Biomechanical variations in patients with flatfoot deformity: Impact of gender, age, and BMI on foot kinetics and kinematics

Abstract

Background

Flatfoot is considered by the collapse of the foot arch, altered biomechanics and impacting functional abilities. The biomechanical gait alteration of foot kinematics and kinetics in individuals with flatfoot, based on gender, age and Body mass index (BMI) in each cohort is unclear. This study explores how gender, age, and body mass index (BMI) impact distinct foot biomechanical characteristics, including ankle joint angle (Jc°), Ground force reaction angle (GFR°), Achilles tendon force (T), Ankle joint force (Jc) and vertical ground reaction force (VGRF) during the gait stance phase, in flatfoot versus normal-foot individuals on Indian Population.

Method

A foot pressure test and sagittal plane motion analysis were performed on 142 individuals with normal-foot arches and 102 with flatfoot, stratified by gender, age, and BMI. Calculations of the magnitude and direction of forces in ankle joint equilibrants relied on inverse dynamic analysis, vertical ground force reaction and mapping motion data of the gait stance phases.

Result

In the midstance phase, females with high BMI (HBMI) in the middle and older age group (p = 0.029 and p = 0.014), and males with HBMI in the older age group (p = 0.039) demonstrate significantly higher $VGRF$. Females and males with HBMI in middle and older age groups, along with males with normal BMI in the older age cohort, show positive and negative ranges of GFR°, indicating gait instability. In the push-off phase, females with HBMI in a middle-aged group exhibit significantly lower $TandJc$ (p = 0.023 and p = 0.026) respectively.

Conclusion

The biomechanical issues in individuals with flatfoot, while accounting for the influence of gender, age and BMI, are crucial for tailored interventions and precise solutions to biomechanical issues, thereby enhancing foot function and reducing discomfort.

Graphical abstract

Highlights

• •Age, gender, and BMI of the patient influence the flatfoot deformity.
• •Cohort allows targeted interventions to improve foot function and reduce discomfort.
• •Flatfoot Middle-aged females with high BMI experience more discomfort than males.
• •Flatfoot-High BMI in middle and older age groups shows instability risk.

1. Introduction

Flatfoot is a condition considered by the collapse of the foot arch, leading to the whole sole making touch with the ground. This occurs due to differences in foot structure and biomechanics resulting in discomfort.1, 2, 3 Research consistently indicates that individuals with flatfoot exhibit significantly poorer subjective physical health compared to those with normal foot.4, 5, 6 The shape of the foot pressure has been shown to impact various aspects such as pain, injury, abnormal gait, and difficulty in walking, potentially leading to excessive tension in the triceps surae, obesity, and ligamentous laxity.7, 8, 9, 10

Plantar pressure measurement and motion analysis have emerged as effective tools for discrimination between normal and pathologic gaits in flatfoot patients.^11,12 We used this method for our study. In gait analysis the mid-stance and push-off phases play a vital role. The mid-stance phase, crucial for stability and body balance, necessitates proper lower body alignment,¹³ while the push-off phase involves the transition from weight-bearing to actively propelling the body forward. Energy release from the stretched Achilles tendon is pivotal for forward movement during this phase.

Moreover, researchers have employed statistical pattern recognition techniques to differentiate between normal and flatfoot populations based on ground reaction force measurements, offering valuable insights for clinical evaluation.¹⁴ Studies have shown significant differences in vertical ground reaction forces during the stance phase, indicating altered plantar loading patterns in individuals with flatfoot.^15,16 Additionally, Prachgosin et al. reported greater eversion deforming force and abnormal ground reaction forces¹⁴ Individuals with flatfoot exhibit increased ankle stiffness and reduced ankle movement compared to those with normal foot arches.^16,17

The association between flatfoot and diminished gait efficiency, noting correlating factors like height, age, weight, foot progression angle, and joint laxity was reported. Particularly in children, these factors contribute to poor performance in physical tasks and slower walking speeds¹⁸

Numerous studies have explored the gait kinematics of adults with flatfoot, revealing notable distinctions in lower limb segments based on age and gender.^19,20 Researchers observed age, gender and obesity have a definite effect on foot arch stiffness and arch structure.²¹ The impact of age and BMI on foot posture alignment and stability was reported only in the normal foot arch cohort. It is unclear in the Flatfoot cohort. Their study revealed that a high BMI notably influences foot posture alignment and stability, particularly among younger age groups.²² To the best of my knowledge, considering the effect of BMI along with age and gender on the kinetic and kinematic of flatfoot has not been so far reported. We felt that the significance of the gait phases, studying foot biomechanics alterations in terms of kinematics and kinetics in normal and flatfoot based on age, gender, and BMI becomes imperative.

