Effects of an Ankle Foot Orthosis With Oil Damper on Muscle Activity

8/9/2019 Effects of an Ankle Foot Orthosis With Oil Damper on Muscle Activity

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Effects of an ankle-foot orthosis with oil damper on muscle activity in adults

after stroke

Koji Ohata a,*, Tadashi Yasui b, Tadao Tsuboyama a, Noriaki Ichihashi a

a Human Health Sciences, Graduate School of Medicine, Kyoto University, Kyoto, Japanb R&D Division, Kawamuragishi Co., Ltd, Osaka, Japan

1. Introduction

For adults with hemiplegia after stroke, regaining the ability to

walk is crucial for performing activities of daily life. Common

problems after hemiplegic strokeinclude decreased gait speed and

an asymmetrical gait pattern [1,2], which increase the energetic

cost [3,4]. After a stroke, individuals often have impaired ankle

function due to muscle weakness [5], increased passive stiffness

[6], and excessive muscle coactivation [7]. These dysfunctions

affect gait because ankle motion and related muscle activities play

important roles in walking.

Wong et al.[8] suggested that the hemiplegic gait after stroke

may lack the typical heel strike and push-off mechanisms, i.e., heel

and forefoot rocker functions as described by Perry [9], thus

altering ground reaction forces and changing the foot contact

pattern to a pathologic shape. The heel rocker uses the heel as a

fulcrum during the loading response phase (LRP). During this

phase, rapid loading of the heel generates plantarflexion torque,

which drives the foot toward the floor. The pretibial muscles

decelerate the foot drop and draw the tibia forward when the foot

rolls into plantarflexion. Insufficient eccentric dorsiflexion muscle

activity after a strokereduces theheel rocker function,as shown by

a positive relationship between dorsiflexor strength and gait

velocity [10]. The forefoot rocker uses the metatarsophalangeal

joint as a fulcrum during the pre-swing (PSw) phase[9]. When the

limb is rapidly unloaded by the transfer of body weight to theother

limb, residual plantarflexion action progresses to the tibia. As a

result, limb progression with knee flexion occurs during the PSw

phase. The hemiplegic gait is characterized by impaired swing

initiation in theaffectedlimb [11] due to inadequate leg propulsion

by the plantarflexor [5]. Thus, these two plantarflexion actions

during the LRP and PSw phase are critical for recovering gait after

hemiplegic stroke.

An ankle-foot orthosis (AFO) can improve the gait of hemiplegic

individuals[12,13]; however, the limited ankle motion associated

with an AFO with plantarflexion stop [14,15], AFO with bilateral

stop[13], or unarticulated AFO[16], seems to be disadvantageous

for ankle function, because both heel and forefoot rocker functions

require adequate plantarflexion range. An AFO with an oil damper

(AFO-OD) was developedto assist the heel rocker function [17,18];

however, differences between an AFO-OD and an AFO with limited

motion are not clear. The aim of this study was to determine

electromyography (EMG) changes of the lower limb muscles in

stroke patients wearing an AFO-OD.

Gait & Posture 33 (2011) 102107

A R T I C L E I N F O

Article history:

Received 9 June 2010Received in revised form 4 October 2010

Accepted 12 October 2010

Keywords:

Stroke

Ankle-foot orthosis

Electromyography

Oil damper

Hemiplegic gait

Muscle activity

A B S T R A C T

Background and objective: An ankle-foot orthosis with an oil damper (AFO-OD) was developed to resist

plantarflexion motion, thereby improving hemiplegic gait performance. The purpose of this studywas to

determinethe effect of AFO-OD on muscleactivity during thegait cycle in individuals affected by stroke.

Methods: Electromyography (EMG) was used to assess gait at a self-selected speed while wearing an

AFO-OD or an AFO with a plantarflexion stop (AFO-PS) worn on the affected side in 11 stroke survivors

and on the right side in 11 age-matched healthy adults. EMG signals were obtained from the tibialis

anterior (TA), gastrocnemius (GAS), and soleus (SOL) muscles. In addition, the ankle joint angle under

both braces and the plantarflexion resistance torque (PFRT) under AFO-OD were monitored.

