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Walking Exercise Simultaneously Combined With Neuromuscular Electrical Stimulation of Antagonists Resistance Improved Muscle Strength, Physical Function, and Knee Pain in Symptomatic Knee Osteoarthritis: A Single-Arm Study

Matsuse, Hiroo1; Hashida, Ryuki1; Takano, Yoshio2; Omoto, Masayuki1; Nago, Takeshi1; Bekki, Masafumi1; Shiba, Naoto1

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Journal of Strength and Conditioning Research: January 2017 - Volume 31 - Issue 1 - p 171-180
doi: 10.1519/JSC.0000000000001463
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Abstract

Introduction

In the developed world, osteoarthritis is the most common articular disease that develops as one ages (15,28). Knee osteoarthritis (KOA) is the most representative osteoarthritis, and its prevalence dramatically increases after the age of 65 (28). Knee osteoarthritis is associated with pain, quadriceps weakness, swelling, instability, and decline of range of motion, physical function, and quality of life (QOL) (20,28). In particular, quadriceps weakness may contribute to incident symptomatic and progressive disease (32,33), cause functional limitations and disability (38), and increase the risk of mortality (23). Anxiety that the increased physical activity causes more knee pain produces the vicious circle. The limited physical activity decreases muscle strength, and the decreased muscle strength limits physical activity. Moreover, physical activity is associated with obesity, cardiovascular heart disease, type 2 diabetes mellitus, and the age-related diseases dementia and Alzheimer's disease (30). Increasing not only muscle strength but also physical activity is required to cut off this vicious circle.

Strength training is recommended for improving physical function and pain for KOA (7,10,14). The American College of Sports Medicine and the National Strength and Conditioning Association show that the minimum resistance training intensity to achieve muscle hypertrophy and strength gain is 65% of 1 repetition maximum (1RM) (14). However, this exercise intensity is often a problem for the people with knee pain or a history of knee injury. It is also a problem for elderly people at elevated risk for symptomatic KOA (e.g., sedentary lifestyle, obesity, knee pain, knee injury, or surgery). Therefore strength training is generally used at a light to moderate intensity for KOA (around 60% of 1RM).

Walking is one of the most basic moderate-intensity physical activity for elderly people. On the other hand, walking training is one of the most basic moderate-intensity aerobic exercise. Aerobic exercise is also recommended for improving physical function and relieving KOA pain (14), because there is a lower risk of injury or pain exacerbation than at a higher intensity. Moreover, aerobic exercise activities with low joint stress including walking, cycling, or swimming are appropriate for arthritis, and high-impact activities such as running and stair climbing and those with stop-and-go actions are not recommended for lower body arthritis (14). There are a few studies to show the effect of muscle strength in elderly people or patients with KOA (12,36). However, walking training is a dynamic physical activity with an intensity sufficient to improve aerobic capacity generally (21). A walking training method that can strengthen the muscle of the knee is desirable for elderly people with knee pain.

Neuromuscular electrical stimulation (NMES) is one of the more effective training methods, even though the exercise intensity is relatively low, and it is widely used to lessen immobilization-associated muscle atrophy, to strengthen muscles, and to improve function in people with neuromuscular disabilities. For patients with KOA, many studies showed that NMES is effective for quadriceps strength, knee pain, and physical function (6). Moreover, the combined application of electrical stimulation (ES) and volitional contractions (VC) is said to be more effective than ES or VC alone (26). A hybrid training system (HTS), which resists the motion of a volitionally contracting agonist muscle with force generated by its electrically stimulated antagonist, was developed as a technique to combine the application of ES and VC (9,18,39). Yanagi et al. (39) showed that HTS with an elbow bending exercise could increase elbow extension torque by about 30% and proximal upper extremity muscle cross-sectional areas by about 15% over a 12-week period. Matsuse et al. (18) reported that elbow flexion torque had increased about 56% and the proximal upper extremity muscle cross-sectional areas had increased about 10% as a result of HTS over an 8-week period, and these increases were better than for isotonic weight training and NMES. Iwasaki et al. studied the efficacy of HTS compared with conventional weight training with 15RM loads for increasing muscle strength around the knee at both slow and fast joint speeds (at 30 and 180° per second), and reported that HTS is comparable with weight training with the exception of high-speed contractions (HTS +25–28%, WT +24–33%, at 30° per second) (9). In elderly people, HTS has been shown to produce improvements in muscle strength by about 40% and mass by about 10% which is as good as or better than those achieved with a knee flexion machine with 30% of the maximum voluntary contraction (9). One of the major advantages of HTS is that ES can be combined with voluntary activity simultaneously. We showed that HTS could be combined with aerobic cycling exercise simultaneously (17). Therefore, this time, we developed the novel training method in which HTS could be performed simultaneously during a walking (HTS with walking [HTSW]) to improve physical function and muscle strength. The HTSW training method is unprecedented, and its effect on KOA has never been reported. We hypothesized that HTSW could not only increase muscle strength but also increase aerobic capacity, improve physical function, and relieve knee pain for patients with KOA. This study was a preliminary research to develop the unprecedented novel exercise method.

