Muscle Activity and Balance in People withand without Flexible Flatfeet Following

a Short Foot Exercise Program

The Graduate School

Yonsei University

Department of Physical Therapy

Jeongah Kim

Muscle Activity and Balance in People withand without Flexible Flatfeet Following

a Short Foot Exercise Program

Jeongah Kim

A Masters ThesisSubmitted to the Department of Physical Therapy

and the Graduate School of Yonsei Universityin partial fulfillment of the requirements

for the degree of Master of Science

June 2014

This certifies that the masters thesis of Jeongah Kim is approved.

Thesis Supervisor: Chunghwi Yi

Ohyun Kwon: Thesis Committee Member #1

Heonseok Cynn: Thesis Committee Member #2

The Graduate SchoolYonsei University

June 2014

Acknowledgements

I attribute this glory to the God who has led me to write this thesis. From his

kindness, I have met these following good people and have received great helps from

them.

I express my sincere gratitude to Professor Chunghwi Yi who has led me to the

best way in all life‟s endeavors and to conduct research always with a benevolent

smile. It is my great honor that he is my supervising professor. To Professor Ohyun

Kwon, I appreciate his passion for academics. He has taught me how to love

academics of physical therapy and to think critically in all areas. To Professor

Heonseock Cynn, I appreciate his thesis guidance from the student‟s point and his

courtesy of education in graduate school with the greatest care. I am also very grateful

that I have met Professor Sanghyun Cho who showed me his lavish life style and

inspired me to keep pursuing my dream. I owe my deepest gratitude to Professor

Hyeseon Jeon who has always understood and supported my plans as a great

counselor throughout my school life. I also appreciate Professor Joshua You who

made me admire his enthusiasm for teaching and studying.

I heartily thank Professor Soyeon Park for her invaluable help with wise solutions,

my senior Onebin Lim for his precious time on teaching me and his sincere advices

for year and a half, and my valued colleague Haerim Han for her supports. I also

appreciate Jia Hong who has led me into the graduate school with great care. My

sincere thanks also goes to Gyuwan Lee, Youngjung Lee, Kisong Kim, Gyoungmo

Kim, Joonhyoung Yong, Sijeong Song, Ilwoo Park and Yonju Sim for their

continuous supports.

To my seniors, I deeply thank Sungdae Choung for his everlasting father-like care,

Kyuenam Park for his powerful advices, Taejin Kim for her great senses, Sihyun Kim

for her continuous consideration, Sunyoung Kang for her wise advices, Incheol Jeon

for his benign smile, Kyungmi Park for her kind mentoring, Silah Choi for her

generous help, and Joohee Park for her encouragement. I would also like to thank

Sunhee Ahn, Uijae Hwang, and my lovely roommate Woojeong Choi. To my last

colleagues, I also thank Jaeik Song, Jiho Park, Sunghoon Jung, and Hyojung Jeong

for their all kindness.

I appreciate Hojoon Jeon of SEED Tech. for his help, Sejin Kong of DooRee

System Tech. for his tremendous assistances in data analysis, and Minhee Lee for his

assistance in using Matlab program. I would like to thank all participants in the

experiment.

I thank Pastor Younghye Kim, Professor Suhong Hwang, Preacher Dosung Kim,

and all members of Epraim Praise in Yousei University Wonju Church for supporting

me spiritually throughout my master‟s course. Moreover, I would like to share this

joy with my dearest friends Hyunna Kim, Hawon Park, and Samsoo Seo.

Last but not the least, I would like to thank my father Joosung Kim, my mother

Eunju Oh, and my younger brother Hwanju Kim for their everlasting love.

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Table of Contents

List of Figures ···································································· iii

List of Tables ····································································· iv

Abstract ············································································ v

Introduction ······································································· 1

Methods ············································································ 4

1. Subjects ······································································ 4

2. Instrumentation ······························································ 6

2.1 Surface Electromyography ············································ 6

2.2 Force Plate ······························································ 7

2.3 Star Excursion Balance Test ·········································· 8

3. Experimental Procedures ··················································· 9

3.1 Pre-test and Post-test ··················································· 9

3.1.1 Single-leg Standing ················································· 9

3.1.2 Star Excursion Balance Test ····································· 11

3.2 Four-week Short Foot Exercise Program ··························· 12

3.2.1 Training ····························································· 12

3.2.2 Short Foot Exercise Progression ································· 12

4. Statistical Analysis ························································· 14

Results ············································································ 15