Results: Peak PFRT under AFO-OD was observed duringthe loading response phase(LRP) in both groups.

AFO-OD promoted adequate plantarflexion during LRP in the stroke group, whereas AFO-PS did not.

Compared with the AFO-PS, the AFO-OD significantly reduced GAS EMG amplitude during LRP in the

stroke group, which was significantly correlated with peak PFRT during LRP.

Conclusion: AFO-OD assisted the heel rocker function and reduced GAS muscle EMG amplitude during

LRP.

2010 Elsevier B.V. All rights reserved.

* Corresponding author at: Department of Health Science, Graduate School of

Medicine, Kyoto University, 53 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507,

Japan. Tel.: +81 75 751 3918; fax: +81 75 751 3918.

E-mail address: [email protected](K. Ohata).

Contents lists available at ScienceDirect

Gait & Posture

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / g a i t p o s t

0966-6362/$ see front matter 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.gaitpost.2010.10.083
http://dx.doi.org/10.1016/j.gaitpost.2010.10.083mailto:[email protected]://www.sciencedirect.com/science/journal/09666362http://dx.doi.org/10.1016/j.gaitpost.2010.10.083http://dx.doi.org/10.1016/j.gaitpost.2010.10.083http://www.sciencedirect.com/science/journal/09666362mailto:[email protected]://dx.doi.org/10.1016/j.gaitpost.2010.10.083


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2. Methods

2.1. Participants

Adult patients with stroke were recruited from the AFO-OD users list of the

manufacturer (Kawamuragishi Co. Ltd.). After providing informed consent, 13 men

with hemiplegia who livedin Osakaor Kyotoand hadwalkedwithan AFO-OD forat

least 1 monthwere selectedfor this study.Inclusioncriteriawere (1)a singlestroke

at least 6 months prior to the study, (2) living at home independently with family

support, (3) ability to walk independently using an ankle foot orthosis and/or T-

cane, (4)no gait symptoms from parkinsonismor ataxia, (5)no pain duringgaitdue

to orthopedic disease, (6) no limitation of activity due to heart disease, (7) restingheart rate


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(AFO-PS), and without a brace. The mean gait speed under each bracing conditionwas recorded for each participant.

The AFO-OD (Gait Solution Design; Kawamura Gishi, Osaka, Japan; Fig. 1A)

carries an oil damper unit on the lateral side of the ankle joint. A small hydraulic

cylinder is inserted in the oil damper unit to provide resistance to plantarflexion as

needed (Fig. 1B). As the ankle joint plantarflexes at initial contact, a piston rod is

pushed upward into an oil-filled cylinder with resistance. A spring returns the

piston to its initial position after plantarflexionmotion. Theresistive force of the oil

damper can be easily changed by adjusting a screw. During measurements, the

screw position was maintained at a constant value that allowed comfortable

walking in the stroke group. In the control group, the screw was set to the same

position for all subjects. The AFO-PS was set to limitankle plantarflexion at 08. Both

braces allowed free dorsiflexion and were worn on the affected side in the stroke

group and on the right side in the control group.

2.3. Measurement procedure

EMG measurements were performed at 1500 Hz with the TeleMyo system

(Noraxon Inc., USA). Bipolar silversilver chloride disposable surface electrodes

were placed over the muscle bellies of the three lower limb muscles that serve as

the main agonist muscles of dorsiflexion and plantarflexion tibialis anterior (TA),

lateral gastrocnemius (GAS), and soleus (SOL) on the paretic in the stroke group

and onthe right side in thecontrol group. Electrodeplacementon theTA was at1/3

on the line 12-cm lateral to the tibia. The GAS electrode was placed at 1/3 on the

line betweentheheadof thefibulaand theheel. The SOL electrodewasplacedat 1/2

to 2/3 on the line between the head of the medial condyles of the femur and the tip

of the medial malleolus. Two foot switches were positioned at the first metatarsal

head and the heel on the paretic side to record the gait cycle.

