Physical Therapy Korea

The foot is a highly complicated structure that is supported by both passive structures, including ligaments and fasciae, and active structures, including the intrinsic and extrinsic foot muscles [1,2]. Furthermore, the effective function of the foot relies on the medial longitudinal arch (MLA), which contributes to the absorption of the external shock during running and walking as well as to transferring the forces through the entire foot [3]. Intrinsic foot muscles have been suggested to play a crucial role in supporting the MLA, as they manage the velocity of the arch deformity, providing critical stabilization and solidity to the foot [1,4,5].

Pes planus, characterized by excessive pronation of the foot, is a medical condition in which the MLA diminishes or flattens [3,6]. Lee et al. [7] compared the activity of intrinsic foot muscle, including the abductor hallucis (AbdH), and extrinsic foot muscle, including the tibialis anterior (TA), between individuals with pes planus and those with a neutral foot alignment; individuals with pes planus showed significantly lower AbdH activity and higher TA activity than those with neutral foot. Previous studies have demonstrated that in individuals with excessively pronated foot, increased extrinsic foot muscle activity functioned as a compensatory mechanism to offset the weakened intrinsic foot muscles [7-9]. Furthermore, Headlee et al. [10] demonstrated that a notable descent of the navicular bone is associated with increased fatigue of the AbdH.

Lowering the MLA can precipitate a range of complications, including plantar fasciitis, hallux valgus, genu valgum, and hip joint instability, thereby resulting in issues across the entire lower extremity [11,12]. To address these problems, passive stability through various orthotic interventions and active stability exercises have been used to alleviate the symptoms [6,7,13]. In a previous study, a foot insole for passive stability was used to support the MLA and decrease AbdH fatigue in individuals with pes planus [14]. However, recent rehabilitation programs have aimed at patients participating spontaneously; at this point, foot orthoses remain passive, and there is a need for clinicians to investigate active exercises to improve excessive pronation [15,16].

Therefore, many researchers have emphasized that intrinsic foot muscle strengthening is essential for the management of excessive foot pronation. In clinical practice, the most widely used intrinsic foot muscle strengthening exercises are the short foot exercise (SFE), toe spread-out exercise (TSO), towel curl, and picking up objects. Of these, the SFE, which can recruit the intrinsic foot muscles separate to the extrinsic foot muscles, including the TA, peroneus longus (PL), and flexor digitorum longus, has become a typical intervention to control excessive pronation [3,17]. Additionally, a new intrinsic foot muscle exercise, the TSO, was introduced specifically for the management of hallux valgus [18]. AbdH activity was found to be significantly higher during the TSO than the SFE in the hallux valgus group [19]. Furthermore, the TSO has been explored as a means of increasing AbdH activity and providing support to the MLA in patients presenting with pes planus [14,20].

However, to date, no study has compared the effects of the SFE and TSO in different weight-bearing conditions in individuals with pes planus. Therefore, the purpose of this study was to compare the effects of the SFE and TSO on AbdH, TA, and PL activity, in addition to the AbdH/TA and AbdH/PL activity ratios in the sitting and standing positions in individuals with pes planus. We hypothesized that there would be differences in the activity of the AbdH, TA, and PL, and the AbdH/TA and AbdH/PL activity ratios, not only between the SFE and TSO, but also between the sitting and standing positions.

1. Participants

G*power software (version 3.1, Heinrich-Heine-Universität) was used to estimate the sample size. An adequate sample size of 10 participants was determined from a pilot study of six participants to achieve a power of 0.80, an effect size 0.041, and α level of 0.05. To consider for potential dropouts and obtain the clinical significance, a total of 20 participants (15 males and 5 females; age: 23.8 ± 3.6 years, height: 171.6 ± 6.5 cm, weight: 76.9 ± 13.6 kg, body mass index: 26.0 ± 4.4 kg/m2, navicular drop: 10.6 ± 1.2 cm) individuals with pes planus were recruited. Pes planus was assessed by measuring the length of the navicular drop; the researcher examined the height of the navicular bone from the floor in the sitting and standing positions, and only participants with a navicular drop > 10 mm participated in this study [21]. Participants with toe deformities, such as hallux valgus or claw toe, repeated lower extremity injuries in the past 6 months, and a history of ankle or foot surgery were excluded [21,22]. Furthermore, participants who were unable to correctly perform the SFE and TSO were excluded.

