Introduction
The medial longitudinal arch is a key anatomical structure of the foot and plays a critical role in foot function such as providing a base of support and shock absorption [1]. The foot can be classified into three types according to the height of the medial longitudinal arch: low-arch, normal-arch, and higharch [2]. It has been shown that foot structure is one of the risk factors for lower extremity injuries [3-5]. For example, previous studies reported that the low-arch foot is associated with a high risk for plantar fasciitis [6], medial tibial stress syndrome [4], and patellofemoral pain syndrome [3]. Also, the high-arch foot is associated with a high risk for lateral ankle sprain and iliotibial band syndrome [5]. Maintaining the center of gravity at the base of support, defined as dynamic balance, is crucial for preventing injuries [7]. Losing dynamic balance can lead to falls and long-term injuries, which may result in reduced mobility and functional independence [8]. Given that the foot provides a base of support during weight-bearing movements, it is reasonable to assume that alterations in the medial longitudinal arch height may change the plantar contact area [9], potentially influencing balance and postural control [10]. Therefore, understanding how the foot arch height affects balance is important.
From this perspective, several studies have examined dynamic balance ability in different foot types [10-12], these results are inconsistent. Furthermore, previous studies suggested that investigating muscle activation patterns can enhance our understanding of the neuromuscular and biomechanical compensation for altered structures in individuals with different foot types [10,13]. However, muscle activation during balance tasks remains unclear. Thus, research on dynamic balance ability and muscle activation patterns in different foot types is necessary to investigate the relationship between foot arch height and balance, as well as the compensatory strategies.
Therefore, this study aimed to investigate whether muscle activation differs based on foot arch height during the dynamic balance test in individuals with different foot types. Furthermore, we aimed to examine the association between the medial longitudinal arch height and the activation of the lower limb muscles. We hypothesized that differences in dynamic balance ability and lower extremity muscle activations among the three groups would be observed during the dynamic balance test.
Methods
Participants
Forty-three healthy young males were recruited for this study. Participants were excluded if they had any history of musculoskeletal disorders, injuries, vestibular, or balance disorders in the last 6 months. All participants provided the study details and signed informed consent. This study was approved by the Institutional Review Board of the Jeju National University (approval number: JJNU-IRB-2022-060).
Arch Height Measurements
For group classification, the navicular drop test was performed [14]. This study has shown good validity and reliability in classifying foot types [15-17]. The navicular tuberosity height was measured from the floor, seated with the hip and knee flexed at 90°. The same measurements were performed in the standing position. Measurements were performed three times in both the sitting and standing positions. Participants were classified into three groups based on the mean difference between the values measured in the sitting and standing positions: an average value of <5 mm was defined as high arch, 5–9 mm as normal arch, and >9 mm as low arch [18]. The intraclass correlation coefficients of the three trials were confirmed in advance to ensure measurement reliability (ICC=0.982). All the measurements were performed on the dominant leg that kicked the ball [19].
Dynamic Balance Measurement
A modified star excursion balance test (mSEBT) was used to measure dynamic balance [20]. Before the test, participants watched an instructional video and practiced four times in each of the three directions to minimize the learning effect [7]. In the anterior direction, the most distal part of the heel was placed at the center of the grid. The most distal part of the great toe was placed at the center of the grid in the posteromedial and posterolateral directions. The participants were instructed to maintain a single-leg stance on their dominant leg barefoot, with their hands on their waist. During the test, participants were instructed to look straight ahead to minimize the influence of visual input. They extended their contralateral leg as far as possible in three directions, touching a line on the floor with the distal part of the great toe, and then returning to the starting position within six seconds (three seconds for reaching and three seconds for returning). The participants performed three trials in each direction in randomized order. The examiner measured the reaching distance in centimeters using a ruler. A 10-second rest was given between tasks, and one minute between trials to minimize fatigue. A trial was discarded and repeated if the participants 1) lifted the heel of the stance limb off the floor, 2) did not keep their hands on the waist, 3) shifted their weight onto the reaching limb, 4) lost balance, or 5) failed to return to their starting position. We used the normalized reach distance (reach distance/leg length × 100) for the analysis [7]. While lying on the bed in the supine position, the leg length from the anterior superior iliac spine to the medial malleolus was measured using tape [7,20].
Muscle Activations
Surface electromyography (EMG) sensors (Trigno sensor; Delsys, Boston, MA, USA) were attached using adhesive tape to the gastrocnemius medialis, gastrocnemius lateralis, soleus, vastus medialis, rectus femoris, vastus lateralis, and biceps femoris long head according to the surface EMG for a noninvasive assessment of muscles guidelines [21]. Before attaching the surface EMG sensors, the participants’ skins were shaved and cleaned with alcohol wipes. The EMG data were collected at a sampling rate of 2,148 Hz and filtered using a 20–450 Hz secondorder Butterworth band-pass filter. EMGworks acquisition and analysis software (Delsys, Boston, MA, USA) was used to acquire the data and perform all analyses.
Statistical Analysis
The Shapiro-Wilk test was used to assess data normality. One-way analysis of variance or the Kruskal-Wallis test was conducted to compare the reach distances and muscle activations within the three arch groups. Bonferroni or Mann-Whitney U tests were used for post-hoc comparisons. Stepwise multivariable linear regression analysis was conducted to investigate the association between the medial longitudinal arch height and lower limb muscle activation during the balance test. Effect sizes were calculated as eta-squared. Effect sizes were defined as small (0.01), medium (0.06). and large (0.14) [22]. All statistical analyses were performed using SPSS version 24 for Microsoft Windows (IBM Corp., Armonk, NY, USA). The statistical significance level was set at 0.05.
