Physical Therapy Korea

Hallux valgus (HV) is a common foot deformity characterized by the lateral deviation of the great toe and medial deviation of the first metatarsal, resulting in a pronounced angle at the metatarsophalangeal (MTP) joint [1,2]. This condition creates aesthetic concerns and leads to significant pain, discomfort, and functional impairment, affecting individuals’ quality of life and mobility [2,3]. The etiology of HV is multifactorial and involves genetic predisposition, biomechanical abnormalities, footwear choices, and environmental factors [3]. HV severity is primarily quantified using the hallux valgus angle (HVA) and the intermetatarsal angle (IMA) [1-3].

Previous studies have demonstrated that individuals with HV often exhibit atrophy and reduced muscle tone in the abductor hallucis muscle (AbdH), contributing to destabilization of the first MTP joint [4-6]. Additionally, compensatory overactivity of the adductor hallucis muscle (AddH) can exacerbate lateral drift of the great toe. These muscle imbalances between the AbdH and AddH affect static alignment and alter dynamic foot mechanics, leading to abnormal plantar pressure distribution and compromised gait patterns [5,6]. Understanding the role of muscle imbalance in HV infections is crucial for developing effective interventions. Targeted exercises aimed at strengthening the AbdH and balancing the activity of foot muscles may prevent the progression of HV and alleviate associated symptoms. However, previous research has primarily focuses on static anatomical changes and has often overlooked the dynamic functional impairments caused by muscle imbalance.

Furthermore, plantar pressure distribution during gait is a critical aspect of foot biomechanics and is often altered in individuals with HV. Abnormal pressure patterns, particularly increased loading on the lateral forefoot and medial heel, can exacerbate the symptoms and contribute to further deformities [7]. Analysis of plantar pressure can provide valuable insights into the compensatory mechanisms employed by individuals with HV and inform targeted interventions to optimize gait and reduce discomfort. Previous research has predominantly focused on static conditions (e.g., standing) rather than dynamic conditions (e.g., walking or running) [2,8,9]. Plantar pressure distribution can significantly differ between static and dynamic activities, and it is vital to understand these variations to develop effective interventions that address real-life functional impairments.

The gastrocnemius (GCM) and soleus muscles are critical for maintaining proper foot biomechanics [10,11]. These muscles are primarily responsible for plantarflexion (P/F) of the ankle during the push-off phase of walking [10,11]. Previous research has suggested that individuals with HV often exhibit altered activation patterns and reduced muscle strength and tightness [6,12]. Limited dorsiflexion (D/F) of the ankle affected by calf muscle tightness not only disrupts the normal rolling motion of the foot during gait but also increases the strain on the great toe MTP joint and promotes HV deformity [6,12]. Restricted D/F and P/F of the ankle along with limited flexion and extension of the great toe MTP joint can impair gait mechanics and exacerbate deformities [1-3]. Therefore, understanding the relationship between HV severity and joint range of motion (ROM) can help to devise comprehensive treatment plans that address both static alignment and dynamic function.

Despite the recognition of these factors, there is a notable gap in the literature regarding the comprehensive evaluation of HV severity in relation to these biomechanical aspects simultaneously. Most existing research focuses on average comparisons between normal and HV groups [13,14]. However, studies examining changes in the static and dynamic mechanical and functional characteristics of the foot and ankle as HV progresses within HV populations are less common.

Therefore, this study aimed to bridge this gap by investigating the correlations between the HVA, IMA, and various factors including the cross-sectional area (CSA) and tone of the AbdH, ROM of the ankle and great toe MTP joint, and plantar pressure distribution during gait. Furthermore, this study provides comprehensive evidence to support integrated interventions.