The purpose of the present study was to assess the efficacy of a 12-week HTSW program to improve muscle strength, aerobic capacity, physical function, knee pain, and QOL in the elderly people with KOA. The primary outcome was change in maximal isokinetic knee extensor torque.

Methods

Experiment Approach to the Problem

A single-arm study was conducted to assess the effects of HTSW on the knee muscle strength in elderly people with KOA. All the subjects belonged to 1 group. The subjects voluntarily performed a 12-week training program with HTSW. The subjects performed a 30-minute walking training session 3 times per week (a total of 36 sessions). Each session was separated by an interval of at least 48 hours. The exercise intensity was set at 3–4 (from moderate to not very intense) using Borg's modified scale. The maximal volitional isokinetic knee torques were measured to assess changes in the muscle strength before and after HTSW. Moreover, physical function, knee pain, and QOL were measured. The subjects were instructed to avoid medical treatment for KOA and any other exercise during the research period, and to carry on their ordinary daily lives, and were not controlled about a daily diet or supplement.

Subjects

The Ethics Committee of Kurume University approved the study protocol (No. 13006). Subjects (60 years or older) who experienced knee pain (unilateral or bilateral) during daily walking were recruited by posters displayed in local community centers (Okawa City, Fukuoka, Japan) from October 6, 2014, to October 31, 2014. The subjects were given oral and written explanations of the study involving the objective of the training method and its risks, and then they read and signed an informed consent form before participation. The study was conducted in adherence to the standards of the Declaration of Helsinki (2008 version) and following the European Community's guidelines for Good Clinical Practice (111/3976/88 of July 1990) as well as the Spanish legal framework for clinical research on humans (Real Decreto 561/1993 on 8 clinical trials). Of a total of 12 people who applied, 1 withdrew voluntarily after the objective of the research was explained to them. The exclusion criteria for the training intervention were cases of acute orthopedic problems besides the knee joint, lower limb surgery, cerebrovascular or heart disease within the past year, dementia, administration of treatment (such as physical therapy, intra-articular injection) or analgesic drugs including external medicine, or participation in regular resistance or aerobic training in the last 3 months before this study. Subjects underwent medical and musculoskeletal examinations conducted by an orthopedic specialist. Standing anteroposterior and lateral knee radiographs were evaluated for each patient for classifying the severity of KOA according to the Kellgren and Lawrence grading system (grade 0–4). Eleven subjects (1 man and 10 women) participated in this study (age 74.0 ± 8.5 years, weight 52.2 ± 12.8 kg, height 151.2 ± 7.1 cm, and body mass index 22.6 ± 3.2, all values represented as mean ± SD). Subjects were evaluated and intervened from November 1, 2014, to March 31, 2015.

Procedures

Hybrid Training System During Walking Protocol

During the walking training, both lower extremities were stimulated using HTS in response to the gait phase of each foot. Electrical stimulation of the quadriceps started gradually from just before heel contact and stopped with heel off (Figure 1). Conversely, ES of the hamstring started gradually from just before heel off and ended with heel contact (Figure 1). The subjects performed the walking training with each stride. During the training, an assistant was always present to provide guidance and monitoring to ensure that the training was performed safely and properly. If the subjects were tired and could not continue walking, they were instructed to take breaks at will. The joint range of motion during walking was not prescribed. Every training began and ended with a 5-minute stretching session supervised by an assistant.

