The effects of combined exercises, short foot exercises, and short foot exercises with isometric hip abduction on navicular drop, static parameters, and postural sway in women with flat foot: A randomized trial

Abstract

Background

Recent research has found that strengthening hip joint stability can considerably affect foot mechanics. The purpose of this study was to determine the effect of short foot exercises (SFEs), combined exercises (CEs), and SFEs with isometric hip abduction (IHA) on navicular drop (ND), static parameters (SP), and postural sway in women with flat foot (FF).

Methods

This study recruited 45 women with flexible FF. The participants were divided into three groups: the CEs group, who performed a series of strengthening, stretching, and balancing exercises, the SFEs group, and the SFEs with IHA group. The groups carried out their assigned regimens daily for six weeks. ND, SP, and postural sway (center of pressure (CoP) parameters) were measured using the ND test and pedoscan device. The data was analysed using a repeated-measures ANOVA statistical test (p≤0.05).

Results

The results showed that all three groups decreased in ND, surface, and foot rotation in the post-test compared to the pre-test (P < 0.05). No difference was observed in the maximum pressure (P = 0.616) and anteroposterior fluctuations (P = 0.065) of the CEs group. Both SFEs and SFEs with IHA groups showed a reduction in all CoP parameters. When comparing the ND (P = 0.22) and mediolateral sway (P = 0.035) of the SFEs with IHA group, a significant difference was observed compared to the CEs group. Additionally, the SFEs with IHA group had a higher percentage of changes in all variables compared to the other two groups.

Conclusions

SFEs with IHA appear more effective than other training methods in reducing ND and decreasing CoP oscillations and stance parameters. Future studies should investigate the long-term effect of this exercise protocol.

Trial registration

Name of the registry: Iranian Registry of Clinical Trials.

Trial registration number

IRCT20220409054456N.

Date of registration

28/09/2022.

URL of trial registry record

https://fa.irct.ir/trial/63065.

Background

Feet, as the final part of the chain of lower limbs, contributes to 5% of our body weight, and the foot’s sole plays a crucial role in daily activities such as standing, walking, and running [1]. This complex structure regulates postural stability on the basis of afferent information from the tactile and proprioceptive receptors of the foot soles and ankle flexor muscles work together in a complementary way [2], thereby maintaining postural control and absorbing shocks in static and dynamic circumstances [3, 4]. Even small biomechanical changes in the foot can impact postural control strategies. Postural control entails managing the body’s position in space to maintain stability (balance) and orientation (proper body part and environmental relationship for a task) [5]. The postural control system involves complex processes that include sensory and motor components. It results from the combination of visual, vestibular, and proprioceptive afferent inputs, creating a framework for maintaining balance. If feedback from any of these methods is disrupted, postural stability is compromised [6].

The combination of the foot’s bones and ligaments forms the three arches of the foot, namely the medial longitudinal arch, lateral longitudinal arch, and transverse arch [4]. The medial longitudinal arch (MLA) is maintained through passive support (by bony and ligamentous structures) and active support (by extrinsic and intrinsic foot muscles), which stabilizes the foot during weight-bearing activities; these forms of support act as a rigid lever that enables muscle contraction to move the body [7]. A decrease in MLA height causes flat foot (FF) [8], of which there are two kinds: flexible and rigid [9]. Flexible FF deformity is a condition in which the MLA flattens during weight bearing and then restores itself during non-weight bearing [10]. MLA deformation can result in excessive foot pronation [8]. This condition can also be related to secondary problems, such as plantar fasciitis, hallux valgus, Achilles tendonitis, patellofemoral pain syndrome, iliotibial band syndrome, and back pain [7, 11, 12]. Moreover, Previous studies have shown that the foot type can affect postural stability [1, 5, 13, 14]. Supination or pronation of foot positions may affect peripheral input through changes in joint mobility or surface contact area [5]. In a study by Ghorbani et al. [1], it was found that people with flexible FF have significantly lower ankle and knee proprioception and postural stability compared to those with normal feet. Postural stability is an indicator of overall movement control and, specifically, of ankle joint control, as reported in previous studies [2, 15], and poor postural stability is considered a risk factor for lower limb injuries. As far as the posture assessment is concerned, recent studies reported that whole-body posture could be quantitatively evaluated using 3D motion analysis in both upright standing [16] and during walking [17]. In addition to postural stability the role of static parameters during standing, such as the correlation between foot rotation and lower extremity alignment, has already been investigated. A study by Khamis and Yizhar [18] found that foot hyperpronation in the standing position was linked to a combination of internal rotation of the shank, internal rotation of the thigh, and anterior pelvic tilt. The study also suggested that differences in foot angle during standing could alter the range of trunk rotational movement.

