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- J Phys Ther Sci
- v.28(8); 2016 Aug
- PMC5011590
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J Phys Ther Sci. 2016 Aug; 28(8): 2332–2336.
Published online 2016 Aug 31. doi:10.1589/jpts.28.2332
PMCID: PMC5011590
PMID: 27630426
Xiaoguang Zhao, MS,1,* Takehiko Tsujimoto, PhD,2 Bokun Kim, MS,1 Yasutomi Katayama, PhD,3 Kyousuke Wakaba, MS,1 Zhennan Wang, MS,1 and Kiyoji Tanaka, PhD2
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Abstract
[Purpose] The purpose of this study was to examine the effects of increasing physicalactivity on foot structure and ankle muscle strength in adults with obesity and to verifywhether the rate of change in foot structure is related to that in ankle muscle strength.[Subjects and Methods] Twenty-seven adults with obesity completed a 12-week program inwhich the intensity of physical activity performed was gradually increased. Physicalactivity was monitored using a three-axis accelerometer. Foot structure was assessed usinga three-dimensional foot scanner, while ankle muscle strength was measured using adynamometry. [Results] With the increasing physical activity, the participants’ feetbecame thinner (the rearfoot width, instep height, and girth decreased) and the archbecame higher (the arch height index increased) and stiffer (the arch stiffness indexincreased); the ankle muscle strength also increased after the intervention. Additionally,the changes in the arch height index and arch stiffness index were not associated withchanges in ankle muscle strength. [Conclusion] Increasing physical activity may be onepossible approach to improve foot structure and function in individuals with obesity.
Key words: Obesity, Foot structure, Ankle muscle strength
INTRODUCTION
Obesity has become a global pandemic and is recognized as a primary public health concernin many countries. In addition to contributing to conditions such as cardiovascular diseasesand diabetes1, 2), obesity is strongly related to lower extremity conditions such asankle and foot pain3,4,5), which lower the quality oflife (QoL) and increase the morbidity of obesity6).
Physical inactivity is considered a major contributor to the development and progression ofoverweight and obesity. A possible explanation for the association between obesity andphysical inactivity is that excess weight has a negative impact on the biomechanicalcharacteristics of the lower extremities7, 8). With regard to the impact of obesity on thefoot and ankle, obesity is reportedly associated with detrimental changes to foot structureand function3, 9, 10). For instance, a recentsystematic review, which analyzed 16 papers, reported that obesity is strongly related withplanus (low-arched) foot structure, pronated dynamic foot function, and increased plantarpressure during walking11). In addition,physical inactivity leads to poor muscle strength in individuals with obesity. Across-sectional study indicated that adults with obesity generally had decreased physicalactivity (PA) and impaired knee strength compared to healthy counterparts12).
PA may be an effective measure to reduce weight, enhance lower extremity function, andimprove QoL in individuals with obesity13). However, to our knowledge, no systematic study has examined theeffects of increasing PA on foot structure and function in adults with obesity. Therefore,the purpose of this study was to examine the effects of increasing PA on foot structure andankle muscle strength in adults with obesity and to verify whether the rate of changes infoot structure is related to that of changes in ankle muscle strength. The results may beuseful to better understand the association of PA with foot structure and function and toclarify whether PA is an effective approach to improve foot structure and increase anklemuscle strength in individuals with obesity.
SUBJECTS AND METHODS
Twenty-seven participants (gender, 15 males and 12 females; age, 53.07 ± 6.17 years;height, 166.75 ± 9.89 cm; weight, 75.35 ± 13.83 kg) were selected for this study. They metthe following inclusion criteria: age between 30 and 64 years; a body mass index (BMI)exceeding 25.0 kg/m2 (on the basis of domestic obesity guidelines)14); a stable weight for at least 3 months; nohabit of regular exercise; and no current or previous lower extremity disorders or otherneuromuscular or musculoskeletal disorders that affect the foot and ankle function. Thisstudy was conducted in accordance with the Helsinki Declaration, and the protocol wasapproved by the Ethics Committee of University of Tsukuba, Japan.
For 12 weeks, participants came to the University of Tsukuba three times a week for a90-min PA session of increasing intensity. Each PA session comprised 10 min of warming upand stretching, 45 min of brisk walking and jogging, and 5 min of cooling down andstretching. In the first 4 weeks, the exercise intensity was set at 50–60% of the maximumheart rate, and it was gradually increased thereafter. In the last 4 weeks, the participantswere finally exercising at 60–70% of the maximum heart rate. On rainy days, indoor exercisewas performed using stationary cycling or stair stepping. In addition to the PA sessions atthe university, participants were also encouraged to perform their preferred type of PA athome or work to the greatest possible extent.