Figure 1.
Figure 1.:
Schematic model of HTSW. During the walking both lower extremities are electrically stimulated in response to the gait phase of each foot. The electrical stimulation of quadriceps is gradually started from just before heel contact and discontinues with heel off. Conversely, the electrical stimulation of the hamstring is gradually started from just before heel off and discontinues with heel contact. The result is that both muscles are trained electrically during walking exercise.

Electrical Stimulation Protocol

Electrical stimulation device: The ES device which has been described previously (9,18) was remodeled for this study by Panasonic Corporation (Home Appliances Development Center Corporate Engineering Division, Appliances Company Panasonic Corporation, 2-3-1-2, Noji-higashi, Kusatsu City, Shiga, Japan). This device is not yet approved by the US Food and Drug Administration, and is not labeled for the use under discussion and the product is still under investigation. The device consists of a custom-designed waveform generator capable of delivering stimulating signals with unique frequencies and waveforms to as many as 4 pairs of electrodes. Acceleration sensors as a joint motion sensor (EWTS9PD; Home Appliances Development Center Corporate Engineering Division, Appliances Company Panasonic Corporation) was placed on the front of each leg 88 mm above the patellar edge. It analyzed the algorithm of each walking pattern and stimulated the antagonist of the motion of each bilateral knee joint during walking. Pairs of 5 × 12-cm low-impedance gel-coated silver fiber electrodes (Nihon Medix Co, Minami-hanashima, Matsudo-shi, Chiba-ken, Japan) were placed to widely cover each motor point of the quadriceps and hamstrings. They were built in to a quick-drying training suit, which the subjects could put on easily.

Stimulation Parameters

The stimulation waveform used in this study consists of a 5,000 Hz carrier frequency with a pulse width of 200 μs modulated at 40 Hz (2.4 milliseconds on, and 22.6 milliseconds off) to deliver a rectangular biphasic pulse. The electrical stimulator gives constant voltage stimulus to the human body (regulated voltage). It has a stimulus pattern with interlock and a limiter for safety. Therefore, the effective current is interlocked at 23mArms when using 500 Ω of the human body equivalent circuit, and the peak voltage and current is limited to under 80 V. Stimulation intensities were redetermined every 2 weeks during the training period. We regulated the stimulation intensity so that the exercise intensities were adjusted to 80% of the maximum comfortable intensity, which successfully improved the muscle strength and mass without causing pain or numbness (35). At these ES intensities, all subjects were able to walk for 30 minutes.

Evaluations

All the evaluations were performed with a blind assessor 1 week before and after the training.

Maximal Isokinetic Torque of Knee Extension Measurement

All the evaluations were performed by 1 physical therapist and 3 assistants. The maximal volitional isokinetic knee extension/flexion torques (KET/KFT) were measured at angular velocities of 60o per second with the Biodex System3-PRO (Biodex Medical Systems, Inc., Shirely, NY, USA). During the strength measurements, the subjects were seated on the Biodex in an upright position. Velcro belts were applied to fixate the trunk and thigh. The seat was adjusted to the same position at each evaluation. Each session began by establishing that the subjects could move their lower extremity comfortably throughout the full 10–100° arc of the exercise range. They then performed 3 practice contractions in the direction and at the speed to be tested. A measurement session consisted of 3 sets separated by 3 minutes after the practice; the 3 measurements from the nondominant lower extremities were pooled, and the mean adjusted by each body weight (kilograms) was used for statistical analysis. The interclass correlation coefficient (ICC) of measurements for KET/KFT was 0.93/0.86 when we measured healthy subjects at our institution.

Muscle Volume of Quadriceps Femoris Muscle Measurement

Ultrasonographic evaluations were performed with an 8-MHz linearprobe (SSA-510A famio5; Toshiba Medical Systems Corporation, Tochigi, Japan) by the same physiatrist, who was blinded to the exercise groups. Measurements were taken on the rectus femoris muscle of the nondominant lower extremities. Subjects were positioned supine with their legs extended and their muscles relaxed. A water-soluble transmission gel was applied to the transducer to aid acoustic coupling and also to eliminate deformations of muscle that can occur when pressure is directly applied to the skin. Images were obtained at the levels of 10 and 15 cm above the patellar superior border. When imaging for pennation angle and fascicle length were performed, the probe was held with a light touch so as not to cause any muscle deformation. Muscle thickness was defined as the distance between the deeper and upper aponeurosis (10MV, 15MV). The ICC of measurements for muscle volume (MV) was 0.68 when we measured healthy subjects at our institution.

