Introduction
Recent global estimates indicate that knee OA affects nearly 375 million people in 2021, and its prevalence is projected to approach 1 billion worldwide by 2050 with ageing populations and rising obesity [1, 2]. However, knee OA poses a significant public health issue in India, particularly among individuals aged 40 and older, with data also suggesting an earlier onset influenced by genetic, lifestyle, and environmental factors [3, 4]. These studies also indicate that more than one-third of the population in rural South India are diagnosed with this condition, with significant associations with obesity, diabetes, hypertension, sedentary lifestyle, menopause, and the practice of squatting for toileting. 
Excessive mechanical loading on joint surfaces significantly contributes to the onset and progression of knee OA, often resulting from abnormal knee joint loading due to disrupted kinematic interactions between the foot and knee [5]. Abnormal foot posture or altered foot movement may contribute to degenerative changes in knee joint cartilage by causing improper loading [6]. As a result, the foot is increasingly considered in lower-limb assessments.
The first metatarsal joint plays a crucial role in maintaining the structural integrity of the foot arch, supporting body weight, and evenly distributing mechanical stress during ambulation [7, 8]. Dysfunction at this joint, such as reduced TGS due to hallux impairment, may disrupt the biomechanical chain influencing proximal joints. When TGS is weak, individuals tend to compensate by increasing ankle dorsiflexion, leading to higher muscular demand around the ankle and elevated torque at the knee and hip during walking [9, 10]. 
TGS plays a vital role in mobility among older adults and may contribute to dysfunction of the proximal joints [11, 12]. Balance impairments are particularly pronounced in individuals with knee OA, likely due to reduced muscle strength and joint position sense. These deficits lead to greater difficulty in walking and an increased risk of falls compared to those without OA [13]. Furthermore, falls in this population can result in severe injuries, fear of movement, or even fatal outcomes, adding a substantial financial strain to healthcare systems [14, 15]. Therefore, assessing balance impairments is essential for optimizing rehabilitation strategies.
A common gait abnormality in individuals with knee OA is the failure to maintain consistent toe-ground contact throughout the stance phase until toe-off, suggesting a possible relationship between compromised toe flexor strength and altered gait mechanics in this population. Given the association between the forefoot, hallux, and rearfoot through the midfoot, hallux dysfunction may indirectly influence knee kinematics in OA by altering rearfoot motion patterns during gait [16]. Uritani et al. highlighted the importance of assessing toe flexor strength as a predictive and preventive measure for walking dysfunction and falls [17]. They also observed a link between preoperative TGS and postoperative functional mobility in patients who underwent total knee replacement (TKR) [18]. Because TKR is an end-stage intervention, this link underscores the critical role of TGS in mobility and recovery.  
Nevertheless, a notable gap remains in the mechanical linkage between TGS and knee biomechanics. While some studies have examined broad association trends, few have explored how toe strength directly contributes to dynamic balance and mobility impairments in knee OA. Moreover, biomechanical explanations tying TGS to compensatory movement patterns in proximal joints are underrepresented, particularly in resource-limited settings where early intervention could be pivotal.
To the best of our current understanding, published evidence examining the correlation among these factors in individuals with knee OA in an Indian context is lacking. As life expectancy rises and obesity becomes more prevalent, India is on the brink of a substantial OA burden, emphasizing the need for region-specific research. In rural areas, limited access to diagnostic imaging and specialized orthopaedic services frequently contributes to delayed identification and management of the condition, often until it has progressed to more advanced stages. The growing prevalence of OA is set to escalate disability and deteriorate quality of life, presenting a significant challenge for India’s resource-limited public health system and imposing a heavy economic strain on the country [3, 19].
Accordingly, the present study aimed to investigate the relationship between TGS and dynamic balance and mobility in patients with knee OA. We hypothesized that TGS deficits would correlate with diminished performance in assessments of dynamic balance and mobility. Understanding these associations, especially their functional and mechanistic underpinnings, may help develop targeted physical therapy strategies to enhance lower limb function and mobility in this population.

Materials and Methods
Study design and setting 
This observational study employed a cross-sectional design and was conducted over a three-month period, from December 2023 to February 2024, recruiting participants from a tertiary care hospital and community setting in a South Indian city. Individuals meeting the inclusion criteria were enrolled after written informed consent was obtained following a comprehensive explanation of the study’s objectives, potential benefits, and associated risks. Reporting adhered to the Strengthening the Reporting of Observational Studies in Epidemiology criteria for observational research.