PA was monitored using an accelerometer (HJA-350IT; Omron Healthcare, Kyoto, Japan), whichwas attached to the waist for over 7 consecutive days (including weekends), except when theparticipants slept or performed PAs in water. Depending on the intensity, PA was classifiedas low (LPA, 1.5–2.9 metabolic equivalents (METs)), moderate (MPA, 3.0–5.9 METs), vigorous(VPA, >6.0 METs), and moderate and vigorous (MVPA, >3.0 METs).
A three-dimensional laser foot scanner (FSN-2100; Dream GP, Osaka, Japan) was used tomeasure foot structure information. Among the many indicators of foot structure, the archheight index and arch stiffness index are especially useful for evaluating the arch andfoot. The arch height index is defined as the instep height divided by the length of theball of the foot15). The arch stiffnessindex, which is a measure of arch flexibility, is defined as the ratio between the standingarch height index and the sitting arch height index16). An arch height index close to 0 indicates a lower arch, and anarch stiffness index close to 1 indicates a stiffer arch.
A Biodex System 4 Dynamometer (Shirley, NY, USA) was used to assess ankle muscle strengthat an angular velocity of 30°/s. In keeping with the manufacturer’s recommendations, thedynamometer orientation, tilt, and seat orientation were set at 90°, 0°, and 90°,respectively, during the plantar flexion to dorsiflexion test and at 0°, 70°, and 90°,respectively, during the eversion to inversion test. After the participants performedsubmaximal repetitions to familiarize themselves with the test procedures, they performed atest comprising 3 maximal repetitions, both plantar to dorsiflexion and eversion toinversion. During ankle muscle strength evaluation, the greatest muscle force output at anymoment during the repetitions was recorded as the peak torque (Nm) and peak torque perkilogram body weight (Nm/kg*100%).
In consideration of the assumption of independence in the statistical analysis, only datapertaining to the structure and ankle muscle strength of the right foot were entered in themain analyses. Because the Shapiro-Wilk test showed that the data of foot structure andankle muscle strength were not normally distributed, the Wilcoxon signed rank test was usedto compare differences in these data before and after the PA intervention. Then, partialcorrelations adjusted for age were used to determine the relationship between the rates ofchanges in foot structure and ankle muscle strength with increasing PA. All data wereanalyzed using SPSS version 22.0, and p<0.05 was considered significant.
RESULTS
Compared to their baseline values, MPA, VPA, and MVPA increased significantly (p<0.01).Foot structure indicators such as the foot length, rearfoot width, length of the ball of thefoot, and instep girth reduced remarkably (p<0.05), while the instep height, arch heightindex and arch stiffness index increased significantly (p<0.05) (Table 1). Moreover, the plantar flexion peak torque increased significantly(p<0.05), although the dorsiflexion, eversion, and inversion peak torques remainedunchanged. In contrast, the values of plantar flexion, dorsiflexion, and eversion peaktorque per kilogram body weight increased remarkably (p<0.05) (Table 2). Additionally, neither the rate of change in the arch height index nor thatin the arch stiffness index was significantly associated with the rate of changes in anklemuscle strength (Table 3).
Table 1.
Foot structure characteristics before and after intervention (n=27)
| Before | After | |
|---|---|---|
| Age (yrs) | 53.07 ± 6.17 | |
| Weight (kg) | 77.05 ± 14.67 | 75.35 ± 13.83** |
| BMI (kg/m2) | 27.42 ± 4.14 | 26.82 ± 3.89** |
| Low PA (min/d) | 244.77 ± 69.37 | 247.95 ± 67.01 |
| Moderate PA (min/d) | 44.79 ± 20.90 | 62.77 ± 21.51** |
| Vigorous PA (min/d) | 1.55 ± 3.37 | 14.29 ± 10.35** |
| Moderate and vigorous PA (min/d) | 46.34 ± 22.67 | 77.06 ± 26.08** |
| Foot length (mm) | 247.43 ± 14.21 | 246.53 ± 14.26* |
| Forefoot girth (mm) | 237.36 ± 15.78 | 237.62 ± 14.29 |
| Forefoot width (mm) | 97.25 ± 6.48 | 97.46 ± 6.22 |
| Rearfoot width (mm) | 63.8 ± 6.02 | 63.08 ± 5.59** |
| Ball of foot length (mm) | 177.8 ± 10.31 | 177.23 ± 10.41* |
| Lateral ball of foot length (mm) | 155.57 ± 9.02 | 155.08 ± 9.11 |
| Instep height (mm) | 60.72 ± 6.18 | 61.52 ± 6.42* |
| Instep girth (mm) | 243.31 ± 18.63 | 241.73 ± 18.05* |
| First toe angle (degree) | 10.05 ± 4.37 | 10.56 ± 4.85 |
| Little toe angle (degree) | 13.87 ± 5.14 | 13.67 ± 5.88 |
| Arch height index (ratio) | 0.342 ± 0.029 | 0.347 ± 0.031* |
| Arch stiffness index (ratio) | 0.906 ± 0.039 | 0.928 ± 0.032** |
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PA: physical activity. *p<0.05, **p<0.01
Table 2.
