INTRODUCTION
Obesity is characterized by an imbalance between energy expenditure and intake, where the energy consumed exceeds that expended, leading to an increase in body fat [1]. Obesity increases the intensity and frequency of the load on joints, which impacts knee pain [2]. The higher the body fat percentage, the greater the physical load on the knee joints, triggering the release of inflammatory substances and pain-inducing hormones from adipose tissue, which promote cartilage damage and lead to joint injury [3-5]. Additionally, obesity has been reported to place strain on the lower limbs during weight-bearing exercises, leading to cartilage and ligament damage [6]. This is considered a risk factor for developing degenerative joint arthritis in weight-bearing joints, such as the knees or hips. Previous studies have indicated that the risk of developing arthritis is nearly three times higher in individuals that are overweight or obese [7].
Weight loss is effective in improving function and reducing pain in patients with obesity and arthritis [8,9]. Sharma also found that reducing body mass index (BMI) by 2 kg/m² (approximately 5 kg of body weight) reduces the risk of developing degenerative arthritis by 50% [4]. Therefore, obesity management is crucial for preventing knee arthritis. Exercise is important not only for weight loss, but also for maintaining reduced weight, specifically lean body mass [10], with positive effects on the cardiovascular system [11-13]. The American and European Rheumatology Societies recommend exercise and weight loss for patients with obesity-related knee arthritis to reduce joint load and delay its progression [14,15]. Notably, resistance exercise has been reported to decrease the percent body fat, increase lean body mass, and improve various physiological factors and functional fitness in individuals with obesity [16]. Resistance exercise directly stimulates the secretion of growth hormone, increasing both its frequency and volume [17]. This hormone plays a crucial role in enhancing muscle strength, hypertrophy, and metabolic regulation in adults [18,19]. Previous studies suggest that resistance exercise alone can increase fat metabolism, improving obesity by increasing muscle mass and reducing body fat, thereby reducing the associated health risks [20]. It has also been reported to improve the blood lipid profiles of patients with obesity, diabetes, and cardiovascular diseases [21].
An effective exercise method for patients with obesity and knee pain is electrical muscle stimulation (EMS) training, which induces effects similar to those of resistance exercise by electrically stimulating muscles without resistance[22]. EMS allows muscle contraction without an external load, offering the potential benefits of resistance exercise [22]. Additionally, because it does not require equipment or weights, it does not strain the joints and allows for adjustable intensity based on the targeted area [23]. Whole-body EMS (WBEMS) training, which utilizes electrical stimulation control technology, enables high-intensity exercise with minimal physical movement in a short time [24,25]. This makes it highly time-efficient and ideal for individuals who are either uninterested in traditional exercise or face challenges with physical movement [24,25]. A previous study using WB-EMS training in men aged 65-75 with metabolic syndrome found significant reductions in abdominal fat, total body fat mass, and waist circumference in the WB-EMS group compared with the group using vibration equipment [26]. Additionally, a study comparing a group of elderly individuals with sarcopenic obesity who underwent WB-EMS training and consumed protein supplements with those who consumed only whey protein supplements and a non-intervention control group reported that the group combining WB-EMS training with protein supplementation showed significant reductions in total body fat, trunk fat mass, and waist circumference, along with a positive change in the ratio of total cholesterol (TC) to high-density lipoprotein cholesterol (HDL-C) [27]. Based on these findings, EMS training has been widely used for individuals unable to engage in regular physical activity and appears to elicit somewhat similar responses to muscle-level exercise.
Therefore, we aimed to verify the effects of lower-extremity strength exercises combined with WB-EMS on body composition, blood lipid levels, pain, and isokinetic muscle function in patients with obesity and knee pain to develop an effective WB-EMS exercise program for this population.
METHODS
Participants
The participants in this study were 20 patients with obesity, with a BMI of 25 kg/m² or higher, who visited M Hospital in Gyeonggi Province, primarily complaining of knee pain. The participants were randomly assigned to either the experimental group (EXP), which performed lower-extremity strength exercises using the WB-EMS equipment, or the control group (CON), which did not use the WBEMS equipment. Both groups engaged in lower-extremity strength exercises three times a week for 12 weeks, and changes in measurements were observed before and after the intervention. However, six participants dropped out (three each from the EXP and CON groups), leaving data from 14 participants for the final analysis. The exclusion criteria were severe pain that made it difficult to perform measurements or exercise movements, sensitivity to electrical stimulation, and the presence of medical implants that conduct electrical currents. The participants’ physical characteristics in each group are presented in Table 1.
