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Clinical Biomechanics 19 (2004) 44–49

Alterations in lower extremity movement and muscleactivation patterns in individuals with knee osteoarthritis

John D. Childs *, Patrick J. Sparto, G. Kelley Fitzgerald, Mario Bizzini, James J. Irrgang

Department of Physical Therapy, School of Health and Rehabilitation Sciences, University of Pittsburgh, Pittsburgh PA, USA

Received 24 February 2003; accepted 15 August 2003

Abstract

Objective. The purpose of this study was to investigate lower extremity movement and muscle activation patterns in individuals

with knee osteoarthritis and healthy age- and gender-matched control subjects.

Design. This study utilized a non-randomized case-control design to compare 24 subjects with unilateral symptomatic knee

osteoarthritis to 24 age- and gender-matched control subjects without knee osteoarthritis.

Background. It is hypothesized that knee osteoarthritis is associated with altered lower extremity movement and muscle acti-

vation patterns.

Methods. A gait analysis was performed to determine the lower extremity movement and muscle activation patterns when

walking on a level surface at 1.12 to 1.34 m/s and while descending a 20 cm step. Paired t-tests were used to compare the average offive trials between the groups.

Results. Subjects with knee osteoarthritis demonstrated less excursion of the knee in the sagittal plane from heelstrike to peak

flexion before midstance (i.e. during loading). Subjects with knee osteoarthritis also demonstrated reduced peak vertical ground

reaction forces relative to body weight. The muscle activity patterns were also different between the groups. The vastus lateralis,

medial hamstrings, tibialis anterior and medial gastrocnemius were on approximately 1.5 times longer than the same muscles in the

control subjects. Additionally, significant increases in muscle co-activation were also observed in individuals with knee osteoarthritis

during walking. Similar findings were observed when the subject descended a 20 cm step.

Relevance

Individuals with knee osteoarthritis utilize different movement and muscle activation patterns when walking and descending a

step. The alterations may interfere with the knee’s ability to dissipate loads, which could lead to progression of the disease.

Published by Elsevier Ltd.

Keywords: Kinematics; Loading; Knee; Osteoarthritis; Gait

1. Introduction

Knee osteoarthritis (OA) is a prevalent condition that

contributes significantly to functional limitations and

disability (van Baar et al., 1998). Common alterations in

gait observed in individuals with knee OA have been

described and include decreased knee excursion (Messier

et al., 1992), altered ground reaction force (Gyory et al.,

1976), and altered muscle activity patterns of key lower

extremity muscles involved in gait (Benedetti et al., 1999;

* Corresponding author. Address: 508 Thurber Drive, Schertz, TX

78154-1146, USA.

E-mail address: childsjd@bigfoot.com (J.D. Childs).

0268-0033/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.clinbiomech.2003.08.007

Suzuki and Takahama, 1979). While these alterations

are believed to be attempts to reduce pain and protectthe knee from further degeneration, over the long term,

they may have adverse effects on the knee joint.

In general, studies assessing kinematics during gait in

individuals with knee OA have demonstrated decreased

excursion of the knee during the stance and swing

phases of gait (Messier et al., 1992). However, these

results must be interpreted cautiously. Brinkmann and

Perry (1985) compared individuals with knee OA toyounger controls who walked at a much faster velocity.

Gyory et al. (1976) allowed some subjects to use assistive

devices, and they did not control velocity. Messier et al.

(1992) controlled gait velocity and found only a 3� dif-ference in total excursion of the knee. Studies investigating

J.D. Childs et al. / Clinical Biomechanics 19 (2004) 44–49 45

the kinetics of gait are also conflicting. Some have re-ported increased ground reaction forces (Radin et al.,

1991), while others have reported similar (Messier et al.,

1992) or decreased (Gyory et al., 1976) ground reactions

forces in patients with knee OA compared to healthy

control subjects.

To facilitate interpretation of these findings, it would

be helpful to understand the muscle activity of key lower

extremity muscles during gait. However, there is limiteddata in the literature that suggests there is altered muscle

activity during gait in individuals with knee OA

(Benedetti et al., 1999). Suzuki and Takahama (1979)

demonstrated increased activity of the quadriceps and

hamstrings during the stance phase of gait. There is also

data to suggest that individuals with knee OA may

utilize increased co-activation patterns (i.e. ‘‘co-con-

traction’’) between antagonistic muscle pairs duringweight bearing (Benedetti et al., 1999). Additional re-

