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Abstract

BACKGROUND:

The abductor hallucis (AbH) is a key muscle in the stability of the foot by supporting the medial longitudinal arch (MLA). Individuals with flat foot show a flattening of the MLA with a decreased selective AbH activity relative to the other foot extrinsic muscles during functional movement.

OBJECTIVE:

To examine the selective muscle activity of the abductor hallucis (AbH) during single-leg mini-squat (SMS) in subjects with flat foot and normal individuals.

METHODS:

Twenty-four healthy adults (13 men and 11 women) with flat or normal feet participated in this study. All subjects performed single minisquat (SMS), and data were collected using sEMG from the AbH, TA, and GCM during SMS.

RESULTS:

An inter-gender comparison of the EMG data revealed no difference. The EMG activity of the AbH in individuals with flat foot was significantly lower than that of individuals with normal feet. Moreover, the AbH/TA ratio in subjects with flat foot, , was significantly lower than that in subjects with normal feet.

CONCLUSION:

In a clinical setting, clinicians should focus on strengthening exercises of the AbH muscle due to reduced activation of the AbH muscle in individuals with flat foot. The selective muscle activity of the AbH muscle during SMS should be emphasized.

Keywords

Flat foot abductor hallucis tibialis anterior selective muscle activation

1. Introduction

Flat foot refers to an altered foot shape with an abnormally low medial longitudinal arch (MLA), resulting in a flat sole of the foot; adult flat foot can be defined as the MLA becoming lower and flatter during body weight support and gait in propulsion phase [1, 2]. Mechanical changes and associative diseases in the lower extremity can develop in relation to flat foot in situations such as heavy loading or walking for a long time. Usually, collapse of the MLA due to flat foot results in problems such as hallux valgus or plantar fasciitis [3]. It can also result in genu valgum, which can cause mechanical changes in the lower extremity [4], and compared to individuals with normal feet, flat foot can also increase fatigue [5]. The most common cause of flat foot is joint hypermobility, but there are also many cases of uncertain etiology. In addition to congenital causes, flat foot can also be caused by fracture or other trauma, joint lesions, such as rheumatoid arthritis, or tibialis posterior dysfunction, and weakening of the muscles supporting the MLA [6]. In terms of muscles, weakening of the intrinsic muscles such as the abductor hallucis (AbH), which contribute to the stability of the MLA, is the most noticeable, and this is known to cause dysfunction in the lower extremities [7].

Abductor hallucis is an important muscle in the stability of the foot, by supporting the MLA [8]. In flat foot, which typically shows pronated foot and flattening of the MLA, the AbH muscle activity is reduced during the stance phase, in which the foot is resting on the ground [9]; there have also been reports of reduced AbH tension, which contributes to both the static and dynamic stability of the foot [10]. In recent studies, when performing gait or other functional movements, individuals with flat foot showed increased activity in the extrinsic muscles of the foot, such as the tibialis anterior and the calf muscles, in order to stabilize the foot [11, 12]. However, there is still a lack of research investigating selective muscle activity of the AbH relative to the extrinsic muscles during functional activity.

The single-leg mini-squat (SMS) tests the functional movements of the lower extremities, in which the examiner assesses the subject’s ability to maintain alignment of the knee and foot while bending one knee in a standing position [13]. In previous research on the muscles of the lower extremities, the SMS has been used to examine knee kinematics [14] and the effects of muscles on the hip and knee joints [15]. However, despite its importance in foot stability, there is a lack of study on the selective activity of the AbH and its contribution to foot stability during functional movements.

Table 1
Demographic characteristics of the subjects ( $n=$ 24, 13 men and 11 women)

Characteristic	Flat foot group ( $n=$ 12) (7 men and 5 women)	Normal foot group ( $n=$ 12) (6 men and 6 women)	$p$
Age (years)	23.41 $\pm$ 2.09	25.58 $\pm$ 7.25	0.410
Height (cm)	169.41 $\pm$ 937	16816 $\pm$ 10.43	0.774
Body weight (kg)	6283 $\pm$ 945	6691 $\pm$ 1540	0.464
Foot posture index	741 $\pm$ 149	2.00 $\pm$ 1.15	$<$ 0.001

Values are means $\pm$ standard deviation (SD).

