), side hop test, and single-leg hop for distance. Pain status was tracked on a biweekly basis using an online survey with an 11-point numerical rating scale. Based on reporting frequency, some athletes were classified into the pain group (pain reported on ≥ 80% of surveys) or into the no-pain group (pain reported on 0%). Inter-limb asymmetry was calculated as |1 − (left/right)| for each performance test.
Results
There was significantly greater asymmetry in the star excursion balance test in the pain group (n = 9; 0.29 ± 0.25) than in the no-pain group (n = 10; 0.04 ± 0.03, p = 0.040). Overall trends towards higher asymmetry were seen in the pain group, but no significant differences between the groups were observed in the side hop test asymmetry index (pain group: 0.13 ± 0.12, no-pain group: 0.06 ± 0.04, p = 0.203) or single-leg hop asymmetry index (pain group: 0.16 ± 0.17, no-pain group: 0.04 ± 0.05, p = 0.117).
Conclusion
Greater inter-limb asymmetry in dynamic balance was associated with frequent pain in a group of volleyball athletes. Our preliminary findings suggest that functional asymmetry screening, especially in dynamic balance, might be useful for early detection of athletes who may be at risk of developing non-traumatic pain.
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Introduction
Injury prevention has long been considered important in the preservation of athlete health and the enhancement of performance sustainability. In high-impact sports such as volleyball, this principle holds particular relevance: athletes are repeatedly exposed to mechanical stressors including vertical jumping, abrupt landings, lateral cutting maneuvers, and overhead arm actions. Together, these are thought to impose cumulative loading on both the lower and upper extremities over time. Recent longitudinal epidemiological investigations targeting male collegiate volleyball players revealed the substantial burden of musculoskeletal injuries in this population. Notably, a five-year cohort study of Japanese university athletes identified lower-limb injuries (particularly those of overuse origin) as the predominant concern, with patellar tendinopathy and ankle sprains accounting for the majority of reported cases, and approximately 40% of all lower extremity complaints were attributed to the knees alone [1].
Elsewhere, in a prospective study of elite male volleyball players, 75% of participants reported symptoms consistent with ‘jumper’s knee’ across a competitive season, and the incidence of clinically significant patellar tendinopathy reached 30% of athletes per season. These figures underscore the pervasive and recurrent nature of knee pathology in these athletes [2].
Similarly, in a recent multicenter survey of university and professional volleyball players spanning multiple countries, 93% of male athletes experienced pain during at least one phase of the season [3]. Among the symptomatic regions, the knees were most commonly affected (31%), followed by the lumbar spine (21%) and the shoulders (19%), suggesting a broader, systemic vulnerability that often does not meet classical injury thresholds yet has profound implications for training capacity, biomechanics, and injury susceptibility.
The etiology of these injuries is thought to be multifactorial and context-specific, but there has been recent attention toward the role of modifiable intrinsic risk factors. Functional asymmetries in neuromuscular control, unilateral strength, joint stability, and sensorimotor integration are notable among them, and they may collectively contribute to altered movement patterns, imbalanced loading, and micro-traumatic accumulation over time [4, 5]. These asymmetries may remain subclinical in many cases, but accumulating evidence suggests that inter-limb asymmetries may reflect latent neuromuscular dysfunction. This could potentially lead to unequal force absorption and compromised frontal plane stability during athletic movements, thereby predisposing athletes to microtrauma, compensatory mechanics, pain, and ultimately, musculoskeletal injuries [6]. The predictive value of asymmetry indices has been widely explored, but results remain inconsistent due to methodological heterogeneity, so rigorous longitudinal analyses in well-defined athletic populations are required.
To operationalize the identification of such asymmetries in field settings, several functional performance tests have gained widespread application in sports science and rehabilitation contexts. The star excursion balance test (SEBT) has been extensively validated for its ability to capture dynamic postural control across multiple planes. Prospective studies have indicated that anterior reach asymmetries > 4 cm or limb symmetry indices < 90% are predictive of increased risk for ankle sprains, anterior cruciate ligament injuries, and generalized lower extremity dysfunction [7, 8]. Meanwhile, unilateral hop tests, such as the single-leg hop for distance and the 10-repetition side hop, have been established as robust indicators of explosive power, neuromuscular coordination, and load-absorption capacity. In clinical and return-to-sport protocols, limb symmetry index thresholds < 85–90% are frequently cited as markers of incomplete functional restoration or elevated risk for re-injury [9, 10].