To measure plantarflexion resistance torque (PFRT) during gait using the AFO-

OD, a load cell was inserted above the hydraulic cylinder in the oil damper unit. As

the hydraulic cylinder produced a resistive force, the hydraulic cylinder pushed the

load cell as a counterforce; therefore, measurement of the counterforce reflected

the PFRT at the oil damper. The angle of the ankle joint with AFO-OD was

simultaneously monitored witha potentiometerattached to thejoint. Fig. 1C shows

a typical change during the gait cycle in a healthy control. Resistance of the oildamper occurred when the joint angle exceeded 08plantarflexion; therefore, PFRT

showed twopeaks duringthe LRPand PSw phases. Theangle of theankle joint with

AFO-PS was also simultaneously monitored with a potentiometer.

Five to 10 gait cycles were used to determine the EMG parameters and obtain

data from the load cell and the potentiometer of each subject. Time during a gait

cycle was expressed as percentage of the gait cycle (%GC). EMG recordings were

band-pass filtered between 16 and 500 Hz. Full wave rectification was performed

using the root mean square smoothing algorithm at a window interval of 50 ms. To

determine the change in muscle activity during LRP, peak EMG amplitudes from

0%GCto 10%GC werenormalizedusing the maximum value from20%GC to 100%GC.

The load cell and the potentiometer were connected to the TeleMyo transmitter

system with EMG and foot switch. EMG signals were recorded at a sampling rate of

1500 Hz and smoothedwith a low-pass filterat 20 Hz.PeakPFRTwas obtained, and

changes in ankle joint angle were calculated from initial contact to plantarflexion

peak during LRP. Dorsiflexion angles were expressed as positive values.

In the stroke group, these measurements were repeated 2 weeks later to

determine testretest reliability. Ten of 11 individuals in the stroke group

participated in the second measurement; one participant was absent for healthreasons. ICCs(1, k) forchangesin anklejoint anglefrominitial contact were0.89for

bothAFO-ODand AFO-PS,and ICC(1, k) forpeakPFRTunder theAFO-ODduringLRP

was 0.95.

2.4. Statistical analysis

One-way repeated measurement analysis of variance and multiple comparisons

(Bonferroni) were used to compare gait speed among brace conditions (AFO-OD,

AFO-PS and without brace) in each group. Peak PFRT and changes in ankle joint

angle during LRP were compared between groups with the MannWhitney Utest.

EMG amplitudes during LRP were compared with the Wilcoxon signed-rank test in

each group. To determine the importance of peak PFRT with the AFO-OD, the

relationship between peak PFRT and gait speed with AFO-OD use and the percent

reduction of EMG amplitude with AFO-OD use compared with AFO-PS use was

determined by partial correlation coefficients adjusted by body weight in each

group. Statistical significance was set atp < 0.05.

3. Results

3.1. Increased gait speed with AFO-PS and AFO-OD

Table 1shows the patient demographic and clinical character-

istics. Age, height, and weight were not significantly different

between the stroke and the control groups (unpaired t-test);

however, ankle muscle strength on both sides was lower in

the stroke group than in the control group. Gait speed without a

brace was 84.1 13.2 m/min (control group) and 28.3 11.0 m/min

(stroke group). Use of AFO-PS increased gait speed to 88.0 13.6 m/

min (control group) and 32.0 11.3 m/min (stroke group), whereas

AFO-OD increased gait speed to 90.1 14.6 m/min (control group)

and 34.8 13.9 m/min (stroke group). A significant difference in gaitspeed among the three bracing conditions was observed in the stroke

group (p = 0.002). Multiple comparison analysis revealed that both

AFO-PS and AFO-OD improved gait speed (p= 0.012 and 0.007,

respectively).

3.2. Brace-dependent change in PFRT, EMG amplitude, and ankle

motion

Fig. 2shows typical PFRT data and ankle joint angle with both

braces in the control and stroke groups, and typical EMG patterns

of each lower limb muscle produced by both braces in the stroke

group. Use of the AFO-OD produced a similar peak in PFRT during

LRP in both groups (Table 2); however, an additional peak in PFRT

was observed during the PSw phase in the control group (Fig. 2A).

