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Clinical Rehabilitation 2002; 1 6: 749–760
© Arnold 2002 10.1191/0269215502cr549oa
Address for correspondence: Professor Christina WY Hui-
Chan, Department of Rehabilitation Sciences, The Hong
Kong Polytechnic University, Hung Hom, Kowloon, Hong
Kong. e-mail: rschris@polyu.edu.hk
Does four weeks of TENS and/or isometric
exercise produce cumulative reduction of
osteoarthritic knee pain?
Gladys LY Cheing, Christina WY Hui-Chan Department of Rehabilitation Sciences, The Hong Kong Polytechnic
University, Hung Hom, Kowloon and KM Chan Department of Orthopaedics & Traumatology, The Chinese University of
Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong
Received 27th December 2001; returned for revisions 15th April 2002; revised manuscript accepted 26th May 2002.
Objective: To evaluate the cumulative effect of repeated transcutaneous
electrical nerve stimulation (TENS) on chronic osteoarthritic (OA) knee pain
over a four-week treatment period, comparing it to that of placebo stimulation
and exercise training given alone or in combination with TENS.
Design: Sixty-two patients, aged 50–75, were stratied according to age,
gender and body mass ratio before being randomly assigned to four groups.
Interventions: Patients received either (1) 60 minutes of TENS, (2) 60 minutes
of placebo stimulation, (3) isometric exercise training, or (4) TENS and
exercise (TENS & Ex) ve days a week for four weeks.
Main outcome measures: Visual analogue scale (VAS) was used to measure
knee pain intensity before and after each treatment session over a four-week
period, and at the four-week follow-up session.
Results: Repeated measures ANOVA showed a signicant cumulative reduc-
tion in the VAS scores across the four treatment sessions (session 1, 10, 20
and the follow-up) in the TENS group (45.9% by session 20, p < 0.001) and
the placebo group (43.3% by session 20, p = 0.034). However, linear regres-
sion of the daily recordings of the VAS indicated that the slope in the TENS
group (slope = –2.415, r = 0.943) was similar to the exercise group (slope =
–2.625, r = 0.935), which were steeper than the other two groups. Note that
the reduction of OA knee pain was maintained in the TENS group and the
TENS & Ex group at the four-week follow-up session, but not in the other
two groups.
Conclusions: The four treatment protocols did not show signicant between-
group difference over the study period. It was interesting to note that
isometric exercise training of the quadriceps alone also reduced knee pain
towards the end of the treatment period.
750 GLY Cheing et al.
have been a confounding factor.
5
In addition, the
age of the subjects, the intensity and duration of
the stimulation were not controlled.
9
It was proposed that repeated applications of
TENS could serve as a form of afferent condi-
tioning that may produce plastic changes in the
central nervous system over time.
10
Cumulative
antinociceptive effects of TENS on the exion
reex and experimental pain were reported rst
in normal subjects,
11
then in patients who suffer
from chronic clinical pain. Cheing and Hui-
Chan
12
found that repeated applications of 60
minutes of conventional TENS in patients who
suffered from chronic low back pain produced a
cumulative analgesic effect over a two-week
period. There was a 48.8% reduction in subjec-
tive pain sensation by the end of the two-week
treatment period (p < 0.01). However, it could be
argued that low back pain involves the axial
joints and is usually caused by mechanical disor-
ders, whereas OA knee pain occurs in a more
peripheral joint and is usually the result of degen-
erative changes. Therefore, the present study set
out to investigate whether TENS produces cumu-
lative antinociceptive effects in chronic
osteoarthritic knee pain.
Knee pain could reduce the exercise tolerance
of people who have OA knee pain. The strength
of the knee muscles of people with OA knee is
usually weaker than that in normal subjects. Our
previous study found that the isometric peak
torque of the quadriceps in the affected leg of the
patients with OA was reduced by about 26%
when compared with that in normal subjects.