1. Participants

Sixty volunteers with mild to moderate bilateral HV (15–40 degrees) who had the ability to walk independently participated in this study. According to established guidelines, HV is classified based on HVA: normal HVA is less than 15°, mild HV ranges from 15° to 20°, moderate HV spans from 20° to 40°, and severe HV is defined as greater than 40° [1-4]. Participants with stage 3 (severe) HV were excluded from this study as non-invasive treatments did not show significant efficacy at this stage, and surgical intervention is typically recommended [15]. Therefore, only individuals with stages 1 and 2 (mild and moderate) HV were recruited. The sample size for the correlation study was calculated using G-power software. The average HVA of both feet in the 60 participants (120 feet) was 25.1°. Individuals with any neurological disorders or musculoskeletal disabilities other than HV, a history of ankle or toe fractures or surgeries, the use of medication, orthotics, or assistive devices for HV, and those with metal inserts in the foot and toe area, were excluded. All participants provided written informed consent, and the study was approved by the Institutional Review Board (IRB) of Yonsei University Mirae Campus (IRB no. 1041849-202303-BM-056-02).

2. Procedure

Participants underwent several measurements as part of the study. First, the HVA and IMA between the first and second metatarsals were measured using radiography imaging. The CSA of the AbdH was assessed using ultrasonography. The muscle tone and stiffness of the AbdH were measured using the Myoton PRO device (Myoton AS). The ankle D/F, P/F, and great toe MTP flexion and extension were measured using a goniometer. Finally, the plantar pressure during walking was measured as the participants walked back and forth 20 times over a distance of approximately 3 m, with a Zebris FDM system (zebris Medical GmbH) placed in the middle of the path.

3. Outcome Measures

1) Hallux valgus angle and intermetatarsal angle

Participants diagnosed with mild to moderate HV had their HVA and IMA measured using radiography (LISTEM, REX-R). These radiographs were obtained at Wonju Yonsei Clinic in Wonju, Gangwon-do, by a single radiologic technologist. The participants were positioned standing in a horizontal stance with weights evenly distributed between both feet. The measured radiographs were analyzed using the ImageJ software (National Institutes of Health) to determine the HVA and IMA. The HVA was measured as the acute angle between the axes of the first metatarsal and proximal phalanx, while the IMA was measured as the acute angle between the axes of the first and second metatarsals.

2) Cross-sectional area of the abductor hallucis muscle

The optimal harmonic imaging feature of an ultrasonography device (HS40; Samsung Medison) was used to measure the CSA of the AbdH. The subjects were seated with their knee joints at 15° of flexion [2]. The anterior border of the medial malleolus was palpated, and a line perpendicular to the footpad was drawn [2]. A 3 MHz linear array probe was used to position the AbdH [2]. To maintain consistency, the pressure applied during measurement has been standardized. Measurements were performed 3 times, and the mean and standard deviation were calculated.

3) Muscle tone of the abductor hallucis muscle

The muscle tone of the AbdH was measured using the Myoton PRO device. According to the manufacturer’s manual, the muscle tone is defined as the vibration frequency (Hz) of the muscle in a relaxed state without voluntary contraction [16]. The participant abducted the great toe while lying down. The examiner palpated the AbdH below the medial malleolus. The participants were instructed to relax and maintain a comfortable position without exertion on the great toe. Muscle tone was measured using Myoton PRO approximately 1–2 cm behind the navicular tuberosity, just in front of the imaginary line passing through the anterior edge of the medial malleolus, parallel to the muscle. The measurements were repeated 3 times, and the mean values were calculated.

4) Plantar pressure measurement during gait

For the analysis of the plantar pressure distribution, the foot was segmented into 10 regions: T, toe region; M1, the 1st metatarsal region representing the medial forefoot; M2, the 2nd metatarsal region representing the medial forefoot; M3, the 3rd metatarsal region representing the middle forefoot; M4, the 4th metatarsal region representing the lateral forefoot; M5, the 5th metatarsal region representing the lateral forefoot; MF, the mid-foot region; HM, medial heel region; HC, central heel region; and HL, lateral heel region. Each region was uniformly sized at 5 × 5 cm². Peak force values in each of these regions were analyzed.

5) Ankle and first metatarsophalangeal joint range of motion

Intra-rater reliability was determined using intraclass correlation coefficients (ICCs) values for a single measure and the associated 95% confidence intervals. Reliability was defined as poor (ICC < 0.40), fair (ICC: 0.40–0.60), good (ICC: 0.60–0.75), or excellent (ICC > 0.75).