2. Method

2.1. Participants

A foot pressure and motion analysis test was conducted on a 244-foot sample of participants. Out of that, 142 were normal foot arch and 102 were flatfoot participants. The sample was classified based on age, gender and BMI. Participants were divided into three age groups: 18 to 35 (A), 36 to 60 (B) and above 60 (C). BMI was categorized into two groups: normal BMI (19–25) and high BMI (above 25), as detailed in Table 1. G*Power software (latest ver. 3.1.9.7; Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany) is used to get the desired sample size. The power level was set at 80 %, the significance level was set at 0.05. The effective size was calculated as 0.55.

Table 1.

Characteristics of the participating subject, Presented as mean value ± Standard deviation.

Variables	NF	FF	P value	NF	FF	P value
Female	F-NBMI			F-HBMI
AGE GROUP 18-35	n = 23	n = 6		n = 10	n = 12
BMI	19.7 ± 2	21.6 ± 2.8	0.277	29.4 ± 2.2	29.5 ± 1.1	0.79
Height (cm)	159.3 ± 5.6	158.3 ± 1.2	0.493	163.8 ± 7	158.9 ± 7.5	0.111
Foot length (cm)	23.3 ± 1	21.7 ± 0.3	0.023	23.8 ± 1.6	23.5 ± 1.2	0.591
Weight (kg)	50.2 ± 6.8	54.1 ± 7.9	0.445	78.9 ± 9.5	74.6 ± 7.5	0.428
AGE GROUP 36-60	n = 9	n = 6		n = 11	n = 30
BMI	20.9 ± 2.9	23.4 ± 0.9	0.235	32.1 ± 3	35.2 ± 8.2	0.517
Height (cm)	154.4 ± 7.5	157.8 ± 7.3	0.905	153.5 ± 5.5	153 ± 7.1	0.637
Foot length (cm)	23.3 ± 0.8	23.2 ± 2.7	0.719	23 ± 0.7	23.5 ± 0.8	0.112
Weight (kg)	49.7 ± 5.4	58.2 ± 4.3	0.024	75.9 ± 11	82.1 ± 17.9	0.303
AGE GROUP 60	n = 3	n = 3		n = 4	n = 10
BMI	23.6 ± 1.5	24.3 ± 8.2	0.102	33.8 ± 0.8	36.7 ± 7.3	0.569
Height (cm)	162 ± 1.69	151 ± 1	0.102	148 ± 2.9	154.7 ± 3.8	0.023
Foot length (cm)	24.5 ± 0.9	23.5 ± 0.5	0.083	23.3 ± 0.3	24 ± 0.7	0.12
Weight (kg)	62 ± 5.3	56.3 ± 5.3	0.083	74 ± 1.2	87.2 ± 14.8	0.059
Male	M-NBMI			M-HBMI
AGE GROUP 18-35	n = 24	n = 6		n = 26	n = 12
BMI	21.5 ± 2.7	22.7 ± 1.1	0.436	31.2 ± 5	29.2 ± 3.3	0.139
Height (cm)	171.8 ± 6.1	168.9 ± 8.9	0.603	171.8 ± 5.9	175.3 ± 7.8	0.346
Foot length (cm)	24.7 ± 1.3	24.8 ± 1.4	0.678	25.4 ± 0.9	25.6 ± 1.9	0.875
Weight (kg)	63.6 ± 8.8	65 ± 9.3	0.958	92.8 ± 19.4	89.4 ± 7.1	0.823
AGE GROUP 36-60	n = 7	n = 3		n = 10	n = 6
BMI	23.5 ± 1.1	24.3 ± 0.3	0.057	29.8 ± 2.8	29.9 ± 3.2	0.92
Height (cm)	167.8 ± 8	160.5 ± 6.9	0.174	167 ± 4.9	167 ± 4.5	0.064
Foot length (cm)	26.4 ± 0.8	25.2 ± 0.6	0.048	24.2 ± 1	25.2 ± 0.3	0.762
Weight (kg)	66.2 ± 5.2	62.6 ± 6.2	0.785	83.3 ± 9.3	83.4 ± 9.6	0.264
AGE GROUP 60	n = 5	n = 4		n = 10	n = 4
BMI	21.2 ± 1.2	22.9 ± 1.3	1	26.2 ± 2.8	28.6 ± 1.5	1
Height (cm)	167 ± 7	169 ± 2.3	0.776	168.2 ± 8.5	159.4 ± 3.6	0.061
Foot length (cm)	25.8 ± 1.3	25.8 ± 0.3	0.765	25.9 ± 0.7	25.5 ± 0.6	0.533
Weight (kg)	59.2 ± 6.8	65.6 ± 5.5	0.569	74.2 ± 9.8	72.6 ± 0.5	0.18