One-Leg Standing Test

The one-leg standing test (OST) is conducted to evaluate the balance function. The subjects were measured according to the length of time they were able to stand on their nondominant lower limb without support with eyes open to assess postural steadiness in a static position (11). The evaluation was performed 2 times, and the scores of the 2 times were averaged for analysis. The ICC of measurements for OST was 0.98 when we measured healthy subjects at our institution.

Functional Reach Test

A functional reach test (FRT) was conducted to evaluate balance function (4). The subjects stood straight with 1 arm stretched out in front at 90° of shoulder flexion with wrists and fingers straight and palms facing down. The starting position was measured at the tip of the middle finger. The subjects were instructed to reach their hand as far forward as possible without taking a step, and the position of the tip of the middle finger at the end of the reach was recorded. The distance between the starting point and the end point was the reach distance automatically measured in centimeters. The subjects performed 1 trial with their dominant hand after 1 practice. The ICC of measurements for FRT was 0.95 when we measured healthy subjects at our institution.

Ten-Meter Maximal Gait Time

For the evaluation of the 10-meter maximal gait time (10 MW), 2 m were added to allow for acceleration before and deceleration after the 10-m gait. The maximal gait time for the 10-m gait was measured to evaluate the gait speed. The gait speed is a functional assessment tool to show individual activity of daily living or physical capacity (27). The subjects were instructed to walk as fast as possible. The evaluation was performed 2 times, and the scores of the 2 trials were averaged for analysis. The ICC of measurements for 10 MW was 0.98 when we measured healthy subjects at our institution.

Timed Up and Go Test

The timed up and go (TUG) test was conducted to evaluate functional mobility (29). The subjects were measured according to the time it took to rise from a standard chair (46 cm seat height), walk a distance of 3 m, walk back to the chair, and sit down. The evaluation was performed 2 times, and the scores of the 2 trials were averaged for analysis. The ICC of measurements for TUG was 0.91 when we measured healthy subjects at our institution.

Six-Minute Walking Test

The evaluation was performed in a 25-m oval walking course at an indoor sports center. The subjects walked at a regular walking speed for 6 minutes, and their walking distances were measured to evaluate aerobic capacity. Six minutes' walk is a good physiologic health predictor (16). Before the test, the subjects rested for 30 minutes, and their blood pressure and pulse were taken. The evaluation was suspended if the subjects were unable to walk or did not feel well. The evaluators did not support them during their walk. The evaluation was performed once. The ICC of measurements for 6-minute walking test (6MWT) was 0.81 when we measured healthy subjects at our institution.

Japan Knee Osteoarthritis Measure

The subjects answered the self-completion questionnaire: Japan Knee Osteoarthritis Measure (JKOM). The Japan Knee Osteoarthritis Measure is a self-administered, disease-specific health measure for Japanese patients with KOA (1).

Visual Analog Scale

Knee pain was evaluated using a Visual Analog Scale (VAS) of 10 cm from no pain to the worst possible pain.

Statistical Analyses

The measurements of KET/KFT, MV, OST, FRT, 10 MT, TUG, and 6MWT in the same sample population were repeatedly measured to test measurement reliability. The ICC was calculated using a 2-way mixed single measures for absolute agreement between the repeated measurements.

All variables are presented as means and SD. Values for KET/KFT, MV, OST, FRT, 10 MT, TUG, 6MWT, VAS, and JKOM were assessed using a paired t-test to compare the differences between pretraining and posttraining. Statistical power using a paired t-test for the primary outcome using total 12 sample size was 0.8. Moreover, we calculated the effect size Cohen's d to know the strength of association between intervention and change of KET of the primary outcome after a paired t-test (Cohen). A paired t-test was performed using JMP Version 9.0 statistical software (SAS Institute, Inc., Cary, NC, USA) and p values ≤0.05 were considered to be statistically significant. The effect size was calculated using G*power software (version 3.0; the Department of Experimental Psychology, Heinrich-Heine-University, Düsseldorf, Germany).