Despite their widespread use, it remains unclear whether baseline inter-limb asymmetries, as quantified by these field-based assessments, are prospectively associated with the development of musculoskeletal pain across an entire competitive season in male volleyball athletes.
This gap is thought to be especially notable given that pain (though distinct from structural injury) can itself lead to altered kinematics, reduced proprioceptive acuity, and a cascade of compensatory strategies that predispose the athlete to more serious pathology.
This study therefore examines whether pre-season asymmetries in performance-based functional tests—specifically the SEBT (anterior direction), single-leg hop for distance, and side hop test—are associated with the development of self-reported pain in male collegiate volleyball players throughout a full competitive season. By dividing athletes into pain and no-pain groups based on repeated biweekly questionnaire responses and comparing their baseline asymmetry indices, we aim to assess the prognostic value of inter-limb performance asymmetry as a low-cost, non-invasive screening tool for identifying athletes at elevated risk of pain-related musculoskeletal compromise.
Methods
Participants
This preliminary study enrolled 27 male collegiate volleyball players (mean age = 20.6 ± 1.0 years) from a University of Commerce volleyball team.
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All athletes were actively participating in competitive team activities, and prior to enrollment, they all provided written informed consent.
Sample Size Estimation
This study was conducted as a preliminary investigation to explore potential associations between inter-limb functional asymmetry and pain development in collegiate volleyball players. Due to logistical constraints, no formal a priori sample size calculation was performed. Instead, the sample size was guided by recommendations for preliminary studies in the literature, which suggest 12–15 participants per group as an acceptable minimum to estimate variability and to inform future research [11]. The current study slightly fell below these benchmarks, but these findings are still thought to provide meaningful trends to inform larger-scale investigations.
Study Design
Baseline performance assessments were conducted during the preseason. Thereafter, participants completed pain surveillance questionnaires on a biweekly basis throughout the volleyball season. Based on the frequency of reported pain, some participants were divided into groups for comparative analysis: a pain group (reported pain in ≥ 80% of the questionnaires), and a no-pain group (no pain across all survey points). Participants with inconsistent or incomplete survey responses were excluded from the final analysis.
Pain assessment
Pain status was assessed every two weeks using a web-based questionnaire administered via Google Forms. Athletes reported the presence or absence of pain, the anatomical site of the pain, and the severity of pain using an 11-point numerical rating scale, ranging from 0 (no pain) to 10 (worst imaginable pain).
Performance assessment
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All performance evaluations were conducted on a non-slip gymnasium floor under the supervision of trained examiners. A standardized warm-up consisting of dynamic mobility drills and submaximal jumping was completed prior to testing. The order of test execution was randomized across participants. Sufficient rest periods were provided between tests to minimize fatigue. Each test was performed bilaterally, and both raw values and inter-limb asymmetry were calculated. To quantify inter-limb asymmetry, we calculated the absolute asymmetry index (AI) for each test using the following formula:
Asymmetry Index = |1 − (Left / Right)|,
as commonly used in sports performance literature [5]. Higher values indicate greater asymmetry between limbs. Considerable variation in asymmetry calculation methods across studies has been reported, but ratio-based indices are thought to remain among the most interpretable and clinically applicable formats [6].
Star excursion balance test
The SEBT was administered in the anterior direction only, which has been shown to strongly correlate with composite reach scores and dynamic balance capabilities [12]. Participants performed three trials per leg while maintaining single-leg stance and reaching forward with the contralateral leg. Reach distance was measured in centimeters and normalized by leg length, defined as the distance from the anterior superior iliac spine to the distal tip of the medial malleolus [13]. The mean of three valid attempts per limb was calculated, and the inter-limb asymmetry was then computed using the above AI formula.