NBMI= Normal Body Mass Index, HBMI = High Body Mass Index, F = Female, M = Male, n = frequency,* indicates a significant difference between the NF and FF (p < 0.05), NF = Normal Foot, FF = Flatfoot.

The inclusion criteria were as follows: (1) Individuals aged 18 years and above (2) male, and female (3) Participants demonstrate the ability to walk independently for at least 1 min without ambulatory or personal assistance. (4) who exhibit flatfoot resulting from Parkinson's Disease, aging, diabetes, foot injury, obesity, or rheumatoid arthritis. The exclusion criteria were: (1) foot wound (2) congenital foot deformities (3) limb length discrepancies (4) pediatric patients with flatfoot. The hospital review board approved this study. The research was carried out following the principles outlined in the 'Declaration of Helsinki'. A written informed consent form was obtained from each participant before their involvement in the study.

2.2. Instrumentation and experimental setup

Foot pressure was measured using an electronic pedometer ‘Footwork Pro Device (Amcube, UK).’ Neon-colored marker stickers were securely fixed on key points of each foot. The participant was allowed to walk over the Pressure pad (Fig. 1a). Simultaneously the camera is set for motion to be captured by videography in the Sagittal plane. Detailed demographic records including gender, age, height, foot length, and weight were maintained for each participant to ensure precise identification and analysis of collected data. Ankle joint mechanics were studied at two gait stance phases: mid-stance and push-off. Five parameters were studied during both stance phases including ankle joint angle, Achilles tendon force angle, ground force reaction angle, ankle joint force, Achilles tendon force, and vertical ground force reaction. In the push-off phase, foot ankle inclination with the ground (F) was also measured. Modelling of the mid-stance and push-off phases involved assumptions that predominant forces are developed by the ankle joint and Achilles tendon during the gait stance phases, while other foot tendons and muscles produce negligible forces. Considering the body is to be in equilibrium at a particular time frame and three (i.e. Ground reaction force, Ankle joint force and Achilles tendon force) non-parallel forces are acting on the body. Then according to the law of concurrency, the line of action of three forces must pass through a single point. Using this equilibrium principle, the partial link model of the foot is prepared at each stance phase (Fig. 1b). The Centroid of foot pressure gives position to the vertical ground reaction force (Fig. 1c). Develop two simultaneous Equations of Equilibrium to get the magnitude and direction of Achilles tendon force and Ankle joint force. Which relies on inverse dynamic analysis, utilizing vertical ground force reaction and mapping motion data of the gait stance phases (Fig. 1).

Fig. 1.

A) Position of the foot in mid-stance and push-off phase with Neon-colored marker stickers were securely fixed on key points of each foot. b) Free body diagram of Partial link Foot model and foot position of Mid-stance and Push-off phases of gait in the sagittal plane. C). Foot pressure and Position Centre of gravity for, Mid-stance, and Push-off. d.) Vertical ground force reaction in stance phase with time.

2.3. Statistical analysis

Statistical Shapiro-Wilk test was conducted to check the normality of the distribution of the sample. For those not satisfying normality, Mann Whitney U test was used. Statistical significance was set as $\alpha =0.05$ SPSS (SPSS, IBM, India) was used for statistical analysis. Differences between foot forces in flatfoot and Normal foot (based on age, gender and BMI) in the mid-stance and push-off phase were found by analysis of variables.

3. Result

Characteristics data of participating patients (male and female) are presented in Table 1. Compared with the normal foot, flatfoot showed no significant difference between BMI, height, foot length and weight. The mean value of BMI in flatfooted males and females is slightly higher than the normal foot.