Based on the findings of Hertel, Gay, and Denegar [13], the single-leg standing position can be used as a reliable criterion for assessing static postural fluctuations in individuals with FF. Since evaluating postural stability and static parameters (SPs) in individuals with FF is crucial, examining the impact of different treatments on these cases can provide valuable insights. The instrumental methodology generally used to evaluate postural control is baropodometry via stabilometric exam in order to evaluate body sway, the center of pressure location in time, sway path and sway area in different conditions (Open and closed eyes or monopodalic and bipodalic support) [19, 20].

Because the MLA is the primary component that influences foot function [21], treating FF deformity may prevent the mentioned secondary injuries [12]. Clinical interventions for the condition include orthopedic surgery, functional foot orthoses, taping, weight loss therapy, non-steroidal anti-inflammatory drugs, physical therapy, and exercise therapy [4, 12]. Exercises such as marble pickup, toe towel curl, the strengthening of the tibialis posterior, the stretching of the iliopsoas, and short foot exercises (SFEs) strengthen foot muscles, prevent MLA loss, and enhance foot stability [4, 7, 12, 22, 23]. In the sports context, FF is treated through toe bending or towel curl exercises, which primarily involve moving the foot’s external muscles, such as the flexor digitorum longus muscle [21]. SFEs constitute sensory motor training that activates the intrinsic foot muscles (IFMs) and enables the active functioning of the longitudinal and transverse arches [3, 24]. These exercises are performed by pushing the head of the first metatarsal toward the heel without bending the toes, resulting in a shortened foot length [12].

Recent research has found that strengthening hip joint stability can considerably affect knee and rear foot mechanics [25]. People with FF have weak abductor halluces (AbdH), which may stimulate the tibialis anterior (TA) as compensation [7]. Choi et al. [7] investigated the electromyography activity of the AbdH, TA, peroneus longus, and gluteus medius (Gmed) muscles under different modes of SFEs and SFEs with isometric hip abduction (IHA). The researchers concluded that SFEs with IHA can effectively reduce TA compensatory activity and increase AbdH muscle activity in people with pes planus [7]. Considering the immediate impact of IHA along with SFEs, it is possible that implementing it as an exercise program can be beneficial for individuals with FF. This aspect has not been addressed in previous studies, so the purpose of this study was to determine the effect of combined exercises (CEs) that encompass a set of foot strengthening, foot stretching, and balancing exercises, SFEs, and SFEs with IHA on navicular drop (ND), SPs, and postural sway in people with FF. We hypothesized that these training methods have different effects on the variables under consideration. The investigation was guided by the following question: Do SFEs with IHA have a better effect on ND, SPs, and postural sway in people with FF than SFEs and CEs?