Side hop test
To assess lateral agility and neuromuscular control, participants performed a 10-repetition side hop test. A 30-cm-wide line was marked with tape on the floor. Participants completed 10 consecutive lateral hops over the line on one leg, as quickly as possible. Two valid trials per leg were recorded, with the fastest time retained for analysis. Trials were invalidated if the participant lost balance or failed to cross the line [14, 15]. Asymmetry in side hop performance was calculated using the absolute difference in hop time between limbs.
Single-leg hop for distance
Unilateral lower-limb power was assessed using the single-leg hop for distance test [16]. Athletes performed a maximal forward hop on one leg, attempting to land and stabilize on the same leg. Distance was measured from the toe at takeoff to the heel at landing. Trials were repeated if the contralateral foot touched the ground or if postural control was lost. Two valid trials per leg were recorded, with the greater distance used for analysis. Asymmetry was calculated using the absolute ratio-based index as described.
Data Analysis
All performance data were processed to compute raw scores for both limbs and inter-limb asymmetry indices. SEBT scores were normalized to leg length and expressed as percentages (%LL). Data distributions were tested for normality using the Shapiro–Wilk test. Independent t-tests were applied to normally distributed variables, while Mann–Whitney U tests were used for non-parametric data. Categorical variables were compared using chi-squared tests. Statistical significance was set at p < 0.05. All statistical analyses were performed using IBM SPSS Statistics version 26.0 (IBM Corp., Armonk, NY, USA).
Results
Of the 27 enrolled volleyball players, one athlete was excluded from the study due to injury at baseline, resulting in a total of 26 participants being included in the final analysis. Nine athletes were allocated to the pain group and 10 to the no-pain group, based on biweekly pain surveillance conducted throughout the competitive season. Seven participants were excluded due to incomplete or ambiguous reporting. There were no significant differences between the groups in age, height, weight, or passive joint range of motion (all p-values > 0.05) (Table 1).
Table 1
Group Comparisons of Demographic Characteristics, Pain Location, and Lower Extremity Performance Measures Between Athletes With and Without Pain
Outcome | All N = 19 | Pain Group N = 9 | No Pain Group N = 10 | p value |
|---|
Age (years) | 20.6 ± 1.0 | 20.8 ± 1.0 | 20.4 ± 1.1 | 0.31 |
BMI (kg/m2) | 21.1 ± 1.8 | 22.6 ± 1.7 | 20.1 ± 0.9 | 0.14 |
Pain intensity | 4.1 ± 1.9 | 4.1 ± 1.9 | 0 | < 0.05 |
Pain location % | | | | |
Low back | 5 (26.3) | 5 (55.6) | 0 | |
knee | 6 (31.6) | 6 (66.7) | 0 | |
ankle | 1 (5.2) | 1 (11.1) | 0 | |
Single Hop (Right) (cm) | 182.3 ± 21.8 | 181.9 ± 25.1 | 182.6 ± 19.8 | 0.24 |
Single Hop (Left) (cm) | 194.0 ± 53.9 | 180.8 ± 20.4 | 207.2 ± 73.5 | 0.46 |