Eligibility criteria
Participants included individuals aged 45 to 75 years with a diagnosis of knee OA confirmed by an orthopaedic specialist, based on the American College of Rheumatology criteria [20], and a history of the condition for more than three months. The orthopaedic surgeon also assessed the severity of knee OA from an anteroposterior weight-bearing film using the Kellgren and Lawrence scale [21], and only those with Grade 2, characterized by definite osteophytes and possible joint space narrowing, were included. This reduced heterogeneity, as advanced grades, is associated with substantial structural damage and functional loss, potentially confounding the relationship between TGS and mobility. Eligible participants were required to ambulate independently without assistive devices or caregiver support and to have a minimum education level equivalent to completion of 4th grade. Exclusion criteria encompassed any neuromuscular or musculoskeletal disorders, prior TKR, lumbar spine surgery, or surgical procedures involving the lower extremities.
After screening, demographic information was recorded, and all outcome measures were evaluated on the same day by a single researcher (MA) to reduce inter-rater variability. The most affected knee was identified through participant-reported symptoms of pain and functional limitation, further confirmed by radiographic findings of grade 2 OA. In cases of bilateral OA, the knee with greater symptom severity as reported by the participant was selected for assessment. 
TGS: The strength of the flexor hallucis longus was assessed with a pinch gauge dynamometer [22, 23], which has demonstrated excellent intra-rater reliability (ICC=0.94–0.98) in previous studies [24, 25]. Participants were seated with their feet flat on the floor, and the dynamometer was positioned beneath the hallux, with its proximal end aligned just below the distal crease of the first metatarsophalangeal joint (Figure 1).

The examiner manually stabilized the device. Participants were instructed to press downward with maximal effort for three seconds, ensuring the heel remained in contact with the ground. Three trials were conducted, with a rest period of less than five seconds between attempts. 
Functional reach test (FRT): Participants stood near a wall without touching it and raised the arm closest to the wall to 90° of shoulder flexion while maintaining a closed fist [26]. The initial position of the third metacarpal head was marked on a yardstick affixed to the wall. The participants then reached as far as possible without taking a step. The final position was recorded, and the reach distance was calculated as the difference between the start and end positions. We averaged three trials.
Timed up and go test (TUG): Participants wore regular footwear and did not use assistive devices during testing. Initially, each participant was seated in a chair with an armrest [27, 28]. At the therapist’s cue, they stood up, started the timer, walked three meters at a comfortable pace, turned, walked back, and sat down completely, at which point timing stopped. The timing ended when the participant was completely seated. Two practice trials were conducted prior to recording one final measurement for statistical analysis.
Visual analog scale (VAS): Pain intensity was assessed using a 100mm horizontal line, marked with the descriptors “no pain” at 0mm and “worst possible pain” at 100mm [29]. The participants indicated their current pain level by placing a mark on the line. The pain score was quantified by measuring the distance in millimeters from the 0mm point to the participant’s mark using a standard ruler.
Western ontario and mcmaster university (WOMAC) OA index: This questionnaire consists of three scales and 24 items, each scored on a 5-point ordinal scale. Higher scores reflect increased symptom severity [30].

Sample size 
The a priori sample size for testing the association between TGS and mobility/balance outcomes was calculated for a two-tailed correlation test using the Fisher z transformation, assuming an expected effect size of r=0.23 (based on the smallest relevant correlation reported by Uritani et al. [31]), with a significance level of 5% and a statistical power of 80%. This indicated a minimum requirement of 146 participants; we recruited 147, thereby exceeding the target.

Statistical methods 
Descriptive statistics, including means and standard deviations, were used to summarize the demographic variables and outcome measures. We applied the Kolmogorov–Smirnov test to assess the normality of the data distribution. Because the variables were not normally distributed, Spearman’s rank correlation was used to examine the relationship between TGS, TUG, and FRT. The correlation coefficient (r) was interpreted as: 0–0.25 (negligible), 0.25–0.50 (fair), 0.50–0.75 (moderate to good), and ≥0.75 (good to excellent) [32]. A partial correlation analysis was conducted to explore the associations between outcome measures while controlling for age, body mass index (BMI), and duration of OA. Scatter plots were used to depict the relationships between TGS and TUG and FRT. Simple linear regression was used to assess the predictive value of FRT and TUG on TGS, while a multiple regression model including both TUG and FRT was constructed to evaluate their combined effect on TGS. Multicollinearity diagnostics were performed using the variance inflation factor (VIF) and tolerance statistics to ensure the validity of regression models. Statistical analyses were performed using IBM SPSS software, version 25.0 (IBM Corp., Armonk, NY, USA), and a P<0.05 was considered statistically significant.