Results
Participants who had musculoskeletal disorders (n=1), injuries (n=2), and refused to participate (n=15) were excluded. In total, 25 healthy young males (low arch=9, normal arch=8, and high arch=8) were included in the analysis. <Table 1> shows the demographic characteristics of participants. There were no significant differences in the demographic characteristics, except for mean arch height.
Differences in Reach Distances and Muscle Activations between Groups
There were no differences in the reach distances in the three directions. In the anterior direction, the high-arch group showed higher activation of the vastus lateralis than the normal- and low-arch groups <Table 2>. Additionally, the high-arch group showed greater activation of the vastus lateralis than the lowarch group in the posterolateral direction <Table 4>. There were no differences in muscle activation in the posteromedial direction <Table 3>. The three groups showed no differences in the activation of other muscles during the test, regardless of the direction.
Results of Stepwise Multivariable Linear Regression Analysis
<Table 5> shows the association between medial longitudinal arch height and muscle activation during the balance test. The medial longitudinal arch height was significantly associated with the vastus lateralis muscle activation (adjusted R2=0.277, β=-0.054, p=0.004).
Discussion
This study aimed to investigate whether muscle activation differs based on foot arch height and to examine the association between arch height and muscle activation during the mSEBT. Our results showed that vastus lateralis muscle activation was significantly higher in the high-arch group than in the normal and low-arch groups in the anterior and posterolateral directions during the test. In addition, the regression analysis showed that a lower navicular drop, indicated by a higher medial longitudinal arch, was associated with increased muscle activation of the vastus lateralis in the anterior direction during mSEBT.
A plausible explanation for these results is the difference in the anatomical structures of the medial longitudinal arch. Increasing or decreasing the foot arch height can change the alignment of the lower extremities, the muscle length-tension relationship, and the biomechanics of movement [13]. These may lead to a change in the muscle activation pattern among the groups. Specifically, individuals with high-arch feet exhibit excessive supination, which shifts the vertical ground reaction force vector laterally [23] and increases the load on the lateral part of the foot [10]. This may explain the increased activation of the vastus lateralis muscle in the high-arch group.
Interaction between the central and peripheral nervous systems is crucial for maintaining balance [10]. To maintain balance, the central nervous system integrates the peripheral information and determines optimal muscular responses [24]. Considering the foot is being fixed on the floor during the test, alteration of foot arch height may affect the muscular response of the central nervous system, due to the change in the peripheral information, particularly plantar sensory information [25,26]. A previous study reported that high-arch feet have reduced plantar sensory input compared to normal or low-arch feet [9]. Thus, the higher activation in high-arch feet reflects the compensation of the muscular response for reducing plantar sensory input to maintain balance.
Also, our study showed that the higher foot arch height was associated with the greater muscle activation of the vastus lateralis in the anterior direction of the mSEBT. It may be related to the characteristics of the anterior direction. In this direction, the quadriceps muscles [27], mechanical restriction, and plantar cutaneous sensation of the ankle complex [28] contribute to reaching. Due to the supination of the subtalar joint, reduced pronation causes greater leg and joint stiffness in individuals with high-arch feet [29]. Previous studies suggested that individuals with high-arch feet exhibited a lack of motion, leading to increased and earlier muscle activations of the lateral aspect of the lower extremity, such as peroneus longus and vastus lateralis, to compensate for the increased vertical loading rate [29,30]. As previously mentioned, the quadriceps muscles primarily contribute to reaching the limb; however, as the foot arch height increases, it may put more demands on the vastus lateralis muscle in response to increased loading in the lateral aspect of the lower extremity.
Our findings emphasize the potential role of foot structure in modulating neuromuscular control strategies during the dynamic balance test. Although altered muscle activation patterns in individuals with high-arch feet function as a compensatory strategy, if sustained over time, we speculate that it may lead to increased risk of musculoskeletal pain and/or injuries. Therefore, further studies are warranted to investigate the long-term effects of altered muscle activation patterns on the risk of injury. Also, intervention studies using foot orthoses are needed to prevent potential injury in this population. This study has some limitations. First, we could not collect kinematic data. Kinematics differ according to the foot type and may affect both the dynamic balance ability and muscle activations [31]. Second, only male participants were included in this study. Considering sex-related differences in anatomical and biomechanical factors, generalizing and interpreting these results in women requires caution. Lastly, this study had a relatively small sample size. Future studies are needed to investigate alteration of the lower extremity muscle activation pattern in response to different foot types with larger sample sizes.
Conclusions
Our study showed that individuals with high-arched feet exhibited a greater vastus lateralis muscle activation than lowand normal-arched feet during the dynamic balance test. Also, increased medial longitudinal arch height may influence muscular response during the dynamic balance test. Our findings are expected to provide a foundation for future research and enhance the understanding of the modulation of muscle activation patterns with different foot arch heights. Expanding on our findings, future studies should investigate the effects of prolonged compensatory patterns and of foot orthoses in preventing and mitigating potential musculoskeletal injury risks.
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