Results

The grade of KOA in subjects was 1.7 ± 0.69 (1–3). All 11 subjects performed a total of 36 exercise sessions and pre/post measurements. There were no adverse events in this study.

In the primary outcome, KET significantly increased from 1.00 ± 0.30 N·m·kg−1 pretraining to 1.23 ± 0.35 N·m·kg−1 posttraining (t = 3.9084, p = 0.002921) (Figure 2A). The effect size d was 0.68, medium. In this study, the statistical power using a paired t-test was 0.94, enough strong.

Figure 2.
Figure 2.:
Changes in knee extension/flexion torque and muscle volume. Changes in knee extension torque (A), knee flexion torque (B), muscle volume at the level of 10 cm and 15 cm above the patellar superior border (C), and muscle volume at the level of 15 cm above the patellar superior border (D). Pre = pretraining; post = posttraining.

The KFT significantly increased from 0.65 ± 0.18 N·m·kg−1 pretraining to 0.78 ± 0.17 N·m·kg−1 posttraining (t = 3.223, p = 0.009128) (Figure 2B).

The 10MV significantly increased from 9.00 ± 2.84 mm pretraining to 10.37 ± 3.16 mm at the end of training (t = 2.3149, p = 0.04315) (Figure 2C). The 15MV significantly increased from 13.80 ± 3.22 mm pretraining to 14.59 ± 3.01 mm at the end of training (t = 3.0393, p = 0.01248) (Figure 2D).

The OST tended to increase from 5.27 ± 6.31 seconds pretraining to 11.55 ± 13.65 seconds at the end of training (t = 2.1702, p = 0.05515) (Figure 3A).

Figure 3.
Figure 3.:
Changes in physical functions. Changes in one-leg standing test (A), functional reach test (B), 10-meter maximal gait time (C), timed up and go test (D), and 6-minute walking test (E). Pre = pretraining; post = posttraining.

The FRT significantly increased from 26.32 ± 7.58 cm pretraining to 28.89 ± 7.48 cm at the end of training (t = 2.4552, p = 0.03395) (Figure 3B).

The 10 MT significantly decreased from 6.56 ± 1.44 seconds pretraining to 5.72 ± 0.99 seconds at the end of training (t = 2.8847, p = 0.01625) (Figure 3C).

The TUG significantly decreased from 9.74 ± 2.12 seconds pretraining to 7.28 ± 2.71 seconds at the end of training (t = 6.0429, p = 0.0001248) (Figure 3D).

The 6MWT significantly decreased from 423.57 ± 107.37 m pretraining to 481.79 ± 97.04 m at the end of training (t = 2.6614, p = 0.02384) (Figure 3E).

The JKOM score improved from 26.73 ± 18.30 pretraining to 17.18 ± 14.02 at the end of training (t = 3.9927, p = 0.002548) (Figure 4A).

Figure 4.
Figure 4.:
Changes in Japan Knee Osteoarthritis Measure and Visual Analog Scale concerning knee pain. Changes in Japan Knee Osteoarthritis Measure (A) and visual analog scale of knee pain (B). Pre = pretraining; post = posttraining.

The VAS score significantly decreased from 35.36 ± 22.59 pretraining to 16.45 ± 19.73 at the end of training (t = 3.1432, p = 0.01045) (Figure 4B). The VAS score remained unchanged in 2 of the 11 subjects, and decreased in 9.

Discussion

The findings of this study show that the simultaneously application of ES to walking training using the HTS technique (HTSW) can increase the muscle strength, and also improve knee pain, physical function, and QOL for people with KOA. Hybrid training system during walking could be a novel effective exercise method for people with KOA.