13
Muscle weakness of the quadriceps may in turn
interfere with the normal mechanics around the
knee joint, thus increasing knee pain. A vicious
cycle could therefore be formed. Exercise is
therefore usually advocated for them.
Strengthening exercise to the quadriceps can
improve the stability of the knee joint.
14
Stronger
knee muscles could theoretically provide better
protection of the knee joint by reducing the
excessive stress and strain on the lax joint cap-
sule where the nociceptors are located, to reduce
knee pain during movement. Indeed, Fisher et
al.
15
demonstrated that quadriceps exercise train-
ing improved muscle strength, which was associ-
ated with a 40% reduction in OA knee pain. Our
previous study
16
showed that a four-week iso-
Introduction
Osteoathritis (OA) is a highly prevalent condi-
tion and one of its major symptoms is pain. In
the United States, it was the leading cause of dis-
ability, with the direct and indirect cost of arthri-
tis estimated to be $68.4 billion in 1992.
1
It has
been projected that 59.4 million people in the
United States, or 18.2% of the population, will
suffer from OA by 2020.
2
Transcutaneous electrical nerve stimulation
(TENS) is a common treatment modality for
musculoskeletal pain,
3
and has been demon-
strated to be effective for managing OA knee
pain.
4–9
Briey, TENS but not placebo stimula-
tion, was found to produce a signicant reduction
in subjective pain and in medication scores.
4
Smith et al.
5
reported that the percentage of
respondents experiencing pain relief was sub-
stantially greater in the TENS group (66.7%)
than the placebo group (26.7%), although this
difference was not signicant due to a lack of sta-
tistical power. The percentage of respondents
remained higher in the TENS group at the four-
week follow-up session. Another study showed
that TENS produced a signicantly longer pain
relief period than placebo stimulation.
6
Other
studies also supported the effectiveness of TENS
in managing OA knee pain,
7,8
but most studies
recorded treatment outcomes only at the end of
a treatment period. None of them documented
the time course of possible cumulative analgesic
effects of TENS during the specic treatment
period. Furthermore, previous studies did not
document the duration and amount of analgesic
effect that could be carried over to post-treat-
ment period by repeated applications of TENS.
Such information is essential in determining the
optimal treatment outcome of a given interven-
tion. Furthermore, there were certain method-
ological problems in these studies. For example,
the stimulation was applied by patients at home,
hence the location of electrodes and the treat-
ment duration might not have been controlled
properly.
4
Moreover, when using a cross-over
design, patients could not have been possibly
blinded when switching from TENS to placebo
stimulation, because of a lack of sensation during
placebo stimulation.
4
Also, patients were allowed
to take NSAID during the study, which might
TENS for OA knee pain 751
dominance were recorded. The subjects were
required to sign a consent form and were advised
to keep their activity level and medication
unchanged throughout the study period. They
were randomly allocated to one of the four
groups, receiving the assigned treatment ve days
a week for four weeks (i.e. a total of 5 ´ 4 = 20
sessions). There was a follow-up assessment four
weeks after the termination of treatment.
Treatment procedures
Group 1: TENS
Subjects received daily conventional TENS to
the affected knee for 60 minutes. A dual-channel
portable TENS unit (Staodyn MAXIMA III;
Staodyn Inc., Longmont Co, USA) was used in
this study. Stimulation was given in continuous
trains of 140 µs square pulses at 80 Hz. Four sur-
face electrodes, 4 ´ 4 cm each, were placed on the
following ve acupuncture points: spleen 9, stom-
ach 35, extra 31, 32 and gallbladder 34 (one elec-
trode pad covering both extra 32 and stomach
35).
5,18,19
As noted by previous investigators, the
local tender points coincided with the acupunc-
ture points in most cases.
20
The intensity of TENS
was adjusted to produce a tingling sensation that
was approximately 3–4 times the subject’s sen-
sory threshold. Each subject received the treat-
ment around the same time of the day
throughout the treatment period, to avoid the
uctuation of pain intensity during the day. In
order to blind the subjects from the placebo
effect, all subjects were told that they might or
might not feel the stimulation.