Results
Of the 186 participants assessed for eligibility, 39 did not meet the inclusion criteria. A total of 147 participants with knee OA were included in the analysis. The mean age was 59.3±7.8 years, and the mean BMI was 25.3±4.2 kg/m². The mean duration of knee OA was 3.8 ± 3.9 years (Table 1).


Consequently, statistical analyses were conducted on data from 147 participants, comprising 70 males (47.65%) and 77 females (52.4%). Of the participants, 38% had right knee involvement, 46% had left knee involvement, and 16% had bilateral knee involvement. Table 2 presents descriptive statistics for outcome measures.  


A good-to-excellent positive correlation was observed between TGS and the FRT (r=0.942; P<0.001), indicating that individuals with lower TGS achieved shorter distances in the reach test. Similarly, a strong negative correlation was found between TGS and the TUG (r=-0.832; P<0.001), suggesting that individuals with greater TGS completed the TUG in less time (Table 3).


Although TGS showed a negligible correlation with both VAS (r=0.001; P=0.989) and WOMAC (r=0.012; P=0.887), these correlations were not statistically significant (Table 3). 
After controlling for age, BMI, and knee OA duration, TGS remained strongly associated with TUG (r=–0.735; P<0.001) and FRT (r=0.892; P<0.001), confirming that these relationships were independent of confounding variables (Table 4).


The scatter plot diagrams illustrate good to excellent linear relationships: A positive association between TGS and FRT (Figure 1), and a negative association between TGS and TUG (Figure 2).

Regression analysis (Table 5) showed that both FRT and TUG strongly predicted TGS.


FRT alone explained 91.3% of the variance in TGS (β=0.628; P<0.001), while TUG accounted for 74% (β=–0.613; P<0.001). In the combined model, both remained significant (R²=0.922), with FRT emerging as the stronger predictor, indicating that balance has a greater influence on TGS than mobility. Collinearity diagnostics confirmed that multicollinearity was not present (all VIF values<2.0; tolerance>0.5). Thus, the high R² values reflect robust associations between functional balance, mobility, and TGS.

Discussion
This study investigated the association between TGS and functional measures of mobility and balance in individuals with knee OA. We found that both the TUG and FRT were significant predictors of TGS. Among these, FRT explained more variance in TGS than TUG, suggesting that balance capacity contributes more strongly to toe flexor strength than mobility performance. The results highlight the potential impact of TGS on functional ability and mobility in patients with knee OA. 
A systematic review of observational studies found that greater toe flexor strength is associated with improved postural stability in individuals aged 60 and above [33]. Yong-Wook et al. showed that elderly non-fallers had significantly stronger muscles in the hallux and second toe, as well as better balance than fallers; toe flexor strength also correlated moderately with FRT, suggesting a role in fall prevention [34]. Similarly, Menz et al. highlighted that toe flexor strength, along with other ankle and foot features, can potentially predict functional performance and balance in older adults [10]. Unsurprisingly, interventions aimed at strengthening toe grip in younger adults have also improved performance on tasks requiring dynamic balance [35, 36].
The foot, a complex and adaptable structure with multiple degrees of freedom, also controls the center of mass. Extrinsic and intrinsic toe flexors play a crucial role in foot function, whether the foot is unloaded or in contact with the ground. In an upright stance, the toe flexors must produce adequate, timely plantar flexor torque to help prevent excessive forward displacement of the centre of mass beyond the base of support [9]. Consequently, balance stability depends on the effective activation of the extrinsic toe flexors to produce plantar flexor force at the ankle joint. This highlights the significance of TGS in maintaining postural control and preventing falls [33]. Toe flexors not only generate mechanical force but also contribute sensory feedback through proprioceptors, aiding neuromuscular control of standing balance and gait. Therefore, impaired proprioception may partly explain the strong association observed between FRT and TUG [37, 38].