Aerobic exercise is recommended for reducing pain and improving physical function for KOA (14). In particular, aerobic exercise activities with low joint stress are appropriate including walking, cycling, or swimming for arthritis (14). Walking exercise is the most basic aerobic exercise. Furthermore, walking exercise is easy for elderly people or clinical patients who have difficulty in performing moderate- or severe-intensity exercise. The intensity of aerobic exercise is light to moderate, because there is lower risk of injury or pain exacerbation than at a higher intensity. Aerobic exercise is commonly performed to improve aerobic capacity or physical activity. Therefore there were many studies about the effect of walking training on aerobic capacity or physical activity, but not many studies about the effect of walking training on muscle strength. Kubo et al. (12) reported that isometric knee flexion strength increased by 19.6% after a progressive program (exercise intensity, comfortable pace, exercise time of 15–40 minutes, and exercise frequency of 3–4 times per week) of walking for 6 months in elderly people, but not extensor muscle. On the other hand, Nemoto et al. (22) reported that in elderly people isometric knee extension strength increased by 7% after walking training at approximately 50% of their peak aerobic capacity, 8000 steps or more per day, for 4 or more days per week for 10 weeks. However, Ozaki et al. reported that there were no changes in either isometric knee extension strength or flexion strength after walking training for 20 minutes by treadmill at an exercise intensity of 45% of heart rate reserve, 4 days per week, for 10 weeks (25). In this way, the effect of walking training on muscle strength is influenced by the training program such as intensity, duration, and frequency (25). In this study, HTSW increased the isokinetic knee extension strength by 23% and isokinetic knee flexion strength by 20% in addition to improvement of the aerobic capacity; although exercise intensity was low (at comfortable pace), exercise frequency was not much (3 days per week), and exercise duration was not long (for 12 weeks). However, physical activity level is an important factor affecting the exercise effect of walking training on muscle strength (25). Physical activity level of people with KOA is low (28). Moreover, guideline shows that aerobic walking training improves muscle strength in the management of osteoarthritis (19). Therefore, even walking training without HTS might have an enough effect on muscle strength. As this study does not have a control group (without HTS), we cannot prove the advantage of HTSW strongly. Moreover, it is difficult to compare the effect of HTSW with other studies because there were few studies about the effect of walking training on muscle strength in the conservative treatment of KOA. The only study shows that walking program that had set individual daily step goals with arthritis education for 24 weeks increased the isokinetic right knee extension strength by 21–30% (the effect size was 0.50–0.66) in those aged 60 and older with symptomatic KOA (36). The increment was either greater than or the same as that we obtained. However, the exercise duration was more than double of our study. Moreover, time of fast walk was not changed, which seemed to show that the effect was limited. On the other hand, HTSW could improve muscle strength, all physical function, knee pain, and QOL. We may make an effect of the walking training more certain by combining HTS. However, weight-bearing exercise such as walking might aggravate knee pain for people with painful KOA (2). Tanaka et al. (37) showed in their systematic review that non–weight-bearing strengthening exercise is more highly recommended than weight-bearing strengthening or aerobic exercise to reduce exercise pain for patients with KOA. In this study, HTSW for 12 weeks did not cause any adverse events. However, a more long-term exercise program with HTSW may aggravate knee pain. Additional long-term prospective studies are needed to determine this.

On the other hand, strength training is also recommended for improving pain, physical function, and physical activity for patients with KOA in the same way as a walking training (13,14). Quadriceps weakness is commonly found together with KOA, and this may contribute to incident symptomatic and progressive disease (32,33). Lower limb muscles, particularly the quadriceps, influence physical function (e.g., gait speed and body balance) (8). Consequently, quadriceps strengthening leads to the improvement of physical function and QOL for KOA (5). In general, strength training intensity is used at light to moderate, around 60% of 1RM or 50–100% of 10RM. We developed HTS as a method of strength training using electrically stimulated eccentric antagonist muscle (39). The eccentric contractions that HTS produce are more conducive to hypertrophy than concentric contractions (3). Hybrid training system could increase muscle strength and mass even in low-intensity exercise (15–20% of 1RM) (9). Takano et al. (35) reported that HTS improved the muscle strength (knee extension +42%) and hypertrophy (rectus femoris muscle +17%), and was as effective as strength training with a big commercial training machine at 40% of 1RM in elderly people. In this study, HTSW could increase knee muscle strength (knee extension +22%, knee flexion +29%) and MV (rectus femoris muscle +8%). In the systematic review, the mean effect size of high- or low-intensity strength training on knee extension strength in people with KOA is 0.76 or 0.47, respectively (40). The effect size of HTSW on knee extension strength was 0.68. Therefore HTSW may be an effective method as strength training even during aerobic walking exercise at low intensity. Both aerobic exercise and strength exercise are equally effective in the treatment of KOA (31). However, strength training is more effective in strength gain and muscle mass than aerobic exercise. Higher quadriceps strength resulted in protection against development of incident symptomatic knee OA (the combination of radiographic knee OA with daily symptoms) (33), as well as progression of tibiofemoral joint space narrowing (32). Furthermore, the quadriceps strength influences physical function (e.g., gait speed and body balance) (8). Therefore it seems that HTSW, which is able to increase muscle strength simultaneously with aerobic exercise, may be appropriate for the treatment of KOA.