Methods

Design

This randomized, single-blind clinical trial was conducted at Arak University’s Sports Rehabilitation Department from September 2022 to January 2023. After posting an announcement of the research in a virtual space and at universities in Arak City, participants were chosen on the basis of the inclusion criteria implemented in this work. After our initial explanation of the research process, women who were willing to participate were asked to sign an informed and voluntary consent form. The sample size for the F family using G*Power software (v. 3.1) for a repeated measures statistical test and within–between interactions among three groups and two measurement periods was determined to be 42, with an effect size of 0.25, a test power of 0.8, and a significance level of 0.05 [26]. Taking into account the possibility of dropouts from the groups, we decided on a group size of 15 people (N = 45). The participants were randomly assigned by an independent party to one of three groups: CEs, SFEs, and SFEs with IHA. RandList software was used to select a code for each subject, and the codes were written on sheets that were then placed in opaque envelopes. After the pre-test, each of the participants was instructed to randomly choose one of the envelopes and was then assigned to the group corresponding to the chosen code.

This study was carried out in compliance with the Declaration of Helsinki, authorized by the ethics committee, and registered in the registration clinical trials database. It should be noted that our study adheres to the CONSORT guidelines. Before beginning the exercises at the university’s sport rehabilitation laboratory, preliminary evaluations were performed. The participants in each group were then informed of the desired interventions to be implemented over a period of six weeks. The grouping of the participants was hidden from the assessors and data analysts.

Participants

The participants were recruited according to the following inclusion criteria: female college students aged 18 to 28 years and with a body mass index of 18 to 25, an ND greater than 10 mm, no lower limb or back injury in the six months preceding the trial, no history of hip and ankle surgery, no neurological disability, and no pregnancy. The dominant foot of all the participants was evaluated. Participants who did not complete the training sessions were excluded (Fig. 1). The evaluators and data analyzer were blinded to participants’ personal information.

Fig. 1

Trial flowchart

Interventions

The participants were directed to perform CEs, which involved a series of strengthening, stretching, and balancing exercises [4, 22, 23] (Table 1; Fig. 2), SFEs [4, 7], and SFEs with IHA [7] (Table 2; Fig. 3) 6 times a week (three times under the supervision of a physical therapist, three times independently) for six weeks. The SFEs with IHA group performed SFEs, for which an elastic band was placed above the knee to provide resistance to the hip abductor muscles. The exercises were divided into three levels (easy, moderate, and difficult) and changed every two weeks. The duration of the training sessions was approximately 30 min in the first week and increased to 45 min in the sixth week. Details regarding the contraction duration of each exercise, relaxation duration between repetitions, and the volume of each exercise session are provided in Tables 1 and 2.

Fig. 2

Combined exercises: (a) Rolling a tennis ball, (b) curling the towel with the toes, c, d) plantarflexion with elastic band, e) eversion with elastic band, f) dorsiflexion with elastic band

Fig. 3

Short foot exercises with and without isometric hip abduction: (a) toe spread, (b) lesser toe extension, (c) doming during double leg standing, (d) doming during single leg standing, (e) doming with isometric hip abduction, (f) doming with isometric hip abduction, (g) toe spread with isometric hip abduction, (h) doming with isometric hip abduction during single leg standing

Table 1 Combined exercises

Table 2 Short foot exercises

Participants engaged in supervised exercises every other day, as well as independent workouts at home. For the independent sessions, they were provided with guidance sheets containing exercise explanations and pictures. Furthermore, a table was prepared for each session, outlining the exercise names, rest duration, contraction duration, and exercise volume. Participants were required to provide feedback on each independent practice session during the subsequent supervised session.

Navicular drop

The Brody method was used to measure the height of the navicular bone in the dominant leg. A subject was asked to sit in an adjustable chair with her hips and knees bent at 90 degrees. The second finger and the knee were arranged in such a way that the subtalar joint was in a neutral, non-bearing position. During this period, the navicular tuberosity’s distance from the ground was measured and re-measured in the weight-bearing position, wherein the participants had their feet shoulder-width apart. The difference between the two values was then calculated and recorded as the ND [9]. The evaluation was conducted three times, and the mean of the repetitions was used to obtain the final data.