The comparative results of the normal foot and flatfoot at mid-stance phase among Ankle joint angle with vertical axis $\left(Jc°\right)\text{,}$ Ground reaction force angle with vertical axis $\left(GFR°\right)$, Vertical ground reaction force in terms of body weight $\left(VGRF\right)\text{,}$ Achilles tendon force in terms of body weight $\left(T\right)$ and Ankle joint force in terms of body weight $\left(Jc\right)$ are shown in Fig. 2 where as that of the normal foot and flatfoot at Push-off phase among $Jc°,VGRF,T\text{,}$ $Jc$ and Foot inclination angle with horizontal axis $\left(F°\right)\text{,}$ are shown in Fig. 3.

Fig. 2.

Ankle joint angle with the vertical axis (Jc°), Ground reaction force angle with the vertical axis (GFR°) Vertical ground reaction force in terms of body weight (VGRF), Achilles tendon force in terms of body weight (T) and Ankle joint force in terms of body weight (Jc) during mid-stance phase of gait stance, * indicates a significant difference between the NF and FF (p < 0.05), NF = Normal Foot, FF = Flatfoot.

Fig. 3.

Vertical ground reaction force in terms of body weight (VGRF), Achilles tendon force in terms of body weight (T), Ankle joint force in terms of body weight (Jc), Ankle joint angle with the vertical axis (Jc°), Foot inclination angle with the horizontal axis (F°) during Push-off phase of gait stance, *indicates a significant difference between the NF and FF (p < 0.05), NF = Normal Foot, FF = Flatfoot.

In the mid-stance phase, particularly in the female population, both those with higher BMI in the younger age group and those with high BMI in the older age group exhibit a reduction in $Jc°$, showing decrements of 14.43 % $\left(p=0.194\right)$ and 15.34 % $\left(p=0.221\right)$ respectively (see Fig. 2). Similarly in males, individuals with normal BMI in the younger age group, those with normal BMI in middle age group, and those with high BMI in middle age group demonstrate decreases in $Jc°$ by 12.72 % (p = 0.101), 14.28 % $\left(p=0.409\right)$, and 13.14 % $(p=0.432$) respectively. The females with high BMI in the middle and older age groups, males with high BMI in the middle age group, and males with normal BMI in the older age group, show there is a mix of positive and negative ranges of GFR°, indicating varied foot mechanics during this phase. These fluctuations highlight the complexity of foot biomechanics across different demographic profiles leading to instability of gait. Moreover, females with high BMI in the middle age group $\left(p=0.029\right)$, females with high BMI in the older age group $\left(p=0.014\right)$, and males with high BMI in the older age group $\left(p=0.039\right)$ demonstrate significantly Higher vertical ground reaction force values, these findings are depicted in Table 2.

Table 2.

Foot forces and angle at mid-stance position during gait stance phase. Presented as mean value ± Standard deviation.