Before the study commenced, the Institutional Review Board (IRB) of Konkuk University approved all procedures and details (Approval No. 7001355-202105-HR-435). Participants were informed of the IRB’s approval, provided with and explanation of the study’s purpose, procedures, and guidelines, and signed a consent form before participating.
Exercise program
The daily exercise program for each group was structured as follows: warm-up, lower-extremity strength exercises, and cool-down. The exercise intensity was adjusted according to the participants’ physical fitness and strength levels. During the first week, which served as an adaptation period, the intensity was set at a light level corresponding to a rating of perceived exertion (RPE) of 11-12 based on Borg’s scale [28]. From weeks 2 to 6, the intensity was increased to a moderate level (RPE 13-14, “somewhat hard”), and from weeks 7 to 12, it was further increased to a hard level (RPE 15-16, “hard”). The lower-extremity strength exercises performed by the CON and EXP groups are shown in Table 2 and Figure 1. Pre-training education was provided to each group to ensure that the participants understood the exercises and precautions before starting the lower-extremity strength exercises.
The warm-up consisted of treadmill walking or stationary cycling at an RPE of 11-12 (light). The lower-extremity strength exercises were based on programs used in the Scali et al. study on anterior knee pain [29] and the Bennell and Hinman study on osteoarthritis [30], with modifications and adjustments made as necessary. The exercises included Straight Leg Raise (SLR)-front, SLR-side, SLR-back, clamshell, hip bridge, knee extension, squat, heel raise, step up and down, and lateral step up and down. Each exercise was performed for 12 repetitions per movement in two sets. The rest periods between exercises lasted from 30 s to 1 min, with a total exercise time of 20 min.
In the EXP group, the current intensity of the WB-EMS was adjusted to 2,500-5,000 Hz (medium frequency), according to the perceived level of stimulation. The intensity was set to the participant’s subjective maximum tolerable level, at which the muscles felt strongly contracted but without discomfort or pain, as the actual electrical stimulation reaching the neuromuscular system varied depending on the individual sensation and resistance in different tissue structures [31]. The WB-EMS equipment used was the MYO MIRROR 43_D (M20, Seoul, Korea), which includes a main unit, scale, belt, power cable, and an EMS suit embedded with electrodes Figure 2. Participants in the EXP group wore an EMS suit, which was moistened with water, and performed lower-extremity strength exercises while connected to the main unit. The equipment operation and exercise sessions were supervised and managed by an exercise specialist from start to end. The participants were asked to maintain their diet and activity levels during the intervention, which were verified through self-reported logs and periodic check-ins.
Measurements
Body composition
The body composition measurements used in this study included weight (kg), BMI (kg/m²), percent skeletal muscle (%), percent body fat (%), waist circumference (cm), hip circumference (cm), and waist-hip ratio (WHR). Body composition was measured using a body composition analyzer (ACCUNIQ BC720; SELVAS Healthcare, Daejeon, Korea) after the participants had fasted for at least 8 h and were lightly dressed.
The participants stood in a relaxed position, and the waist and hip circumferences were measured at the widest part of the hips, between the anterior superior iliac spine and the anterior inferior iliac spine, using a 150 cm measuring tape. WHR was calculated by dividing the waist circumference by the hip circumference.
Blood lipid levels
Blood lipid levels were measured using a Lipidocare analyzer (Standard Lipidocare; SD BIOSENSOR, Suwon, Korea). Blood samples were collected using the fingertip method after the participants fasted for at least 8 h before the test. Lipid profile measurements included triglycerides (TG), TC, HDL-C, and low-density lipoprotein cholesterol (LDL-C).
Pain scale
The visual analog scale (VAS) was used to assess the subjective pain level in patients with knee pain before and after the exercise intervention. VAS is a widely used method for quantifying pain, as it is easy to score and has high validity and reliability in clinical and research settings. On the VAS, a score of 0 represents no pain, and a score of 10 indicates extreme pain.
The Kujala Patellofemoral Score (KPS) measures pain during functional activities. This is a self-reported questionnaire comprising 13 items related to knee function. The maximum score is 100 points, with higher scores indicating better leg function [32].
