OBJECTIVE: This study examined the effects of obesity on plantar pressure distributions in prepubescent children.
DESIGN: Field-based, experimental data on BMI (body mass index), foot structure and plantar pressures were collected for 13 consenting obese children and 13 non-obese controls.
SUBJECTS: Thirteen obese (age 8.1±1.2 y; BMI 25.5±2.9 kg/m2) and 13 non-obese (age 8.4±0.9 y; BMI 16.9±1.2 kg/m2) prepubescent children, matched to the obese children for gender, age and height.
MEASUREMENTS: Height and weight were measured to calculate BMI. Static weight-bearing footprints for the right and left foot of each subject were recorded using a pedograph to calculate the footprint angle and the Chippaux–Smirak index as representative measures of the surface area of the foot in contact with the ground. Right and left foot plantar pressures were then obtained using a mini-emed® pressure platform to calculate the force and pressure experienced under each child's foot during static and dynamic loaded and unloaded conditions.
RESULTS: Obese subjects displayed significantly lower footprint angle (t=4.107; P=<0.001) values and higher Chippaux–Smirak index values (t=−6.176; P=<0.001) compared to their non-obese counterparts. These structural foot changes were associated with differences in plantar pressures between the two subject groups. That is, although rearfoot dynamic forces generated by the obese subjects were significantly higher than those generated by the non-obese subjects, these forces were experienced over significantly higher mean peak areas of contact with the mini-emed® system. Therefore, rearfoot pressures experienced by the two subject groups did not differ. However, the mean peak dynamic forefoot pressures generated by the obese subjects (39.3±15.7 N·cm−2; q=3.969) were significantly higher than those generated by the non-obese subjects (32.3±9.2 N·cm−2).
CONCLUSIONS: It is postulated that foot discomfort-associated structural changes and increased forefoot plantar pressures in the obese foot may hinder obese children from participating in physical activity and therefore warrants immediate further investigation.
Feet, as the body's base of support, continually endure often high ground reaction forces generated during activities of daily living. The component primarily responsible for absorbing and dissipating these forces in the feet is the longitudinal arch. Although this arch comprises bony articulations, ligaments and muscles, it is primarily the ligaments that support and stabilise the longitudinal arch, as well as acting as powerful energy-storing mechanisms.1,2 Muscles provide secondary support by maintaining the arch during dynamic tasks. Ligaments rarely incur physiological fatigue and therefore offer a greater resistance to stress compared to muscles.3 However, repeated excessive loading may stretch ligaments beyond their elastic limit, damaging soft tissues and increasing the risk of foot discomfort and subsequent development of foot pathologies.
Increased loading of the feet may be classified according to time-frame and described as temporary, short-term or long-term. A temporary loading effect occurs, for example, when carrying a backpack or wearing a weighted belt that temporarily increases body mass. The increased loading caused by this temporary body mass increase can be terminated immediately by removing the external item. Short-term loading increases result from more ‘permanent’ body mass changes such as the body mass increase associated with pregnancy. This increase in body mass, although applied for up to 9 months, is then terminated with birth. In contrast, a long-term loading effect occurs over an extended period, such as in obesity, where the increase in mass is continuous. Although studies pertaining to temporary and short-term loading effects on lower limb and foot mechanics are available,4,5,6,7 minimal research has examined the long-term loading effects of obesity on the musculo-skeletal system, particularly in reference to the feet.