Normalized Single Hop (Right) | 0.95 ± 0.08 | 1.01 ± 0.17 | 0.94 ± 0.09 | 0.24 |
Normalized Single Hop (Left) | 0.98 ± 0.11 | 0.99 ± 0.13 | 0.96 ± 0.06 | 0.45 |
Side Hop (Right) (sec) | 7.3 ± 1.2 | 7.0 ± 1.6 | 7.6 ± 0.6 | 0.44 |
Side Hop (Left) (sec) | 7.8 ± 0.8 | 7.9 ± 1.0 | 7.8 ± 0.6 | 0.14 |
Normalized SEBT (Right) | 0.6 ± 0.1 | 0.7 ± 0.2 | 0.6 ± 0.1 | 0.84 |
Normalized SEBT (Left) | 0.6 ± 0.1 | 0.6 ± 0.1 | 0.6 ± 0.1 | 0.39 |
Hip Flexion (Right) | 123.1 ± 10.9 | 121.8 ± 14.6 | 124.3 ± 6.2 | 0.67 |
Hip Flexion (Left) | 123.8 ± 8.5 | 122.50 ± 10.7 | 125.0 ± 6.0 | 0.57 |
Hip Extension (Right) | 18.8 ± 7.4 | 21.2 ± 9.2 | 16.2 ± 4.4 | 0.19 |
Hip Extension (Left) | 21.6 ± 8.9 | 26.2 ± 9.5 | 16.8 ± 5.3 | 0.08 |
Hip External Rotation (Right) | 45.6 ± 10.1 | 43.1 ± 11.0 | 48.1 ± 9.1 | 0.35 |
Hip External Rotation (Left) | 45.0 ± 9.8 | 43.1 ± 11.63 | 46.8 ± 8.0 | 0.47 |
Hip Internal Rotation (Right) | 38.8 ± 9.6 | 33.1 ± 3.72 | 44.3 ± 10.5 | 0.07 |
Hip Internal Rotation (Left) | 38.4 ± 7.9 | 36.2 ± 3.54 | 40.6 ± 10.5 | 0.29 |
Ankle Dorsiflexion (Right) | 16.9 ± 8.3 | 18.8 ± 9.9 | 15.0 ± 6.6 | 0.39 |
Ankle Dorsiflexion (Left) | 15.6 ± 7.7 | 16.8 ± 10.0 | 14.3 ± 5.0 | 0.54 |
Ankle Plantarflexion (Right) | 44.1 ± 7.6 | 41.9 ± 9.6 | 46.2 ± 4.4 | 0.27 |
Ankle Plantarflexion (Left) | 43.8 ± 8.5 | 42.5 ± 7.1 | 45.0 ± 10.0 | 0.57 |
Ankle Inversion (Right) | 22.5 ± 7.8 | 24.9 ± 7.5 | 20.1 ± 5.3 | 0.17 |
Ankle Inversion (Left) | 23.1 ± 10.1 | 25.6 ± 7.8 | 20.6 ± 12.1 | 0.34 |
Ankle Eversion (Right) | 11.9 ± 5.7 | 13.1 ± 4.6 | 10.6 ± 6.8 | 0.4 |
Ankle Eversion (Left) | 13.4 ± 6.5 | 15.6 ± 6.2 | 11.2 ± 6.4 | 0.19 |
| Average ± SD, SEBT: the Star Excursion Balance Test |
Functional performance and asymmetry
Analysis revealed marked inter-limb asymmetry in the pain group across functional tasks. In SEBT, the AI, calculated as |1 − [Left/Right]|, was significantly greater in the pain group (0.29 ± 0.25) than in the no-pain group (0.04 ± 0.03; p = 0.04), despite these groups demonstrating comparable normalized reach distances. These results suggest that dynamic postural control may be less consistent between limbs among athletes who developed pain (Table 2).
In the side hop test, the pain group exhibited greater asymmetry (AI: 0.13 ± 0.12 vs. 0.06 ± 0.04), but this difference was without statistical significance (p = 0.203). Hop duration appeared to be longer in the pain group, but timing data were not consistently recorded and thus are not reported.
In the single-leg hop for distance, the pain group again demonstrated numerically greater asymmetry (AI: 0.16 ± 0.17) than the no-pain group (0.04 ± 0.05), with a moderate effect size (Cohen’s d = 0.75), although this difference did not reach statistical significance (p = 0.12).
Collectively, we suggest these findings indicate that athletes who developed pain during the season exhibited significantly greater inter-limb asymmetry in dynamic balance (SEBT). There were non-significant but moderate-to-large trends toward greater asymmetry in lateral agility and unilateral power.