The ankle D/F, P/F, and great toe MTP flexion and extension were measured using a goniometer. The participants lay supine with their legs extended and their ankles off the edge of the bed, facing the ceiling [17,18]. The goniometer was aligned with the lateral aspect of the lower leg and centered on the lateral malleolus [17,18]. The stationary arm of the goniometer was aligned with the fibula and the movable arm was aligned with the fifth metatarsal [17,18]. Ankle D/F was measured by instructing the participant to move the foot upward, whereas ankle P/F was measured by instructing the participant to move the foot downwards [17,18]. Each measurement was performed 3 times, and the average value was recorded.

The goniometer was placed along the medial aspect of the foot with the fulcrum centered over the great toe MTP joint [17,18]. The stationary arm was aligned with the first metatarsal, and the movable arm was aligned with the proximal phalanx of the hallux [17,18]. MTP flexion was assessed by instructing the participant to bend the hallux downwards and extension was assessed by instructing the participant to lift the hallux upward [17,18]. Each measurement was performed 3 times, and the mean values were recorded. The ICC values indicated high reliability (ICC = 0.97), demonstrating that the measurements were consistent across different assessors.

4. Statistical Analysis

The collected data were analyzed using IBM SPSS Statistics for Windows (version 27.0; IBM Co.). The Shapiro–Wilk test was employed to determine the normality of participant characteristics and dependent variable data. Pearson’s correlation coefficient was used to evaluate the relationship among the HVA, CSA of the AbdH, and plantar pressure during gait. The correlation coefficients (r) were interpreted as follows: 0.00–0.25 indicated poor correlations; 0.25–0.50 indicated fair correlations; 0.50–0.75 indicated moderate to good correlations; and greater than 0.75 indicated good to excellent correlations [19]. A significance level of α = 0.05 was set for all statistical tests.

The demographic and clinical characteristics of the participants, including age, sex, and other relevant variables, are presented in Table 1.

Table 1 . General characteristics of the participants (N = 60, 120 feet).

Variable	Value
Age (y)	29.50 ± 4.16
Height (cm)	168.13 ± 7.01
Weight (kg)	63.23 ± 11.91
BMI (kg/m2)	22.31 ± 2.80
Sex (male/female)	24/36
Shoe size (mm)	249.83 ± 16.01
Dominant foot (Rt./Lt.)	58/2
HV classification (mild/moderate)	16/104

Values are presented as mean ± standard deviation or number. BMI, body mass index; Rt., right; Lt., left; HV, hallux valgus..

1. Hallux Valgus Angle and Intermetatarsal Angle

The average HVA was 24.32° and the average IMA was 17.22° (Table 2). The correlation analysis revealed a strong positive relationship between the HVA and IMA (r = 0.858, p < 0.05), indicating that as the severity of the HV deformity increased, represented by a higher HVA, the IMA also increased (Table 3).

Table 2 . Summary of average value (N = 60, 120 feet).

Variable	Value
HVA (°)	24.32 ± 6.06
IMA (°)	17.22 ± 6.16
CSA (cm2)	1.61 ± 0.38
Muscle tone (Hz)	20.76 ± 0.99
Muscle stiffness (N/m)	458.30 ± 15.31
Ankle D/F (°)	16.70 ± 4.58
Ankle P/F (°)	47.75 ± 3.64
1st MTP flexion (°)	44.50 ± 1.63
1st MTP extension (°)	66.66 ± 4.62

Values are presented as mean ± standard deviation. HVA, hallux valgus angle; IMA, intermetatarsal angle; CSA, cross-sectional area; D/F, dorsiflexion; P/F, plantarflexion; MTP, metatarsophalangeal..

Table 3 . Summary of correlation analysis (N = 60, 120 feet).