Table 1

Participant characteristics in the stroke and control groups.

Stroke group (n = 11) Control group (n =11) p

Age (years) 52.113.6 52.19.3 n.s.

Height (cm) 169.45.3 169.75.6 n.s.

Weight (kg) 63.06.9 70.510.7 n.s.

Etiology (n): ischemia/hemorrhage 4/7

Time post-stroke (months): median (range) 20 (6116)

Affected side (n): right/left 7/4

Modified Rankin scale (n): I/II/III 5/5/1Brunnstrom stage (n): III/IV/V/VI 1/6/2/2

Ankle strength

DF on paretic (non-dominant) side (Nm/kg) 0.210.13 0.640.13


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In most subjects in the stroke group, PFRT was low during the PSw

phase with no clear peak. Furthermore, peak PFRT during the PSw

phase using the AFO-OD was significantly correlated with gait

speed in the control group (r= 0.78,p = 0.008).

The peak plantarflexion angle after initial contact did not differ

between stroke and control groups when using the AFO-OD

(Table 2). In contrast, the ankle joint angle was significantly

different between the groups when using the AFO-PS (p= 0.033).

In particular, the stroke group showed dorsiflexion immediately

after initial contact (Fig. 2B).

In the control group, there was no significant brace-dependent

difference in EMG amplitude of the three lower limb muscles

(Table 2). However, in the stroke group, GAS muscle activity was

significantly lower with AFO-OD than with AFO-PS during LRP

[

(uV)

f. AFO-PS

0

-500

500 Tibialis anterior

0

-500

500 Gastrocnemius

0

-500

500 Soleus

LR

(uV)

-500

0

-500

500

0

-500

500

500

0

Tibialis anterior

Gastrocnemius

Soleus

LR e. AFO-OD

A. PFRT

C. Electromyography

B. Ankle joint angle

d. AFO-PS

c. AFO-OD

-10

0

10

20

30

40 Stroke

Control

100%GC

(degree)

-5

0

5

10

15

20

20%GC

Stroke

Control

-10

0

10

20

30

40

100%GC

(degree)

-5

0

5

10

15

20

20%GC

0

5

0

5

0 100%GC 0 100%GC

0

5

0

5

0 100%GC

a. Control b. Stroke(Nm) (Nm) (Nm)

Fig. 2. Typical plantarflexion resistive torque (PFRT), ankle joint angle, and electromyography (EMG) amplitudes during gait cycle. (A) PFRT in the control group (a: left

column) shows two peaks during the loading response (LR) and pre-swing (PSw) phases. However, PFRT in the stroke group (b: middle and right columns) shows low or no

peak duringPSw.(B) Theankle joint angleduringthe entire gait cycle(left) and from 0%to 20%of gait cycle (right) usingAFO-OD(c) andAFO-PS(d). Positivevalues represent

dorsiflexion. Solid and dash lines indicate the stroke and control group. (C) Raw EMG data using AFO-OD (e) and AFO-PS (f).

K. Ohata et al. / Gait & Posture 33 (2011) 102107 105


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(p= 0.041,Fig. 2C andTable 2). This difference in EMG amplitude

between AFO-OD and AFO-PS (percent reduction) was significantly

correlated with peak PFRT during LRP (Table 3).