Ankle muscle strength before and after intervention
| Before | After | |
|---|---|---|
| Peak torque (Nm) | ||
| Plantar flexion (Nm) | 60.03 ± 31.71 | 68.24 ± 25.65* |
| Dorsiflexion (Nm) | 24.24 ± 9.58 | 25.20 ± 10.59 |
| Eversion (Nm) | 16.96 ± 5.03 | 18.00 ± 5.20 |
| Inversion (Nm) | 23.02 ± 9.05 | 22.92 ± 7.75 |
| Peak torque per body weight (%) | ||
| Plantar flexion (%) | 83.80 ± 37.72 | 98.80 ± 30.38* |
| Dorsiflexion (%) | 33.41 ± 9.19 | 35.62 ± 11.29* |
| Eversion (%) | 23.93 ± 5.61 | 26.21 ± 6.63* |
| Inversion (%) | 32.34 ± 9.96 | 33.47 ± 9.97 |
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*p<0.05
Table 3.
Correlation between the rate of changes in the arch height index and archstiffness index and that of changes in the ankle peak torque per kilogram body weightadjusted for age
| ∆ Arch height index | ∆ Plantar flexion | ∆ Dorsiflexion | ∆ Eversion | ∆ Inversion |
| r | −0.05 | 0.07 | −0.17 | −0.09 |
| ∆ Arch stiffness index | ∆ Plantar flexion | ∆ Dorsiflexion | ∆ Eversion | ∆ Inversion |
| r | 0.02 | 0.03 | −0.06 | 0.28 |
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DISCUSSION
The main findings of this study were that the feet became shorter (the foot length andlength of the ball of the foot decreased), thinner (the instep girth and rearfoot widthdecreased), higher (the instep height and arch height index increased), and stiffer (thearch stiffness index increased) with increasing PA. Further, the peak torque per kilogrambody weight of all ankle muscles, except the invertor, increased with the increasing PAintervention. Additionally, it seems that the changes in foot structure and ankle musclestrength associated with increasing PA were independent of each other.
The presence of a lower arch is reportedly related to increased body weight or BMI17, 18). It is known that obesity is always accompanied by decreased PA.However, increasing PA may have a positive impact on arch structure and function inindividuals with obesity. One study19)found that PA increased the arch height in school-aged children with obesity, but thisobservation may have been confounded by the regular growth and development of children. Thefindings of the present study further proved that increasing PA alone significantly enhancedarch height. Additionally, arch stiffness, whose value reduces in the case of soft-tissueinjuries of the foot and ankle20), wasalso found to improve remarkably after PA intervention. On the basis of these results, it issuggested that the structure of the arch has reversible characteristics, and low PA may beone of the important reasons underlying the detrimental changes in arch structure andfunction in individuals with obesity.
On the other hand, a cross-sectional study reported that children with obesity generallyhave fatter feet than age- and gender-matched children with normal body weight21). Another study investigated theassociation of body weight with foot parameters in 872 adults and found that BMI is relatedpositively to foot indicators such as height, width, and girth22). In the present study, as expected, foot indicators suchas the rearfoot width, instep height, and girth reduced significantly with increasing PA inadults with obesity. Taken together, the results indicate that increasing PA can make thefeet of individuals with obesity thinner. Therefore, increasing PA should be recommended tosuch individuals for improving arch and foot structure.
It is widely accepted that obesity is associated with decreased muscle strength12, 23). With regard to the effects of obesity on ankle muscle strength, aprevious study showed that compared to individuals of normal weight, adults with obesity aremore likely to have reduced ankle muscle strength, which is related to declined functioningof the ankle and foot, instability of the ankle joint24), and even sprains25). However, the present study found that almost all indicators ofankle muscle strength adjusted for body weight increased with the increasing PAintervention. This suggests that the improved ankle muscle strength induced by increasing PAmay improve the function of ankle and foot in individuals with obesity.
This study has some limitations that should be acknowledged. Although foot structurecharacteristics were measured, the pathological bone deformation of flat foot was notinvestigated. Therefore, it was not clear whether increasing PA can improve arch height inindividuals with flat foot. In addition, since only middle-aged adults (30–64 years) wereinvolved in the study, it is not known whether the research findings are equally applicableto children and older adults. Further studies need to be conducted to examine theseaspects.
In conclusion, the results of the present study indicate that with increasing PA, the feetbecome thinner and arches become higher and stiffer; further, ankle muscle strengthincreased. Additionally, the changes in arch height and arch stiffness are not associatedwith changes in ankle muscle strength. Increasing PA may be one possible approach to improvefoot structure and function in individuals with obesity.
Acknowledgments
The authors thank the Pigeon Company for providing the three-dimensional foot scanner usedin this study and appreciate the help of Ms. Kaori Itagaki with operating this device.
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