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1. Subject Characteristics ···················································· 15

2. Effects of Short Foot Exercise Program ································ 16

3. Difference of Effects between Neutral Feet and Flexible Flatfeet ··· 18

4. Relationship between Muscle Activity and Balance ··················· 19

Discussion ········································································ 20

Conclusion ········································································ 25

References ········································································ 26

Abstract in Korean ······························································· 33

- iii -

List of Figures

Figure 1. Single-leg standing ···················································· 10

Figure 2. Star excursion balance test (A: anterior, B: posterolateral,

C: posteromedial directions) ········································· 11

Figure 3. Progression of short foot exercise program

(A: stage 1, B: stage 2, C: stage 3, D: stage 4) ····················· 13

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List of Tables

Table 1. Subject characteristics ················································ 15

Table 2. Results of paired t-tests between the pre-test and post-test ······· 17

Table 3. Results of analyses of covariance in the post-test ················· 18

Table 4. Results of Pearson‟s correlation coefficients between muscle

activity and balance using the difference between the pre-test

and post-test ····························································· 19

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ABSTRACT

Muscle Activity and Balance in People with

and without Flexible Flatfeet Following

a Short Foot Exercise Program

Jeongah Kim

Dept. of Physical Therapy

The Graduate School

Yonsei University

The aim of this study were to investigate the effects of a 4-week short foot

exercise (SFE) program on abductor hallucis (AbdH) activity, static balance, and

dynamic balance in people with neutral feet (NF) and flexible flat feet (FF), compare

the effects between NF and FF, and confirm correlations between AbdH activity and

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balance. A total of 24 subjects participated in this study (12 with FF and 12 with NF).

AbdH activity was measured using surface electromyography, static balance [center

of pressure (COP) sway area and speed] was measured using a force plate, and

dynamic balance was measured using the star excursion balance test (SEBT) score.

The paired t-test was used to investigate SFE effects, the analysis of covariance to

investigate group differences, and the Pearson‟s correlation coefficient to investigate

relationships. Both groups exhibited significantly improved AbdH activity and static

and dynamic balance. Significant group differences were observed in AbdH activity

and COP sway area but not in COP speed and SEBT score. Moderate correlations

were observed between AbdH activity and static balance but not dynamic balance.

These results demonstrate that the 4-week SFE program improved foot intrinsic

muscle activity and balance in both people with NF and FF. Therefore, SFE may be

considered as an effective exercise for FF as well as rehabilitation for people with

ankle instability.

Key Words: Dynamic balance, Flexible flatfeet, Muscle activity, Short foot exercise,

Static balance.

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Introduction

Short foot exercise (SFE) involves making a small change to the shape of the foot

by drawing the toes and metatarsals toward the heel without toe flexion. SFE is a

form of sensorimotor training that is intended to correct muscle imbalances and

improve body posture and equilibrium by stimulating proprioceptive pathways (Janda

1996; Kim et al. 2013). The sole of the foot is one of the main targets of sensorimotor

training. The sole receptors can be activated through contraction of intrinsic foot

muscles or stimulation of the sensory system. Ultimately, the goal of SFE is to

improve functional balance by activating the intrinsic foot muscles, such as the

abductor hallucis (AbdH) (Mulligan, and Cook 2013).

Previous SFE studies have focused on subjects with flexible flatfeet (FF) (Mulligan,

and Cook 2013; Park, Cynn, and Choi 2011). Mulligan, and Cook (2013) investigated

the effects of SFE on muscle strength and dynamic balance in healthy subjects,

including those with a lowered arch. Park et al. (2011) investigated the visual

feedback effect of pinch gaze during SFE in subjects with NF and FF. One

characteristic of FF is excessive pronation in the weight-bearing foot (Cote et al. 2005;

Franco 1987; Kayano 1986). Weakness and dysfunction of the AbdH may be a

contributing cause for excessive pronation (Gray, and Basmajian 1968; Headlee et al.

2008; Kelly et al. 2012) and may be linked with balance impairments (Menz, Morris,

and Lord 2005; Mickle et al. 2009). Finally, FF may be related to an increased risk of

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secondary injuries, including ankle sprains (Briem et al. 2011), plantar fasciitis

(Williams, McClay, and Hamill 2001), and falling in the elderly (Menz, Morris, and

Lord 2005), as well as deformities, including hallux valgus (Coughlin, and Jones

2007). SFE may be prescribed for FF to prevent secondary injuries or deformities.