Kellgren-Lawrence grading scale (K-L grade)
Knee radiographs were obtained using XGEO GC80 (Samsung, Suwon, Korea) to assess the joint space and degree of arthritis in both knees. A radiologic technologist performed the scans at a specialized medical institution. For an accurate interpretation of the radiographs, the images were reviewed by a medical expert capable of providing a diagnostic assessment. The degree of osteoarthritis was classified according to the Kellgren-Lawrence grading system (K-L grade) [33]: grade 0 indicates normal joints; grade 1 represents the presence of minor osteophytes and suggests early osteoarthritis; grade 2 shows definite osteophytes with normal joint space, indicating mild osteoarthritis; grade 3 features definite osteophytes with moderate joint space narrowing, indicating moderate osteoarthritis; and grade 4 indicates severe osteoarthritis with subchondral bone sclerosis and significant joint space narrowing.
Isokinetic muscle function
The knee muscle strength and endurance were measured using an isokinetic dynamometer (Cybex 770; HUMAC NORM, MA, USA). Muscle strength was assessed by performing four maximum contractions of extension and flexion at an angular velocity of 60°/s, from which the peak torque and relative peak torque (peak torque/body weight) values were calculated. Muscle endurance was evaluated by performing 11 repetitions at an angular velocity of 180°/s, and the average power and total work were determined.
Dynamic balance
Dynamic balance was assessed using the Y-Balance Test in the lower quarters (Y-Balance Test Kit; Functional Movement Systems Inc., Virginia, USA) [34]. The Y-Balance Test (YBT) measures the maximum reach distance in the anterior, posteromedial, and posterolateral directions. Each direction was measured twice, and the maximum distance reached was recorded. Before testing, the length of the participant’s leg (from the anterior superior iliac spine to the medial malleolus) was measured using tape. This measurement was used to calculate the composite score.
Statistical analysis
All data obtained in this study were analyzed using SPSS 28 (SPSS Institute, IBM Corp., Armonk, NY, USA). The means and standard deviations were calculated for each variable. A repeated two-way ANOVA was conducted to assess the effectiveness of the 12-week intervention on the dependent variables. When significant interaction effects or main effects were found between the two groups before and after the intervention, post-hoc analyses were performed using independent and paired t-tests. The effect size was computed as partial eta-squared values (ηp2; small: ≥0.01, medium: ≥0.06, large: ≥0.14). The statistical significance level was set at p < 0.05.
RESULTS
Body composition
Changes in body composition in the CON and EXP groups after 12 weeks of intervention are shown in Table 3. Although no significant interaction effects were observed for any body composition variables, significant main effects of the treatment were found for weight (p=0.010, ηp2=0.441), BMI (p=0.006, ηp2=0.474), percent body fat (p=0.028, ηp2=0.341), waist circumference (p<0.001, ηp2=0.868), and hip circumference (p=0.001, ηp2=0.631).
Post-hoc analyses revealed significant reductions in weight (CON, p=0.035; EXP, p=0.048), BMI (CON, p=0.035; EXP, p=0.042), waist circumference (CON, p=0.001; EXP, p=0.001), and hip circumference (CON, p=0.001; EXP, p=0.041) in both groups. However, no significant differences were found between the CON group performing lower-extremity strength exercises alone and the EXP group undergoing combined WB-EMS and lower-extremity strength exercises.
Blood lipid levels
Changes in blood lipid levels in the CON and EXP groups after 12 weeks of intervention are shown in Figure 3. Although no significant interaction effects were observed for any blood lipid concentration variables, significant main effects of the treatment were found for TG (p<0.001, ηp2=0.676), TC (p=0.015, ηp2=0.399), HDL-C (p<0.001, ηp2=0.841), and LDL-C (p=0.012, ηp2=0.424).
Post-hoc analyses revealed that TG levels were significantly decreased in the CON group performing lower-extremity strength exercises (p=0.002) and the EXP group undergoing combined WB-EMS and lower-extremity strength exercises (p=0.009). HDL-C levels were significantly increased in both the CON (p=0.001) and EXP (p=0.001) groups. Significant reductions in TC (p=0.003) and LDL-C (p=0.003) levels were observed only in the CON group. However, the reductions in TC (CON vs. EXP = 23.1 mg/dL vs. 23.0 mg/dL) and LDL-C (CON vs. EXP = 24.3 mg/dL vs. 30.6 mg/dL) showed similar trends in both groups.
Pain scale and K-L grade
Changes in pain scales and K-L grades in the CON and EXP groups after 12 weeks of intervention are shown in Figure 4. Significant interaction effects were observed in the KPS (p=0.002, ηp2=0.560), while significant main effects of the treatment were found in the VAS (p<0.001, ηp2=0.893).
Post-hoc analyses revealed that the KPS significantly decreased in both the CON (p=0.004) and EXP (p=0.001) groups, with a greater decrease observed in the EXP group than in the CON group. Additionally, the VAS showed significant reductions in both the CON (p=0.001) and EXP (p<0.001) groups, with a greater reduction observed in the EXP group.