Postural sway

A DIERS pedoscan (DIERS International GmbH, Germany) composed of 2304 high-sensitivity capacitive sensors and having an acquisition frequency of 120 Hz was used to record the SPs and center of pressure (CoP) trajectories during a single-leg stance. The participants were instructed to stand on their dominant foot with their hands on their waist, bend the other leg at the knee, and hold it up for 30 s. Each test was carried out three times, and each variable’s mean of the repetitions was recorded as a participant’s final data. SPs and CoP oscillations were recorded during the standing session. The following variables were reported:

SPs variables:

• I. Surface (cm²): This is representing the average amount of foot sole surface touching the ground throughout data assessment.

• II. Foot rotation (°): Acquired longitudinal foot axis starting from the middle of the rear edge of the heel and pointing towards the interdigital space of MII and MIII in relation to the y-axis (vertical axis) of the platform.

• III. Max pressure (N): Displays the highest overall pressure value that a sensor detected for the foot.

CoP variables:

• I. AP sway [mm]: The distribution of the amount CoP sway in anterior/ posterior direction.

• II. ML sway [mm]: The distribution of the amount CoP sway in lateral direction.

• III. Trace [mm]: Shows the full length of the CoP trajectory – independently from horizontal or vertical directions.

• IV. Sway area [mm²]: The area that is covered by the CoP trajectory throughout the assessment time.

• V. Average speed [mm/s]: The average sway velocity of the CoP.

Statistical analyses

The Statistical Package for the Social Sciences (version 26) was used to analyze the data. The Shapiro–Wilk test was first conducted to confirm that the data were distributed normally. The repeated-measures ANOVA statistical test was carried out with consideration for the data’s normal distribution (P > 0.05). The pre-test versus post-test times served as the within-subject component, whereas the group type (CEs, SFEs, SFEs with IHA) was the between-subject factor. The outcomes of the Bonferroni’s post hoc test were examined to compare groups and times and determine whether there was significant interaction between time and group as well as a main effect for both variables. All the statistical analyses of the main effects and interactions were significant (P < 0.05). The variables’ percentage change (PC) was estimated using the following formula: ((post-test – pre-test) / pre-test )*100.

Results

The means and standard deviations of the demographic information of the participants are listed in Table 3.

Table 3 Demographic characteristics of the participants (Mean ± SD)

The results of the repeated-measures ANOVA test are presented in Table 4. The main effects of time (P < 0.001, ES = 0.704) and group (P = 0.039, ES = 0.143), as well as the interaction effect of time*group (P = 0.018, ES = 0.174) on ND, were significant. The results indicated a significant difference between the pre-test and post-test outcomes of the three groups. The findings of the post hoc test that compared the groups revealed no significant difference in the pre-test ND among the groups, but after the interventions, a significant difference in ND was found between the SFEs with IHA and CEs groups (P = 0.022).

Table 4 Results of the repeated-measures ANOVA. Mean and Standard Deviation (SD) of all parameters in each condition, Statistic (F), p-value (P) and effect size (Pη²) were reported. Significant differences were marked via “*”

The main effect of the group did not show significant differences in any of the CoP variables. However, the main effect of time (P < 0.001) for all variables was significant. Furthermore, the interaction effect of time*group (P = 0.005, ES = 0.319, P = 0.026, ES = 0.378, and P = 0.046, ES = 0.229) on max pressure, AP, and ML oscillations was significant, respectively. After six weeks, there was a significant difference in the ML sway of the center of pressure between SFEs with IHA and CEs group (P = 0.035).

Additionally, the post hoc test revealed that after six weeks of intervention, except for AP oscillations in the CEs group (P = 0.065), other variables in the three groups showed a decrease in postural oscillations between the pre-test and post-test (Table 4).