Various authors have suggested that excessive increases in weight bearing forces caused by obesity may negatively affect the lower limbs and feet.8,9,10 However, despite the potential negative consequences of obesity on lower limb structure, only one investigation, from the same laboratory, has considered the effects of obesity on foot structure in children. Riddiford–Harland et al11 examined the foot structure of 62 obese (body mass index (BMI)>95th percentile12) and 62 non-obese (10th percentile<BMI<90th percentile) prepubescent children (mean age=8.5±0.5 y). Foot structure was characterised using the same techniques as in the current study, the footprint angle (FA) and Chippaux–Smirak index (CSI).13 The authors found a significant difference between the FA of the obese and non-obese subjects for both the left (t=3.663; P<0.001) and right feet whereby the obese children displayed a reduced angle. Furthermore, CSI scores for the left (t=−6.362; P<0.001) and right (t=−5.675; P<0.001) feet were significantly greater in the obese children. A decreased FA and an increased CSI are characteristics of structural foot changes, such as lowered longitudinal internal arches, a flatter cavity and a broader midfoot area of the footprint, that have been associated with compromised foot function. For example, lower arches have been associated with a decrease in the integrity of the foot as a weight-bearing structure.11 From these results Riddiford–Harland et al11 concluded that excess body mass appeared to negatively affect the foot structure of prepubescent children whereby obese children as young as 8 y of age were displaying structural foot characteristics which may develop into problematic symptoms if excessive weight gain continued. It was also postulated that foot discomfort associated with higher plantar pressures caused by these structural changes in the obese foot may have hindered obese children from participating in physical activity and therefore warranted immediate further investigation.11 Although providing comprehensive data pertaining to the effects of obesity on foot contact area, the Riddiford–Harland et al11 study was restricted to a static analysis of external foot structure and did not examine the pressures exerted on the plantar surfaces of the children's feet. Consequently it is unknown whether the significant changes to the external foot structure noted by Riddiford–Harland et al11 were associated with an increased loading per unit area under the feet of these obese children. Therefore, the purpose of the present study was to examine the effects of obesity on plantar pressures under the feet of prepubescent children.
Seven female and six male obese prepubescent children (age 8.1±1.2 y, height 133±8 cm; BMI>95th percentile for age and gender12) without other pathologies unassociated with their obesity were selected as experimental subjects. Thirteen non-obese children (BMI=50th percentile12), matched to the obese subjects for gender, age and height (age 8.4±0.9 y; height 132±7 cm) were selected as controls. Before commencing the study, all procedures and methods were approved by the University of Wollongong Human Research Ethics Committee and the Ethics Committee of the New Children's Hospital, Westmead. All testing was conducted according to the Statement of Human Experimentation.14
Body mass index
Each subject's height was measured to the nearest millimetre using a portable, calibrated Hadlands Photonics stadiometer while the subjects stood barefoot in the anatomical position. Body mass was measured to the nearest 0.05 kg using calibrated UC-300 Precision Medical Scales (capacity 150 kg) while the subjects stood motionless and wearing minimal clothing (shorts or skirt and a shirt). Body mass index (BMI) was then calculated using the Quetelet index: body mass divided by height squared (kg·m−2) as an indicator of obesity. Subjects were classified by their BMI score according to percentile range cut-off points12 that accounted for effects of developmental stages of maturation on body weight and height.
Foot structure assessment
Prior to testing, the children's feet were screened by a podiatrist to identify and exclude subjects with any external factors that may have contributed to variations in plantar pressures, such as calluses. The area of contact of the plantar surface of the foot with the ground was then determined through recording each subject's footprints using a Productos Suavepie pedograph and replicating the procedures of Riddiford–Harland et al,11 thereby allowing later between-study data comparisons. From the footprints, FA and the CSI were calculated and classified following the protocol described by Forriol and Pascual13 and Riddiford–Harland et al11 (see Figure 1). The purpose of the footprint analysis was to quantify the surface area of each child's foot that was in contact with the ground during static weight bearing and to quantify external characteristics of their medial longitudinal arch.15
Plantar pressure distribution
The mini-emed® system (Novelgmbh, Munich) was used to quantify the static and dynamic pressures exerted on the plantar surface of each subject's feet. The platform was placed on a firm and level surface, surrounded by a custom-designed dense foam walkway so that the plate was flush with the surrounding walkway. The purpose of assessing plantar pressure during static weight bearing and dynamic gait was to quantify the actual forces and pressures applied to each region of the plantar surface of the children's feet during typical activities of daily living—standing and walking.
Static plantar pressure measurement.
To assess static plantar pressure distribution, each subject stood barefoot and relaxed in the anatomical position, adopting the same posture used to collect footprint data with the pedograph. Two static plantar pressure trials were recorded (16 Hz; N·cm−2) for each subject's left and right foot. Pressure data were collected using the mini33 (Novelgmbh, Munich) program, transferred and saved to a Compac Armada 153OD laptop computer for later analysis. The threshold at which the mini-emed® platform was triggered to collect data (1 N·cm−2) was high relative to the mass of some of the smaller children, making static pressure data difficult to collect. From the static footprint generated by the mini-emed® system, the variables of peak force (N), peak area (cm2) and peak pressure (N·cm−2) for the whole foot during weight bearing were calculated using Novel® emed-SF® software.