Isokinetic muscle function and dynamic balance
Changes in isokinetic muscle function and dynamic balance in the CON and EXP groups after 12 weeks of intervention are shown in Figure 5. While no significant interaction effects were observed for any of the measures, significant main effects of the treatment were found for knee extension peak power (p=0.002, ηp2=0.577), knee extension relative peak power (p=0.003, ηp2=0.541), knee extension total work (p=0.001, ηp2=0.616), knee extension average power (p=0.029, ηp2=0.338), knee flexion average power (p=0.017, ηp2=0.390), and the YBT (p<0.001, ηp2=0.658).
Post-hoc analyses revealed significant increases in quadriceps peak power (p=0.004) and relative peak power (p=0.003) only in the EXP group. Additionally, significant improvements in YBT scores were noted in both the CON (p=0.017) and EXP (p=0.012) groups.
DISCUSSION
This study was conducted assuming that a 12-week combined treatment of WB-EMS and lower-extremity strength training would yield greater improvements in body composition, blood lipid levels, pain, and isokinetic muscle function than standard lower-extremity strength training alone.
Regarding body composition, prior research targeting obese individuals, such as that by Kemmler et al., reported that a combined treatment group using WB-EMS showed a significant reduction in waist circumference compared to a control group that performed exercises without electrical stimulation. Similarly, Reljic et al. found that all exercise groups experienced reductions in body weight and body fat, with significant decreases in waist circumference in both the resistance training and WB-EMS groups [35]. In our study, as reported by Reljic et al. [35], improvements in body composition were observed across all groups; however, no significant differences were found between the two groups. This lack of significant difference may be due to our study’s subjective setting of WB-EMS current intensity, which may not have increased the energy expenditure sufficiently to show significant differences in body composition.
Both the CON and EXP groups showed significant reductions in TG and increased HDL-C levels after 12 weeks of exercise. Although significant reductions in TC and LDL-C were observed only in the CON group, the reduction rates of TC and LDL-C were similar between the two groups, indicating no difference in the overall blood lipid levels between the two interventions. Regular exercise controls weight, increases HDL-C, and decreases TG, TC, and LDL-C, thus positively affecting metabolic syndrome and blood lipid levels [36]. Reljic et al. reported significant reductions in TC levels in the WB-EMS combined treatment and control groups, with LDL-C showing a statistically significant decrease only in the control group [37]. Our findings are consistent with those of Reljic et al. [37], who showed similar trends in TC and LDL-C levels in both groups, suggesting that positive changes might also be expected in the EXP group.
Regarding pain-scale changes, the KPS and VAS scores significantly improved in both the CON and EXP groups, with the EXP group demonstrating greater improvement. VAS scores decreased significantly in both groups, with a greater reduction in the EXP group. Reduced joint use due to pain is a major factor in functional decline [38-40], leading to a cycle of decreased muscle strength and atrophy, resulting in reduced activity and ongoing pain. This study used the VAS to measure pain levels and complemented it with the KPS to diversify the pain assessment. Kuru et al. found a correlation between KPS and VAS, with an average KPS score of 76.8 correlating with a VAS score of approximately 6, indicating that KPS accurately measures individual pain levels and knee function [41].
The isokinetic muscle function showed significant increases in knee extension peak power and relative peak power only in the EXP group. Human skeletal muscle consists of type I, type IIa, and type IIx fibers, and the distribution of fiber types is largely determined by genetic factors [42]. However, various external stimuli, such as exercise, induce significant plasticity [43]. Previous research has found that currents below 50 Hz primarily activate type I fibers, whereas currents above 50 Hz activate type II fibers more effectively [44,45]. High-frequency electrical stimulation, such as that used in this study, is believed to be effective in activating type II muscle fibers, thereby increasing muscle strength. Our study used a subjective maximum intrinsic intensity of 2,500-5,000 Hz for WB-EMS, which likely resulted in the observed improvement in quadriceps strength, consistent with previous findings. This suggests that WBEMS can significantly enhance quadriceps strength. Since muscle strength is independently associated with morbidity and mortality [46,47], the significant improvement in muscle strength observed in the EXP group is expected to provide additional health benefits. In conclusion, the results of this study indicated that the combined application of WB-EMS and lower-extremity strength training leads to positive changes in pain and knee extension strength in patients with obesity and knee pain. Additionally, as no adverse effects of electrical stimulation were observed during the study, this suggests that WB-EMS can be safely applied in patients with obesity and knee pain.
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