Group 2: Placebo stimulation
Subjects received placebo stimulation at the
same sites for the same duration and period as
the TENS group, except that placebo units were
used for the placebo group. These units were
identical to the real TENS units, and the indica-
tor lamp was lit up when the unit was switched
on. However, the internal circuit had been dis-
connected by the manufacturer (Staodyn Inc.) for
the purpose of our study.
Group 3: Exercise
The Cybex II+ isokinetic dynamometer
(Cybex, Division of Lumex, Inc., New York,
USA) used in the present study was calibrated
metric exercise training of knee muscles pro-
duced a 20.9% gain (p < 0.05 in three selected
knee positions) in the isometric extensor peak
torque, and the addition of TENS to the exercise
training generated a 26.6% gain (p < 0.05 in three
selected knee positions) in the extensor’s peak
torque. Would such an improvement in muscle
strength be associated with any pain reduction?
More importantly, would the addition of TENS
to exercise training produce a greater pain reduc-
tion than TENS or exercise alone?
Therefore, the objectives of the present exper-
iment were twofold:
1) To compare the time course and the effec-
tiveness of a single session of the following
four treatment protocols on the reduction of
OA knee pain intensity: (a) TENS for 60
minutes, (b) placebo stimulation for 60 min-
utes, (c) isometric exercise training for 30
minutes, (d) TENS for 60 minutes and iso-
metric exercise training for 30 minutes
2) To examine the possible cumulative effect of
repeated daily application of the four treat-
ment protocols on OA knee pain over a four-
week period.
Methods
Sixty-six patients with OA knee, aged 50–75, par-
ticipated in the study; 56.0% of the patients had
bilateral knee OA. For patients with bilateral
knee OA, data obtained from the more affected
knee were identied as the affected side. Subjects
were diagnosed and referred by an orthopaedic
surgeon from the Prince of Wales Hospital in
Hong Kong, based on both clinical and radi-
ographic ndings. The inclusion criterion was
grade II (or above) OA changes according to
Kellgren and Lawrence.
17
Eligible subjects had
had OA for more than six months, been stable
on their medication for three weeks before enter-
ing the study, and received no paramedical treat-
ment within the previous two weeks before the
experiment. The exclusion criteria were: prior
knee surgery, prior experience with the use of
TENS and/or having received a steroid injection
within the previous three weeks.
All subjects were able to walk on their own for
10 minutes. Their age, body mass index and leg
752 GLY Cheing et al.
Recording
Arthropometric data such as subjects’ body
weight (kg) and height (m) were collected using
standard clinic scales. The body mass index
(BMI) (kg/m
2
) was calculated by dividing the
body weight by the square of body height. All
subjects were asked to provide information on
their medical history, use of medications, recent
injuries, activity level and history of knee pain.
A visual analogue scale (VAS) was used to
measure the intensity of subjective pain sensa-
tion. The VAS consisted of a 10-cm horizontal
line, anchored with ‘no pain’ at the left end (i.e.
threshold intensity), and ‘pain as bad as it could
be’ at the right (i.e. maximally tolerable inten-
sity). The VAS scores were recorded daily from
session 1 to session 20 and in the follow-up ses-
sion. Subjects were requested to move a mechan-
ical curser along the horizontal line up to a point
that represented their present intensity of knee
pain. After each VAS estimate, the subject was
requested to return the mechanical cursor back
to the left end of the scale. A build-in variable
resistor was attached to the VAS. Any movement
of the cursor generated a voltage output, which
was then fed into a computer to produce a digi-
tal output. In each session, the VAS score was
measured before and 20 minutes after treatment
for all groups. The VAS scores were also mea-
sured at 20, 40 and 60 minutes during stimulation
for groups 1, 2 and 4. Therefore, a total of ve
VAS scores were collected from these three
groups. Only two VAS scores were recorded for
group 3 (before and after the exercise), because
no stimulation was applied to this group.