This study was approved by the Institutional Review Board of Yonsei University Mirae campus (IRB no. 1041849-202212-BM-235-01). All procedures were explained by the researcher before data collection and each participant signed a written informed consent form after a detailed explanation.

2. Instrumentation

To assess the amplitudes of AbdH, TA, and PL activity, surface electromyography (EMG; Noraxon TeleMyo DTS Wireless System, Noraxon Inc.) was utilized with a wireless system. The sampling rate was 1,000 Hz, and we proceeded with root mean square data. Raw EMG signals were notch filtered at 60 Hz and bandpass filtered at 10 to 450 Hz.

3. Procedures

The researcher demonstrated the SFE and TSO correctly to familiarize all participants with both exercises. Participants who presented with bilateral navicular drops selected the worse limb for data collection. All four conditions (SFE + sitting, SFE + standing, TSO + sitting, TSO + standing) were performed in a randomized order generated using Microsoft Excel (Microsoft Corp.).

Each participant performed every exercise thrice, and each task was held for 5 seconds. A 2-minute rest period was given between trials, and a 10-minute rest period was given between exercises to avoid muscle fatigue and learning effects. The mean value of middle 3 seconds of the data, excluding the first and last seconds, was recorded in three trials. The mean of three trials was calculated for data analysis. The gathered EMG amplitudes for the activity of the AbdH, TA, and PL during the SFE and TSO in the sitting and standing positions were expressed as a percentage of the mean maximal voluntary isometric contraction (%MVIC).

Participants placed both feet on a wooden box at a width similar to their pelvic width. Participants sat on the edge of the table, and their hip and knee were maintained at 90° by using a height controller of the table [22]. Participants were instructed to keep their spine upright and neutral position of the pelvis, in addition to positioning the second toe and calcaneus in a straight line to maintain exact foot position during the exercise. Participants drew the heads of the metatarsals towards the heel to elevate the MLA and were not allowed to flex their toes [3]. They were instructed to hold their forefoot and rearfoot in contact with a wooden box during the entire exercise program [22] (Figure 1A).

Figure 1. (A) SFE-SIT and (B) SFE-STA. SFE, short foot exercise; SIT, in the sitting position; STA, in the standing position.

Participants placed both feet on a wooden box at a width similar to their pelvic width. Participants were asked to stand while maintaining their spine upright and neutral position of the pelvis. The following procedures were exactly the same as those performed for the SFE-SIT (Figure 1B).

Participants placed both feet on a wooden box at a width similar to their pelvic width. Participants sat on the edge of the table and their hip and knee were maintained at 90° by using a height controller of the table [22]. Participants were instructed to keep their spine upright and neutral position of the pelvis, in addition to positioning the second toe and calcaneus in a straight line to maintain exact foot position during the exercise. Participants extended all five toes and abducted them while remaining the heads of the metatarsals and heels on a wooden box. The first toe was then flexed in the medial direction, and the fifth toe was flexed in the lateral direction by pushing a wooden box. They also kept extending the second to forth toes [1,18,20]. They were instructed to hold their forefoot and rearfoot in contact with a wooden box during the entire exercise program [22] (Figure 2A).

Figure 2. (A) TSO-SIT and (B) TSO-STA. TSO, toe spread-out exercise; SIT, in the sitting position; STA, in the standing position.

Participants placed both feet on a wooden box at a width similar to their pelvic width. Participants were asked to stand while maintaining their spine upright and neutral position of the pelvis. The following procedures were exactly the same as those performed for the TSO-SIT (Figure 2B).

4. EMG Data Collection and Analysis

The skin was cleaned with alcohol pad before attaching the EMG electrodes to ensure exact electrode contact and transmission. Skin hair was also shaved to minimize disturbance while collecting the EMG data. Two disposable surface electrodes (Ag/AgCl) were attached in the direction of each muscle fiber with 2 cm distance between the two electrodes. The locations of the electrodes were as follow: For the AbdH, two electrodes (2 cm apart) were placed 2 cm behind the navicular tuberosity, anterior to the perpendicular axis from the front of the medial malleolus [23]. For the TA, two electrodes (2 cm apart) were located parallel to and just lateral to the medial shaft of the tibia, approximately 1/4 to 1/3 of the distance between the knee and ankle. For the PL, two electrodes (2 cm apart) were attached at 1/4 of the axis between the lateral malleolus and the fibular head [24].