Table 2
Comparison of Inter-Limb Asymmetry Indices in Dynamic Balance, Lateral Agility, and Unilateral Power Between Pain and No-Pain Groups
Outcome | Pain Group | No Pain Group | p value |
|---|
SEBT asymmetric ratio | 0.29 ± 0.25 | 0.04 ± 0.03 | 0.04 |
Side hop test asymmetric ratio | 0.13 ± 0.12 | 0.06 ± 0.04 | 0.20 |
Single hop test asymmetric ratio | 0.16 ± 0.17 | 0.04 ± 0.05 | 0.12 |
SEBT: the Star Excursion Balance Test | | | |
Discussion
Athletes who reported persistent pain demonstrated significantly greater asymmetry in dynamic balance, as measured by the SEBT. Although asymmetries in lateral agility (side hop test) and unilateral power (single-leg hop for distance) did not reach statistical significance, the observed effect sizes nonetheless suggest potentially meaningful trends.
These results partially align with those in existing literature. A previous systematic review of 28 prospective studies concluded that inter-limb asymmetry is a potential risk factor for sports injuries [6], though findings vary significantly due to heterogeneity in test protocols, injury definitions, and asymmetry calculations. This methodological variability may also explain the non-significant differences observed in the present hop-based assessments.
In particular, while not statistically significant, the side hop test revealed a trend toward performance decrement and asymmetry in the pain group. This finding is consistent with published research into soccer and basketball populations, where lateral hop asymmetry has been linked to compromised knee valgus control and altered neuromuscular mechanics [15]. Furthermore, although a reported study focused on single-leg hop for distance [17], they emphasized that distance alone may not detect underlying functional deficits—particularly in knee power absorption and landing control—which are crucial during high-load, multidirectional movements such as the side hop.
The underlying mechanisms linking asymmetry to pain development are thought to be multifactorial. Asymmetrical loading may impair frontal-plane stability and lead to uneven force distribution, neuromuscular compensation, and cumulative microtrauma [6]. These mechanisms are thought to be particularly salient in volleyball, a sport characterized by frequent single-leg landings, rapid lateral shifts, and high plyometric demand. Asymmetries in dynamic balance may reflect proprioceptive deficits, whereas disparities in hop performance may signify impairments in joint power, rate of force development, or neuromuscular coordination.
In addition, functional asymmetries may impair movement efficiency, requiring increased neuromuscular effort and metabolic cost to maintain performance. Over time, such compensation may manifest as chronic overuse symptoms, particularly in commonly affected areas such as the patellar tendon, ankle ligaments, and lumbar spine.
The findings from this study underscore the value of preseason asymmetry screening. Tests like SEBT and side hop are low-cost, field-based tools capable of detecting latent functional imbalances. Standardized asymmetry indices (e.g., |1 − [Left/Right]|) provide an objective means of monitoring athletes longitudinally. However, the lack of consensus on asymmetry definitions (absolute, percentage, ratio-based) remains a barrier to cross-study comparison [6]. Establishing standardized metrics and reporting reliability indices (e.g., ICC, coefficient of variation) will be critical for advancing the clinical utility of asymmetry-based screening.
Limitations and future directions
This study has several limitations. The sample size was modest and restricted to a single collegiate volleyball team, limiting generalizability. Pain was assessed via self-report rather than clinical diagnosis, which may have introduced potential bias. Also, failure to account for limb dominance or standardized injury classification may further complicate interpretation [6].
Despite these limitations, the observed asymmetry trends and the use of validated performance assessments provide preliminary evidence that may inform future prospective studies. To clarify the predictive validity of these indices in volleyball and other sports contexts, larger multicenter investigations that incorporate biomechanical and clinical markers, alongside asymmetry profiling, are warranted.
Conclusion
Greater inter-limb asymmetry in dynamic balance was found to be associated with frequent pain in a group of volleyball athletes. Functional asymmetry screening, especially in dynamic balance, might be useful for early detection of athletes at risk.
Acknowledgements:
We would like to acknowledge Ben for his assistance with English proofreading and editing.
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Author Contribution
R.I. conceived and designed the study, performed data collection and analysis, and wrote the main manuscript text. A.K. contributed to data collection. T.K. contributed to data collection and analysis. D.B. contributed to methodology, provided critical advice, and contributed to the writing and revision of the manuscript. All authors reviewed and approved the final manuscript.
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