Statistical analysis
Repeated measures ANOVA using the SPSS
statistics package (version 10.0) was used to
analyse the VAS scores recorded in session 1,
session 10, session 20 and the follow-up session.
The between-subject factor was treatment
‘groups’ (group 1 to group 4). The within-subject
factor was ‘time’ (before and after stimulation)
and ‘sessions’. To examine the cumulative effects
of each treatment protocol, the pre-treatment
VAS scores of session 10, session 20 and the fol-
low-up session were normalized with respect to
those recorded in session 1. Signicant results
were then analysed by post-hoc tests (LSD – least
monthly by the dealer during the study period.
Subjects received isometric exercise training for
about 20 minutes on each workday for four
weeks. Isometric contraction referred to a negli-
gible movement of the relevant joint. Patients
who participated in the pilot study indicated that
isometric contractions elicited less pain than iso-
tonic or isokinetic exercises. The subjects were
seated with their hip exed at 80° and their back
supported by a backrest. The axis of the
dynamometer arm was aligned with that of the
tested knee. One strap each was used to stabilize
the thigh and the lower leg.
Training was carried out only for the involved
leg (or the more involved leg in cases of bilateral
involvements), and the goal was to strengthen
mainly the knee extensors (quadriceps) with less
focus on the knee exors (hamstrings). The treat-
ment protocol consisted of 10 isokinetic warm-up
knee extension exercises, starting from 90° of
knee exion through the available pain-free
range, at a speed of 180°/s. Three submaximal
isometric quadriceps contractions of increasing
intensity were followed by six maximal 5-second
isometric quadriceps contractions. Most patients
achieved their peak torque within 5 seconds. The
isometric quadriceps contractions were repeated
with the knee exed at 30°, 60° and 90° (E
30°
, E
60°
and E
90°
) respectively. The isometric peak torque
for the hamstrings was then performed with the
knee exed at 90° (F
90°
). Training was done in the
various knee positions because different muscle
lengths and joint angles are required for various
functional activities such as walking on level
ground, climbing stairs or rising from a chair.
There was a 30-second rest after each contraction
and a 1-minute rest after completing a set of con-
tractions at each knee position. The sequence of
which leg to be tested and the knee positions to
be tested were randomly assigned on session 1
and then maintained for the whole treatment
period. Each training session usually lasted for
about 30 minutes.
Group 4: TENS and exercise (TENS & Ex)
The treatment protocol received by group 1
was combined with that of group 3. Briey, sub-
jects received 60 minutes of TENS followed by
20 minutes of rest, then ended with 30 minutes
of isometric exercise training on the affected leg.
TENS for OA knee pain 753
signicant difference). Finally, linear regression
was used to analyse the changes in the daily
recording of the VAS scores across the 20 treat-
ment sessions. The level of statistical signicance
was set at p < 0.05 for all the tests.
Results
Patient characteristics
Sixty-six patients with OA knee attended the
rst session, and 62 of them completed the four-
week treatment period. Two patients each from
the placebo group and the TENS & Ex group
dropped out of the study due to time conicts and
medical reasons. Table 1 shows the characteris-
tics of patients with OA knee who participated
in the four-week treatment programme. No sig-
nicant differences in the demographic data were
found among the four groups (p > 0.05), except
for the body mass index. The body mass index of
the exercise group was signicantly higher than
that of the TENS & Ex group (p < 0.05). There-
fore, this factor was adjusted in the subsequent
analyses.
Effectiveness of the four treatment protocols
on reducing OA knee pain in session 1
Table 2 shows the VAS scores of the four treat-
ment groups recorded in the various sessions.
The inuence of the four treatment protocols on
the VAS scores recorded in session 1 is shown in
Figure 1a. For between-group comparisons, the
post-treatment VAS scores in session 1 tended to
be lower than those of the pre-treatment with in
each group, except for the exercise group (p =
0.055). The difference in the pre- and post-treat-
ment pain level reached statistical signicance
(p = 0.031). Post-hoc test (LSD) showed that the
difference came from comparing the TENS group
with the exercise group (p = 0.011), and from
comparing the TENS & Ex group with the exer-
cise group (p = 0.008).