All EMG signals were normalized to the MVIC. Each MVIC was calculated using standard manual muscle-testing method [25]. For the AbdH, in the sitting position, the heel was fixed tightly with the ankle in the neutral position, resistance was subsequently applied to the medial side of the first proximal phalanx by the researcher. Participants were instructed to abduct the first toe maximally against the applied resistance [18]. For the TA, in the sitting position, the heel was fixed tightly in neutral alignment with the ankle joint, and resistance to the dorsomedial side of the foot was applied by the researcher. Participants were instructed to perform maximum dorsiflexion against the applied resistance. For the PL, the researcher applied resistance on the plantar and lateral aspects of the foot in the supine position with neutral alignment of the ankle joint. Participants were instructed to perform plantar flexion and maximum eversion against the applied resistance [26]. Participants performed three trials of MVIC data collection with a 5-second hold. They had a 2-minute rest period between trials to minimize muscle fatigue. Data from the middle 3 seconds were used to determine the mean values. The gathered EMG amplitudes of the activity of the AbdH, TA, and PL during the SFE and TSO in the sitting and standing positions were recorded as %MVIC.

5. Statistical Analysis

IBM Statistics 26.0 software (IBM Corp.) was used to perform all statistical analyses, and the level of statistical significance was set at 0.05. The normality of the distribution was examined using the Shapiro–Wilk test. Two-way repeated analyses of variance with two within factors (exercise: SFE and TSO; position: sitting and standing) was performed to assess the statistical significance of AbdH, TA, and PL activity, in addition to the AbdH/TA and AbdH/PL activity ratios. If a significant interaction was not revealed, the main effects of exercise and position were identified. If a significant interaction was revealed, the simple effects were determined by pairwise comparisons with Bonferroni correction (α = 0.05/4 = 0.0125).

No significant interaction was observed regarding the activity of the AbdH (F = 0.086, p = 0.773) and TA (F = 1.982, p = 0.175), and the AbdH/TA (F = 1.866, p = 0.188) and AbdH/PL (F = 2.150, p = 0.159) activity ratios. However, there was a significant interaction in terms of PL activity (F = 6.060, p = 0.024).

There was a significant main effect regarding the exercise (F = 23.109, p < 0.001) and position (F = 23.747, p < 0.001) on AbdH activity. AbdH activity was significantly higher during the TSO than the SFE (58.976 %MVIC and 32.622 %MVIC, respectively; Figure 3A), in addition, it was significantly higher in the standing position than in the sitting position (53.209 %MVIC and 38.389 %MVIC, respectively; Figure 3B). TA activity had a significant main effect of position (F = 6.771, p = 0.018), but no significant main effect of exercise (F = 0.271, p = 0.609). The activity of the TA was significantly higher in the standing position than in the sitting position (15.392 %MVIC and 10.302 %MVIC, respectively; Figure 4A). PL activity significantly increased during the TSO-STA compared to the SFE-STA (p = 0.005); however, there was no significant simple effect between the SFE-SIT and TSO-SIT (p = 0.188). In addition, it was significantly higher in the SFE-STA and TSO-STA conditions than in the SFE-SIT and TSO-SIT conditions, respectively (both p < 0.001) (Figure 4B). The AbdH/TA activity ratio was not significantly affected by the exercise (F = 0.363, p = 0.554) or position (F = 0.025, p = 0.877). The AbdH/PL activity ratio showed a significant main effect of position (F =7.202, p = 0.015); however, there was no significant main effect of exercise (F = 0.255, p = 0.620). The AbdH/PL activity ratio was significantly higher in the sitting position than in the standing position (Figure 5).

Figure 3. Comparison of AbdH activity during (A) the SFE and TSO and (B) in the sitting and standing positions. %MVIC, percentage of the mean maximal voluntary isometric contraction; AbdH, abductor hallucis; SFE, short foot exercise; TSO, toe spread-out exercise. *p < 0.05, significant difference in main effect.