The results for the SPs variables showed that the main effects of time on the surface (P < 0.001, ES = 0.773), foot rotation (P < 0.001, ES = 0.598), and maximum pressure (P < 0.001, ES = 0.723) were significant. The post hoc test findings revealed that the surface and foot rotation significantly decreased in all three groups after the exercises. However, only the SFEs and SFEs with IHA showed a significant decrease in maximum pressure after the intervention, while CEs didn’t show any differences (P = 0.616). There were no differences between groups for the SPs variables, but the PCs varied between the groups. According to the results, the PCs in the SFEs with IHA was higher than in the other two groups, and in the SFEs, it was higher than in the CEs group.

Discussion

This study investigated the effects of CEs, SFEs, and SFEs with IHA on ND, SPs, and postural sway in patients with flexible FF. Six weeks of daily exercises significantly alleviated the ND in all three groups, with PCs of 40.14%, 28.71%, and 22.78% for SFEs with IHA, SFEs, and CEs, respectively. ND was alleviated to a greater extent in the SFEs with IHA group than in the CEs group.

Strengthening the foot muscles can provide dynamic support for the MLA [7]. Exercise therapy for FF comprises strengthening the extrinsic and intrinsic muscles of the foot to maintain the MLA. In addition to common exercises (e.g., toe towel curls) and toe bending exercises (e.g., toe curls and picking up small objects with toes), we implemented ankle dorsiflexion and plantarflexion exercises, strengthening exercises, and hip and knee stretching in the CEs program. These exercises engage the IFMs, but they also involve a significant activation of extrinsic foot muscles [11, 12]. Because patients with FF have a higher prevalence of IFM atrophy than people with normal MLA, we focused on strengthening the IFMs, specifically through SFEs [27, 28]. SFEs increase MLA height via the contraction of IFM muscles without over-activating the extrinsic foot muscles, such as the TA and gastrocnemius. Patients can actively perform these exercises under weight-bearing circumstances.

Prior research found that people performing SFEs exhibit stronger activation of the AbdH than individuals performing traditional finger curls [11], but SFEs with IHA enhance AbdH muscle activation to a greater degree than SFEs alone. In the current work, TA muscle activity in sitting and standing SFEs with IHA was significantly lower than that in SFEs only, while Gmed muscle activity was significantly higher in the standing position than in the sitting position [7]. Strengthening the gluteal muscles indirectly strengthens the kinetic chain and aids in the treatment of FF [29]. In our study, a combination of these factors appeared to have been responsible for the larger PCs and the presence of a significant difference in ND reduction in the SFEs with IHA group. These exercises offer simultaneous benefits from SFEs and IHA.

After performing the exercises, a decrease in foot surface and rotation was observed in all three groups. These results may indicate an improvement in FF and are likely related to the reduction of ND. The reduction in surface seems to indicate positive changes in the improvement of FF. Although the PCs do not seem high, considering that corrective changes often require a long time, this level of change is promising. It is possible that by increasing and continuing the exercises for a more extended period of time, better results can be achieved.

In the study, it was found that there was a decrease in the angle of foot rotation after six weeks. The angle of the foot plays a crucial role in both static and dynamic activities. According to Yamamoto, Ito, and Shinkaiya [30], foot rotation affects the range of motion (ROM) of the hips and trunk. The researchers overestimated the impact of foot rotation angle on hip rotation. In mathematical terms, a 1-degree change in the foot rotation angle from the central angle results in approximately a 1-degree decrease in pelvic ROM. It is important to note that participants were not instructed on how to position their foot during the pre and post-test; they were only asked to stand on the plate with their dominant leg. All participants in our study exhibited an outward rotation of the foot. Previous research suggests that a greater foot rotation angle externally leads to a decreased free moment acting on the rotating side of the foot. This not only restricts trunk or pelvis rotation but also creates a mechanical imbalance during trunk rotation [30]. Hence, even a slight reduction in foot rotation could be extremely significant. In both surface and foot rotation variables, the SFEs with IHA showed higher PCs. SFEs have been suggested to strengthen the intrinsic foot muscles in managing FF. In both the SFEs and SFEs with IHA groups, it was observed that the maximum pressure decreased significantly. Consistent with our findings, previous studies have indicated that SFEs exercises can change plantar pressure distribution. Engaging in these exercises for four weeks can lead to a significant reduction in ND and improved balance [31].