Individuals with flat foot could show even greater reduction in selective activation of the AbH muscle relative to the foot extrinsic muscles, and there is a need to investigate changes in selective muscles activity of the AbH during functional movements. In this study we aimed to measure the extrinsic and intrinsic muscle activity of the foot in individuals with flat feet and normal feet performing SMS, with a specific focus on analyzing the selective muscle activity of the AbH.

2. Methods

2.1 Subjects

The participants in this study consisted of 24 subjects (13 men and 11 women) currently enrolled at Gimhae College in Gimhae-si. Among the participants, individuals who had a history of orthopedic surgery on the foot or ankle, or who reported lower extremity pain or ankle instability were excluded. The participants’ mean age, height, and weight are shown in Table 1. All participants consented to participate in the study based on a thorough understanding of the study’s intentions and signed a consent form, which had been approved by the Gimhae College institutional review board (IRB approval #GHCIRB-2019-001 of 31 October 2019).

Before the sEMG participants were divided into two groups (flat feet vs. normal feet) based on the results of assessment using the foot posture index. The foot posture index is a means of classifying flat foot by testing the following six items for the dominant foot: 1) talar head palpation, 2) supra and infra lateral malleolar curvature, 3) inspection of calcaneus in the frontal plane, 4) talonavicular joint margin, 5) MLA height, and 6) examination of the forefoot and rearfoot on abduction and adduction. Each item is scored on a scale from –2 to 2; the test is performed on the dominant side, and the sum of the scores is calculated. The results are classified as highly supinated: $<$ $-$ 5, supinated: –4 to $-$ 1, normal: 0 to $+$ 5, pronated: $+$ 6 to $+$ 9, or highly pronated: $>$ $+$ 10. In our study, participants with a classification of pronated or highly pronated were placed in the flat foot group [16]. Table 1 shows demographic data for the flat foot (7 men and 5 women) and normal foot groups (6 men and 6 women).

2.2 Instrumentation

Surface EMG was measured from the TA, GCM, and AbH while performing the SMS movement. A wireless telemetry system (TeleMyo Desktop DTS, Noraxon, Scottsdale, AZ, USA) was used for data collection, and MyoResearch Master Edition 3.10 (Noraxon, Scottsdale, AZ, USA) was used to analyze collected data. The sampling frequency for the EMG signal was 2000 Hz, the signal was bandpass-filtered at 20–350 Hz to eliminate noise, and a notch filter of 60 Hz was used. The EMG signals measured during contraction of each muscle in the SMS test were processed as root mean squares (RMS). All mean RMS data was expressed as the %MVIC, and the mean %MVIC values were used in data analysis to compare the flat and normal foot groups.

Table 2
Comparison of muscle activities and ratio between men and women within each group

Characteristic	Flat foot group ( $n=$ 12) (7 men and 5 women)	$p$	Normal foot group ( $n=$ 12) (6 men and 6 women)	$p$
AbH (%MVC)
Men	59.82 $\pm$ 21.85	0.172	79.45 $\pm$ 28.72	0.358
Women	43.98 $\pm$ 17.25		96.33 $\pm$ 38.55
TA (%MVC)
Men	28.62 $\pm$ 12.14	0.146	27.27 $\pm$ 7.31	0.209
Women	51.81 $\pm$ 29.26		32.51 $\pm$ 22.29
GCM (%MVC)
Men	31.31 $\pm$ 8.12	0.098	33.27 $\pm$ 14.16	0.141
Women	48.81 $\pm$ 18.99		39.38 $\pm$ 22.59

Values are means $\pm$ standard deviation (SD). AbH: Abductor hallucis muscle, TA: Tibialis anterior, GCM: Gastrocnemius. %MVC: Percentage of maximal voluntary contraction.