Dynamic plantar pressure assessment.
After familiarisation trials to limit targeting of the pressure platform and to ensure the children were comfortable with the experimental procedures, each subject walked barefoot over the mini-emed® platform at a consistent walking pace set by an accompanying walker's speed16 and using the two-step method.17 For the two-step method, the subject stood approximately 1.2 m in front of the mini-emed® platform, stepped onto the platform with their second foot strike and continued to walk over and past the plate for approximately 2 m. The two-step method was selected in preference to the one-step or mid-gait methods18 as the young subjects tested in the present study had a greater chance of striking the platform without the need for excessive repeated trials. Furthermore, the two-step method has been shown to elicit pressure values in the fore-, mid- and rearfoot areas which are representative of the pressures recorded in the traditional mid-gait method.17 Data collection was restricted to three successful trials per foot for all subjects during the dynamic condition to minimise fatigue. Dynamic pressure measurements were triggered when the force generated by the first foot contact within the first third of the plate exceeded the threshold of 1 N·cm−2.
For the dynamic plantar pressure measurements each footprint was divided into two anatomical regions to ensure that the same areas were compared between subjects. These anatomical areas included the forefoot (50% of the length of the footprint as defined by the Novel® emed-SF® software) and rearfoot. Peak dynamic force (N), peak area (cm2) and peak pressure (N·cm−2) were then derived for the two foot divisions using the Novel® emed-SF® software. Peak dynamic pressures and relative forces were averaged across the three trials completed for each foot during each condition before further statistical analysis.
The artificial loading effect data collection.
Static and dynamic plantar pressure distribution assessments were repeated with the subjects loaded with an additional 20% of their body mass. For example, if a child weighed 40 kg they were required to carry an additional 8 kg by wearing a weighted jacket (see Figure 2). The additional mass was distributed evenly around the subject's torso in order to minimise gait alterations. Twenty percent was selected as the additional mass increase as this is the percentage increase that occurs typically in other short-term mass increases such as pregnancy. Furthermore, pilot testing confirmed 20% as the maximal additional mass that an 8-y-old child could ‘comfortably’ wear during walking. The purpose of this artificial loading condition was to determine whether an instantaneous increase in mass influenced the dependent variables (static and dynamic plantar pressure distribution) compared to long-term loading effects created by obesity.
Means and standard deviations were calculated for the total subject sample for the dependent variables of FA and CSI derived from the pedograph and for the static and dynamic plantar pressure dependent variables recorded during both the loaded and unloaded conditions. As paired t-test results indicated there were no significant differences between each variable when comparing right and left limbs, all variables were pooled across test limbs in subsequent analyses. The FA and CSI data derived for the non-obese (n=13) and the obese (n=13) subjects were then analysed using independent t-tests to ascertain whether there was any significant effect (P<0.05) of obesity on the traditional footprint parameters. Before analysis, normality of the data was tested using a Kolmogorov–Smiranov test (with Lillefors' correction) whereas equal variance was tested using a Levene median test.
All plantar pressure dependent variables were analysed using an ANOVA design with one between-factor (subject group: obese and non-obese) and one within-factor (loading condition: loaded and unloaded). When a main effect was found post hoc comparisons of the means were conducted using a Tukey HSD test. The purpose of this design was to determine whether there were any significant main effects of either subject group or loading condition on plantar pressures generated during standing and walking. A level of significance of P<0.05 was selected in all analyses to limit the chance of a type I error to 5%.
t-Tests for independent means revealed the obese subjects displayed significantly lower FA (33.1±13.9; t=4.107; P≤0.001) values and higher CSI values (46.3±13.3; t=−6.176; P=<0.001) compared to their non-obese counterparts (FA=45.0±5.1; CSI=23.8±13.0; see Figures 3 and 4).
Significant main effects of both obesity and loading were found for the static plantar pressure results when the data were pooled across loading condition and body type, respectively, although no significant interactions were noted. Post hoc tests confirmed that the loading (weighted jacket) condition resulted in significantly higher mean static peak forces (q=3.937) compared to the unloaded condition (see Table 1). Furthermore, the obese subjects displayed significantly higher mean peak forces (q=11.322) than the non-obese subjects. A significant main effect of obesity was also found for static peak area data whereby obese subjects displayed a significantly larger peak contact area (q=12.80) compared to the non-obese subjects (see Table 1). However, there was no main effect of loading condition on static peak area, nor any subject group×loading condition interactions. Despite main effects of obesity on both force and area, there was no main effect of obesity on static peak pressure. In contrast, a significant main effect of loading condition on static peak pressure was found. Post hoc tests confirmed that the mean static peak pressure data were significantly higher in the loaded condition (q=5.403) compared to the unloaded condition (see Table 1).