Female	F-NBMI		P value	F-HBMI		P value
Variables	NF	FF		NF	FF
AGE GROUP 18–35 (A)
Jc◦	5.26 ± 1.54	5.33 ± 0.58	0.705	5.11 ± 2.01	4.58 ± 1.31	0.459
GFR ◦	5.72 ± 2.7	3.07 ± 0.29	0.085	3 ± 2.16	3.57 ± 3.12	0.373
VGFR	0.88 ± 0.1	0.9 ± 0	0.701	0.94 ± 0.11	1.03 ± 0.09	0.029*
T	1.62 ± 0.48	1.3 ± 0.17	0.146	1.4 ± 0.52	1.36 ± 0.57	0.974
Jc	2.47 ± 0.54	2.2 ± 0.17	0.243	2.33 ± 0.55	2.35 ± 0.62	0.766
AGE GROUP 36–60 (B)
Jc◦	6.56 ± 1.81	6.5 ± 2.59	0.589	6.08 ± 2.43	5.2 ± 1.99	0.194
GFR ◦	3.8 ± 2.43	3.43 ± 3.29	0.953	3.77 ± 3.66	3.17 ± 3.95	0.492
VGFR	0.94 ± 0.09	1.05 ± 0.15	0.156	0.98 ± 0.09	1.03 ± 0.14	0.13
T	1.46 ± 0.27	1.78 ± 0.42	0.169	1.52 ± 0.64	1.8 ± 0.62	0.233
Jc	2.38 ± 0.24	2.82 ± 0.54	0.15	2.49 ± 0.69	2.81 ± 0.64	0.368
AGE GROUP 60 (C)
Jc◦	7.5 ± 2.12	5.67 ± 1.53	0.22	5.5 ± 2.08	5 ± 1.41	0.716
GFR ◦	3.85 ± 0.07	3.67 ± 3.1	0.12	1.95 ± 3.82	1.12 ± 3.05	0.085
VGFR	0.85 ± 0.07	1.1 ± 0.1	0.121	0.9 ± 0.14	1.09 ± 0.09	0.014a
T	1.2 ± 0.28	1.57 ± 0.47	0.439	0.93 ± 0.64	1.02 ± 0.46	0.476
Jc	2 ± 0.14	2.67 ± 0.5	0.368	1.8 ± 0.62	2.07 ± 0.47	0.226
Male	M-NBMI			M-HBMI
AGE GROUP 18–35 (A)
Jc◦	6.88 ± 1.6	6 ± 1.23	0.101	5.08 ± 1.96	5.08 ± 1.88	0.774
GFR ◦	3.68 ± 2.88	4.85 ± 3.66	0.659	6.11 ± 1.79	5.45 ± 3.82	0.875
VGFR	0.89 ± 0.12	1.02 ± 0.15	0.052	1.02 ± 0.13	0.98 ± 0.07	0.688
T	1.76 ± 0.62	1.75 ± 0.49	0.697	2.01 ± 0.54	1.96 ± 0.58	0.78
Jc	2.61 ± 0.68	2.78 ± 0.55	0.775	2.99 ± 0.62	2.94 ± 0.61	0.987
AGE GROUP 36–60 (B)
Jc◦	7 ± 2.08	6 ± 1	0.409	7.1 ± 2.03	6.17 ± 2.14	0.432
GFR ◦	4.56 ± 3.44	5.1 ± 3.55	0.731	4.11 ± 2.81	−1.25 ± 8.33	0.104
VGFR	0.87 ± 0.08	0.93 ± 0.12	0.334	0.94 ± 0.08	1.03 ± 0.08	0.039*
T	1.8 ± 0.66	1.73 ± 1	0.818	1.84 ± 0.39	1.32 ± 1.08	0.549
Jc	2.66 ± 0.66	2.67 ± 1.11	1	2.75 ± 0.43	2.37 ± 1.05	0.585
AGE GROUP 60 (C)
Jc◦	4.6 ± 1.14	4.5 ± 1.73	0.706	4.7 ± 2.41	4.5 ± 1	0.561
GFR ◦	4.28 ± 3.24	3.23 ± 4.43	0.806	3.35 ± 3.98	0.25 ± 8.65	0.396
VGFR	0.9 ± 0	0.95 ± 0.21	0.467	0.97 ± 0.11	0.95 ± 0.06	0.765
T	1.32 ± 0.34	1.3 ± 0.74	0.537	1.31 ± 0.8	1 ± 0.92	0.62
Jc	2.22 ± 0.34	2.25 ± 0.84	0.539	2.27 ± 0.78	1.98 ± 0.97	0.619

NBMI = Normal Body Mass Index, HBMI = High Body Mass Index, F = Female, M = Male, n = frequency.

^aindicate a significant difference between the NF and FF (p < 0.05), NF = Normal Foot, FF = Flat foot.

In the push-off phase, it's noted that the ankle joint angle remains relatively consistent across these groups, with no significant change observed except in females with high BMI in the older age group. In this particular subgroup, there is a smaller $Jc°$ observed, which is significant (p = 0.01). The foot inclination angle $\left(F°\right)$ shows significantly smaller in females with a normal BMI in the younger age group $\left(p=0.021\right)$ and males with normal BMI in the younger age group $\left(p=0.018\right)$. Additionally, the mean absolute value of $F°$ tends to be smaller in both females and males across the groups, as observed in Fig. 3. Furthermore, there are notable differences in vertical ground reaction force $\left(VGRF\right)$ values. Female with normal BMI in the younger age group $\left(p=0.016\right)$ and female with high BMI in a older age group $\left(p=0.047\right)$ exhibit significantly higher $VGRF$ values as indicated in Table 3. Moreover, the Achilles Tendon force and ankle joint force show significantly higher values in females with high BMI in the middle age group and male with normal BMI in the younger age group $\left(p=0.023andp=0.026\right)$ respectively, as depicted in Table 3.

Table 3.

Foot forces and the angle at the Push-Off Position during the gait stance phase. Presented as mean value ± Standard deviation.

