In sports science, bilateral asymmetry in lower-limb function is recognized as a risk factor for sports injuries (1). This asymmetry is prevalent among athletes due to sport-specific movement patterns (e.g., kicking in football or shooting in basketball) and unilateral dominance training. Cheng et al. (2) indicated that functional limb asymmetry not only increases injury risk but can also be exacerbated by injuries themselves, creating a positive feedback loop that amplifies bilateral differences. Nie et al. (3) noted that muscular imbalances in one part of the human movement system inevitably lead to compensatory mechanisms in other areas to regulate the body’s center of gravity and maintain postural stability. Similarly, Petch et al. (4) suggested that both under- and over-activation of muscles can impair motor control. Thus, such muscle strength imbalances are both a manifestation of abnormal movement patterns and a factor that exacerbates them, leading to increased limb asymmetry. Numerous studies have investigated the use of bilateral lower-limb asymmetry to assess athletic performance. For example, Madruga-Parera et al. (5) identified a correlation between inter-limb differences and declines in performance among adolescent tennis players. Similarly, Bishop et al. (6) found that vertical and horizontal asymmetry in female soccer players was strongly negatively correlated with performance in jumping and sprinting. Furthermore, researches have reported the use of bilateral asymmetries across various functional tests as predictive risk factors for non-contact injuries: Read et al. (7) identified asymmetry in the peak vertical ground reaction force (pVGRF) during single-leg countermovement jumps (SLCMJ) as the most significant risk factor for lower-limb injuries in adolescent footballers. Smith et al. (8) reported that anterior (ANT) reach asymmetry in the Y-balance test was significantly associated with non-contact injuries, while Mokha et al. (9) found that asymmetric or low FMS scores were effective predictors of musculoskeletal injuries. However, findings across the literature are mixed, as the observed associations vary depending on the population, injury type, and asymmetry metric used. For example, Fort-Vanmeerhaeghe et al. (10) found that uninjured young team athletes exhibited lower asymmetry in SLCMJ height compared to their injured counterparts. In contrast, Brumitt et al. (11) found that jumping asymmetry was unrelated to injury.

In specialized professions such as emergency rescue, mine rescue personnel frequently operate in environments characterized by high physical exertion, elevated risks, and significant psychological pressure (12). During training and rescue operations, they perform demanding tasks such as load-bearing ascents, heavy lifting, and long-distance casualty transport. Their physical capabilities are critical determinants of both operational success and personal safety. However, Buoncristiani et al. (13) noted that over time, the combined effects of complex hazardous environments and unilateral movement patterns substantially increase the likelihood of asymmetry in this population. Supporting this, Miratsky et al. (14) observed in a study of firefighters that repetitive unilateral loading movements during training—such as carrying equipment and operating ladders—promote limb dominance and asymmetry. Furthermore, Vaulerin et al. (15) found, through Y-balance tests on 39 firefighters, that bilateral lower-limb asymmetry had high diagnostic value for injury prediction. Consequently, detailed research on limb asymmetry among mine rescue personnel is warranted. Wu et al. (16) observed that low back injuries accounted for the highest proportion (37.2%) of occupational injuries among mine rescue personnel, with most being non-contact injuries (e.g., strains and sprains). However, whether the occurrence of these prevalent injuries is associated with bilateral asymmetry in lower-limb explosive strength among rescue personnel remains to be investigated.

In study on identifying limb asymmetry in young female soccer players, Bishop et al. (6) suggested that vertical jumps might be more sensitive than horizontal ones for detecting asymmetry, and that unilateral jump tests are easy to administer and ecologically valid. Guan et al. (17) noted that unilateral jumping, which relies on power generated from one side, provides a more direct measurement of bilateral differences compared to bilateral jumping. Similarly, Benjanuvatra et al. (18) and Heil et al. (19) argued that unilateral support and propulsion are more prevalent in activities like sprinting, jumping, and directional changes—movements closely associated with rescue and training tasks for mine rescuers. Additionally, landing mechanisms in unilateral jumping are often linked to non-contact injury mechanisms such as anterior cruciate ligament (ACL) tears (20, 21).