Dynamic peak, rearfoot and forefoot force, area and pressure data obtained for the obese and non-obese subjects under both loading conditions are displayed in Table 2. Significant main effects of both obesity and loading condition on dynamic peak force for the whole foot were found when data were pooled across loading condition and body type, respectively. Post hoc analysis confirmed that the mean dynamic peak forces were significantly higher in the loaded condition (q=5.186) compared to the unloaded condition (see Table 2). Furthermore, dynamic peak force generated by the obese subjects (q=14.298) were significantly higher than those generated by the non-obese subjects. A significant main effect of obesity on dynamic peak area was also found (see Table 2), whereby obese subjects displayed significantly higher mean peak areas of contact dynamically with the mini-emed® system (q=15.426) compared to the non-obese subjects. In contrast, no main effect of loading condition was found on dynamic peak area. However, a significant main effect of loading condition on peak dynamic pressure data was found when the data were pooled across body type. That is, the dynamic peak pressure generated during the loaded condition was significantly higher (q=2.890) compared to those generated when unloaded (see Table 2).
Although rearfoot dynamic force, area and pressure data (see Table 2) mirrored the same data trends as were noted for the peak dynamic data, differences were evident in the forefoot dynamic plantar pressure data. That is, similar to the peak dynamic and rearfoot data, significant main effects of both obesity and loading condition on forefoot force were found when the data were pooled across loading condition and body type, respectively (see Table 2). Furthermore, consistent with the previous data no main effect of loading was found for the forefoot area. However, in contrast to the peak dynamic and rearfoot results, significant main effects of both obesity and loading condition on forefoot dynamic pressure were found. That is, the mean peak dynamic forefoot pressure was significantly higher in the loaded condition (q=3.008) compared to the unloaded condition. Furthermore, the mean peak dynamic forefoot pressures generated on the mini-emed® system by the obese subjects were significantly higher (q=3.969) than those generated by the non-obese subjects (see Figure 5).
The obese prepubescent children displayed significantly flatter feet when assessed using FA and CSI compared to their non-obese counterparts. These results, consistent with the findings of Riddiford–Harland et al,11 confirm that obese prepubescent children display changes to the structure of their feet. The functional significance of these structural foot changes was highlighted in analyses of both the static and dynamic plantar pressure results.
In the static condition increased mass, whether it is temporary (weighted jacket) or long-term (obesity), caused a greater peak force to be exerted on the soles of the feet of prepubescent children while they were standing. Although a significant effect of obesity was found for the static peak areas, increasing mass temporarily in the loaded condition did not significantly alter the peak contact area between the plantar surface of the feet and the platform during static trials. It is therefore postulated that it is the long-term effect of increased mass (obesity), and not just increasing load per se, which increases the area of foot contact with the mini-emed® system. This increased foot contact area is also consistent with the flatter feet noted for the obese children via the FA and CSI data.
Despite main effects of obesity on both static force and area, there was no main effect of obesity on the peak pressures exerted on the plantar surface of the children's feet while standing. That is, the increased forces generated by the obese subjects were distributed over a larger surface area of their feet. In contrast, a significant main effect of loading condition on static peak pressure was found whereby static peak pressure data were significantly higher in the loaded condition compared to the unloaded condition. That is, as the higher forces were experienced over the same area during the loaded condition, static peak pressures also increased. Higher forces and pressures associated with artificial loading have also been reported by Nyska et al.7 This effect of the temporary loading condition was in contrast to the effect of obesity where the higher static forces were distributed over a larger area, thereby having minimal effect on pressures experienced by the obese children under the plantar surface of their feet.