HTSW has a few advantages of using NMES against antagonist. Neuromuscular electrical stimulation is effective for quadriceps strength, knee pain, and physical function for people with KOA (6). The combined application of NMES and VC is said to be more effective than ES or VC alone (26). In the only trial to evaluate the effects of exercise with NMES to improve QOL for people with KOA, subjects experienced improvement in knee pain and physical function over the duration of the trial (34). However, there were no differences between the combined method and conventional exercise alone. This study was not simultaneous with VC. On the other hand, in HTS, NMES is simultaneous with VC (39). We showed that HTS could combine with aerobic cycling exercise simultaneously in young men and could create a workload during aerobic exercise (17,24). In this study, HTS could be combined with aerobic walking exercise. Electrical stimulation of HTSW stimulated the antagonist muscle of the knee joint bending motion during walking (quadriceps and hamstrings alternately). This added load by HTS would enhance an exercise effect more. Then the antagonist muscles have the role of maintaining joint stability. Hybrid training system might contribute to the stabilization of the knee joint. Moreover, NMES is effective for pain relief (6). Indeed, in this study, all the subjects were able to perform the walking exercise for 30 minutes with light or no pain. Therefore, we suppose that HTSW would be effective even for patients with symptomatic KOA.

There are a few potential limitations of this study. The number of participants were few and the gender ratio was uneven, although statistical power was good. The main purpose of this study was to be a pilot study for developing the novel technique of HTSW. However, this study could not show superiority when compared with other exercises because of it being a single-arm study. A long-term randomized control study using HTSW is necessary to evaluate its effectiveness for the treatment of KOA when compared with conventional training methods. The grades of KOA in the participants of this study were not severe, and there were no people with severe KOA. This study could not show whether HTSW was effective for the treatment of severe KOA.

Practical Applications

This is a novel training method to stimulate antagonist electrically as resistance exercise during aerobic walking exercise (HTSW). The results show that HTSW could improve knee muscle strength, physical function, knee pain, and QOL for people with KOA.

Note: Hiroo Matsuse, Ryuki Hashida, Masayuki Omoto, Takeshi Nago, Masafumi Bekki, and Naoto Shiba are now with the Department of Orthopedics, Kurume University School of Medicine, Asahimachi, Kurume, Fukuoka 8300011, Japan.

Note: Yoshio Takano is now with the Division of Physical Therapy, Fukuoka International University of Health and Welfare, Okawa, Fukuoka 8318501, Japan.

Acknowledgments

Financial Disclosures: The authors have declared that competing interests exist by Panasonic Corporation. However, the staffs of Panasonic Corporation were not involved in this study enforcement and analysis of results at all. We studied to develop the novel effective training method for the treatment of the people with knee osteoarthritis. This study was supported by Panasonic Corporation $45,000. These funds were used for the subject's compensation and the stimulator purchase primarily. This funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Author Contributions: Conceived and designed the experiments: H. Matsuse and R. Hashida; performed the experiments: H. Matsuse, Y. Takano, M. Omoto, T, Nago, and M. Bekki; analyzed the data: H. Matsuse, Y. Takano, and N. Shiba; contributed reagents/materials/analysis tools: N. Shiba; wrote the paper: H. Matsuse. H. Matsuse and R. Hashida contributed equally to this work.

Institutional Review: The study was designed in accordance with the ethical standards of the Helsinki Declaration of 1975 and received the approval of the Ethics Committee of Kurume University (13006).

All procedures were fully explained to the participants who gave their written informed consent to participate.

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Keywords:

aerobic exercise; elderly people; rehabilitation; resistance exercise

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