Variable	HVA	IMA	CSA	Muscle tone	Muscle stiffness	Ankle D/F	Ankle P/F	Toe flexion	Toe extension
HVA (°)	1	0.858*	–0.337**	–0.889**	–0.847**	–0.307**	0.312**	–0.197*	–0.182*
IMA (°)	0.858*	1	–0.295*	–0.786**	–0.721**	–0.298*	0.304**	–0.186*	–0.191*
CSA (cm2)	–0.337*	–0.295*	1	0.332**	0.303**	0.421**	0.219	–0.105	–0.167
Muscle tone (Hz)	–0.889**	–0.786**	0.332**	1	0.924**	0.205*	0.155	–0.113	–0.118
Muscle stiffness (N/m)	–0.847**	–0.721**	0.303**	0.924**	1	0.198*	0.132	–0.107	–0.167
Ankle D/F (°)	–0.307**	–0.298*	0.421**	0.205*	0.198*	1	0.380**	0.371**	0.514**
Ankle P/F (°)	0.312**	0.304**	0.219	0.155	0.132	0.380**	1	0.359**	0.331**
1st MTP flexion (°)	–0.197*	–0.186*	0.105	0.113	0.107	0.371**	0.359**	1	0.336**
1st MTP extension (°)	–0.182*	–0.191*	0.167	0.118	0.167	0.514**	0.331**	0.336**	1

HVA, hallux valgus angle; IMA, intermetatarsal angle; CSA, cross-sectional area; D/F, dorsiflexion; P/F, plantarflexion; MTP, metatarsophalangeal. *p < 0.05, **p < 0.01..

In addition, we analyzed 120 feet, divided them into mild and moderate-severity groups according to the criteria [3], and performed correlation analyses for each group separately. The results showed a positive correlation between the HVA and the IMA in both groups: r = 0.840 (p < 0.05) in the mild group and r = 0.506 (p < 0.05) in the moderate group, with a more pronounced correlation in the moderate group.

2. Cross-sectional Area of the Abductor Hallucis Muscle

The average CSA was 1.61 cm² (Table 2). A significant negative correlation was observed between HVA and CSA (r = –0.337, p < 0.05), indicating that an increase in HVA was associated with a decrease in the CSA of the AbdH. Similarly, the IMA showed a negative correlation with CSA (r = –0.295, p < 0.05), suggesting that a higher IMA corresponds to a smaller CSA of the AbdH muscle.

3. Muscle Tone of the Abductor Hallucis Muscle

The average muscle tone of the AbdH was 20.76 Hz, and the average muscle stiffness was 458.30 N/m (Table 2). A very strong negative correlation was found between HVA and muscle tone (r = –0.889, p < 0.01), indicating that higher HVA values are significantly associated with reduced muscle tone. Additionally, the IMA exhibited a strong negative correlation with muscle tone (r = –0.786, p < 0.01).

4. Range of Motion of the Ankle and Great Toe

The average ankle D/F and P/F values were 16.70° and 47.75°, respectively (Table 2). Ankle D/F was negatively correlated with HVA (r = –0.307, p < 0.01) and IMA (r = –0.298, p < 0.05), indicating that higher HVA and IMA values were associated with reduced ankle D/F. However, ankle P/F was positively correlated with HVA (r = 0.312, p < 0.01) and IMA (r = 0.304, p < 0.01), suggesting that as HVA and IMA increased, ankle P/F increased.

The average great toe flexion and extension angles were 44.50° and 66.66°, respectively (Table 2). MTP flexion decreased as the HVA (r = –0.197, p < 0.05) and IMA (r = –0.186, p < 0.05) values increased. The MTP joint extension decreased as the HVA (r = –0.182, p < 0.05) and IMA (r = –0.191, p < 0.05) values increased.

5. Plantar Pressure During Gait

Plantar pressure distribution analysis indicated higher pressures in the M2 and HM regions than in the other zones. Specifically, the M2 region exhibited an average pressure of 166.43 N, and the HM region showed an average pressure of 142.05 N. Pearson correlation analyses revealed significant negative correlations between both HVA and IMA with Toe, M1, M5, MF, and HC, while significant positive correlations between both HVA and IMA with M2, M3, M4, HL, and HM (Table 4). This result indicates more weight-bearing on the medial aspect of the forefoot than on the central and lateral aspects of the stance foot. The relationship between the mean plantar pressure during walking and the HVA for each part of the foot is shown in Table 4.