The mean VAS scores of the TENS group
dropped gradually over time during session 1,
from a normalized value of 100% before stimu-
lation, to 67.3% (±46.7%) at 60 minutes into
TENS (Figure 1). This score dropped slightly to
64.1% (±40.7%, Table 2) at 20 minutes after
TENS stopped. In contrast, the VAS score of the
placebo group reached 86.3% (±37.5%) at 60
minutes into stimulation (Figure 1), and 84.5%
(±39.6%, Table 2) at 20 minutes after stimula-
tion. For the exercise group, the VAS score
increased to 122.0% (±101%, Table 2) after exer-
cise. The results in the TENS & Ex group were
similar to the TENS group, the VAS score being
decreased to 69.0% (±46.9%, Table 2) at 20 min-
utes after stimulation.
The inuence of the four-week treatment
programme on OA knee pain
The VAS scores recorded in session 10, session
20 and the follow-up session are presented in Fig-
ure 1b–d. Since there was signicant interaction
between ‘group’ and ‘session’, subsequent analy-
sis was performed separately. For the TENS
group, the pre-stimulation VAS score decreased
to 54.1% of the control value by session 20, and
further to 51.5% in the follow-up session (p =
0.000) (Table 2). For the placebo group, the pre-
stimulation VAS score decreased to 56.7% by
session 20. However, it increased to 67.9% in the
follow-up session (p = 0.034). For the exercise
group, the pre-treatment VAS score was reduced
to 70.7% by session 20 and returned to 93.2% in
Table 1 Patient characteristics
a
Group TENS Placebo Exercise TENS & Ex p-value
n 16 16 15 15 –
Age 65.3 ± 8.3 64.1 ± 6.1 60.9 ± 7.3 64.3 ± 9.2 0.446
Height (cm) 151.9 ± 7.3 151.8 ± 6.8 155.2 ± 6.9 155.7 ± 6.6 0.243
Weight (kg) 62.6 ± 13.4 66.0 ± 7.7 71.5 ± 12.1 61.7 ± 8.7 0.059
Body mass index (kg/m
2
) 26.8 ± 4.0 28.8 ± 3.7 29.6 ± 4.3 25.5 ± 3.1 0.016*
Gender (female %) 87.5 93.8 86.7 73.3 0.434
*As the body mass index (BMI) was signicantly different between groups (p < 0.05), BMI was adjusted in subsequent
analyses.
754 GLY Cheing et al.
steeper than that of the placebo group, but the
between-group difference was just short of sta-
tistical signicance (p = 0.078).
From session 20 to the follow-up session, none
of the patients received any treatment. Interest-
ingly, the VAS scores were slightly reduced in the
TENS group (by 2.6%) and the TENS & Ex
group (by 7.6%). In contrast, the knee pain
rebounded by 11.2% in the placebo group and by
22.5% in the exercise group.
Discussion
Time course of the analgesic effect of 60
minutes of TENS or placebo stimulation on
OA knee pain
In session 1, TENS analgesia was found to be
developed gradually. These effects peaked at the
end of the stimulation period, and outlasted the
treatment. Such a gradual onset and offset of
TENS analgesia was also demonstrated in people
suffering from chronic low back pain.
21
It is inter-
esting to note that the progressive and prolonged
time course of TENS analgesia is similar to that
produced by endogenous opioids.
22,23
In fact,
acupuncture-like TENS and conventional TENS
the follow-up session (p = 0.407). For the TENS
& Ex group, the pre-treatment VAS score
decreased to 70.6% by session 20 and further to
63.0% in the follow-up session (p = 0.074).