Figure 4. (A) Comparison of TA activity in the sitting and standing positions. (B) Comparison of PL activity during the four conditions (SFE-SIT, SFE-STA, TSO-SIT, and TSO-STA). %MVIC, percentage of the mean maximal voluntary isometric contraction; TA, tibialis anterior; PL, peroneus longus; SFE, short foot exercise; TSO, toe spread-out exercise; SIT, in the sitting position; STA, in the standing position. *p < 0.05, significant difference in main effect. **p < 0.01, significant difference in simple effect.

Figure 5. Comparison of the AbdH/PL activity ratio in the sitting and standing positions. AbdH, abductor hallucis; PL, peroneus longus. *p < 0.05, significant difference in main effect.

This study aimed to examine the most effective intervention for pes planus by assessing the activity of the AbdH, TA, and PL across four conditions. Our findings revealed a significant increase in the activity of the AbdH during the TSO and in the standing position compared to during the SFE and in the sitting position, respectively. No significant difference was observed in the activity of extrinsic foot muscles (the TA and PL) between the SFE and TSO; however, a significant increase was observed in the standing position compared to the sitting position. In addition, a significant increase in the AbdH/PL activity ratio was observed in the sitting position compared to the standing position. These results support the underlying hypothesis of this study and indicate significant differences between each exercise and position. This is the first study to compare the activity of intrinsic and extrinsic foot muscles in addition to activity ratios during a typical exercise ‘SFE’ and a new exercise ‘TSO’ in different weight-bearing positions for people with excessive pronated foot.

In this study, AbdH activity was significantly higher during the TSO than the SFE (58.976 %MVIC and 32.622 %MVIC, respectively). Several researchers have explored the effects of various exercises targeting intrinsic foot muscles [18,20]. Gooding et al. [1] used magnetic resonance imaging (MRI) to investigate the effects of several intrinsic foot muscle exercises on muscle activity. Muscle activity was calculated by the number of activated pixels on MRI images before and after each exercise, which revealed that the TSO elicited the highest percentage activity in key intrinsic foot muscles such as the adductor hallucis oblique, abductor digiti minimi, and flexor digiti minimi. Furthermore, the previous researcher observed that the TSO resulted in a significant increase on AbdH activity compared to the SFE in individuals with a mild hallux valgus, suggesting that the TSO has the potential for correction and prevention of hallux valgus [18]. The TSO entails movements in which the first toe is abducted and flexed with the AbdH inserted on the medial aspect of the proximal phalanx of the great toe. Consequently, the TSO stands out as a preferred option for activating the AbdH more effectively than other exercises because it can reflect the anatomical movements of the AbdH [1,20]. Furthermore, there was a significant increase on AbdH activity in the standing position compared to the sitting position in the current study (53.209 %MVIC and 38.389 %MVIC, respectively). Previous studies [9,22] have demonstrated a significant increase in AbdH activity in the weight-bearing position compared to the non-weight-bearing position. In addition, several researchers have reported that the AbdH supported the MLA against gravitational forces in the stance phase, which was a fully loaded foot [23]. The AbdH plays a principal role in foot stabilization as it is located in the long axis of the foot and perpendicular to the transverse tarsal joints [5]. Furthermore, Jung et al. [9] compared the difference in AbdH activity and MLA between the SFE and towel curl exercise in the sitting and one-leg standing positions. There was a significant increase in AbdH activity during the SFE in the one-leg standing position compared to that in the sitting position, although there was no significant change in the MLA during the SFE between the two positions. This observation suggested that AbdH activity was increased as a protective response to lowering of the MLA owing to gravity from the body weight instead of promoting the elevation of the MLA.