In addition, only the SFEs and IHA group showed a reduction in CoP sway in the ML direction. Excessive pronation of the foot during the stance phase has been related to an increase in internal rotation of the tibia and hip, resulting in increased hip adduction [7]. IHA not only improves hip and trunk stability, but it also helps enhance hip joint stability during SFEs. Koh et al. found that external rotator weakness and disorder in the hip can lead to hip joint adduction and internal rotation, as well as dynamic valgus of the knee—problems that ultimately affect foot pronation [29]. Conversely, gluteal muscle weakness causes the internal rotation of the hip joint and foot pronation. As a result, reactivating the gluteal muscles restores proper muscle use patterns. According to the findings of Choi et al., the activity of the Gmed muscle in the standing position increases significantly [7]. In the present study, the exercises in the SFEs and SFEs with IHA groups were carried out in a sitting position only in the first two weeks so that the participants could learn and master the correct execution of the exercises. In the next four weeks, the exercises were performed in double-leg and single-leg standing positions. After six weeks of intervention, it appears that IHA contributed to the strengthening of the hip strategy and the reduction in ML sway.

The results of the AP oscillations variable indicated no improvement in the CEs group. However, both SFEs and the SFEs with IHA showed a decrease in AP oscillations after six weeks. These findings are likely linked to the specific exercises carried out by each group. The SFEs and the SFEs with IHA focused on exercises targeting the plantar region. Furthermore, in the final four weeks, their exercises were performed while standing, and in the last two weeks, they were performed while standing on one leg, which was similar to the single-leg test condition used to assess postural sway.

All three groups improved in other variables of postural fluctuations (sway area, trace, and average speed of CoP), which indicates their effectiveness. In general, the SFEs group with IHA showed better results regarding the variables of postural fluctuations, and the PCs in all variables were higher than the other two groups.

In this study, for the first time, we simultaneously performed IHA with SFEs to engage both the intrinsic and extrinsic foot muscles during exercise. The increased effectiveness of SFEs combined with IHA may be due to the strengthening of the core muscles in the soles of the feet as well as the extrinsic foot muscles. According to McKeon et al. [32], IFMs primarily contribute to the local stability of the foot arches, while extrinsic foot muscles are responsible for providing dynamic stability to the foot arches. Involving both intrinsic and extrinsic foot muscles in these exercises has likely led to better results in this group. The study suggests that performing SFEs along with IHA may help reduce ND, SPs, and postural fluctuations. Therefore, these exercises are recommended to reduce ND and improve posture fluctuations and SPs in women with FF.

In our study, participants completed exercises in three supervised sessions and three independent sessions at home. Despite receiving feedback and reports from each exercise session at home, this can be considered as one of the limitations of our study. Moreover, our evaluations (ND, SPs, and postural fluctuations) were only conducted in the static state; we suggest that future studies assess the effectiveness of these exercises in dynamic states, such as during gait. It’s important to note that our study is short-term, and we acknowledge that longer-term exercise regimens may produce different results, which should be considered in future studies. Additionally, we did not examine the durability of the intervention’s effect, and we recommend that this be explored in future research.

Conclusions

The findings showed that all three training methods effectively decreased foot rotation, surface, and ND. Both the groups SFEs and SFEs with IHA had a more significant impact on CoP parameters. Additionally, SFEs with IHA had significantly lower ND and ML postural sway than CEs. Moreover, the PCs in the examined variables were higher in the SFEs with IHA group compared to the other two groups. As a result, SFE with IHA is recommended for reducing ND and SPs and improving postural stability in women with FF.