Based on these findings, this study selected the SLCMJ test and employed a prospective cohort study design to determine the risk relationship between bilateral asymmetry in lower extremity explosive strength and non-contact low back injuries in mine rescuers over a 12-month follow-up period. The study hypothesized that greater interlimb asymmetry in takeoff peak force during the SLCMJ would be significantly associated with an increased risk of sustaining a non-contact low back injury in this occupational group.

During the 12-month follow-up period, 15 out of the 87 team members (17.2%) sustained non-contact low back injuries. Among these 15 injuries, the majority were primarily Chronic Lumbar Muscle Strain (n = 11), with Acute Lumbar Muscle Strain (n = 4). The median time lost from training or duty due to injury was 7 days (interquartile range: 5–14 days). The basic characteristics of the team members are presented in Table 1. No significant differences were observed between the injured and uninjured members in terms of age, length of service, height, or weight (p > 0.05).

As an initial exploratory step to identify candidate predictors from the multiple asymmetry metrics collected, univariate regression analyses were performed. The log-binomial model failed to converge for several variables; therefore, Poisson regression with robust standard errors was employed consistently for all univariate analyses, as per the pre-specified analytical plan. This analysis identified four asymmetry indicators with p < 0.1 for inclusion in subsequent multivariable modeling: eccentric peak force (N), vertical velocity at takeoff (m/s), peak takeoff acceleration (N), and takeoff peak force (N). Details are presented in Table 2.

To identify the most discriminative indicator for subsequent multivariate analysis, the bilateral asymmetry indices of these four indicators were compared between the injured and uninjured groups using the Mann–Whitney U test (Table 3).

The results revealed that, among all indicators, only the asymmetry of takeoff peak force showed a statistically significant difference between the two groups. The injured group exhibited a significantly higher degree of asymmetry in takeoff peak force compared to the uninjured group (mean rank: 41.01 vs. 60.22; Z = −2.721, p = 0.007). Based on this result, the bilateral asymmetry of takeoff peak force was selected as the primary variable for further investigation into the risk of non-contact low back injuries.

The study assessed potential variables for multicollinearity. First, any factor with a variance inflation factor (VIF) greater than 5 was excluded. Second, based on a comparison of the −2 log-likelihood values between nested models, the following variables were also excluded: height, weight, BMI, peak acceleration at jump, peak centrifugal force, and concentric impulse. Consequently, the following variables were included in the final multifactor Poisson regression model: vertical velocity at takeoff, takeoff peak force, weight, and length of service.

In the final multivariate Poisson regression model (Table 4), multicollinearity was assessed. The VIF values for each variable were as follows: takeoff peak force (VIF = 1.106), vertical velocity at takeoff (VIF = 1.088), weight (VIF = 1.014), and length of service (VIF = 1.028). All VIF values were well below the exclusion threshold of 5, indicating no serious multicollinearity among the factors and that redundant information would not be introduced for predicting the dependent variable. The Hosmer-Lemeshow test was not significant (χ² = 13.052, df = 8, p = 0.110), indicating a good model fit. The Omnibus test of model coefficients was significant (χ² = 12.324, df = 4, p = 0.015), indicating a significant difference between the model and the null hypothesis model.

Based on this, the ROC curve for the asymmetry of takeoff peak force was analyzed (AUC = 0.731; 95% CI: 0.614–0.847, p = 0.004), indicating a moderate predictive ability. The optimal threshold, determined by the Youden’s index, was 7.05% (sensitivity = 0.688, specificity = 0.718). Using this cut-off point, participants were categorized into two groups: those with a takeoff peak force asymmetry greater than 7.05% were defined as the high-risk group, and those with asymmetry of 7.05% or less were defined as the low-risk group. This continuous variable was thus transformed into a dichotomous variable for subsequent analysis. The data were input into the final multivariate Poisson regression model. The results showed that the high-risk group was significantly associated with non-contact lumbar injury (Table 5).