Female	F-NBMI		P value	F-HBMI		P value
Variables	NF	FF		NF	FF
AGE GROUP 18–35 (A)
F◦	12.76 ± 5.06	5.67 ± 1.16	0.021a	10 ± 3.56	7.17 ± 3.81	0.06
Jc◦	22.13 ± 4.04	21.67 ± 2.31	0.968	19.9 ± 1.91	19.5 ± 2.43	0.367
VGFR	1.22 ± 0.12	1.03 ± 0.06	0.016a	1.1 ± 0.08	1.17 ± 0.13	0.255
T	3.4 ± 0.66	3.83 ± 0.95	0.334	2.99 ± 0.54	3.18 ± 0.42	0.53
Jc	4.55 ± 0.73	4.83 ± 0.95	0.573	4.04 ± 0.57	4.3 ± 0.44	0.354
AGE GROUP 36–60 (B)
F◦	10.56 ± 3.21	8.67 ± 4.08	0.476	10.08 ± 4.66	6.77 ± 5.93	0.2
Jc◦	20.11 ± 2.32	19.67 ± 1.21	0.388	18.69 ± 3.59	17.6 ± 5.22	0.912
VGFR	1.22 ± 0.08	1.13 ± 0.15	0.318	1.12 ± 0.13	1.12 ± 0.16	0.914
T	2.97 ± 1.21	2.92 ± 0.59	0.315	2.86 ± 0.62	2.31 ± 0.71	0.023a
Jc	4.1 ± 1.21	4 ± 0.69	0.405	3.92 ± 0.69	3.4 ± 0.71	0.026a
AGE GROUP 60 (C)
F◦	10.5 ± 3.54	9 ± 7.07	0.102	9.75 ± 5.5	1.75 ± 2.55	0.075
Jc◦	21 ± 0	19 ± 1.41	1	16.75 ± 0.96	13.5 ± 5.04	0.01a
VGFR	1.23 ± 0	1.2 ± 0	0.083	1.03 ± 0.1	1.2 ± 0.11	0.047a
T	2.75 ± 0.35	3.2 ± 0.42	0.136	2.23 ± 0.3	2.15 ± 0.73	1
Jc	3.6 ± 0.42	4.3 ± 0.42	0.136	3.23 ± 0.33	3.33 ± 0.69	0.619
Male	M-NBMI			M-HBMI
AGE GROUP 18–35 (A)
F◦	12.5 ± 4.85	7.33 ± 3.2	0.018a	11.23 ± 4.42	9.27 ± 8.33	0.106
Jc◦	21.88 ± 3.51	23 ± 2.61	0.347	20.96 ± 2.68	20.08 ± 3.58	0.65
VGFR	1.23 ± 0.15	1.27 ± 0.1	0.439	1.15 ± 0.08	1.18 ± 0.08	0.434
T	3.85 ± 0.75	4.87 ± 1	0.021a	3.63 ± 0.7	3.75 ± 1.86	0.441
Jc	5.03 ± 0.79	6.08 ± 1.13	0.026*	4.74 ± 0.75	4.88 ± 1.8	0.446
AGE GROUP 36–60 (B)
F◦	6 ± 3.65	10.67 ± 8.15	0.409	8.82 ± 6.06	3.5 ± 6.12	0.059
Jc◦	22.29 ± 5.09	23.33 ± 6.66	0.73	20.55 ± 2.25	18.5 ± 5.05	0.608
VGFR	1.04 ± 0.08	1.03 ± 0.12	0.673	1.16 ± 0.08	1.15 ± 0.08	0.744
T	3.59 ± 1.61	2.03 ± 0.71	0.136	3.66 ± 0.7	2.32 ± 1.91	0.13
Jc	4.61 ± 1.59	3.03 ± 0.55	0.136	4.77 ± 0.67	3.47 ± 1.84	0.13
AGE GROUP 60 (C)
F◦	6.4 ± 3.17	4.67 ± 5.69	0.539	3.8 ± 4.92	2.5 ± 3	0.226
Jc◦	21.4 ± 4.48	18.67 ± 2.08	0.898	16.4 ± 2.07	16.5 ± 5.26	0.132
VGFR	1.14 ± 0.08	1 ± 0.14	0.081	1.12 ± 0.05	1.03 ± 0.1	0.078
T	3.73 ± 1.15	3.15 ± 0.84	0.462	3.04 ± 0.81	2.1 ± 1.44	0.119
Jc	4.84 ± 1.16	4.13 ± 0.87	0.142	4.14 ± 0.76	3.1 ± 1.35	0.065

NBMI = Normal Body Mass Index, HBMI = High Body Mass Index, F = Female, M = Male, n = frequency.