search is needed to clarify the timing and magnitude of

lower extremity muscle activity during gait in this pop-

ulation. To date, this information has not been reported

in the literature for individuals with unilateral symp-

tomatic knee OA compared to age- and gender-matched

control subjects that do not have symptomatic or ra-

diographic evidence of OA.Research related to gait in individuals with knee OA

has been conducted under a variety of conditions that

frequently do not control for potential confounding

factors such as age (Brinkmann and Perry, 1985), gait

velocity (Brinkmann and Perry, 1985), and the use of

assistive devices (Gyory et al., 1976). For example,

controlling for gait velocity has been recommended and

utilized by researchers to account for potential differ-ences in range of motion and vertical ground reaction

forces that could simply be attributed to differences in

self-selected velocity between those with knee OA and

those without (Messier et al., 1992). The variety of

testing conditions makes comparisons between studies

problematic and likely contributes to the conflicting

findings between studies. Therefore the purpose of this

study was to concomitantly investigate lower extremitysagittal kinematics, kinetics and muscle activation pat-

terns in individuals with knee OA and healthy age- and

gender-matched control subjects during two functional

activities, walking on a level surface and negotiating a

step 20 cm in height. An improved understanding of the

alterations during gait in individuals with knee OA may

lead to the development of more effective rehabilitation

strategies.

2. Methods

This study utilized a non-randomized case-control

design to compare the gait characteristics of 24 subjects

with unilateral symptomatic knee OA to 24 age- and

gender-matched control subjects without knee OA. Thedata were collected as part of a pilot study to determine

important sagittal kinematic, kinetic and electromyog-

raphy variables that distinguish subjects with OA from

healthy controls when walking at a controlled speed on

a level surface and when descending a 20 cm step.

3. Subjects

Subjects with OA consisted of volunteers, 62 (47–78)

years of age (56% female). Subjects were previously di-

agnosed with unilateral knee OA involving the tibio-

femoral and/or patellofemoral compartments. All subjects

signed an informed consent document approved by the

University of Pittsburgh Institutional Review Board for

Biomedical Research prior to participating in the study.Subjects were included if they were at least 40 years of

age, met the 1986 American College of Rheumatology

clinical and radiographic criteria for knee OA (Altman

et al., 1986), and had grade II or greater Kellgren and

Lawrence radiographic changes on the symptomatic side

(Kellgren and Lawrence, 1957). Subjects with knee OA

were allowed to have bilateral radiographic evidence of

knee OA so long as their symptoms were unilateral.Subjects with knee OA were excluded if they: (1) were

unable to safely walk distances greater than 200 feet

without the use of assistive devices; (2) had a history of a

ligament injury of the involved knee, (3) had symp-

tomatic knee OA of the uninvolved knee or (4) had

undergone total knee arthroplasty.

Control subjects consisted of healthy age- and gen-

der-matched individuals, 62 (47–78) years of age (56%female), who: (1) had no history of knee OA or other

pathology involving the knees or other joints of the

lower extremity; and (2) did not have greater than Grade

I Kellgren and Lawrence radiographic changes of either

knee. Subjects in either group who had uncontrolled

hypertension, a history of cardiovascular disease, were

pregnant, or were cognitively impaired to the extent they

could not understand the instructions for the testingprocedures were also excluded. Demographic charac-

teristics, key clinical findings, and self-report measures

of function for both groups are listed in Table 1. These

include range of motion of the knee, quadriceps strength

expressed as a percentage of body mass index, an 11-

point numeric pain rating scale (Downie et al., 1978),

the Activities of Daily Scale (ADLS) of the Knee Out-

comes Survey score (Irrgang et al., 1998), and theWestern Ontario and McMaster Universities (WO-

MAC) Osteoarthritis Index score (Bellamy et al., 1988).

Subjects with knee OA had greater mass and body mass

index and significantly less range of motion and quad-

riceps strength. In addition, they had significantly

greater pain and WOMAC scores and lower ADLS

scores. The number of subjects with medial, lateral, and

Table 1

Descriptive statistics (mean (SD)) of knee OA and control subjects

Knee OA ðn ¼ 24Þ Controls ðn ¼ 24Þ p-value

Age (years) 62 (10) 62 (10) 0.366

Gender (percent female) 56% 56% n/a

Height (cm) 167 (14) 166 (8) 0.372

Mass (kg) 84 (24) 74 (16) 0.046

Body Mass Index (kg/m2) 30 (7) 27 (6) 0.033

Knee extension (degree): A negative sign corresponds to hyperextension. 6 (4.4) )1.7 (3) <0.001

Knee flexion (degree) 125 (10) 138 (6) <0.001

Total knee range of motion (degree) 119 (13) 139 (8) <0.001

Knee extensor strength (Nm/BMI) 5.2 (1.9) 7.0 (1.9) <0.001

NPRSa (average of best, worst, and current levels of pain over a 24-hour period) 2.1 (1.3) 0 (0) <0.001

ADLSb 70.1 (16) 99.8 (1) <0.001

WOMACc 24.6 (15) 0.3 (1) <0.001

aNumeric pain rating scale.bActivities of daily living scale of knee outcome survey (higher scores indicate higher levels of function).cWestern Ontario and McMasters University Osteoarthritis Index (lower scores indicate lower levels of disability).

