Impact of foot pronation on postural stability: An observational study

Abstract

OBJECTIVE:

To investigate the effect of foot pronation on the postural stability through measuring the dynamic balance including overall stability index (OAI), anteroposterior stability index (APSI) and mediolateral stability index (MLSI).

METHODS:

Forty participants from both sexes were selected from the Faculty of Physical Therapy, Cairo University, with a mean age of 23.55 $\pm$ 1.74 years. Subjects were divided into two groups: group A (8 males and 12 females) with foot pronation, and group B (9 males and 11 females) with normal feet. The Navicular Drop Test (NDT) was used to determine if the feet were pronated and Biodex Balance System was used to assess dynamic balance at level 8 and level 4 for both groups.

RESULTS:

No significant difference was found in dynamic balance, including OAI, APSI and MLSI at stability level 8 ( $p>$ 0.05) but, there was a significant difference at stability level 4 ( $p<$ 0.05) between the two groups with lower stability in group A.

CONCLUSION:

Foot pronation affects the postural stability at stability level four and not affects stability level eight compared with those in the control group.

Keywords

Flat feet foot pronation dynamic balance biodex balance system

1. Introduction

Flat foot is the term used to describe a condition of the foot, which consists of an absent or abnormally low longitudinal arch. In other words, the normal foot arches have partially or completely collapsed [1, 2]. When the foot is put on the ground, its inner or middle side comes down to the floor rather than remaining raised [3], causing it to roll inwards (over pronation of the foot) [4]. Pronation is a natural motion of the foot as it rolls inward after the foot makes contact with the ground. So, the foot acts as a shock absorber and adapt to the contour of the ground. Therefore, normal pronation is required for the foot to function properly [5]. However, too much pronation will cause the arch of the foot flatten excessively, placing more stresses and pressures on the muscles, tendons and ligaments underneath the foot [4, 6].

Flat foot or over pronation of the foot causes the tibia to be medially rotated. This rotation affects the femur, patella, ilium, sacrum and the entire musculoskeletal system [7]. This over pronation is responsible for up to 60–90% of all foot and lower extremity pathologies associated with overuse injuries [8]. The body balance maintained in the closed kinetic chain depends on the closely integrated feedback and movement strategies among the hip, knee, and ankle joints [9]. Therefore, body balance can be disrupted by diminished afferent feedback or deficiencies in the strength and mechanical stability of any joint or structure along the lower extremity kinetic chain [10].

Considering that the foot is the most distal segment in the lower extremity chain and represents a relatively small base of support upon which the body maintains balance, it seems reasonable that even minor biomechanical alterations in the support surface may influence postural-control strategies [11]. Specifically, excessively pronated foot postures may influence peripheral (somatosensory) input via changes in joint mobility or surface contact area or, secondarily, through changes in muscular strategies to maintain a stable base of support [12]. Therefore, the current study intended to investigate the effect of foot pronation on dynamic balance including the overall stability index (OAI), anteroposterior stability index (APSI) and mediolateral stability index (MLSI).

2. Materials and methods

2.1 Study design

A cross sectional study design was utilized to assess the effect of flat feet or over pronation on dynamic balance including 3 indices: OAI, APSI and MLSI.

2.2 Study location and time period

This study was conducted at the balance laboratory, Faculty of Physical Therapy, Cairo University in a 3 months’ period between 1st of January and 31st of March, 2014.

2.3 Sample size and study population

Forty participants (17 males and 23 females) were included in this study. They were equally divided into two groups: an experimental group A that includes 20 participants with bilateral flat feet (8 males and 12 females), and a control group B involving 20 participants with normal feet (9 males and 11 females). Their ages ranged between twenty and thirty years and their Body Mass Index (BMI) ranged from 18.5 to 25 kg/m ${}^{2}$ . The sample size (40 participants) was calculated to yield an 85% power and $\alpha=$ 0.05.

2.4 Eligibility

The participants were excluded when they had one of the following criteria: history of lower extremity injuries as fractures or deformities, history of surgery to the lower extremity, history of cerebral concussions, visual or vestibular disorders, and/or any neurological deficit affecting balance.