^aindicate a significant difference between the NF and FF (p < 0.05), NF = Normal Foot, FF = Flat foot.

4. Discussion

This research work examined the effect of gender, age and BMI on flatfoot deformity. The observations with control/normal and flatfoot were derived considering the above parameters. The foot pressure measurement in the plantar fasciitis foot was carried out. Relation for foot equilibrium, the kinetics and kinematics (from motion analysis) of the foot during gait is presented.

The findings of our study suggest that individuals with flatfoot exhibit smaller ankle joint angles $\left(Jc°\right)$ values, leading to increased ankle stiffness compared to those with normal foot. Notably, observations indicate that individuals with flatfoot and a higher $BMI$ tend to have slightly higher ankle stiffness, as evidenced by smaller $Jc°$ values, compared to those with the normal foot. This observation suggests a further increase in $Jc°$ promoting further joint stiffness. This is a typical observation in patients with obesity factors.

It is evident in previous research that gait kinematics of flatfoot in adults shows notable distinctions in lower limb segments based on age and gender.^19,20,23 The results presented here are consistent with these authors' findings, which indicate that individuals with flatfoot exhibit increased ankle stiffness and reduced ankle movement compared to those with normal foot arches.^16,17 Yan et al. found that obese children with flatfoot and normal-weighted, the obese group showed a significantly larger contact area under the midfoot region, indicating that obesity has a greater impact on foot biomechanics and thus in plantar fascia deformity²⁴

Ground force reaction (GFR) inclination, particularly in individuals with flatfoot, high BMI, and in older age groups, can have both negative and positive values with respect to the vertical axis. These variations suggest a combined influence of aging and flatfoot morphology. Such variations can contribute to instability during walking and an elevated risk of balance loss, as implied by equilibrium equations in biomechanics. This observation aligns well with findings from previous studies, indicating that flatfoot deformity significantly affects balance and stability.^7,22,25 The study suggests that in individuals with high BMI and flatfoot, and normal foot, smaller GFR inclinations were observed compared to individuals with normal BMI and normal foot. This phenomenon is likely attributed to the forward shift in the body's center of gravity required for maintaining balance in individuals with higher BMI. The findings are consistent with Chehab et al., which indicated a correlation between higher BMI and forward pelvic tilt. This further supports the notion that body weight distribution and BMI can influence biomechanical factors related to balance and stability¹³

The increased vertical Ground Reaction Force (VGRF) observed in mid-stance flatfoot individuals may be a consequence of reduced variation in the vertical acceleration of the body's centre of mass, which can be attributed to the loss of foot arch and being overweight.²⁶ This finding is supported by Boozari et al., who noted a flatter VGRF graph during the stance phase in flatfooted individuals, suggesting a greater shock absorption behaviour in this group.²⁷ Additionally, significantly higher VGRF values were noted in older age groups during the mid-stance phase. This could be due to altered load transfer mechanisms in flatfooted individuals, leading to decreased walking stability or imbalance, which subsequently reduces velocity. Jons et al. proposed that as this increased VGRF might also be indicative of age-related declines in neuromuscular control.²⁸ Furthermore, research by Okamur et al. on lower extremities highlights that as the severity of flatfoot increases, the efficiency of foot ground force transmission decreases. This inefficiency could potentially contribute to a higher incidence of lower-extremity injuries among individuals with severe flatfoot compared to those with mild flatfoot deformity.⁷

In individuals with flatfoot during the push-off phase, the complete sole contacts the ground due to collapsed arches. This structural difference alters foot biomechanics, diminishing propulsion effectiveness and fostering dynamic instability, thereby reducing forward momentum compared to individuals with normal foot structure. In flatfoot, it is observed that the inclination of the foot with the ground ($F°$) does not vary significantly, but the angle of the foot with the leg (F°) is notably smaller compared to normal foot, indicating excessive inward rolling of the foot shaft during movement. This inward rolling can disrupt foot and ankle alignment, potentially compromising force transmission efficiency during push-off and resulting in reduced VGRF. This observation aligns with Kim et al.'s findings, which showed a decrease in ground reaction force in flatfoot compared to normal foot during the push-off phase.¹⁶ Similarly, Fan et al. reported significant differences ($p=0.01)$ in the distribution of plantar vertical ground reaction force during stance phases between individuals with flatfoot and those with normal foot structure.¹⁵ Boozari et al. suggested that the increased flexibility of foot joints, possibly due to inadequate action of foot muscles, might impair the formation of a proper lever arm for the propulsion phase of gait, contributing to reduced push-off force in individuals with flatfoot.²⁷