Table 2

Summary of radiographic findings in subjects with knee OA

No. of

subjects

ðn ¼ 24ÞIsolated medial compartment knee OA 12

Medial and patellofemoral compartment knee OA 9

Lateral and patellofemoral compartment knee OA 2

Tri-compartmental knee OA 1

46 J.D. Childs et al. / Clinical Biomechanics 19 (2004) 44–49

patellofemoral compartment knee OA is summarized in

Table 2.

4. Equipment

A gait analysis was performed to determine the knee

excursion range of motion, ground reaction force, andmuscle activity patterns when walking on a level surface

and while descending a step 20 cm in height. A 20 cm

step was chosen because it is the height of a normal step

in the United States. Knee excursion was measured us-

ing an electromagnetic motion analysis system (Motion

Monitor, Innovative Sports Training Inc., Chicago, IL,

USA). Ground reaction force data were obtained using

a six degrees-of-freedom force platform (AdvancedMechanical Technology Inc., Newton, MA, USA). A

surface electromyography (sEMG) recording system

(Bagnoli-2 EMG, Delsys Inc., Boston, MA, USA) was

used to measure activity of the vastus lateralis, medial

hamstrings, tibialis anterior, and medial gastrocnemius.

Angular position of the knee was sampled at 50 Hz and

ground reaction force and surface electromyography

data were sampled at 1000 Hz.

5. Testing procedures

Subjects wore comfortable walking shoes, with the

restriction that the shoes did not have a heel height

greater than 1 in. Electromagnetic sensors were placed

using neoprene cuffs around the shanks and thighs. The

cuffs have connectors that firmly attach the sensor to the

body segment and are designed to minimize movementbetween the sensor and body segment. The primary axis

of the electromagnetic sensor was aligned with the long

axis of the respective body segment. Prior to application

of the sEMG electrodes, the hair underlying the elec-

trodes was removed using a standard disposable safety

razor, and the skin was cleansed with rubbing alcohol.

Surface electromyography electrodes were placed over

the vastus lateralis, medial hamstrings, tibialis anterior,and medial gastrocnemius muscles. The electrodes were

oriented on the center of the muscle belly in a longitu-

dinal fashion in the direction of the muscle fibers.

Placement of the electrode was facilitated by palpat-

ing the muscle as the subject contracted the muscle

against resistance. Additionally accurate placement of

the electrode was verified during the normalization

process. If a good signal was not obtained when theindividual performed a maximal isometric contraction

of the muscle, then the electrode was repositioned.

A reference electrode was placed over the head of the

fibula.

Subjects were required to walk along a level surface

approximately 3.7 m in length at a velocity ranging from

1.12 to 1.34 m/s, which is considered to be the normal

walking velocity for individuals in the age group thatwas enrolled in this study (Murray et al., 1970). Gait

velocity was controlled by the use of two infrared sig-

nals, one placed before and after the force platform.

Subjects were instructed to strike the force platform with

their involved side. For control subjects, the side used

for comparison was determined a priori by the involved

side of the subject with knee OA with whom the control

subject was matched. Data were collected over five validtrials. A valid trial was defined as one in which the

subject struck the force platform with the involved ex-

tremity at the required velocity without adjusting his/her

J.D. Childs et al. / Clinical Biomechanics 19 (2004) 44–49 47

stride length. With practice, subjects rarely had tocomplete more than 2–3 additional trials.

Subjects also descended a step 20 cm in height. From

a level surface standing next to the step, the subject was

asked to step up on the step with the asymptomatic

extremity and then step down onto the force platform

with the symptomatic extremity. After stepping down,

the subject continued to walk forward several steps.

Subjects performed five repetitions of this activity anddid not use a handrail or any other device to assist ne-

gotiation of the stair. Subjects were permitted to per-

form 1–2 practice trials to insure they understood the

instructions and initiated the task with the appropriate

foot.

6. Data management and analysis

The knee flexion/extension angle, and ground reac-

tion force data were smoothed using a zero-lag Butter-

worth lowpass filter. The magnitude of the ground

reaction forces was normalized by body weight. The

sEMG were smoothed using a moving root-mean-

square (RMS) filter with a 25 ms window. To facilitate

comparison between groups, the sEMG was normalizedto the mean of the RMS values obtained during three 3-s

maximum voluntary isometric contractions of each of

the muscles (Rudolph et al., 2000).

