Does four weeks of TENS and/or isometric exercise produce cumulative reduction of osteoarthritic knee pain?

Transcription

1 Clinical Rehabilitation 2002; 1 6: 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 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 strati ed 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 signi cant cumulative reduction 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 regression 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 signi cant betweengroup 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. Address for correspondence: Professor Christina WY Hui- Chan, Department of Rehabilitation Sciences, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. rschris@polyu.edu.hk Arnold / cr549oa

2 750 GLY Cheing et al. Introduction Osteoathritis (OA) is a highly prevalent condition and one of its major symptoms is pain. In the United States, it was the leading cause of disability, with the direct and indirect cost of arthritis estimated to be $68.4 billion in It has been projected that 59.4 million people in the United States, or 18.2% of the population, will suffer from OA by Transcutaneous electrical nerve stimulation (TENS) is a common treatment modality for musculoskeletal pain, 3 and has been demonstrated to be effective for managing OA knee pain. 4 9 Brie y, TENS but not placebo stimulation, was found to produce a signi cant reduction in subjective pain and in medication scores. 4 Smith et al. 5 reported that the percentage of respondents experiencing pain relief was substantially greater in the TENS group (66.7%) than the placebo group (26.7%), although this difference was not signi cant due to a lack of statistical power. The percentage of respondents remained higher in the TENS group at the fourweek follow-up session. Another study showed that TENS produced a signi cantly 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 speci c treatment period. Furthermore, previous studies did not document the duration and amount of analgesic effect that could be carried over to post-treatment period by repeated applications of TENS. Such information is essential in determining the optimal treatment outcome of a given intervention. Furthermore, there were certain methodological problems in these studies. For example, the stimulation was applied by patients at home, hence the location of electrodes and the treatment 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 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 conditioning that may produce plastic changes in the central nervous system over time. 10 Cumulative antinociceptive effects of TENS on the exion re ex 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 subjective 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 disorders, whereas OA knee pain occurs in a more peripheral joint and is usually the result of degenerative changes. Therefore, the present study set out to investigate whether TENS produces cumulative 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 capsule where the nociceptors are located, to reduce knee pain during movement. Indeed, Fisher et al. 15 demonstrated that quadriceps exercise training improved muscle strength, which was associated with a 40% reduction in OA knee pain. Our previous study 16 showed that a four-week iso-

3 TENS for OA knee pain 751 metric exercise training of knee muscles produced 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 reduction than TENS or exercise alone? Therefore, the objectives of the present experiment were twofold: 1) To compare the time course and the effectiveness 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 minutes, (c) isometric exercise training for 30 minutes, (d) TENS for 60 minutes and isometric exercise training for 30 minutes 2) To examine the possible cumulative effect of repeated daily application of the four treatment protocols on OA knee pain over a fourweek period. Methods Sixty-six patients with OA knee, aged 50 75, participated 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 identi ed 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 radiographic 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 entering the study, and received no paramedical treatment 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 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 surface electrodes, 4 4 cm each, were placed on the following ve acupuncture points: spleen 9, stomach 35, extra 31, 32 and gallbladder 34 (one electrode pad covering both extra 32 and stomach 35). 5,18,19 As noted by previous investigators, the local tender points coincided with the acupuncture 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 sensory threshold. Each subject received the treatment 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 indicator lamp was lit up when the unit was switched on. However, the internal circuit had been disconnected 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

4 752 GLY Cheing et al. 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 negligible movement of the relevant joint. Patients who participated in the pilot study indicated that isometric contractions elicited less pain than isotonic 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 treatment 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 contractions 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. Brie y, subjects received 60 minutes of TENS followed by 20 minutes of rest, then ended with 30 minutes of isometric exercise training on the affected leg. 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 sensation. 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 intensity). The VAS scores were recorded daily from session 1 to session 20 and in the follow-up session. Subjects were requested to move a mechanical 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 digital output. In each session, the VAS score was measured before and 20 minutes after treatment for all groups. The VAS scores were also measured 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 follow-up session were normalized with respect to those recorded in session 1. Signi cant results were then analysed by post-hoc tests (LSD least

5 TENS for OA knee pain 753 signi cant difference). Finally, linear regression was used to analyse the changes in the daily recording of the VAS scores across the 20 treatment sessions. The level of statistical signi cance 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 fourweek treatment period. Two patients each from the placebo group and the TENS & Ex group dropped out of the study due to time con icts and medical reasons. Table 1 shows the characteristics of patients with OA knee who participated in the four-week treatment programme. No signi cant 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 signi cantly higher than that of the TENS & Ex group (p < 0.05). Therefore, 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 treatment groups recorded in the various sessions. The in uence 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-treatment pain level reached statistical signi cance (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 exercise 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 stimulation, 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 stimulation. For the exercise group, the VAS score increased to 122.0% (±101%, Table 2) after exercise. 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 minutes after stimulation. The in uence 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 Figure 1b d. Since there was signi cant interaction between group and session, subsequent analysis 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 prestimulation 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 Age 65.3 ± ± ± ± Height (cm) ± ± ± ± Weight (kg) 62.6 ± ± ± ± Body mass index (kg/m 2 ) 26.8 ± ± ± ± * Gender (female %) *As the body mass index (BMI) was signi cantly different between groups (p < 0.05), BMI was adjusted in subsequent analyses.