2.3 Procedure

The age, height, and weight of the 24 participants were measured. sEMG electrodes were placed on the tibialis anterior (TA), gastrocnemius (GCM), and AbH muscles. The position of the TA electrode was lateral to the medial shaft of the tibia, one-third of the distance from the fibular head to the medial malleolus [17]; the position of the GCM electrode was one-third of the distance from the fibular head to the calcaneus [17]; the position of the AbH electrode was 1–2 cm posterior to the navicular tuberosity, anterior to the medial malleolus [17]. Before attaching the electrodes, the attachment sites were shaved to minimize resistance, wiped with alcohol-soaked cotton, and dried. After attaching the sEMG device, the maximal voluntary isometric contraction (MVIC) of each muscle was measured [18].

Table 3
Comparison of muscle activities and ratio between flat foot and normal foot during small knee bending

Characteristic	Flat foot group ( $n=$ 12)	Normal foot group ( $n=$ 12)	$p$
AbH (%MVC)	52.82 $\pm$ 22.36	9289 $\pm$ 3817	0.005
TA (%MVC)	38.23 $\pm$ 2501	29.88 $\pm$ 17.53	0.351
GCM (%MVC)	3860 $\pm$ 1694	3632 $\pm$ 1994	0.766
AbH/TA ratio	191 $\pm$ 0.31	445 $\pm$ 397	0.440
AbH/GCM ratio	1.85 $\pm$ 1.15	3.34 $\pm$ 2.53	0.078

Values are means $\pm$ standard deviation (SD). AbH: Abductor hallucis muscle, TA: Tibialis anterior, GCM: Gastrocnemius. %MVC: Percentage of maximal voluntary contraction.

To measure the MVIC for TA, in a sitting position, the participant first performed slight ankle dorsiflexion and then, while giving the start signal, the examiner applied resistance in the direction of plantar flexion and eversion. To measure the MVIC for GCM, lying in a prone position, the participant first performed ankle plantar flexion and then the examiner applied maximal resistance in the direction of dorsiflexion. To measure the MVIC for AbH, with the participant sitting in a straight position, the examiner first held the participant’s heel and then used the other hand to apply maximal resistance on the first digit in the adduction direction while instructing the participant to try to perform abduction. To prevent muscle fatigue, participants were given 2 min of rest between movements. For each muscle, maximal contraction was maintained for 5 s, and after repeating the procedure three times, the mean value was measured and used in normalization data for the %MVIC. After attaching the EMG electrodes, participants stood on their dominant leg on a Y-balance board (Move2Perform, Evansville, IN), and maintained a position of 30 ${}^{\circ}$ flexion of the supporting knee; the non-dominant leg was stretched forward as far as possible and this position was maintained for 5 s. This process was repeated three times in total. Before data collection, the participant was provided ample opportunity to practice. If the participant lost their balance or their strength failed during the SMS, the movement was repeated. Participants were given 2 min of rest between each repetition.

2.4 Statistical analysis

Mean $\pm$ standard deviation (SD) was calculated for the muscle activity of each muscle. To examine the inter-gender differences in each group, independent t-test were used. To investigate selective AbH activity relative to the extrinsic muscles, the AbH/TA and AbH/GCM ratios were calculated. Independent $t$ -tests were used to compare the results between the flat and normal foot groups. The significance level was set to $p=$ 0.05. PASW (Predictive Analytics Software) for Windows version 18.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analysis.

3. Results

Table 1 shows the participants’ general characteristics. All participants were right-foot dominant. Using the %MVIC of each group, it was confirmed that there were no significant differences in all variables upon inter-gender comparison within each group ( $p>$ 0.05) (Table 2) The AbH activity in flat foot group (52.82 $\pm$ 22.36 %MVC) was significantly lower than that in normal footed group (92.89 $\pm$ 38.17 %MVC) ( $p<$ 0.05). In addition, no significant differences in each TA and GCM muscle activity data were observed between flat foot and normal footed group (TA: $p=$ 0.351; GCM: $p=$ 0.766).