However, it is still not clear whether SFE can produce a desired effect in FF similar to

that in neutral feet (NF).

Recent research revealed that SFE activated the AbdH in healthy subjects as

measured by surface electromyography (EMG) (Jung et al. 2011; Kelly et al. 2012;

Kim et al. 2013; Park, Cynn, and Choi 2011). Jung et al. (2011) confirmed that in NF

SFE increased AbdH activity more than sitting, especially during single-leg standing.

Kim et al. (2013) demonstrated that SFE activated the AbdH of subjects with mild

hallux valgus. Other studies have revealed that an 8-week SFE program increased the

cross-sectional area of the AbdH, as measured via ultrasound (Jung, Koh, and Kwon

2011) and that a 4-week SFE program increased the performance grade of foot

intrinsic muscles from fair to good, as assessed by the Intrinsic Foot Musculature Test

(Mulligan, and Cook 2013). However, it is still not clear whether the effects of SFE

can be applied to various positions or functional movement by increasing the

recruitment of the AbdH.

Several studies have established the effects of SFE in relation to sensorimotor

training for subjects with NF. The results of those studies demonstrated that SFE can

stimulate cutaneous sensation on the sole, lessen motion, and thus ensure a stable base

for the body (Greeman 2003; Rothermel et al. 2004). Lynn et al. (2012) reported that

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a 4-week SFE program yielded significant improvement in a dynamic balance test but

not in a static balance test, based on decreased mediolateral center of pressure (COP)

displacement, which is a balance ability index. A recent study demonstrated that a 4-

week SFE program increased dynamic balance as measured by the Star Excursion

Balance Test (SEBT) (Mulligan, and Cook 2013). However, to date, most previous

studies have focused on muscular changes and have excluded the sensorimotor

training effect. Although Mulligan, and Cook (2013) investigated both musculature

and balancing performance, they did not find a clear relationship.

Therefore, the purposes of this study were to investigate the effects of a 4-week

SFE program on AbdH activity, static balance (COP sway area and COP speed), and

dynamic balance (SEBT score) in NF and FF groups, to compare the effects between

the NF and FF groups, and to confirm the correlation between AbdH activity and

balance. The hypotheses were that a 4-week SFE program would improve AbdH

activity and static and dynamic balance in both the NF and FF groups, that the

improvements in the NF and FF groups would be similar, and that AbdH activity and

balance would be correlated.

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Methods

1. Subjects

The required sample size was determined by using a power analysis based on data

from the pilot experiment. A sample size of 11 subjects per group was confirmed as

achieving an effect size of 0.82, an alpha level of 0.05, and a power of 0.80. In total,

30 volunteers were recruited between November and December 2013 via

advertisements around the university.

The navicular drop (ND) test and resting calcaneal stance position (RCSP) were

used for group allocation. To quantify the amount of ND, the examiner first marked a

dot on the subject‟s aspect of the navicular tuberosity. The first placement was

marked on an index ruler at the level of the dot when the examiner gripped the

subtalar joint to achieve neutrality in a sitting position, the second placement was

marked when in the standing position, and the distance between the two placements

on the ruler was recorded (Brody 1982; Cote et al. 2005; Denyer, Hewitt, and

Mitchell 2013). To measure the RCSP, the examiner drew a bisecting line on the

calcaneus of the subject in the prone position. The angle between the bisecting line

and the ground was measured using a goniometer placed on a 20-cm high box, with

the subject in a relaxed standing position (Sell et al. 1994).

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The NF group included subjects with an ND of 5–9 mm and within 2° of inversion

and eversion of both feet in the RCSP. The FF group included subjects with an ND

greater than 10 mm and 4° or more of eversion of both feet in the RCSP. Subjects

were excluded from either group if they had less than 5 mm of ND, non-symmetric

ND between feet (Tsai et al. 2006), more than 20 mm discrepancy in leg length

(Murrell, Cornwall, and Doucet 1991), an orthopedic malady, a history of lower-

extremity surgery (Ledoux, and Hillstrom 2002), a neurological diagnosis that would

limit muscle activation or disturb balance (Lynn, Padilla, and Tsang 2012),

continuous pain in a lower extremity within the past six months, failed the single-leg

standing criteria more than seven times, were taking medication that might cause

dizziness or loss of balance (Park, Cynn, and Choung 2013), or were engaged in

training requiring balance (Tsai et al. 2006).