Table 4 . Correlation analysis between plantar pressure and HVA and IMA.

Variable		Value (N)	Correlation
			HVA (r)	IMA (r)
	Toe	84.01 ± 12.02	–0.247**	–0.157
	M1	82.37 ± 13.51	–0.347**	–0.258**
	M2	166.43 ± 21.98	0.457*	0.370**
	M3	86.66 ± 16.03	0.398**	0.366**
	M4	60.50 ± 8.82	0.195*	0.142
	M5	44.94 ± 3.97	–0.248**	–0.193*
	MF	51.23 ± 11.46	–0.335**	–0.254**
	HL	139.36 ± 28.02	0.380**	0.355**
	HC	74.53 ± 13.36	–0.197*	–0.194*
	HM	142.05 ± 29.18	0.221*	0.172

Values are presented as mean ± standard deviation. HVA, hallux valgus angle; IMA, intermetatarsal angle; M1, the 1st metatarsal region representing the medial forefoot; M2, the 2nd metatarsal region representing the medial forefoot; M3, the 3rd metatarsal region representing the middle forefoot; M4, the 4th metatarsal region representing the lateral forefoot; M5, the 5th metatarsal region representing the lateral forefoot; MF, the mid-foot region; HM, medial heel region; HC, central heel region; HL, lateral heel region. *p < 0.05, **p < 0.01..

The primary purpose of this study was to determine whether there is a relationship between the HV severity and the thickness and tone of the AbdH. The secondary objective was to examine the relationship between HV severity and plantar pressure during the stance phase of the gait. Third, we aimed to investigate the relationship between HVA and the ROM of ankle joint and the first MTP joint.

The HVA and IMA have been widely regarded as reliable standard measurements for diagnosing HV since Mann and Coughlin [3] proposed their diagnostic guidelines. In this study, the mean HVA was 24.32 ± 6.06, and the mean IMA was 17.22 ± 6.16, indicating that our subjects generally fall into the moderate severity category. In addition, the results showed a high correlation between the two values, confirming that both measurements were valuable diagnostic factors, as proposed by Mann and Coughlin [3].

Muscle strength is a general term representing the ability of a muscle to generate force, whereas muscle tone refers to the baseline level of muscle tension during relaxation [20]. The CSA of a muscle, which is a direct measure of muscle mass, is closely associated with its capacity to generate force. A larger CSA generally indicates greater force production capability [21]. Maintaining normal foot biomechanics and joint alignment requires adequate muscle strength and tone in the foot and ankle [20,21]. The AbdH is a key intrinsic muscle of the foot responsible for abducting and flexing the great toe, supporting the medial longitudinal arch and stabilizing the first MTP joint [22,23]. In the context of HV, a reduction in the CSA of the AbdH suggests muscle atrophy, which corresponds to decreased muscle strength [22,23]. Muscle tone, defined as the continuous and passive partial contraction of muscles, contributes to posture and joint stability [16,24,25]. Therefore, a decrease in the CSA and muscle tone of the AbdH indirectly reflects a loss of muscle strength and an impaired ability to support the medial arch and stabilize the great toe [24,25]. This underscores the importance of the CSA and muscle tone as functional elements for evaluating muscle health and performance.

Our study found a moderately negative correlation between the severity of the HVA and CSA of the AbdH (r = –0.337, p < 0.05), which aligns with previous research indicating that increased HV severity is associated with reduced muscle thickness and CSA, reflecting the impact of altered anatomical positioning of the muscle [2,22,23]. We also found a strong negative correlation between HVA and AbdH muscle tone (r = –0.889, p < 0.05) and stiffness (r = –0.847, p < 0.05). Previous studies have shown that weakened AbdH, indicated by reduced muscle tone and CSA, impairs medial arch support and MTP joint stability; the well-established relationship between CSA and muscle tone underscores that larger CSA and higher tone generally reflect greater muscle strength and better function, with atrophy in the HV leading to decreased CSA and tone, impacting muscle functionality and alignment [22,26]. Therefore, reduced CSA and muscle tone are indicative of AbdH weakness and functional capacity of the AbdH and HV severity.