Figure 2 shows the regression lines of the daily
recording of the pre-stimulation VAS scores over
the 20 sessions of treatment. Note that the results
of the follow-up session were not included. The
mean VAS scores in all groups decreased almost
linearly across sessions (all p = 0.000). Using
regression analysis, the slope of the regression
lines for each of the four groups was signicantly
different (p = 0.000). The slope of the regression
line for the TENS group (slope = –2.415, r =
0.943; Figure 2a) and the exercise group (slope =
–2.625, r = 0.935; Figure 2c) was similar (p =
0.565), and were steeper than the other two
groups. However, the pre-stimulation VAS
scores for the exercise group increased by about
27.6% from session 1 to session 3, then decreased
from session 4 onward (Figure 2c). From session
6 to session 20, the knee pain in this group
dropped below the baseline value. The slope of
the placebo group (slope = –1.850, r = 0.878; Fig-
ure 2B) was steeper than the TENS & Ex group
(slope = –1.176, r = 0.809; Figure 2d) (p = 0.037).
The regression line of the TENS group was
Table 2 Summary of the intensity of pain as measured by VAS scores across sessions
Group
a
Session TENS Placebo Exercise TENS & Ex
1 Before 100.0 ± 0 100.0 ± 0 100.0 ± 0 100.0 ± 0
After 64.1 ± 40.7 84.5 ± 39.6 122.0 ± 101 69.0 ± 46.9
Difference
b
35.9 ± 40.7 15.5 ± 39.6 –21.6 ± 101 31.0 ± 46.9
10 Before 68.4 ± 36.1 69.6 ± 42.9 96.4 ± 112 76.0 ± 61.6
After 60.5 ± 35.0 72.2 ± 70.7 87.3 ± 86.0 67.8 ± 51.7
Difference
b
7.9 ± 16.8 –2.7 ± 33.4 9.1 ± 40.7 8.2 ± 18.0
20 Before 54.1 ± 34.3 56.7 ± 36.3 70.7 ± 59.7 70.6 ± 72.7
After 42.2 ± 27.0 50.4 ± 42.4 63.2 ± 64.0 55.6 ± 50.6
Difference
b
11.9 ± 16.1 6.2 ± 25.4 7.6 ± 48.3 15.0 ± 27.7
Follow-up Before 51.5 ± 32.9 67.9 ± 78.6 93.2 ± 111 63.0 ± 55.8
After 43.7 ± 30.3 48.6 ± 42.2 95.2 ± 118 61.1 ± 57.9
Difference
b
7.8 ± 10.6 19.3 ± 2.9 –42.0 ± 20.1 1.9 ± 10.2
p-value ‘session’
c
p = 0.000 p = 0.034 p = 0.407 p = 0.074
a
’BMI’ was not a signicant covariate (p = 0.244). Unadjusted means and standard deviations are therefore presented.
Values are mean ± SD.
b
Difference = before – after.
c
Signicant interaction occurred between ‘group’ and ‘session’. Hence, subsequent analysis was carried out separately
for each group, and each session.
TENS for OA knee pain 755
Figure 1 The inuence of the four treatment protocols on the VAS scores in (a) session 1, (b) session 10, (c) session 20 and (d) the follow-up
session. Each data point represents the group mean of the VAS scores normalized with respect to the control value. In session 1, the post-treatment
VAS scores for each group tended to be lower than the pre-treatment scores except for the exercise group (p = 0.055). The change in the pre- and
post-treatment pain scores reached signicant between-group difference (p = 0.031). By session 20, the TENS group tended to show the lowest VAS
score among the four groups.
(a) Session 1 (b) Session 10
(c) Session 20 (d) Follow-up
756 GLY Cheing et al.
Figure 2 The regression lines of the daily recording of the pre-stimulation VAS scores over the 20 treatment sessions. Regression analysis showed
that the slope of the regression lines of each of the four groups was signicantly different (p = 0.000). The slope of the regression line for (a) the
TENS group (slope = –2.415, r = 0.943) and (c) the exercise group (slope = –2.625, r = 0.935) was similar (p = 0.565), and was steeper than the other
two groups. The slope of (b) the placebo group (slope = –1.850, r = 0.878) was steeper than that of (d) the TENS & Ex group (slope = –1.176,
r = 0.809) (p = 0.037).
