Relationship between Body Mass Index (BMI) and Foot Posture Index (FPI) Among Healthy Adults Aged 20-30 Years - An Observational Cross-Sectional Study

Introduction

Obesity is the new epidemic globally. Body mass index (BMI) is used to define Obesity. Obese is defines as BMI of ≥ 30 kg/m² and Overweight is defined as BMI of ≥ 25 kg/m² Studies have reported an increased prevalence of obesity in developing countries with a noteworthy increase in the prevalence of non-communicable diseases including diabetes, hypertension, Polycystic ovary syndrome (PCOS), etc.^1-3 As per the BMI classification, about 39% of the adult population worldwide come under the overweight category.⁴

The findings from a National survey estimated that in India, about 27.8% of the population would be overweight and 5.0% would be obese by the year 2030.⁵ Increased BMI values are associated with an increased severity of joint pains. Pain is reported to be more prevalent in the load-bearing joints of the lower extremities and can also manifest in the upper extremities, resulting in gait abnormalities, weakness of muscles, and impaired quality of life.^6-7

The foot plays a major role in daily human activities like standing, walking, and maintaining an erect posture. The foot mechanics help to correct postural sways in single-limb stance and support the normal gait by controlling the ground reaction force and push-off mechanism from the ground.^8-10

It is expected that shortfalls like the foot curve's congruity, adaptability, and strength affect the capacity of the foot functions and predispose subjects to standing balance disturbances. Foot discomfort has lately been linked to obesity, with both fat mass and higher BMI being significantly connected with foot pain. As a result, the mechanism behind the link between obesity and foot discomfort must be investigated further. In people with a higher BMI, there has been a link between defective foot anatomy, function and foot pain. One proposed explanation for this link is that obese people develop alterations in foot structure and function due to increased lower-limb mechanical load when walking. Obesity has also been linked to increased leg swing velocity, higher metabolic expenditure, and poorer postural stability, all of which might affect foot function. Therefore, this study was planned to determine the relationship between body mass index and foot posture index (FPI) among healthy adults in the age group of 20-30 years studying at a medical university.

Material and Methods

This cross-sectional study was conducted in the outpatient department of orthopaedics and physiotherapy, SDM College of Medical Sciences and Hospital, Dharwad, India. A total of 90 healthy adults were screened, of which 50 healthy participants in the age group of 20-30 years were recruited for the study as per the eligibility criteria. Healthy male or female subjects aged 20 to 30 years who were able to speak, read, and understand English were included in the study. Individuals with prior surgical history involving the right or left lower limbs, those with history of pain, trauma, or prevalent lower limb deformity, history of reconstructive surgery in the foot, and those who refused to participate in the observational study were excluded.

A total of 90 subjects were screened for participation in the study. About 10 subjects did not meet the inclusion criteria for age, 15 did not consent to participate due to various reasons, 10 had non-specific foot pain, while five subjects had recent foot trauma. Thus, 50 subjects who met the criteria were assessed for their foot posture using the foot posture index scale. The data on 100-foot postures were obtained from 50 subjects.

The demographic details of the participants were obtained. Written informed consent was obtained before the commencement of the study, and the entire information sheet containing all the details regarding the testing protocol was given to all participants before testing. Individuals with any prior history of surgeries, limb deformities, or reconstructive surgeries of the foot were excluded from the study. The ethical clearance was obtained by the Institutional Review Board prior to the conduct of the study.

Study parameters

Age in years of all the participants was recorded. The height of participants was measured in cm using a stadiometer with a calibrated analogue scale in an upright position. The weight of the participants in light-weight clothes and bare feet was measured in kg. BMI was calculated using the Quelet index, i.e., body weight in kg divided by height in meters squared, and was classified as per WHO and Asia-Pacific guidelines. Foot Posture Index-6 FPI-6 is a novel method of rating foot posture using six criteria and is a simple scale. It is devised to report deviations in foot postures with respect to supinated, pronated, or neutral foot postures.¹⁰

Procedure

Before conducting the initial testing procedure, the primary investigator demonstrated the procedure to the participants. Once the participants understood the task, the procedure was initiated. The participants were instructed to stand still with their arms by the side, and look straight ahead for 120 seconds. During the assessment, the participants were advised not to swivel in an attempt to see what was happening to themselves, as this would significantly affect their foot posture. The participants had to stand still for approximately two minutes in total for the assessment to be conducted. The forefoot and the rear foot were viewed in all three planes: transverse, frontal, and sagittal. The foot was assessed in six different components and scored accordingly. The values ranged from -2 to +2, where 0 suggested neutrality.