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2. Instrumentation

2.1 Surface Electromyography

AbdH activity data were collected using a Noraxon TeleMyo 2400T system

interfaced with Noraxon MyoResearch 1.06 XP software (Noraxon, Inc., Scottsdale,

AZ, USA). The raw EMG data were sampled at 1500 Hz, band-pass filtered between

20 and 450 Hz, notch–filtered at 60 and 120 Hz, and processed into the root mean

square with a window of 50 ms. The device was zeroed each time before recording.

To attach a disposable Ag/AgCl electrode, the skin was shaved and cleaned with

isopropyl alcohol to minimize skin resistance. For the AbdH, electrodes were placed

on the foot dorsum as a reference and approximately 1–2 cm posterior to the

navicular tuberosity (Arinci Incel et al. 2003). The electrodes were placed parallel to

the muscle fiber, 2 cm apart. The data were normalized to a maximal voluntary

isometric contraction (MVIC) recorded three times for 5 s. To measure the MVIC of

the AbdH, the examiner gripped the heel at a neutral position of the subtalar joint and

applied resistance to the medial side of the first metatarsal and proximal phalanx in

the direction of abduction of the first toe. The subject was asked to adduct the

forefoot against the resistance (Kendall 2005). After excluding the initial and last

seconds, the middle 3 s were used to determine the mean values.

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2.2 Force Plate

Static balance data were collected using a six-component AMTI-OR6-7-2000 force

plate (AMTI, Inc., Watertown, MA, USA) synchronized with EMG software to match

the recording time using an external trigger. The device was zeroed each time before

recording and the data were sampled at 1500 Hz. Data were collected for 9 s during

single-leg standing, and the initial 2 and last 2 s were excluded. From the raw data of

the middle 5 s, COP sway area and COP speed were calculated using MATLAB

version 7.10.0 software (MathWorks Inc., Natick, MA, USA). COP sway area was

calculated using COP 95% confidence ellipse area, which included 95% of the COP

trajectories by obtaining the radius of major and minor axes (Doyle et al. 2007). COP

speed was calculated by adding the distances traveled and dividing by the total time

(Doyle et al. 2007). The calculated mean values of three trials were normalized to

foot size, defined as the distance from the most posterior calcaneus aspect to the most

anterior toe aspect (Tsai et al. 2006).

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2.3 Star Excursion Balance Test

Dynamic balance data were collected using the modified SEBT, a sensitive and

reliable measure to evaluate dynamic balance that uses three reaching directions

(anterior, posterolateral, and posteromedial) (Hertel et al. 2006; Kinzey, and

Armstrong 1998; Olmsted et al. 2002). SEBT scores were calculated by adding the

distances reached in the three directions. The calculated mean of three trials was

normalized to leg length, defined as the distance from the anterior superior iliac spine

to the medial malleolus (Murrell, Cornwall, and Doucet 1991).

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3. Experimental Procedures

This study had a 2 x 2 repeated measures design. The experimental procedures

were approved by the Yonsei University Wonju Institutional Review Board, Republic

of Korea. The experimental procedures were explained in detail to all participants,

who provided written informed consent prior to the experiment.

3.1 Pre-test and Post-test

All subjects performed the single-leg standing task and SEBT prior to the SFE

program and again 4 weeks later. All tasks were conducted with bare feet to remove

additional influences from shoes (Gribble, and Hertel 2004). Subjects were limited to

two familiarization trials of each task to minimize the learning effect. After the

familiarization trials, subjects performed the following tasks three times each, in a

random order, with a 5-min rest between the tasks.

3.1.1 Single-leg Standing

The single-leg standing task was performed in the same way in both groups. A

cross line was placed on the force plate to control foot position (Tarantola et al. 1997).

Subjects began by crossing both arms over the chest and focusing the eyes on a circle

placed on the wall 3 m in front of them. On the verbal cue “start,” subjects flexed

their dominant leg to a target bar placed at 90° of knee flexion for 4 s. The „dominant

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leg‟ was defined as the leg used to kick a ball (Edwards et al. 2008). At 90° of knee

flexion, subjects were asked to remain as motionless as possible for 7 s while

avoiding contact between the flexed leg and the ground or the stance leg (Cobb et al.

2004) (Figure 1). The time was measured using a metronome. Then, the subjects

lowered their leg to the ground for 4 s in response to the verbal instruction “lower the

leg.” If a subject dropped a hand from the chest, averted the eyes from the circle, or

moved the stance foot from the starting stance, this was defined as a failure: failure

data were discarded and the subject repeated the trial.