6 754 GLY Cheing et al. 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 ± ± ± ± 0 After 64.1 ± ± ± ± 46.9 Difference b 35.9 ± ± ± ± Before 68.4 ± ± ± ± 61.6 After 60.5 ± ± ± ± 51.7 Difference b 7.9 ± ± ± ± Before 54.1 ± ± ± ± 72.7 After 42.2 ± ± ± ± 50.6 Difference b 11.9 ± ± ± ± 27.7 Follow-up Before 51.5 ± ± ± ± 55.8 After 43.7 ± ± ± ± 57.9 Difference b 7.8 ± ± ± ± 10.2 p-value session c p = p = p = p = a BMI was not a signi cant covariate (p = 0.244). Unadjusted means and standard deviations are therefore presented. Values are mean ± SD. b Difference = before after. c Signi cant interaction occurred between group and session. Hence, subsequent analysis was carried out separately for each group, and each session. 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 signi cantly 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; Figure 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 steeper than that of the placebo group, but the between-group difference was just short of statistical signi cance (p = 0.078). From session 20 to the follow-up session, none of the patients received any treatment. Interestingly, 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 interesting 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

7 TENS for OA knee pain 755 (a) Session 1 (b) Session 10 (c) Session 20 (d) Follow-up Figure 1 The in uence 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 signi cant between-group difference (p = 0.031). By session 20, the TENS group tended to show the lowest VAS score among the four groups.

8 756 GLY Cheing et al. (a) TENS group (b) Placebo group (c) Exercise group (d) TENS & Ex group 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 signi cantly 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).

9 TENS for OA knee pain 757 Clinical messages A single treatment session of TENS or TENS & Ex produced signi cantly greater pain reduction than exercise alone. Over the four-week treatment period, various degree of pain reduction was found in different groups, but the four treatment protocols did not show signi cant betweengroup difference at the end of the study period. has been shown to increase the release of endogenous opioids, more speci cally Metenkephalin-Arg-Phe and dynorphin A respectively 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 suggested that the gradual offset of dorsal column analgesia may be due to the prolonged after-discharges in the cells of the super cial dorsal horn cells. Romita and colleagues 10 indicated that intense peripheral electrical stimulation produced persistent inhibition on the withdrawal re ex in rats, which lasted for more than an hour after the stimulation. 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 present study, we applied lower intensity (3 4 times the sensory threshold) but more prolonged stimulation to human subjects (see also ref. 26). Nevertheless, both types of stimulation patterns appeared to produce a gradual onset and prolonged offset of stimulation-produced analgesia. The in uence of the four-week treatment programme on pain intensity The linear regression lines of the pre-stimulation VAS scores were plotted against treatment sessions. There were signi cant changes in the VAS scores with in each of the four groups (all p = 0.000). However, each group performed differently between session 20 and the follow-up session. Speci cally, the VAS scores were maintained in the TENS group and the TENS & Ex group, but signi cantly 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 signi cantly greater than that of placebo stimulation (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 stimulation, 6 10 hours/day for four weeks, produced improvements in knee pain, knee exion and knee function, and reduced the duration of morning 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 contribute 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

10 758 GLY Cheing et al. placebo effect may be attributable to cultural factors. 30,31 For instance, Johnson and Din 31 demonstrated that the effect of placebo TENS produced a signi cantly greater increase in cold-pain threshold and a signi cantly greater reduction in pain intensity rating in Asian subjects than in Caucasian subjects. Exercise and OA knee pain The in uence of the isometric exercise training on knee pain varied during the course of treatment. 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 follow-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 contractions could have increased the compression force on the knee joints thereby increasing knee pain. The new exercise regime might have produced 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 excitation 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 sessions were not reported by subjects in the TENS & Ex group. From session 4 onwards, most patients appeared to have adapted to the exercise training, 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 de ciency, 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 cushioning effect. This may further irritate the nociceptors around the knee joint and increase knee pain. However, strengthening exercise can reverse the situation. In a companion study, we found that a four-week exercise programme produced a signi cant 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, signi cant 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 sustained 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 preserving 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 exercise 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 signi cantly greater pain reduction than the exercise group. Over the four-week treatment period, various degree of pain reduction was found in the different groups, but the four treatment protocols did not show signi cant between-group difference at the end of the treatment 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 session 4 to session 20. However, the pain reduction

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