Inference

The score with a positive value meant pronated foot, while the negative value meant supinated foot presentation. A normal of neutral foot reported neutral or zero value on FPI scoring.

Statistical analysis

SPSS software version 28.0 was used for statistical analysis. Descriptive analysis was done and presented as mean, standard deviation, and P values.

Results

A total of 50 enrolled participants had a mean age of 22±1.38 years and the mean BMI value was 23.53±4.42 kg/m² . The participant group included 12 males and 38 females. The mean FPI for left foot was 1.34 and for right foot was 1.32. The positive value of FPI reflected pronation of the feet (Table 1).

A statistically significant positive correlation was noted between BMI and left foot FPI. It indicated that increase in BMI causes increase in left foot posture index significantly (Table 2).

A statistically significant positive correlation was noted between BMI and right foot FPI with Pearson’s correlation value of 0.5, which meant increase in BMI further increases foot posture index (Table 3).

Right foot FPI showed high statistical significant correlation between BMI and FPI (P value <.001) as compared to left foot (P value <.10).

Discussion

The foot has three arches: two longitudinal (medial and lateral) arches, one transverse arch. The shape of these arches allow them to act like a spring on bearing weight. They are the important shock absorbers during locomotion. Heads of metatarsals along with the tuberosity of the calcaneum are the weight bearing areas of the foot. Adults with higher BMI take slow, short, and wide steps to increase the stability while walking. Obese individuals have lower arches which are more flexible during step off phase leading to abnormal mechanical loading, and are consequently at high risk for developing chronic foot pain disorders.¹¹

The aim of the study was to determine the relationship between body mass index and foot posture index among healthy adults in the age group of 20-30 years studying at a medical university. The mean body mass index of all 50 participants was 23.53, with a higher mean foot posture index in the left foot (1.34) compared to the right foot (1.32). Out of the 50 subjects included, four were underweight, 30 were healthy, five were overweight, and 11 were obese (Table 2).

The findings of our study suggest that higher BMI is significantly related to the foot posture index of both the feet (Table 3 and 4). The foot posture index is influenced by an overweight BMI. These findings are in line with the previous study which reported that obesity influences the foot mechanics. Obesity exerts abnormal influence on structure of the foot.^12-13 The foot is subdivided into the rear foot, midfoot, and forefoot. It functions as a rigid structure for weight bearing, and can also function as a flexible structure to conform to uneven terrain. The foot mechanics are very important for supporting and balancing body weight, shock absorption, and transferring ground reaction forces. Good foot stability is needed to bear body weight and act as a stable lever to propel the body forward. There is more pressure on the mechanical loading of the foot in obese individuals, which may be directly due to increased body weight, which in turn changes the foot mechanics. This explains the effect of a high BMI on FPI.^13-15 Foot posture is a major factor influencing mechanical alignment and, thus the functions of lower limbs. Understanding the effect of increased BMI on the foot posture index can provide timely intervention and also prevent further dysfunction.

In contrast to our findings, various studies have reported a negative correlation between BMI and foot posture index. These studies have explained that increased weight also leads to increased contact pressure with the ground, but at the same time, physiologically, plantar distribution and location of pressure get adjusted to the increased weight, creating the illusion of a flat foot rather than a pronated foot.^16-19

Butterworth et al. reported the strong association of increased fat mass with chronic foot pain and increased body mass with poor foot function. In their study, they demonstrated that the adults with high body mass index are at risk for general foot pain and plantar heel pain and obesity is associated with low arched pronated foot with decreased range of movements at the subtalar joint and high plantar pressure while walking.²⁰ Unver et al. demonstrated reduction in the muscle strength of plantar flexors, dorsi flexors, and invertors in obese individuals. There was also decreased range of plantar flexion and inversion. The FPI scores were higher in the obese individuals. These findings are in agreement with the findings of our study.²¹

Limitations of the study

Our study has not taken various other factors that can affect the foot posture index into consideration, like gender, types of footwear used by individuals, and history of footwear usage on the ground while playing and also at home.

Future directions

Further studies can take various factors affecting foot mechanics into consideration to gain more comprehensive knowledge.

Conclusion

Increased BMI leads to pronation of the feet. This can further cause the individual to walk slightly on the outside of the foot instead of the feet flat on the ground.

Conflict of interest

None