Overall, in the younger age group (A) despite the observed differences in $GFR°$ and $Jc°$; other variables such as VGRF, $Jc$ and T did not show significant differences between individuals with normal and flatfoot arches. This is typically due to strong muscle strength behaviour observed in younger age groups. This lack of significant differences may be attributed to factors as a younger age group with flatfoot biomechanics suggesting that foot biomechanics or load transfer mechanisms are not substantially altered in this demographic and it is necessarily not causing discomfort.

In the cohort of middle-aged females with elevated BMI, there is a consistent pattern of $VGRF$ observed during Push-off, accompanied by a significant decrease in Achilles tendon force and Ankle joint angle. Consequently, there is a likelihood of increased load placed on other tendons, suggesting potential biomechanical inefficiencies or adaptations to mitigate excessive loading on these structures, particularly in comparison to individuals with normal foot mechanics. These findings highlight the cumulative impact of childbirth, menopause, and hormonal fluctuations on muscle strength loss and biomechanical disruptions over time, particularly accentuated in individuals with flat-footedness. Such individuals may consequently experience augmented discomfort over the years.

Females typically have a wider pelvic bone compared to males, which can lead to deviations in ground reaction force distribution during flat-footed stance phases, potentially exacerbating biomechanical alterations associated with flatfoot and resulting in greater disruption in lower limb function and potentially increased discomfort.

Tomasiak et al. found that women with flatfoot lack lower limb propulsion leading to side sway while walking, while men exhibit altered lower limb, resulting in painful gait necessitating the use of external assistive devices. It is suggested that gender-specific factors should be considered in flatfoot analysis and treatment.²³

Future studies should aim to compare the kinetics and kinematics of flatfoot patients before and after implementing group-specific clinical interventions, integrating concurrent designs and outcome measures capturing functional and patient-reported outcomes to bridge the gap between biomechanical research and clinical practice. This research has limitations of sample size in some cohorts. The authors are working on increasing the sample size.

This study, the first of its kind, examines the gait stance phase kinematics and kinetics of flatfoot patients across three major cohorts, considering stratification based on age, gender, and BMI to analyze foot biomechanics comprehensively.

5. Conclusion

The research findings show that females with higher BMI experience increased vertical ground reaction forces during mid-stance, and middle-aged females may experience discomfort due to hormonal changes and altered load transfer mechanisms compared to males during push-off. Additionally, those with flatfoot and higher BMI demonstrate increased ankle stiffness. Furthermore, older individuals with flatfoot encounter instability in walking and a heightened risk of falls and middle-aged females with high BMI may experience greater instability compared to males in the same group.

The research highlights the heightened risk of instability and altered gait mechanics related to flatfoot, particularly with age and higher BMI. Personalized interventions considering gender, age, and BMI are crucial for precise solutions to biomechanical issues, enhancing foot function, and reducing discomfort. Customized footwear design and interventions are crucial in optimizing stability and redistributing loading effectively and make necessary interdisciplinary collaboration among healthcare professionals. This interventional research provides comprehensive care and optimizing outcomes for individuals with altered foot biomechanics.

Ethical approval

Experimental trials were carried out following approval from Deenanath Mangeshkar Hospital and Research Centre, Pune (IHR_2023_Feb_AS_490).

The study conducted in accordance with the ethical principles mentioned in the Declaration of Helsinski (2013).

All participants gave written informed consent.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Patient's consent

All participants are 18 and above age. All participants gave written informed consent.

Conflicts of interest

The authors declare no conflicts of interest of any kind during experimentation or otherwise for this study.

CRediT authorship contribution statement

Mrudula Patil: Writing – original draft, Investigation, Methodology, Data curation. Mrudula S. Kulkarni: Conceptualization, Methodology, Writing – review & editing, Supervision, Visualization. Avijan Sinha: Visualization, Supervision, Conceptualization. Ratnakar R. Ghorpade: Methodology, Writing – review & editing, Conceptualization.