This prospective cohort study aimed to investigate the relationship between bilateral asymmetry in lower limb explosive strength and the risk of non-contact low back injuries among mine rescue personnel. Additionally, it sought to identify predictive factors for these injuries. The findings are crucial for developing scientifically sound prevention strategies and training programs. The results indicate that peak vertical jump force metrics in the SLCMJ test are significantly correlated with non-contact low back injuries. As shown in Table 3, the degree of asymmetry in peak vertical jump force among rescue personnel who sustained low back injuries was significantly higher than among those who did not (p = 0.007). According to the results of the multifactorial Poisson regression analysis (Table 4), bilateral asymmetry in peak jump force was the sole risk factor. Specifically, for each 1% increase in takeoff peak force asymmetry, the relative risk of lumbar injury increased by approximately 18.5% (RR = 1.185, 95% CI: 1.091–1.288), which corresponds to an absolute risk increase of 4.7%. More practically, when using the identified 7.05% screening threshold, rescue workers classified as high-risk faced a near four-fold increase in relative risk (RR = 3.647) and an absolute injury incidence that was 24.1% higher than their low-risk counterparts. These risk-based metrics (RR and RD) provide clearer and more actionable information for prevention program design, as OR tends to exaggerate the apparent strength of the association between exposure and outcome (whether positive or negative) compared to RR (26). In summary, the takeoff peak force during the SLCMJ test is a predictive risk factor for non-contact low back injuries among mine rescue personnel.

To assess the potential impact of unmeasured confounding on the observed associations, we calculated E-values for the primary risk estimates. For the association per 1% increase in continuous asymmetry (RR = 1.185, 95% CI: 1.091–1.288), the E-value for the point estimate was 1.58, and for the lower confidence limit (RR = 1.091) it was 1.29. For the association based on the dichotomized asymmetry (high-risk vs. low-risk group, RR = 3.647, 95% CI: 1.353–9.832), the E-value for the point estimate was 7.27, and for the lower confidence limit (RR = 1.353) it was 1.94.

In addition to relative risk (RR), we reported risk differences (RD) to convey absolute risk increases. For each 1% increase in asymmetry, the absolute risk of injury rose by 3.2% (RD = 0.032). When using the 7.05% cutoff, high-risk individuals had an absolute injury rate 24.1% higher than low-risk individuals. These RD estimates are clinically informative, as they quantify the actual burden of injury attributable to asymmetry and can guide the prioritization of interventions.

This study aligns with the broader literature linking inter-limb asymmetry to injury risk, yet the optimal asymmetry threshold of 7.05% is notably lower than the 10–15% often cited in sports medicine research (27–30). This discrepancy underscores the critical influence of population-specific characteristics. Brumitt et al. (27) suggested that optimal thresholds may vary across athletic populations. This perspective directly extends to specialized occupational groups like mine rescuers For instance, while a Y-balance test (YBT) reach asymmetry threshold of 4 cm is often reported for injury prediction in athletic populations (8, 31, 32), research by Vaulerin et al. (15) identified a lower optimal threshold (2–3 cm) for firefighters. Therefore, the optimal thresholds established in general research may not be directly applicable to mine rescue personnel due to their unique characteristics (e.g., age, weight), demanding working environment (e.g., high pressure, high temperature), and the complex, hazardous nature of their duties. However, as this study was conducted within a single rescue squadron in China, the generalizability of the 7.05% threshold requires further validation.

Bilateral asymmetry in lower limb strength and power is commonly assessed using isometric/isokinetic muscle strength tests or unilateral/bilateral jump tests (17). For example, Hietamo et al. (33) evaluated lower limb asymmetry by measuring peak torque during isometric testing. Other studies have used metrics such as single-leg countermovement jump height or distance to calculate asymmetry and validate its association with injury (34–36). Similarly, Read et al. (7) identified peak landing vertical ground reaction force as a strong predictor of injury in football players. Correspondingly, the present analysis revealed that takeoff peak force also has good predictive efficacy for non-contact low back injuries in mine rescue personnel. It is widely used to assess lower limb muscular capabilities, such as explosive power and absolute strength (37).

Previous research has suggested that factors like training history and body mass index (BMI) can influence injury risk. For instance, Guan et al. (38) reported an increased risk of lower limb sports injuries with accumulated training years in adolescent athletes, and Kobayashi et al. (39) highlighted BMI’s significant impact on lateral ankle sprain risk. In the present study, however, the multivariate Poisson regression model—after screening for and adjusting multicollinearity—revealed that neither body weight (p = 0.584) nor length of service (p = 0.358) was significantly associated with non-contact low back injuries. In contrast, the bilateral lower limb takeoff peak force asymmetry index was significantly correlated with these injuries (p = 0.013).