Figure 1. Single-leg standing.

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3.1.2 Star Excursion Balance Test

In the SEBT, subjects began by placing both hands on their hips. Next, they

extended a leg over a fixed tape as far as possible without touching the ground and

then with the first toe slightly touching the tape. The test was performed in anterior,

posterolateral, and posteromedial directions, in that order (Fitzgerald et al. 2010)

(Figure 2). A 2-min rest was permitted between trials. If a subject dropped a hand

from the hip, did not return to the starting position, put weight on the first toe of the

reaching foot, or moved the stance foot from the starting position, this was defined as

a failure: failure data were discarded and the subject repeated the trial (Coughlan et al.

2012).

Figure 2. Star excursion balance test (A: anterior, B: posterolateral, C:

posteromedial directions).

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3.2 Four-week Short Foot Exercise Program

All SFE training was provided and monitored by a single physical therapist. During

the 4-week SFE program, no participants in either the NF or FF group participated in

any other foot exercise program.

3.2.1 Training

All subjects practiced SFE in a sitting position during the week. For the SFE

position, they were asked to shorten the foot by drawing the first metatarsal head

towards the heel without toe flexion or extension with maximal force and hold it for 5

s. Five repetitions were considered a set. Three sets were repeated with a 5-min rest

between sets, once a day, with both feet alternately. To ensure accurate exercise

positioning, the physical therapist taught subjects about SFE by using visual feedback,

including a sheet of paper under the first metatarsal bone and EMG biofeedback to

ensure AbdH activation.

3.2.2 Short Foot Exercise Progression

After the training, subjects performed SFE at the laboratory three times and at

home four times per week. SFE progression involved visual feedback using paper

during all stages. The first stage started in a sitting position. A set was defined as

holding SFE for 5 s and repeating it five times. Five sets were performed per day,

with a 5-min rest between sets. After a week, the subjects moved to the next stage if

they could perform all five sets correctly. If subjects had difficulty in performing the

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sets or their foot position was incorrect, they remained at the current stage and were

retested after one week. The second stage was performed standing on both legs. The

third stage was performed standing on one leg. Finally, the fourth stage was

performed standing on an unstable board on one leg (Figure 3).

Figure 3. Progression of short foot exercise program

(A: stage 1, B: stage 2, C: stage 3, D: stage 4).

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4. Statistical Analysis

One-sample Kolmogorov–Smirnov tests were conducted to confirm the assumption

of normal distribution. Independent t-tests were conducted to compare general

characteristics between the NF and FF groups. Paired t-tests were conducted to

compare pre-test and post-test values of dependent variables in both groups. Analyses

of covariance (ANCOVA) were conducted to compare the two groups in post-test

values of dependent variables using the pre-test value as a covariate. Pearson‟s

correlation coefficient was used to evaluate any correlation between AbdH activity

and balance (COP sway area, COP speed, and SEBT score) using the pre-test and

post-test difference data. All statistical analyses were performed using SPSS version

21 software (SPSS Inc., Chicago, IL, USA). The statistical significance level was set

at 0.05 for all tests.

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Results

1. Subject Characteristics

In total, 24 of the 30 subjects screened participated in the study: 6 subjects did not

fulfill the inclusion criteria and were excluded. The NF and FF groups each included

12 subjects. No significant differences were found between groups except for the

amount of ND and RCSP (Table 1).

Table 1. Subject characteristics (N=24)

Parameters

Neutral feet (n1=12;

8 men, 4 women)

Flexible flatfeet (n2=12;

8 men, 4 women) p

Age (year) 23.8 ± 3.3a 22.8 ± 1.7 0.356

Height (cm) 170.2 ± 7.1 173.4 ± 8.2 0.311

Weight (kg) 66.4 ± 14.5 64.8 ± 10.7 0.752

BMIb (kg/m

2) 22.8 ± 4.1 21.4 ± 1.9 0.306

Foot length (mm) 237.1 ± 13.6 244.3 ± 21.3 0.331

NDc (mm) 6.1 ± 0.8 12.2 ± 2.7 < 0.001

*

RCSPd (°) 1.7 ± 0.4 5.4 ± 1.9 < 0.001

*

aMean ± standard deviation.

bBMI: Body mass index.

cRCSP: Resting calcaneal stance position.

dND: Navicular drop.

*p < 0.05.