(a) TENS group (b) Placebo group
(c) Exercise group
(d) TENS & Ex group
TENS for OA knee pain 757
has been shown to increase the release of
endogenous opioids, more specically Met-
enkephalin-Arg-Phe and dynorphin A respec-
tively in the cerebrospinal uid (CSF) of human
subjects.
24
The gradual onset of TENS analgesia
could thus be explained by the time lag observed
in the release of the endogenous opioids. The
gradual offset could be due to the well-known
prolonged effects of these opioid substances
before decaying. Moreover, Dubuisson
25
sug-
gested that the gradual offset of dorsal column
analgesia may be due to the prolonged after-dis-
charges in the cells of the supercial dorsal horn
cells.
Romita and colleagues
10
indicated that intense
peripheral electrical stimulation produced persis-
tent inhibition on the withdrawal reex in rats,
which lasted for more than an hour after the stim-
ulation. However, these investigators applied
high-intensity electrical stimulation (20 times
the sensory threshold) for 20 minutes, which
recruited probably Ad bres in rats. In the pre-
sent study, we applied lower intensity (3–4 times
the sensory threshold) but more prolonged stim-
ulation to human subjects (see also ref. 26). Nev-
ertheless, both types of stimulation patterns
appeared to produce a gradual onset and pro-
longed offset of stimulation-produced analgesia.
The inuence of the four-week treatment
programme on pain intensity
The linear regression lines of the pre-stimula-
tion VAS scores were plotted against treatment
sessions. There were signicant changes in the
VAS scores with in each of the four groups (all
p = 0.000). However, each group performed dif-
ferently between session 20 and the follow-up
session. Specically, the VAS scores were main-
tained in the TENS group and the TENS & Ex
group, but signicantly increased in the placebo
group and the exercise group. This indicated that
the carryover effect of real TENS was longer
than that of the placebo stimulation, or exercise
alone.
Our previous studies demonstrated that two
weeks of daily application of TENS produced
cumulative inhibition on experimental pain in the
normal subjects (p < 0.01),
27
as well as on chronic
low back pain in the patients (p < 0.01).
12
In both
studies, the antinociceptive effect of TENS was
signicantly greater than that of placebo stimu-
lation (all p < 0.05). Zizic and collaborators later
examined the effectiveness of repeated electrical
stimulation on knee pain.
28
They showed that
repeated applications of pulsed electrical stimu-
lation, 6–10 hours/day for four weeks, produced
improvements in knee pain, knee exion and
knee function, and reduced the duration of morn-
ing stiffness for patients with OA knee. There
was a 31.3% pain reduction as measured by VAS
in the treatment group, but only 19.01% for the
placebo group. The present study found a 48.5%
reduction in the TENS group and 43.3% in the
placebo group after four weeks of treatment.
Even though we used only 60 minutes of TENS,
compared with 6–10 hours/day in Zizic’s study,
28
we found a greater analgesic effect but also a
greater placebo effect.
Why did we nd a greater placebo effect as
compared with that of our previous studies on
experimental pain
11
or low back pain;
12
or of
Zizic and colleagues’ study
28
on OA knee pain?
Placebo effect
It was suggested that various factors such as
age, diagnosis, study design, therapist and patient
relationship or cultural differences could con-
tribute to the extent of placebo response. The
placebo effect in this study seems to be stronger
than that reported in our previous studies, which
used a similar treatment protocol and study
design to examine experimental pain in young
normal subjects or chronic low back pain patients
in the patients in Canada.
11,12
Clinical pain could
be more susceptible to placebo effect than
experimental pain.
29
Alternatively, the stronger
Clinical messages
A single treatment session of TENS or
TENS & Ex produced signicantly greater
pain reduction than exercise alone.
