Physical Therapy Korea

Pronated foot is a common foot problem affecting 2% to 25% of adults [1-4]. Several factors, such as muscle imbalance, structurally incorrect alignment at ankle joint, abnormal gait pattern, and various other compensatory mechanisms, can cause foot pronation [5-8]. Structural deformities associated with pronated feet include partial or complete collapse of the medial longitudinal arch [1,9,10]. Along with functional issues, such as limited dorsiflexion of the first metatarsal, pronation of the first metatarsal and subtalar joint [9,11,12]. These structural and functional issues of a pronated foot can be the underlying cause of various foot disorders, such as hallux valgus, hallux limitus, and rigidus [11,13,14].

In clinical settings, foot pressure distribution (FPD) is widely recognized as an assessment tool that uses pressure distribution to estimate foot posture and alignment [15,16]. Initial attempts have often focused on comparing FPD based on foot posture in the standing position [17-19]. Subsequently, several studies have explored FPD during human movement. In Buldt et al. [15]’s study, plantar pressure was compared among three groups with normal foot posture normal, pes planus deformity and pes cavus deformity during gait. In addition, a systematic review reported that foot posture is associated with plantar pressure during gait [20]. Sneyers et al. [21] investigated FPD based on foot posture during barefoot running.

In daily activities, numerous lower extremity uni-lateral movements, and control is required during stair descent, stair ascent, single-leg jump, and single-leg landing. Therefore, various lower extremity uni-lateral training and assessments have been performed [22-26]. The single-leg squat (SLS) is widely used as a uni-lateral closed-kinetic clinical assessment tool to identify potential risk factors for musculoskeletal pathologies [27]. It involves coordinated motions in the hip, knee, and ankle and is used as a tool for screening faulty movements, such as dynamic knee valgus [25]. Additionally, it is used to evaluate the movement of the lower extremities, facilitating assessment of patellofemoral pain syndrome [28,29]. Furthermore, it has been recommended for knee rehabilitation exercises including anterior cruciate ligament reconstruction, because of the co-contraction of the hamstring and quadriceps brachii, which could produce less shear force in the knee joint [30-32].

Few researchers have investigated FPD during closed kinetic tasks. Telarolli et al. [33] compared the FPD among five different closed kinetic tasks and between each task. Koh et al. [34] examined the differences in FPD between normal and pronated feet based on static squat depth. However, studies comparing FPD between groups with and without pronated feet, both within each group and across different phases during SLS, is lacking. Therefore, the purpose of this study was to compare FPD between pronated foot and normal foot groups. We hypothesized that the pronated foot group would have greater foot pressure on hallux or the medial side of the forefoot than that of the normal foot group.

1. Participants

A total of 30 participants (19 males and 11 females) were recruited for this study, with 15 each included into the pronated foot, and normal foot groups. The criterion of foot position was based on the foot posture index (FPI), a simple assessment tool used to classify foot position in a bipedal standing posture. An FPI score of 0 to +5 is considered normal, and a score greater than +5 indicates pronated foot [35]. Participants were excluded if they had: (1) a previous diagnosis of hip, knee and ankle pathology or trauma; (2) a history of surgeries or injuries to the lower limb during the past 6 months; (3) ankle or knee pain during any of the activities; (4) ankle or foot musculoskeletal disorders such as plantar fasciitis, ligament injury, tendinopathy, or bursitis; (5) rheumatic pathology, such as gout, rheumatic arthritis, or osteoarthritis in the lower extremity; and (6) systemic diseases, such as epilepsy, lupus, or diabetes [36]. Before the study, the procedure was explained to all participants and a signed informed consent form was obtained from them. The study was approved by the Institutional Review Board of the Yonsei University Mirae campus (IRB no. 1041849-202303-BM-037-01).

2. Foot Pressure Distribution

Measurement of FPD was conducted using a 158 × 50.5 × 2.1 cm (length × width × height) Zebris FDM (Zebris Medical GmbH) with 11,264 sensors at a sampling rate of 100 Hz. The Zebris FDM is an equipment used to assess the FPD during static and dynamic movements [37]. The collected FPD data were processed using WinFDM software (Zebris Medical GmbH). The FPD was segmented into the five sections: the hallux, medial forefoot, central forefoot, lateral forefoot, and rearfoot (Figure 1). The pressure data for each section were obtained as average values. For the hallux, the average value was calculated based on the peak pressure of four adjacent sensors. For the forefoot, the central part, located in the middle of both ends of the medial and lateral sides of the forefoot was designated as the standard. Medial and the lateral part were assigned based on the central part. The average pressure was calculated using the nine sensors for the medial and central parts, and six sensors for the lateral part. Additionally, for the rearfoot section, nine adjacent sensors based on the middle of the calcaneus were used [38].