The data from each of the five valid trials were nor-

malized in time to the duration of stance phase. Fur-

thermore, onset and offset times of the muscle activity

were determined without normalization to time by vi-

sually identifying the time when the sEMG had a phasicincrease in activation above baseline. The amount of

muscle co-activation between antagonistic muscle pairs

in the lower extremity (vastus lateralis vs. medial ham-

strings and anterior tibialis vs. gastrocnemius muscles)

Table 3

Summary of the alterations in knee kinematics and vertical ground reaction

Walking task

OA C

Kinematic parameters (units)

Knee angle at initial contact (degree) 4.5 (4.5) 1

Knee flexion excursion during loading response phase

(heelstrike to peak flexion before midstance) (degree)

15.7 (5.7) 1

Time to peak during loading response phase (%stance) 27.9 (4.9) 2

Kinetic parameters (units)

Loading rate to 1st peak of vertical ground reaction

force (%/s)

650 (170) 7

Maximum vertical ground reaction force during loading

(% body weight)

106 (12) 1

Maximum vertical ground reaction force during pushoff

(% body weight)

104 (7) 1

All values represent parameters during the stance phase of gait––mean (SD)

was determined using the method of Rudolph et al.(2000)

ðEMGL þ EMGMÞ � EMGL=EMGM;

where EMGL is the level of activity in the less active

muscle and EMGM is the level of activity in the moreactive muscle. Muscle co-activation was averaged over

the stance phase, and the mean value from the five trials

was calculated. For all dependent measures, paired t-tests were used to compare the average of five trials

between the involved limb of the knee OA subject and

the same side limb of the matched control subject

(Portney and Watkins, 2000). Because this study was

exploratory in nature, the alpha-level was not adjusteddownwards to control for the Type-I error rate.

7. Results

A summary of selected kinematic and kinetic vari-

ables appears in Table 3. Of note, subjects with unilat-

eral knee OA exhibited less flexion of the knee fromheelstrike to the angle of peak knee flexion before

midstance (i.e. during the loading response phase),

during both level walking and stair descent. The de-

crease in excursion during level walking was partly due

to subjects with knee OA landing in less extension

ðp ¼ 0:04Þ. During stair descent, subjects with knee OAlanded with the same amount of extension, but flexed

less during the loading period. Subjects with knee OAalso demonstrated reduced peak vertical ground reac-

tion force relative to body weight. The muscle activity

patterns showed some of the most dramatic changes

(Table 4). The vastus lateralis, medial hamstrings, tibi-

alis anterior and medial gastrocnemius were on ap-

proximately 1.5 times longer than the same muscles in

the control subjects (Fig. 1). Additionally, significant

forces when walking and descending a step

Step task

ontrol p-value OA Control p-value

.4 (5.7) 0.04 9.4 (4.6) 10.2 (4.8) 0.56

9.5 (5.2) 0.007 14.4 (6.2) 20.5 (5.2) <0.001

7.7 (4.4) 0.87 28.6 (7.2) 30.7 (4.1) 0.14

50 (170) 0.04 1520 (640) 1650 (430) 0.131

14 (7) 0.01 152 (3) 169 (2) 0.002

09 (6) 0.002 103 (12) 103 (14) 0.487

.

Table 4

Summary of the alterations in muscle activation patterns when walking and negotiating a step

Walking task Step task

OA Control p-value OA Control p-value

Duration of muscle activity

Vastus lateralis (ms) 447 (187) 282 (84) <0.001 469 (202) 299 (95) <0.001

Hamstrings (ms) 417 (130) 250 (106) <0.001 343 (134) 182 (53) <0.001

Tibialis anterior (ms) 382 (107) 224 (60) <0.001 407 (181) 174 (79) <0.001

Gastrocnemius (ms) 502 (156) 362 (128) <0.001 492 (122) 319 (90) <0.001

Muscle co-activation

Vastus lateralis-hamstrings (%) 28 (25) 15 (9) 0.03 24 (22) 15 (12) 0.07

Tibialis anterior-gastrocnemius (%) 20 (11) 11 (5) <0.01 20 (15) 12 (5) 0.05

All values represent parameters during the stance phase of gait––mean (SD).

Muscle coactivation¼ (EMGL +EMGM) *EMGL/EMGM, where EMGL is the average level of activity in the less active muscle during stance phase

and EMGM is the average level of activity in the more active muscle during stance phase.