Interestingly, the lack of significant correlation between TGS and WOMAC may highlight a dissociation between neuromuscular performance and subjective symptom perception in knee OA. This aligns with prior literature suggesting that functional impairments may not always parallel pain reports [39]. This observation underscores the need to explore both biomechanical and psychosocial dimensions of OA.
Our findings align with previous research showing that TGS is significantly weaker in individuals with knee OA than in control subjects. While the causal link between TGS and knee OA has not yet been established, diminished TGS may compromise dynamic balance during standing and reduce propulsive force, which could subsequently increase mechanical stress on the knee joint [40]. This observation underscores the importance of TGS in the functional biomechanics of osteoarthritic knees. While diminished TGS may contribute to altered biomechanics and increased knee joint loading, this hypothesis requires confirmation in mechanistic and longitudinal studies. A previous investigation in 665 Japanese older adults found that TGS was independently correlated with TUG performance but not with FRT. These findings highlight the potential of TGS-focused physical therapy to improve functional mobility [17]. Evidence from European and North American cohorts similarly supports the association between foot muscle strength and functional performance, suggesting that the observed relationships are not restricted to Asian populations [37, 41].
Prior studies, such as those by Poojari et al. have also demonstrated a correlation between reduced TGS and fear of pain in patients with knee OA, suggesting a potential psychosomatic link that influences TGS [23]. Furthermore, a multicenter cross-sectional study found that TGS negatively correlated with age and positively with isometric strength of the knee extensors in patients with OA. In the control group, TGS was similarly associated with age and the TUG test, emphasizing the interplay between forefoot function and knee OA progression [31]. These findings suggest that improving TGS could be a practical therapeutic strategy to mitigate knee OA progression. 
Our findings align with emerging evidence supporting functional, muscle-targeted interventions in knee OA management. For instance, Tedeschi highlighted the role of neuromuscular coordination and proprioceptive training in restoring function and performance, emphasizing the importance of strengthening specific muscle groups for optimal recovery [42]. Our results reinforce this perspective by identifying TGS as a potential marker of dynamic balance and mobility, suggesting that distal muscle activation and control may influence overall lower limb performance. Similarly, a randomized controlled trial demonstrated that structured remote interventions can significantly improve functional outcomes in patients with OA [43].
Further evidence suggests that decreased TGS on the affected limb correlates with a history of falls in older adults with knee OA, reinforcing the importance of incorporating TGS strengthening in physical therapy interventions to reduce fall risk [36]. Furthermore, longitudinal studies have established a relationship between preoperative TGS and postoperative functional performance in patients undergoing TKR, with stronger preoperative TGS being associated with better postoperative TUG outcomes [19]. These findings suggest that TGS may serve as a functional marker and therapeutic target for improving the outcomes of knee OA.

Limitations and recommendations
The study was limited to individuals with mild-to-moderate knee OA, suggesting that future investigations should include cases with severe OA to achieve a comprehensive understanding. Participant recruitment from both hospital and community settings may introduce selection bias, limiting generalizability. Additionally, the influence of other foot muscles should be investigated, and comparisons between males and females are necessary to explore potential gender-based differences. Replication of these findings in independent cohorts across diverse populations is warranted to strengthen external validity and confirm the robustness of the observed associations. Although our study was conducted in an Indian cohort, the biomechanical role of TGS is universal. Nevertheless, caution is warranted when extrapolating these findings to populations with differing lifestyles, footwear habits, or genetic predispositions. Lastly, as this was a cross-sectional study, causality cannot be inferred. Whether reduced TGS contributes to functional decline, or vice versa, requires longitudinal research.

Conclusion
TGS was significantly correlated with dynamic balance and mobility, with individuals showing lower TGS taking longer to complete the TUG and achieving shorter reach distances in the FRT. TGS did not show any significant relationship with pain levels or knee function. These findings suggest that incorporating TGS screening into clinical assessments and considering toe flexor strengthening in rehabilitation programs may be feasible strategies to improve functional outcomes.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of Father Muller Institution, Mangalore, India (Code: FMIEC/CCM/553/2023) on 09/10/2023. All procedures were conducted in compliance with the principles outlined in the Declaration of Helsinki (2000).Written informed consent was obtained from all participants following a detailed explanation of the study’s purpose. Consent included permission for participation as well as for the use of anonymized data for research and educational purposes.  

Funding
This study was financially supported by Rajiv Gandhi University of Health Sciences, Bengaluru, India (Grant No.: UG23PHY529).

Authors' contributions
All authors contributed equally to the conception and design of the study, data collection and analysis, interpretation of the results, and drafting of the manuscript. Each author approved the final version of the manuscript for submission.

Conflict of interest
The authors declared no conflicts of interest.

Acknowledgments
The authors sincerely thank all participants for their valuable participation and the institution for its support and cooperation throughout the study.

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