Our study also examined the relationship between the HVA and ROM of the ankle and great toe. We observed a negative correlation between HVA and ankle D/F (r = –0.307, p < 0.01), indicating that as the severity of HV increased, the ability to dorsiflex the ankle decreased, and vice versa. Conversely, a positive correlation was found between HVA and ankle P/F (r = 0.312, p < 0.01), suggesting that greater HVA severity is associated with increased P/F. Mann and Coughlin [3] supported the hypothesis that tense Achilles tendons are prone to HV. Additionally, the ROM of the great toe MTP joint was assessed, which revealed a decrease in both flexion and extension with increasing HVA. These results suggest that the deformity associated with HV not only affects the structure of the first MTP joint but also limits its functional mobility. A reduced ROM of the great toe can lead to a compromised toe-off phase during walking, which in turn affects gait efficiency and stability.

This study explored the relationship between the HVA and plantar pressure distribution during the stance phase of walking. We observed that, as the HVA increased, the pressure exerted on the M1 area decreased, whereas the pressure exerted on the M2 and M3 heads increased. This finding corresponds with Moon et al. [2] one-leg standing study, which found that individuals with HV exhibited reduced weight-bearing on the first metatarsal and increased weight-bearing on the lateral side while standing on one leg. Togei et al. [7] also reported that compared to healthy individuals, those with HV showed increased weight-bearing in the central forefoot during walking. This could be due to a lack of ROM and pain in the great toe, which hinder the toe-off action during walking, resulting in decreased weight bearing. This finding supports the concept that as the HVA increases, compensatory pressure increases in the central forefoot due to medial deviation of the first metatarsal and MTP joints.

One notable limitation of this study was the relatively small sample size and lack of direct measurements of the AddH, which plays a significant role in HV biomechanics. Due to its anatomical position in a deeper layer, the AddH is not directly measurable in terms of muscle tone and CSA [27].

Future research should include assessments of the AddH and the incorporation of electromyography (EMG) to gain deeper insights into muscle activity and neuromuscular control. Furthermore, EMG was not directly performed in this study because of the difficulty subjects with HV experience in accurately performing isolated great toe abduction movements, making it challenging to obtain meaningful measurements [28].

This study provides information on the relationship between HV severity and morphological changes in muscle and plantar pressure distribution during gait. Furthermore, longitudinal studies are suggested to track changes over time and evaluate the effects of comprehensive interventions, including AbdH strengthening exercises, AddH stretching, and self-management education. To address muscle imbalances associated with HV, previous studies have suggested that toe-spread-out exercises strengthen the weakened AbdH [23]. Moon et al. [2] proposed that stretching the shortened adductor hallucis is beneficial. Buchanan et al. [29] also indicated that shortening of the GCM and soleus muscles can exacerbate plantar fascia tightness, further aggravating the muscle imbalance between the AbdH and adductor hallucis [30].

Based on this evidence, incorporating combination exercises that aim to improve ankle ROM, stretch the shortened adductor hallucis, and strengthen weakened AbdH is recommended. Such research is crucial for developing evidence-based guidelines for the management and treatment of HV and ultimately improving patient outcomes.

This study highlights the significant relationships between HV severity and various biomechanical factors, including muscle atrophy, reduced muscle tone, limited ROM, and altered plantar pressure distribution. These findings underscore the importance of comprehensive assessments and targeted interventions to address the multifaceted impairments associated with HV. Strengthening exercises for the AbdH, stretching the AddH, and self-management education are crucial components of effective treatment plans aimed at improving foot function and alleviating symptoms in individuals with HV.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: KAM. Data curation: KAM, YJK. Formal analysis: KAM. Investigation: KAM. Methodology: KAM. Project administration: KAM, YJK. Resources: KAM, YJK. Software: KAM. Supervision: HSJ. Writing - original draft: KAM, HSJ. Writing - review & editing: KAM, HSJ.