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2. Effects of Short Foot Exercise Program

AbdH activity significantly increased in both the NF (p < 0.001) and FF (p = 0.001)

groups. COP sway area significantly decreased in both the NF (p < 0.001) and FF (p

= 0.001) groups. COP speed significantly decreased in both the NF (p = 0.002) and

FF (p = 0.026) groups. SEBT score significantly increased in both the NF (p = 0.001)

and FF (p = 0.003) groups (Table 2).

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Table 2. Results of paired t-tests between the pre-test and post-test

Variable Group Pre-test* Post-test

* ES

a t p

AbdHb activity,

%MVICd

NFc 64.30

(19.99)

70.55

(20.30)

2.96 -7.331 < 0.001†

FFe 51.84

(13.13)

71.51

(14.71)

1.91 -4.664 0.001†

COPf sway area,

% of foot size

NF 52.50

(9.08)

48.80

(8.32)

2.59 5.907 < 0.001†

FF 56.98

(11.72)

47.62

(14.01)

1.83 4.245 0.001†

COP speed,

% of foot length

NF 97.10

(26.59)

95.41

(26.79)

1.43 3.929 0.002†

FF 143.16

(96.82)

131.11

(87.88)

1.25 2.578 0.026†

SEBTg,

% of leg length

NF 77.55

(5.58)

84.51

(5.72)

1.76 -4.322 0.001†

FF 75.67

(10.71)

82.29

(10.51)

1.54 -3.768 0.003†

aES: Effect size.

bAbdH: Abductor hallucis.

cNF: Neutral feet.

dMVIC: Maximal voluntary isometric contraction.

eFF: Flexible flatfeet.

fCOP: Center of pressure.

gSEBT: Star excursion balance test.

*Values are mean ± standard deviation.

†p < 0.05.

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3. Difference in Effects between Neutral Feet and Flexible Flatfeet

Significant group differences were observed in AbdH activity (F = 6.408, p = 0.019)

and COP sway area (F = 15.238, p = 0.001). No significant group differences were

observed in COP speed (F = 2.597, p = 0.122) or SEBT score (F = 0.121, p = 0.731)

(Table 3).

Table 3. Results of analyses of covariance in the post-test

Variable Covariate NFa*

FFb*

η F p

AbdHc activity,

%MVICd

58.07 65.22

(20.30)

76.83

(14.71)

0.234 6.408 0.019†

COPe sway area,

% of foot size

57.02 52.20

(9.41)

42.10

(9.28)

0.420 15.238 0.001†

COP speed,

% of foot length

120.52 117.36

(25.57)

111.02

(93.13)

0.110 2.597 0.122

SEBTf,

% of leg length

76.61 83.80

(5.72)

83.00

(10.51)

0.006 0.121 0.731

aNF: Neutral feet.

bFF: Flexible flatfeet.

cAbdH: Abductor hallucis.

dMVIC: Maximal voluntary isometric contraction.

eCOP: Center of pressure.

fSEBT: Star excursion balance test.

*Values are mean ± standard deviation.

†p < 0.05.

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4. Relationship between Muscle Activity and Balance

Moderate correlations were observed using the pre-test and post-test difference

data between AbdH activity and COP sway area (r = 0.50, p = 0.013) and AbdH

activity and COP speed (r = 0.56, p = 0.005). No significant relationship was

observed between AbdH activity and SEBT score (r = -0.181, p = 0.396) (Table 4).

Table 4. Results of Pearson‟s correlation coefficients between muscle activity and

balance using the difference between the pre-test and post-test

AbdHa activity

COPb sway area* 0.50 (0.013

†)

COP speed* 0.56 (0.005†)

SEBTc score* -0.18 (0.396)

aAbdH: Abductor hallucis.

bCOP: Center of pressure.

cSEBT: Star excursion balance test.

*r (p).

†p < 0.05.

- 20 -

Discussion

In this study, we investigated the effects of a 4-week SFE program on four

dependent variables (AbdH activity, COP sway area, COP speed, and SEBT score).

AbdH activity significantly increased not only in the NF group (6.25%MVIC) but

also in the FF group (19.67%MVIC). This result may be due to the increased

awareness and utilization of the AbdH during SFE in various positions for 4 weeks.

Previous cross-sectional studies have reported increased EMG activity of the AbdH

during SFE (Jung et al. 2011; Kelly et al. 2012; Kim et al. 2013; Park, Cynn, and

Choi 2011). Instead of this kind of immediate effect, we focused on improving AbdH

functioning via training by increasing the recruitment of the AbdH as a foot stabilizer

during single-leg standing.