In this study, there was a significant increase in TA activity in the standing position compared to the sitting position (15.392 %MVIC and 10.302 %MVIC, respectively). The weight-bearing position requires continuous muscle activity, particularly anti-gravity muscles, such as the TA, because individuals must sustain their position against the force of gravity [27]. Furthermore, load receptors, including proprioceptive sensors in the joints and cutaneous mechanoreceptors in the foot, play a pivotal role in providing the necessary sensory inputs. Vertical load, which serves as a stimulus for load receptors, has traditionally been regarded as the important factor in managing the continuous activity of anti-gravity muscles [28]. Consequently, in response to an increase in the vertical load through body weight on the foot in the standing position, the activity of the TA also increases via a sensory-input motor-output response mechanism. In addition, previous researchers have suggested the necessity of a new intervention that can effectively enhance AbdH activity while reducing TA activity to mitigate compensatory mechanisms by overactivity of the TA for the weakened AbdH in patients with pes planus [7]. Choi et al. [22] reported that isometric hip abduction during the SFE helped to elevate the MLA and did not result in the compensatory overuse of the TA. Furthermore, in this study, no significant difference in the activity ratio of the AbdH/TA between exercises in addition to positions was observed, although AbdH activity was significantly higher during the TSO and in the standing position than during the SFE and in the sitting position, respectively.

PL activity was significantly different between the two positions in addition to the two exercises in the standing position; however, there was no significant difference between the two exercises in the sitting position. PL activity was increased approximately two-fold from sitting to standing in our study (17.226 %MVIC and 35.514 %MVIC, respectively). Studies have consistently highlighted that peak PL activity occurs while foot stabilization is maintained [29,30]. In particular, Louwerens et al. [30] demonstrated there was strong activity of the PL in the stance phase with bursts up to 65 %MVIC while maintaining balance during a gait. Under typical circumstances, individuals primarily depend on the somatosensory system for balance [31,32]. Furthermore, in the standing position, the center of mass is located in front of the ankle joint, resulting in continuous forward acceleration induced by gravity from the vertical position. The muscle spindles of the plantar flexor muscles, including the PL, play a crucial role in generating corrective torque and counteracting gravitational forces to maintain balance [28]. The transition from sitting to standing imposes greater stress on the foot, consequently eliciting higher PL activity. The activity ratio of the AbdH/PL was significantly higher in the sitting position than in the standing position.

Regarding the muscle activity ratio, exercises in the sitting position may be the proper intervention to increase intrinsic foot muscle activity while simultaneously decreasing extrinsic foot muscle activity, although AbdH activity did not reach its maximum potential in the sitting position. Considering the significant increase in AbdH activity observed during the TSO compared to the SFE, we can recommend that the TSO-SIT is the optimal method among the four conditions for individuals with pes planus.

This study has several limitations. First, the involvement of certain intrinsic and extrinsic foot muscles, including the flexor hallucis longus and posterior longus, was excluded owing to their deep anatomical compartments because muscle activity was measured using a surface EMG device. Second, the cross-sectional design of this study impeded conclusions regarding the long-term effects of the four exercises. Thus, a future longitudinal study is required to evaluate the effects of the SFE and TSO on various intrinsic and extrinsic foot muscle activity to investigate the optimal treatment in individuals with pes planus.

This study investigated the activity of the AbdH, an intrinsic foot muscle, and the TA and PL, extrinsic foot muscles, during the SFE and TSO in the sitting and standing positions to suggest an optimal intervention for individuals with pes planus. Our findings showed a significant increase in AbdH activity during the TSO compared to the SFE. Therefore, the TSO was shown to be a superior intervention for activating the AbdH. There was a significant increase in AbdH, TA, and PL activity in the standing position compared to the sitting position. The TA and PL activity was increased to maintain balance by sustaining anti-gravity muscle activity and preventing forward instability against gravitational forces in the standing position. Furthermore, the AbdH/PL activity ratio was significantly increased in the sitting position, although there was a significant increase in AbdH activity in the standing position. Consequently, we recommend the TSO-SIT as a clinical strategy to increase the activity of the AbdH and decrease the activity of the TA and PL in individuals presenting with excessively pronated foot.

None.

None to declare.

No potential conflict of interest relevant to this study are reported.

Conceptualization: HL, SK, SB, HC. Data curation: HL, SK, SB, HC. Formal analysis: HL, SK, SB, HC. Investigation: HL, SK, SB, HC. Methodology: HL, SK, SB, HC. Project administration: HL, SK, SB, HC. Resources: HL, SK, SB, HC. Software: HL, SK, SB, HC. Supervision: HL, SK, SB, HC. Validation: HL, SK, SB, HC. Visualization: HL, SK, SB, HC. Writing - original draft: HL, SK, SB, HC. Writing - review & editing: HL, SK, SB, HC.