Figure 1. Segment of foot pressure distribution.

3. Procedure

All participants performed SLS barefoot using the dominant leg, and the FPD was assessed using the Zebris FDM. Before the task, all participants were provided with guidance on appropriate movement and underwent sufficient practice for familiarization. In addition, during SLS practice and the study, a co-researcher ensured knee flexion of 60° using a goniometer and provided verbal cues to maintain adequate movement. The angle of knee flexion of 60° was chosen based on the balance ability of general population and the knee joint’s low anterior shear force during SLS [39]. The starting position involved standing on one leg with the dominant leg and the other maintained 90° of knee flexion. Both hands remained behind the back to minimize compensatory movements in the upper extremity. The SLS lasted for 6 seconds at 60 bpm and divided into three phases, each of 2 seconds: descent, hold, and ascent. The descent phase was defined as the step from the starting position to knee flexion at 60°. In the hold phase, the participants maintained 60° knee flexion for 2 seconds. The ascent phase involved return to the starting position from the 60° knee-flexed position (Figure 2). The mean values of measurements from the two trials were used for data analysis.

Figure 2. Process in three different phases during a single-leg squat. (A) Descent phase, (B) hold phase, and (C) ascent phase.

4. Statistical Analyses

Statistical analysis was performed using IBM SPSS version 25.0 (IBM Co.). The Kolmogorov–Smirnov test was used to assess data normality. A two-way mixed model analysis of variance was used to compare the FPD between the two groups (pronated foot vs. normal foot) and across different phases (descent phase vs. hold phase vs. ascent phase). In case of a significant interaction, a post-hoc t-test was used to compare the differences between the groups based on the phase. The level of significance was set at p < 0.05.

Table 1 summarized characteristics of the study participants. No significant differences were observed between the groups in terms of age, height, and body mass (p > 0.05). The mean FPD values during SLS and results of the two-way mixed-model ANOVAs are presented in Table 2. The results for the hallux and central forefoot were significantly different between the groups (p < 0.05) (Figure 3). The results for the hallux; medial, central, lateral segments of forefoot; and rearfoot were significantly different across various phase (p < 0.05). For the hallux, a significant difference was observed between the group and across different phase (p < 0.05). The results of post-hoc t-test are presented in Figure 4. The mean pressure of hallux in the descent and hold phases were significantly higher in pronated foot group than in the normal foot group (p < 0.05). In central forefoot, the mean pressure was significantly higher in the normal foot group than in the pronated foot group (p < 0.05).

Table 1 . General characteristics of the study participants.

Variable	Total (N = 30)	Pronated foot group (n = 15)	Normal foot group (n = 15)	p-value
Age (y)	26.03 ± 3.07	26.40 ± 3.11	25.67 ± 3.37	0.930
Height (cm)	171.41 ± 7.43	169.87 ± 6.76	173.07 ± 7.51	0.497
Body mass (kg)	67.62 ± 10.11	68.13 ± 9.69	67.07 ± 9.20	0.483

Values are presented as mean ± standard deviation..

Table 2 . Foot pressure distribution during single-leg squat in three different phases.

Foot pressure	Pronated foot group				Normal foot group				p-value
	Descent	Hold	Ascent		Descent	Hold	Ascent		Group	Time	Group x Time
Hallux	19.69 ± 4.47	22.63 ± 5.61	16.27 ± 4.89		11.33 ± 4.93	13.70 ± 6.88	12.13 ± 6.23		0.001*	< 0.001*	0.015*
Forefoot medial	7.42 ± 1.56	8.77 ± 2.63	7.35 ± 2.48		7.78 ± 2.58	8.74 ± 2.45	8.80 ± 1.81		0.435	0.024*	0.187
Forefoot central	9.20 ± 2.91	9.59 ± 2.91	9.05 ± 3.09		12.51 ± 3.70	12.77 ± 2.30	11.03 ± 1.73		0.007*	0.073	0.312
Forefoot lateral	7.59 ± 1.60	7.50 ± 1.94	8.00 ± 2.23		8.30 ± 1.49	8.39 ± 2.05	7.95 ± 2.70		0.453	0.997	0.352
Medial/lateral ratio	1.03 ± 0.29	1.25 ± 0.44	1.01 ± 0.48		0.98 ± 0.33	1.14 ± 0.48	1.22 ± 0.42		0.923	0.054	0.096
Rearfoot	9.15 ± 2.56	7.48 ± 3.61	9.88 ± 3.63		10.17 ± 3.57	8.87 ± 3.66	11.45 ± 3.96		0.263	0.001*	0.903

Values are presented as mean ± standard deviation. *Statistical significance is set at the 0.05 level..