The post-test MVIC values of AbdH activity were higher (11.61%MVIC) in the FF

group than in the NF group. This difference may be explained by AbdH weakness or

dysfunction in individuals with FF. Lynn et al. (2012) suggested that young and

healthy individuals may not require SFE because their intrinsic foot muscles already

function optimally. Additionally, Mulligan, and Cook (2013) proposed that SFE may

affect individuals with a relatively high ND; this could be explained if SFE increased

navicular height in subjects with a relatively high ND while subjects with normal

range had no significant change. Hence, the results of the current study support

- 21 -

previous research reporting greater improvement after the SFE program in FF than in

NF subjects, who may already use the AbdH correctly.

Second, COP sway area decreased significantly not only in the NF group (3.70% of

the foot size) but also in the FF group (9.36% of the foot size) after the SFE program,

indicating improvement in static balance. In a previous study, the effect of a 4-week

SFE program was investigated in subjects with NF, but no significant differences

were reported in mediolateral COP displacement measured by a force plate during

single-leg standing (Lynn et al. 2012). Hypothetically, COP variables such as

displacement or excursion could be influenced significantly by maximum and

minimum data outliers. Additionally, COP variables may not account for foot

movement directions along vertical, anteroposterior, and mediolateral axes. For these

reasons, we used COP sway area to mitigate error: COP sway area may be more

appropriate to evaluate static balance for FF by considering the outliers and the

movement directions of the foot along the three axes. This is the first study using

COP 95% confidence ellipse area (COP sway area) to investigate the effects of a SFE

program in subjects with FF. Thus, data of this study cannot be compared with those

of other studies, but the results support previous research indicating that SFE can

improve balance performance (Greeman 2003; Mulligan, and Cook 2013; Rothermel

et al. 2004).

The post-test values for COP sway area showed a greater decrease in the FF group

(10.01% of the foot size) than in the NF group. One possibility for this finding is the

excessive flexibility of FF, which could induce a wider COP area than in NF. This

- 22 -

finding could be supported by the pre-test data of this study revealing larger COP

sway areas in the FF group than in the NF group, which also supports the results of a

previous study (Chao, and Jiang 2012). Consequently, the initial difference in COP

sway areas could explain the greater improvement in the FF group than in the NF

group, who may already have had a stable and small COP sway area. Additionally,

the partial eta squared (η ) of 0.420 could explain the group difference of

approximately 42% in the COP sway area. Therefore, COP sway area can be used as

a reliable variable that reflects the morphological and functional differences between

NF and FF.

Third, COP speed significantly decreased not only in the NF group (1.69% of the

foot size) but also in the FF group (12.05% of the foot size). As with COP sway area,

reductions in COP speed could indicate improved balance motor control. However, a

learning effect should be considered. Although the interval between pre-test and post-

test was 4 weeks, the repeated tests and SFE position such as single-leg standing

during stages 3 and 4 might have influenced balance performance.

The post-test values of COP speed did not differ between the groups. This result

indicates that individuals with FF can obtain COP speed effects equal to those of

individuals with NF through a SFE program. However, the η of COP speed (0.110)

was lower than the value of COP sway area (0.420). These results indicate that COP

speed may not explain the difference in the effects of the SFE program between

subjects with NF and those with FF.

- 23 -

Fourth, SEBT score significantly increased not only in the NF group (6.96% of the

leg length) but also in the FF group (6.62% of the leg length), indicating that SFE can

improve functional abilities. These findings support previous research involving

subjects with NF, which demonstrated that SFE improves dynamic balance (Lynn et

al. 2012; Mulligan, and Cook 2013). However, numerous factors in addition to SFE

can influence SEBT performance, including co-stability, flexibility, and strength of

the lower extremities (Filipa et al. 2010). Therefore, further studies should consider

contributing factors such as hips, knees, ankle joints and muscles, foot extrinsic joints

and muscles, and the sensory system.

The SEBT scores of the two groups were similar; the morphological differences

between NF and FF may be insufficient to explain the discrepancy in SEBT scores.

Further studies need to consider more sensitive measurements to reflect the

morphological differences in feet.