-400 -200 0 200 400 600 800Time (ms), where 0 = Heelstrike

Vastus lateralis

Hamstrings

Tibialis anterior

Gastrocnemius

Control

Knee OA

Fig. 1. Duration of muscle activity in subjects with knee OA vs. con-

trols during walking task.

48 J.D. Childs et al. / Clinical Biomechanics 19 (2004) 44–49

increases in muscle co-activation were also observed in

individuals with knee OA during walking. Similar find-

ings were observed during the step task.

8. Discussion

These results indicate that individuals with knee OA

utilize different movement and muscle activation pat-

terns when walking and negotiating a step compared to

healthy age- and gender-matched control subjects

without knee OA. Our results confirm and clarify someof the previous research that has assessed gait parame-

ters in individuals with knee OA. Compared to healthy

control subjects, individuals with knee OA exhibit a

decreased excursion of flexion of the knee during the

loading response phase of gait (Messier et al., 1992). In

our study, during the loading response phase of gait,

individuals with knee OA exhibited approximately 4–6�less flexion of the knee compared to age- and gender-matched control subjects (Table 3). During level walk-

ing, the decrease in excursion could be explained by

subjects with knee OA landing with the knee flexed.

These changes are consistent with the decreased knee

extension range of motion that is observed clinically inthe subjects with knee OA (Table 1). Future work will be

directed at more fully exploring the relationships be-

tween impairments and function of the knee during gait.

Reduced knee flexion excursion combined with in-

creased muscle co-activation during the loading re-

sponse phase of gait may represent a stiffening of the

joint. The combination of these two factors may lead to

increased compressive loading and reduce the potentialfemoral contact area over which the force can be dis-

tributed, which may in turn contribute to increased cu-

mulative loading in localized areas.

Subjects with knee OA in our study demonstrated an

increased duration of muscle activity during stance for

all four lower extremity muscles that were assessed dur-

ing both the walking and step tasks. Fig. 1 illustrates this

finding during the walking task for all muscle groups. Weobserved that the muscles of the subjects with knee OA

turn on sooner and turn off later than the same muscles

of the control subjects. In addition, significant increased

co-activation of antagonistic muscle pairs in the lower

extremity was observed between the vastus lateralis and

hamstrings and between the anterior tibialis and gas-

trocnemius muscles during walking. The results of our

study support the findings of previous studies thatdemonstrated increased muscle activity and the possi-

bility of increased co-activation of antagonistic muscle

pairs in key lower extremity muscles during gait in in-

dividuals with knee OA (Benedetti et al., 1999). Future

studies should concomitantly investigate movement and

muscle activation patterns in the frontal and transverse

planes and address the mechanism for the increased

muscle activity observed in the subjects with knee OA.Additionally, it is possible that a different step height

could alter the amount of knee motion needed to ne-

gotiate a step depending on the subject’s height, which

could in turn alter muscle activity patterns and perhaps

loading upon landing. However, a 20 cm step is the

height of a normal step in the United States, and no

J.D. Childs et al. / Clinical Biomechanics 19 (2004) 44–49 49

differences in height of the subjects were observed be-tween the groups ðp ¼ 0:37Þ (Table 1).Decreased excursion of the knee combined with in-

creased muscle co-activation and duration of muscle

activity may represent an attempt to avoid pain and/or

stabilize the knee during the loading response phase of

gait. Although we did not have a measure of load dis-

tribution (which is difficult to measure experimentally), it

is possible these alterations may act to stiffen the knee,thus increasing compressive loads across the knee. This

may make the knee less capable of dissipating potentially

harmful loads, increasing susceptibility of the knee to the

development and progression of the disease. If this is the

case, rehabilitation strategies that increase knee excur-

sion and decrease muscle co-activation patterns may

need to be developed to maximize the effectiveness of

rehabilitation that is traditionally focused on quadricepsstrengthening. Although the preliminary results of this

study are encouraging, future work is currently being

conducted to investigate the implications of our findings

on the rehabilitation of patients with knee OA.

Acknowledgements

The authors would like to acknowledge Dr. Chris

Harner from the University of Pittsburgh Medical

Center (UPMC) Health System’s Center for Sports

Medicine for referring patients as potential subjects for

this study. The authors would also like to acknowledgeDr. Chester W. Oddis from the UPMC Department of

Rheumatology for interpreting the radiographs that

were performed in this study.

This study was supported in part by a grant from the

Orthopaedic Physical Therapy Section of the American

Physical Therapy Association, Inc. and by a scholarship

from the Foundation for Physical Therapy, Inc.

The opinions or assertions contained herein are theprivate views of the authors and are not to be construed

as official or as reflecting the views of the U.S. Air Force

or Department of Defense.

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