2.5 Ethical aspects

Prior to participating in this study, all participants were required to read and sign a written human subject informed consent form. All participants were chosen by their willingness to participate. Guidelines of Declaration of Helsinki on conduction of human research were followed, and ethical approval was granted by the Institutional Review Board at Cairo University (reference number “No.P.T.REC/012/00258”) prior to commencement of the study.

2.6 Data collection

Participants reported demographic data (age, weight, and height), and medical and health statuses were documented for all participants.

Table 1
Demographic characteristics of study participants (Groups A and B)

Demographic characteristics	Group A $n=$ 20	Group B $n=$ 20	t-value	P-value
Age [years]	23.55 $\pm$ 1.74	22.67 $\pm$ 1.68	1.61	0.88
Weight [kg]	74.7 $\pm$ 5.34	73.07 $\pm$ 5.83	1.63	0.91
Height [cm]	170.1 $\pm$ 8.25	170.4 $\pm$ 6.84	$-$ 0.3	0.12

Values of age, weight, and height are expressed as mean $\pm$ SD.

2.6.1 Navicular Drop Test

The Navicular Drop Test using the Brody method has been chosen to characterize foot structure and function. It consists of locating the navicular tuberosity while the subject is standing in bipedal stance. Vertical distance to the ground is measured two times. First with the foot placed in subtalar joint neutral position and second in relaxed standing position. Navicular Drop is defined as the difference between the two values recorded. According to the Navicular Drop Test, the participants were classified into one of two groups: an experimental group with bilateral flat feet (more than 10 mm of navicular drop) and a control group with normal feet (between 5 and 9 mm of navicular drop) [13].

2.6.2 Assessment of the dynamic balance

Biodex Balance System was used for the assessment of the dynamic balance of all participants in the two tested groups. The system utilizes dynamic multi axial platform. This platform allows approximately 20 degrees inclination in 360 degrees range and is interfaced with computer software. It measures the participant’s ability to control the platform’s angle of tilt, which is quantified as a variance from the center, as well as the degree of deflection over time at various stability levels [14].

Stability levels allowed by the system ranged from one to eight [15]. Stability level eight, allows the highest level of stability as it makes the platform to be the least tilted and is easier for the subject to maintain stability on. On the other hand, stability level one represents the least level of stability as it makes the platform to be the highest tilted and is more difficult for the subject to maintain stability [16].

The participant’s ability to control the platform’s angle of tilt was measured by the system. The participant’s performance was noted as a stability index. The stability index represents the variance of platform displacement in degrees from level. A high number is an indicative of a lot of motion, which indicates balance problem. The data regarding the balance of the tested participant were supplied from the system. These data include overall stability index (OSI), antero-posterior stability index (APSI), and medio-lateral stability index (MLSI). Overall stability index (OSI) represents the participant’s ability to control the balance in all directions, antero-posterior stability index (APSI) represents the participant’s ability to control the balance in sagittal plane, and medio-lateral stability index (MLSI) represents the participant’s ability to control the balance in frontal plane [17].

Before the testing procedures, the participant’s weight, height and age were introduced into the system. All participants were tested on two stability levels, stability level eight (most stable) and stability level four (less stable) for 20 seconds for each of these stability levels according to Aydog et al. [18]. Firstly, each participant has been received verbal explanation about the testing steps. The participant was asked to assume the test position (standing on both feet without footwear) with arms held at both sides. Then each participant was asked to center himself on the foot platform before starting the test. Then the participant was asked to try to control his/her balance as much as possible during the testing procedures [19].

Table 2
Descriptive statistics and multiple pairwise comparison tests (post hoc tests) between both groups for OAI, APSI and MLSI at different levels of stabilit

Stability indices	Level 4 stability			Level 8 stability
	Group A	Group B	p-value	Group A	Group B	p-value
OAI	4.13 $\pm$ 1.4	2.17 $\pm$ 0.79	0.0001 ${}^{*}$	2.01 $\pm$ 0.36	1.91 $\pm$ 0.64	0.736
APSI	2.84 $\pm$ 0.92	1.71 $\pm$ 0.72	0.0001 ${}^{*}$	1.88 $\pm$ 0.69	1.63 $\pm$ 0.49	0.280
MLSI	2.94 $\pm$ 1.33	1.49 $\pm$ 0.48	0.0001 ${}^{*}$	1.54 $\pm$ 0.45	1.36 $\pm$ 0.19	0.439

OAI: overall stability index; MLSI: mediolateral stability index; APSI: anteroposterior stability index; *Significant at the alpha level ( $p<$ 0.05).