There was a significant difference in the AbH/TA ratio between flat foot (1.91 $\pm$ 0.31) and normal footed group (4.45 $\pm$ 3.97) ( $p<$ 0.05), but no difference in the AbH/GCM ratio between the former (1.85 $\pm$ 1.15) and latter group (3.34 $\pm$ 2.53) ( $p>$ 0.05), respectively (Table 3).

4. Discussion

The purpose of this study was to investigate selective contraction of the AbH in providing foot stability while performing functional movements, such as the SMS, and to compare the results between flat-footed and normal-footed participants. The results showed that, compared to normal-footed participants, flat-footed participants showed significantly less activity in the AbH while performing the SMS, and also showed a significantly lower AbH/TA ratio.

Regarding the EMG results, participants with flat foot showed significantly reduced AbH activity compared to normal-footed participants. These results are similar to those of a previous study that compared AbH activity between flat- and normal-footed participants in a one-leg standing task [15]. This study indicates that flat-footed participants are less able to use AbH activity to support the height of the medial longitudinal arch and maintain foot stability during functional movements [19]. Although we did not observe a significant difference in TA activation, there was a trend-level increase; similar results have been reported in a previous study [20]. This could be a cause of stress fracture [21], Achilles tendon disorder [22], or tibialis anterior overuse syndrome [23] in flat-footed individuals, as persistent contraction of TA results in increased fatigue.

Although flat-footed participants did not show a significant difference in GCM or TA activity compared to normal-footed participants, they showed a significant decrease in selective AbH activity relative to TA activity (AbH/TA). The SMS position requires participants to maintain a 30 ${}^{\circ}$ flexion of the knee, which is accompanied by ankle dorsiflexion. As an ankle dorsiflexor, the TA performs reverse-mover action during knee flexion, which can result in even greater concentric contraction activity [24]. To maintain active support of the MLA, which flattens as it supports the body weight, there may be activation of muscles such as the TA, tibialis posterior, flexor hallucis longus and brevis, and AbH [25]. As we observed in our results, in flat-footed participants, reduced AbH activity resulted in less support for the MLA; it is thought that to provide a counterforce to shift from a pronated foot position towards a supinated foot position, activity in the foot extrinsic muscles such as the TA increases, resulting in a decreased selective AbH activity ratio.

Because this study was conducted with a relatively small number of subjects, there are some limitations in generalizing the results; we were also unable to account for differences in muscle strength between males and females. Another limitation of the study is that we were unable to measure the MLA angle to identify the flat foot condition more precisely. These limitations will need to be improved upon in future studies.

5. Conclusions

This study discovered that flat-footed individuals show a deficiency in selective contraction of the AbH during activation of the foot extrinsic muscles to provide stability to the foot during gait and other functional movements. This shows that selective contraction of the AbH is important in interventions for individuals with flat foot.

Author contributions

CONCEPTION: Dong-Chul Moon.

PERFORMANCE OF WORK: Jun-Seok Kim.

INTERPRETATION OR ANALYSIS OF DATA: Jun-Seok Kim.

PREPARATION OF THE MANUSCRIPT: Dong-Chul Moon.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Jun-Seok Kim.

SUPERVISION: Dong-Chul Moon.

Ethical considerations

All participants consented to participate in the study based on a thorough understanding of the study’s intentions and signed a consent form, which had been approved by the Gimhae College institutional review board (IRB approval #GHCIRB-2019-001 of 31 October 2019).

Funding

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015R1D1A1A01056853).

Footnotes

Acknowledgments

The authors have no acknowledgments.

Conflict of interest

The authors declare that they have no competing interest.

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