4. Discussion

In the present study, a peak in PFRT was observed with AFO-OD

use in stroke patients and control subjects during LRP. Most of the

subjects in the stroke group dorsiflexed the ankle immediately

after initial contact when using the AFO-PS. With the AFO-OD,

however, the ankle joint in the stroke group showed adequate

plantarflexion from initial contact, similar to the control group. In

the stroke group, the AFO-OD decreased GAS muscle activity

compared with the AFO-PS during LRP. Further, the percent

reduction in GAS muscle EMG amplitude with AFO-OD, compared

with AFO-PS, was significantly correlated with peak PFRT during

LRP. The control group produced an additional peak in PFRT during

the PSw phase when using AFO-OD, whereas the stroke group did

not. PFRT during the PSw phase was significantly correlated to gait

speed.Eccentric contraction of the TA muscle decreases the rate of

plantarflexion during LRP. This contraction draws the tibia forward

as the foot drops. In the hemiplegic gait, dorsiflexion inabilityleads

to insufficient toe clearance during the swing phase; the TA muscle

is required for toe clearance during the swing phase and to

progress the tibia during LRP. The AFO-PS can compensate for the

TA to improve the toe clearance during the swing phase through

limited plantarflexion; however, this leads to excessive tibial

progression during LRP. In fact, this study showed that the ankle

joint began to dorsiflex immediately after initial contact in stroke

patients using the AFO-PS, indicating that the AFO-PS blocks

adequate plantarflexion. In contrast, adequate plantarflexion was

observed during LRP in stroke patients using the AFO-OD,

consistent with results of a case series study reporting that

AFO-OD achieves sufficient plantarflexion of the ankle by proper

PFRT during LRP[18].

In the present study, the triceps surae produced high EMG

amplitudes during LRP in the stroke group wearing the AFO-PS,

probably because of an excessive stretch reflex due to dorsiflexion

immediately after initial contact. This dorsiflexion may also

produce higher plantarflexor activity. AFO-OD reduced the EMG

activity of the GAS muscle during LRP in the stroke group.

Furthermore, this reduction in EMG amplitude was related to thepeak PFRT during LRP. The smooth plantarflexion motion achieved

with the AFO-OD may reduce excessive activity caused by the

stretch reflex.

PFRT is a convenient method to assess plantarflexion torque

during the PSw phase. Peak PFRT during the PSw phase was

significantly correlated with gait speed in the control group;

however, in the stroke group, PFRT did not show a definite peak

during the PSw phase. The loss of plantarflexor force during the

PSw phase and its relationship with gait function in patients after

stroke have been previously reported[5,11,20].

The present study has several limitations. Muscle activity was

not measured at the peroneus longus or extensor hallucis longus.

Kinematic analysis of the knee or other joints is also lacking in this

study; thus it is not known whether the AFO-OD influences theseparameters. Furthermore, the difference in gait speed between the

stroke and control groups probably influenced the presented

results, such as the peak PFRT during PSw and ankle joint motion.

Further study with proper adjustments of gait speed is necessaryto

evaluate the difference between both groups more strictly. In

addition, participants in the present study had been using the AFO-

OD for at least one month. It is not clear whether similar changes

would be observed immediately with initial use of the AFO-OD.

In conclusion, the AFO-OD assists the heel rocker function by

producing adequate plantarflexion. The main effect of PFRT during

LRP is not to reduce TA activity but to decrease GAS activity to

avoid an excessive stretch reflex; however, the deficit in ankle

plantarflexion torque during the PSw phase remains a major

problem.

Acknowledgements

This study was supported by a grant from Kawamuragishi Co.

Ltd., Japan. We wish to thank the participants who volunteered.

Conflict of interest

The authors declared a potential conflict of interest as follows:

Tadashi Yasui is employed by Kawamuragishi Co. Ltd.

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Table 2

Brace-dependent changes in stroke patients and control subjects.

Stroke group (n = 11) Control group (n =11)

AFO-OD AFO-PS p AFO-OD AFO-PS p

Peak PFRT during LR (Nm) 1.51.1 1.30.9

Peak PFRT during PSw N.A. 2.81.4

Change of ankle angle from initial contact (8) 1.81.4 0.91.2# 1.71.1 0.10.5

TA activity during LR (%) 62.537.3 56.025.9 0.508 55.215.9 53.719.7 0.328

GAS activity during LR (%) 73.735.3 120.568.9 0.041* 8.44.0 9.95.0 0.182

SOL activity during LR (%) 84.626.0 128.192.6 0.594 13.64.7 13.55.6 0.534

Data are expressed as meanSD. Group differences were determined by Wilcoxon signed-rank test. PFRT: plantar flexor resistive torque, LR: loading response phase, PSw: pre-

swing phase, and N.A.: not available. Plantar flexion angle in Change of ankle angle from initial contact was expressed as a negative value.# p


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Effects of an Ankle Foot Orthosis With Oil Damper on Muscle Activity