Each participant was asked to perform two test trials before the actual testing procedures for the purpose of instrument familiarity prior to data collection [20]. The participant was instructed that the platform was unstable just after the alarm. Each participant was instructed to maintain a level platform for the period of the test. Instructions were given for the participants to focus on a visual feedback screen directly in front of them and attempt to maintain the cursor, which represents the center of the platform, at the center of the bulls’-eye on the screen equated to a level platform. Finally, after conducting the two actual tests at the two testing stability levels, a printout report was obtained. This report included the information regarding the OSI, APSI and MLSI. The values of the two tests at both stability levels was recorded for each participant at the two tested groups.

2.7 Data analyses

All statistical measures were performed using the Statistical Package for Social science (SPSS) program version 20 for Windows. Prior to final analysis, data were screened for normality assumption, and presence of extreme scores. This exploration was done as a pre-requisite for parametric calculation. Normality test of data using Shapiro-Wilk test was used, that reflect the data were normally distributed for all dependent variables. Additionally, testing for the homogeneity of covariance revealed that there was no significant difference with p values of $>$ 0.05. All these findings allowed the researchers to conduct parametric analysis. So, between subjects MANOVA test was used to compare the tested variables of interest in both tested groups. The alpha level was set at 0.05.

3. Results

There were no statistically significant differences ( $P>$ 0.05) between subjects in both groups concerning age, weight, and height (Table 1). The mean age, weight, and height was presented at Table 1 for both groups.

Considering the effect of tested group on all dependent variables, between subjects MANOVA revealed that there was a significant difference between both groups for all dependent variables (F $=$ 9.852, P $=$ 0.0001). Table 2 represents mean $\pm$ SD and multiple pairwise comparisons (post hoc tests) for the 3 dependent variables in both groups. Multiple pairwise comparison tests revealed that there were significant higher values of overall stability index, anteroposterior stability index, and mediolateral stability index ( $p<$ 0.05) in group A (lower stability) in comparison to group B at level 4 stability, while there were no significant differences at level 8 stability between both groups ( $p>$ 0.05).

4. Discussion

Flat feet are the most common foot disorders in which alteration in its function can be followed by a series of biomechanical changes [21]. This produces a wide variety of signs and symptoms through the interrelated structures and systems of the body [12, 22]. The findings of the current study revealed that there was no significant difference in the dynamic balance in the experimental group (Flat feet) compared with the control one at stability level 8. However, there was a significant deficit in the dynamic balance in the experimental group compared with the control one at stability level 4.

Stability level 8 allows the highest level of stability as it makes the platform to be the least tilted. This allows minimal foot displacement and so it is easier for the individual to maintain his stability. Therefore, there was no significant difference in the dynamic balance in the experimental group compared with the control one at this stability level. In contrast, stability level 4 represents low level of stability as it makes the platform to be highly tilted. This allows more foot displacement and so it is difficult for the individual to maintain his stability [16].

In pronated foot, there is flattening of the medial longitudinal arch causing hypermobility of the midfoot [12]. This affects the foot alignment and increases the soft tissue stress and demand on the postural control [23]. In addition, this biomechanically abnormal foot posture alters the normal transitions of the subtalar joint required during dynamic activities [24]. The foot therefore loses the ability to maintain a rigid support in full weight bearing and shock absorption activities [25]. Moreover, the foot is a flexible structure that required to perform very diverse functions, particularly during weight bearing activities [26]. Therefore, deviations of the foot from its normal posture place the foot under excessive stress and demand on the postural control [27].