Finally, the relationship was proved between AbdH activity and balance (COP

sway area, COP speed, and SEBT score). This is the first study to demonstrate a

direct relationship between AbdH activity and balance parameters. AbdH activity was

moderately related to COP sway area (r = 0.50, p = 0.013) and COP speed (r = 0.56, p

= 0.005), which were recorded concurrently. Therefore, activation of foot intrinsic

muscles is closely related to static balance. Because the single-leg standing position

does not provide enough postural demand for AbdH activation, lower values (0.50

and 0.56) were observed. Kelly et al. (2012) reported that activation of the foot

intrinsic muscles increases with increased postural demand. Therefore, further studies

- 24 -

need to include test positions that involve greater postural demand, such as heel rising,

running, and jumping.

Contrary to static balance, no significant relationship was observed between AbdH

activity and SEBT scores (r = -0.18, p = 0.396). As mentioned above, many factors

affect SEBT performance. For example, AbdH activity may be too small to affect

SEBT scores directly. Additionally, the increased muscle activation may be

insufficient to produce functional changes compared with increased muscle strength.

This study had several limitations. First, the 4-week period was not long enough for

a majority of subjects to complete all four stages: only six subjects reached the final

stage within the given period while the others reached the third stage. Although

Mulligan, and Cook (2013) mentioned that a significant change can occur in as little

as 4 weeks, a longer period or more intensive training is required to complete the

program successfully. Second, the repeatedly measured MVIC at post-test might be

incorrect compared with the pre-test value due to replacement of an electrode or the

amount of resistance or different activation after the SFE program. Differences

between the pre-test and post-test MVIC values of AbdH might influence the results.

Finally, this study did not examine other musculature factors that may compensate for

the AbdH or its co-activation. Further studies are recommended to examine

relationships between SFE and extrinsic muscles, such as the tibialis posterior, which

may influence the formation of FF.

- 25 -

Conclusion

The current study demonstrated that a 4-week SFE program improved activation of

foot intrinsic muscles and static and dynamic balance. These findings prove that SFE

can be a clinically adequate program for individuals with FF as well as those with NF.

A moderate relationship was observed between intrinsic foot muscle activation and

static balance.

- 26 -

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국문 요약

작은 발 운동 프로그램 후의

유연성 평발 유무에 따른 근활성도와 균형

연세대학교 대학원

물리치료학과

김 정 아

이 연구의 목적은 4주 동안 정상발을 가진 사람과 유연성 평발을 가진

사람을 대상으로 작은 발 운동프로그램을 적용한 후 첫째, 엄지벌림근의

근활성도, 정적 균형, 동적 균형에 미치는 효과를 알아보는 것, 둘째,

정상발을 가진 사람과 유연성 평발을 가진 사람 간의 효과를 비교하는 것,

셋째, 엄지벌림근의 근활성도와 균형간의 상관관계를 확인하는 것이다. 이

연구에는 총 24명(유연성 평발 12명, 정상발 12명)의 대상자들이

참여했다. 엄지벌림근의 근활성도는 표면근전도를 이용하여 측정되었고,

정적 균형(압력중심 동요면적과 압력중심 속도)은 힘판을 이용하여

- 34 -

측정되었으며, 동적 균형은 스타 동작(star excursion) 균형 검사 점수로

측정되었다. 작은 발 운동 프로그램 전과 후의 효과를 알기 위하여 짝비교

t-검정이 사용되었고, 집단 간 차이를 알아보기 위하여 공분산분석을,

근활성도와 균형과의 관계를 알아보기 위하여 피어슨 상관계수를 구하였다.

두 집단 모두 엄지벌림근의 근활성도, 그리고 정적, 동적 균형 능력이

유의하게 향상되었다. 엄지벌림근의 근활성도와 압력중심 동요면적에서 두

집단 간에 유의한 차이를 보였으나, 압력중심 속도와 스타 동작 균형 검사

점수에서는 두 집단 간에 유의한 차이가 없었다. 엄지벌림근의 근활성도와

정적 균형 간에는 중등도(moderate)의 상관관계가 관찰되었으나 동적

균형과는 유의한 상관성이 없었다. 이러한 결과들은 4주 동안의 작은 발

운동 프로그램이 유연성 평발을 가진 사람과 정상발을 가진 사람 모두에게

엄지벌림근의 근활성도와 정적, 동적 균형 능력을 향상시킨다는 것을

의미한다. 따라서 지금까지 주로 발목의 불안정성을 가진 환자의 재활을

위해 사용되었던 작은 발 운동은 유연성 평발을 가진 사람에게도 효과적인

운동 방법으로 고려될 수 있을 것이다.

핵심 되는 말: 근활성도, 동적 균형, 유연성 평발, 작은 발 운동, 정적 균형.