According to Cheng et al. (2), sports injuries constitute one cause of functional asymmetry in athletes’ lower limbs. Consequently, a history of previous injuries may also represent a potential risk factor. However, given that the subjects’ average length of service was 12.97 years—an excessively long period to accurately gather details of their past injuries—a history of previous injuries was not included in this study. Future research should investigate the duration of injuries among specialized occupational groups, incorporating prior injury histories into analyses to yield more accurate findings. Brumitt et al. (11, 27) incorporated training and match duration (i.e., sports exposure time) as a risk factor in their risk prediction study for female university volleyball players. The inclusion of exposure time effectively controls and filters for confounding variables, enhancing model accuracy and enabling more precise quantification of risk probabilities. Simultaneously, it standardizes injury occurrence by expressing it as injuries per 1,000 h of exposure time, facilitating objective analysis of injury trends over time.

This study focused on bilateral asymmetry in lower limb explosive strength and did not assess other potential factors like flexibility or balance. Future research could incorporate assessments such as the YBT, Star Excursion Balance Test (SEBT), and range of motion (ROM) measurements to provide a more comprehensive analysis of lumbar injury risk factors. Combining these with lower limb explosive strength testing could lead to the development of robust risk prediction indicators. The practical application of asymmetry measures is supported by studies such as Paterno et al. (40), who used bilateral lower limb strength asymmetry as a criterion for return-to-play decisions in ACL reconstruction. Similarly, bilateral lower limb asymmetry could serve as one of the key criteria for determining when mine rescue personnel are fit to return to training and duty following lumbar or other non-contact injuries, thereby safeguarding their long-term health.

This study did not statistically analyze the training and working hours of mine rescue personnel as potential risk factors for inclusion in the analysis. However, as all personnel were drawn from the same rescue squadron, their rescue training and duties exhibited homogeneity, thereby controlling for the relevant impact of this factor to a certain extent. Nevertheless, future research should incorporate statistical data on subjects’ training and working hours, alongside documentation of the stage and timing of injury occurrence, to enhance the study’s accuracy.

As all participants in this study were drawn from the same region, their risk factor profiles may be influenced by shared living environments and occupational duties. In addition, as this study was a single-center study conducted within a specific mine rescue team in China, and there may be sampling bias, it is difficult to accurately reflect the actual situation of rescue teams in other regions. Future research should incorporate heterogeneity analyses, including rescue personnel from diverse regions and units, to better reflect the injury risk factors across the entire mining rescue community.

The estimates derived from regression models (e.g., RR, RD) are presented as associational measures and should not be interpreted as causal effects. While the study adjusts for several covariates, unmeasured confounding may still exist. The selection of covariates was guided by both statistical criteria (e.g., VIF) and subject-matter relevance (e.g., weight, length of service) to optimize model stability and predictive performance, rather than to fully establish causality.

To assess the potential impact of selection bias due to loss to follow-up, baseline characteristics (age, height, weight, length of service, and baseline takeoff peak force asymmetry) were compared between participants who completed the study and those lost to follow-up using Mann–Whitney U tests. No significant differences were found (all p > 0.10), suggesting that the data may be missing at random (MAR) with respect to these measured variables. Given the relatively low proportion of loss to follow-up (13.9%) and the lack of evidence for systematic differences in key baseline measures, primary analyses were conducted on the complete-case cohort (n = 87). The assumption of non-informative censoring, while not verifiable for unmeasured factors, is considered plausible in this context. For sensitivity, inverse probability of censoring weights (IPCW) were considered but not applied in the primary model, as the conditions for MAR appeared reasonably met and the added complexity was deemed unnecessary for the primary predictive aim of this study. However, the potential for residual selection bias cannot be entirely ruled out.

Assessment of bilateral asymmetry in lower limb explosive force indexes (e.g., peak force of jump) effectively evaluates the risk of non-contact low back injuries in mine rescue personnel. These findings supplement existing risk factors for mine rescue workers, providing evidence for applying lower limb asymmetry assessments in specialized occupations such as emergency rescue. This offers healthcare professionals and physical trainers in relevant fields additional options for developing comprehensive, systematic preventive measures and training programs.

Future research should incorporate additional limb asymmetry assessments alongside potential risk factors such as prior injuries and exposure duration to further explore the relationship between bilateral asymmetry in lower limb explosive strength and non-contact low back injuries.