Furthermore, the human body receives information about its position in space and about the environment. The body receives this information throughout 2 systems: the neural system, which integrates the sensory information to access the position and the movement of the body in space; and the musculoskeletal system, which generates forces to control the position of the body, known as postural control system [28]. In overpronated foot, the ligament deficiencies and joint damage could disrupt joint proprioception mechanisms and lead to local changes in motor pattern that may affect the ability of the individual to respond effectively to perturbations [29]. In addition, excessive foot pronation, whether in one foot or bilaterally, interferes with the carefully coordinated movement during gait and causes problems throughout the musculoskeletal system [6].

Decreased OSI may be due to that overpronated foot that causes poor foot position sense, which could hinder the accommodation between the plantar surface of the foot and the supporting surface, thus requiring postural adjustments more proximally to maintain the upright posture and balance [30]. The affection of APSI values may contributed to poorer standing postural control and abnormal displacement of the center of pressure particularly in anteroposterior direction in participants with flat feet [31] and had shorter single-limb stance duration [32]. Regarding the results of MLSI values, it has been attributed to that excessive pronated foot tends to collapse toward the medial aspect of the foot and have a reduced ability to maintain a rigid support in mediolateral direction in full weight bearing activities [24].

In the current study, the finding of the dynamic balance deficits in participants with flat feet at stability level four come in agreement with different previous research work. Rozzi et al. [19] concluded that the participants with flat feet showed a high number for motion in in all directions, the frontal plane and sagittal plane. A high number referred to a lot of motion, which indicated balance problem.

Our results came in accordance with Otman et al. [33] who found that when one or both feet spend too much time in pronation, many muscles throughout the body and around the spine do not turn on and shut off in proper sequence. This will affect the postural balance, raise the work effort for all activities, and even increase the amount of oxygen consumed during normal walking. Tsai et al. [32] also stated that subjects with pronated foot have poorer standing postural control compared with subjects with normal feet. This may be due to the reduced stability within the foot joints.

In addition, Charrette [6] cleared that if one or more of the foot’s arches are not able to provide the necessary support, or if there has been a breakdown of the plantar fascia, abnormal postural adaptations are created, additional stress is then placed on many joints, ligaments and muscles involved in helping to maintain upright posture. Whenever there is an unequal amount of support from each leg during weight bearing stance, the posture will definitely suffer.

Moreover, Subotnick [34] proved that the pronated foot has increased recruitment of motor units during stance as the stability of the foot decreases. Overloading is placed on the extrinsic and intrinsic dynamic stabilizers. These muscles try to stabilize the foot, leading to fatigue and insufficiency, and overuse injuries. Furthermore, Ross and Guskiewicz [35] revealed that individuals with functional ankle instability took significantly longer time to stabilize than individuals with stable ankles after a single-leg jump landing.

Also, Ho and Tan [36] investigated the effect of foot structure and functional foot stability on the gait patterns of the foot. The results showed a significant difference in total excursion of rear foot inversion/eversion of the flat foot group compared to the other groups. However, the sample’s mean age was about 65 years and there is balance impairment associated with aging. Age-related decline in the ability of the somatosensory, vestibular and visual systems to receive and integrate sensory information contributes to poor balance in older adults [37].

However, our finding is contradicted with the finding obtained by Hertel et al. [38] who noted that there is no postural deficit in the adults with pronated foot posture. This contradiction may be due to the nature of the balance testing, as their findings were limited to the static balance testing with eyes open. However, in the current study our findings were obtained from the dynamic balance testing with a difficult stability level. This study was limited by physical and psychological condition of the participants during recording period, possible human error in the application of measurement and personal or individual differences between subjects. More researches are recommended to study the effect foot over pronation on dynamic balance deficit using single-limb stance as another objective measurement to support dynamic balance as the test can give more functional assessment for subjects. Also other future studies may discuss the possible interventions that help solving the dynamic balance deficit in those subjects.

5. Conclusion

It can be concluded that there was a dynamic balance deficit in the participants with flat feet or foot over pronation at stability level four using the Biodex Balance System compared with those in the control group.

Conflict of interest

None to report.

Footnotes

Acknowledgments

We would like to express our thanks and gratitude to all individuals who contributed to the completion of this work, especially study participants.

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