Figure 3. Typical foot pressure distribution. (A) Pronated foot and (B) normal foot.

Figure 4. Comparison of foot pressure distribution in the (A) descent, (B) hold, and (C) ascent phase between the pronated foot and normal foot groups. *p < 0.05.

The aim of this study was to compare the foot pressure between pronated foot group and normal foot group during the SLS in three different phases (descent, hold, and ascent phase). The results of this study showed that there were significant differences in descent and hold phase in hallux and central forefoot. Compared with the normal foot group, the pronated foot group exhibits significantly greater pressure in the hallux and significantly lower pressure in the central forefoot during the descent and hold phases.

A pronated foot is accompanied by the abnormal lower-extremity kinematics during walking or while engaging in exercises [23,25,40]. For instance, in the hip joint, there is a tendency for hip internal rotation and hip adduction moment in pronated feet compared with normal feet [41,42]. Kim et al. [43] reported that, compared with the normal foot group, the pronated foot group exhibit greater dynamic knee valgus accompanied by medial side displacement of the knee in the frontal plane during a single-leg step down. As a single-leg movement, the single-leg step-down is similar to SLS in the descent-to-hold phase. These abnormal movements of the lower extremities, observed in the pronated foot may cause higher pressure in the hallux. Furthermore, Buldt et al. [15] found that the FPD of the pronated foot group demonstrated greater pressure in the hallux than that of the normal foot group during gait.

When analyzing FPD from a temporal perspective, different FPD patterns were revealed during the ascent phase, contrasting with the descent and hold phases. Each phase exhibited distinct patterns of contraction and control strategies, including eccentric control during the descent phase, isometric control during the hold phase, and concentric control during the ascent phase. From a mechanical perspective, eccentric control requires higher absolute forces than concentric control [3,4] which may pose greater difficulty in controlling, particularly evident in pronated feet. In addition, different strategies are adopted in each phase, which can result in differences in joint motions and muscles activation [44-47]. During the ascent phase, the plantar flexor muscle activation is higher than that in the descent and hold phases [46,47]. The plantar flexor muscles, such as the gastrocnemius and tibialis posterior, support the medial longitudinal arch [48]. Thus, during the ascent phase, unlike the descent and hold phases, the medial longitudinal arch is supported in the pronated foot, which may have led to a reduction in the differences between the groups.

SLS is a representative weight-bearing unilateral movement, primarily used for the screening of lower extremity injuries and as an exercise in rehabilitation [24-26,49]. In particular, it mimics daily-life activities, such as single-leg step-down, single-leg stance, and single-leg jump landing. Previous studies have shown increased dynamic knee valgus during a single-leg step-down [42], and an increase in medial displacement of the knee in pronated foot group [50]. The results of the present study demonstrated an increased pressure on the hallux during the descent and holding phases in the pronated foot group. Therefore, when performing SLS in the descent-to-hold phase, caution must be exercised by individuals with a pronated foot to prevent excessive pressure on the hallux, and efforts should be made to transmit the pressure on the central part of the forefoot. In addition, it is necessary to provide support for the medial longitudinal arch, such as insoles, for individuals with pronated feet during SLS. This would be helpful into preventing excessive pronation in individuals with pronated feet.

This study has a few limitations. First, an imbalance exists in the sex ratio between the groups. Second, the study was limited to young people in their 20s and 30s. Third, the participants were restricted to individuals without pain or disorders affecting the lower extremities. Further research is needed to conduct studies on individuals with lower extremity pathologies, such as patellofemoral pain syndrome, and anterior cruciate ligament reconstruction. Furthermore, there is a necessity for research targeting the elderly population to be conducted.

This study compared the FPD between pronated foot and normal foot groups during the representative uni-lateral closed-kinetic movement, SLS. In comparison to the normal foot, the pronated foot exhibited significant differences in FPD in the hallux and central forefoot during the descent and hold phases. Therefore, individuals with a pronated foot during the descent and hold phases of the SLS should be cautions to prevent excessive pressure in the hallux.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: IA, UH, OK. Data curation: IA. Formal analysis: IA, GG. Investigation: IA, HY. Methodology: IA, OK. Project administration: IA. Resources: IA, HY. Supervision: GG, UH. Visualization: IA. Writing - original draft: IA. Writing - review & editing: GG, OK.