Over the four-week treatment period, vari-
ous degree of pain reduction was found in
different groups, but the four treatment pro-
tocols did not show signicant between-
group difference at the end of the study
period.
758 GLY Cheing et al.
the situation. In a companion study, we found
that a four-week exercise programme produced a
signicant increase in isometric peak torque of
knee extensor (an overall of 20.9% gain in the
exercise group; 26.6% gain in the TENS and
exercise group). Now, signicant gain in knee
muscle strength could improve the stability of the
knee joint.
14
With less stress and strain on the
joint capsule where the nociceptors are located,
less pain could be triggered by movement. The
shock-absorbing potential of the muscles around
the knee joint could also be increased. Therefore,
although the introduction of a new exercise
regime may initially increase the knee pain, a sus-
tained exercise training protocol can reduce knee
pain. In addition to the reduction of knee pain,
appropriate exercise could help to maintain bone
mineral content and ultrastructure, therefore pre-
serving the compliance of the subchondral bone.
From session 20 to the follow-up, there
appeared to be some detraining effect and the
strength of the quadriceps could be reduced. The
exercise group reported an increase in knee
pain by 22.5%. Note that such a return of knee
pain was not found in the TENS & Ex
group. Therefore, the addition of TENS to exer-
cise training could alleviate an increase in knee
pain at the beginning of exercise training.
Also, a four-week TENS & Ex treatment tended
to produce a longer carryover effect on pain
relief than exercise alone. It lasted for up to four
weeks after the termination of the treatment
period.
Conclusion
A single treatment session of TENS or TENS &
Ex produced signicantly greater pain reduction
than the exercise group. Over the four-week
treatment period, various degree of pain reduc-
tion was found in the different groups, but the
four treatment protocols did not show signicant
between-group difference at the end of the treat-
ment period, at least with in the patient sample
studied. It was interesting to note that isometric
exercise training temporarily increased knee pain
during the initial 2–3 treatment sessions, but
reduced pain below the baseline value from ses-
sion 4 to session 20. However, the pain reduction
placebo effect may be attributable to cultural fac-
tors.
30,31
For instance, Johnson and Din
31
demon-
strated that the effect of placebo TENS produced
a signicantly greater increase in cold-pain
threshold and a signicantly greater reduction in
pain intensity rating in Asian subjects than in
Caucasian subjects.
Exercise and OA knee pain
The inuence of the isometric exercise training
on knee pain varied during the course of treat-
ment. During the rst three sessions, exercise
training actually increased knee pain by 20%.
From session 4 to session 20, exercise reduced
knee pain to 70.7%. From session 20 to the fol-
low-up session, the cessation of exercise training
resulted in a return of knee pain to 93.2% of the
baseline value.
During the rst three sessions, quadriceps con-
tractions could have increased the compression
force on the knee joints thereby increasing knee
pain. The new exercise regime might have pro-
duced excessive stretch on the joint capsule or
periarticular structures where the nociceptors are
located.
32
There are substance P bres around the
knee joint including fat pads, periosteum and
subchondral bone.
33
Mechanical or chemical exci-
tation of the periarticular nociceptors located in
various soft tissues could explain the increase in
osteoarthritic knee pain at this stage. Note that
such an increase in pain during the rst few ses-
sions were not reported by subjects in the TENS
& Ex group.
From session 4 onwards, most patients
appeared to have adapted to the exercise train-
ing, which could explain the gradual reduction of
pain observed. The quadriceps muscles are
important in supporting a exed knee, and play
an important role in the stability of the knee
joint. To compensate for quadriceps deciency,
patients with OA knee tend to avoid exing their
knees when they walk.
34
Unfortunately, walking
with a rigid limb will reduce the shock-absorbing
effect of the knee joint. The impact of the body
weight will thus be transmitted directly from the
femur to the tibia with out any muscular cush-
ioning effect. This may further irritate the noci-
ceptors around the knee joint and increase knee
pain.
However, strengthening exercise can reverse
TENS for OA knee pain 759
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