In the present study obese children generated significantly higher dynamic peak forces when walking than the non-obese children. Previous studies18,19 have reported similar findings for adults whereby increases in body mass have been associated with increases in the load exerted on the longitudinal arch. Although obese subjects in the present study displayed a significantly higher mean peak area of contact dynamically with the mini-emed® system compared to the non-obese subjects, there was no main effect of loading condition on dynamic peak area. Therefore, similar to the static results discussed previously, only long-term increases in mass, and not temporary mass increases, influenced the dynamic area of foot contact. However, the dynamic peak pressures generated during the loaded condition were significantly higher than those generated when unloaded. That is, the higher peak dynamic forces generated when wearing the weighted jacket were experienced over the same area of foot contact, resulting in higher dynamic peak pressures. However, there was no main effect of obesity on dynamic peak pressure as the obese children were able to distribute the higher peak forces they generated during walking over a greater foot contact area compared to their non-obese counterparts, resulting in minor or no changes in the pressures generated under their feet.
Based on these static and dynamic plantar pressure results it is postulated that the increased foot contact area caused by the long-term bearing of additional mass is due to a flattening of the longitudinal arch of the foot. In contrast, in the temporary loading condition, despite higher pressures, the foot complex appears to be able to maintain the longitudinal arch through compensatory mechanisms in both the obese and non-obese groups.7 However, the actual mechanism of the increased area of contact in the obese subjects is speculative. Although it may be due to a flattening of the longitudinal arch, as supported by the FA and CSI data presented, it could also be due to differences in foot length/breadth or the existence of a plantar fat pad in the midfoot region. Further research is warranted to identify the precise mechanism.
Although the rearfoot dynamic values obtained in the present study mirrored the dynamic peak data, the obese children experienced significantly higher pressures under their forefoot, despite displaying increased areas of forefoot contact. That is, the increased forefoot contact area of the obese children was not able to compensate for the increased forces generated under this region of their feet. These increased forefoot pressure values are of major concern as the forefoot region of the foot is composed of small bones and has a decreased ability to dissipate forces associated with dynamic weight-bearing tasks. Such excessive weight-bearing forces might cause damage to the foot.20 Therefore, obese prepubescent children appear to be at an increased risk of developing stress fractures and other foot pathologies, particularly in the forefoot region, as a consequence of increased dynamic weight-bearing pressures. Based on these results, it was postulated that peak dynamic forefoot pressures might have detrimental health implications for the obese prepubescent child.
Riddiford–Harland et al11 speculated that structural changes in the foot associated with obesity may be a factor that hinders the participation of obese children in physical activity. Results of the present study have furthered this speculation, providing evidence that these structural foot changes are associated with increased pressure under the forefoot of obese children during the most common activity of daily living, walking. Whether these increased forefoot pressures are associated with sufficient increases in foot discomfort to deter the obese children measured from participating in physical activity, and thereby perpetuating the cycle of obesity and exacerbating foot pathologies, requires further investigation. Furthermore, whether these structural and functional changes to the feet of obese prepubescent children influence the children's ability to perform gross motor tasks, as assessed for example by a comprehensive gait analysis, and whether any such changes can be resolved following weight reduction, is also unknown. Continued investigation of the effects of obesity on the musculoskeletal structure and biomechanical function of young children as they perform activities of daily living is therefore recommended.
Temporary mass increases resulted in increased static and dynamic plantar pressures, but no significant change to the structure of the feet of prepubescent children. However, long-term mass increases associated with obesity appeared to flatten the medial longitudinal arch of the children as confirmed by an increased area of foot contact with the ground, a decreased FA and increased CSI. Whether this flattening of the longitudinal arch associated with obesity is permanent or reversible by mass reduction and/or whether it is associated with foot discomfort or foot pathologies is unclear. Furthermore, whether this change in foot structure may in turn hinder participation in physical activity, in either childhood or adulthood, is speculative and requires further investigation. However, obese prepubescent children generated significantly higher dynamic pressures under the forefoot and thereby may be at an increased risk of developing foot pathologies in this area of the foot. With an increasing number of children being diagnosed as obese it is imperative that more research is conducted to characterise the structure and function of the lower limb and feet of prepubescent children and provide meaningful improvements in quality of life.
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Cite this article
Dowling, A., Steele, J. & Baur, L. Does obesity influence foot structure and plantar pressure patterns in prepubescent children?. Int J Obes 25, 845–852 (2001). https://doi.org/10.1038/sj.ijo.0801598
- body mass index
- foot structure
- prepubescent children
- plantar pressure distribution
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