Physical Therapy Korea

Electrical muscle stimulation (EMS) involves artificially activating muscles using a program designed to modulate a variety of electrical waveforms, resulting in an electrical current that can be used comfortably to stimulate innervated muscles. EMS has been applied as a supplement or substitute for voluntary muscle activation, for the re-education of muscle action, facilitation of muscle contraction, muscle strengthening, and maintenance of muscle mass and strength during prolonged immobilization in various rehabilitation settings [1,2]. EMS programs have also been used to improve muscle strength in healthy individuals [3,4] and athletes [5,6].

Neuromuscular adaptations due to EMS training can be as large as those due to voluntary contractions, but they are rarely greater than those resulting from voluntary contractions [7,8]. Previous studies comparing the effectiveness of different training methods showed that EMS induced comparable amounts of neuromuscular adaptation as voluntary contractions in healthy subjects [9-11]. Whether EMS or voluntary contraction is more effective is still debated, but the two can be considered as complementary stimuli eliciting different physiological effects. For example, EMS tends to reverse the order of motor unit recruitment observed during voluntary contractions [12,13].

In the superimposing technique (ST), EMS is applied during voluntary muscle contraction. With the ST, there may be an increase in the physiological effects attributed to each stimulus [14]. ST can recruit more motor units than EMS or voluntary contraction alone because EMS primarily stimulates large motor units, whereas voluntary contraction first recruits small motor units [14]. Moreover, although EMS has limited functional applications (because it is usually applied in an open kinetic chain position [15]), it can be supplemented with ST. Previous studies on the effects of ST have concentrated on the quadriceps muscle (increasing muscle strength and muscle size and improving motor performance in sport) [16-19]. However, no study has determined the effect of the application of ST in squats, which are most commonly used for quadriceps strengthening.

It has been postulated that the musculature of most people is weakened owing to the effects of the sedentary modern lifestyle. The quadriceps, when considered as one, is the largest muscle in the human body [20], and strengthening it is a major factor in knee joint stabilization or physical rehabilitation [21]. Although various EMS devices for the quadriceps muscle are being marketed to the general public, there is still a lack of research regarding whether ST training can provide significant advantages for improving quadriceps muscle strength or thickness compared with EMS alone. Therefore, this study aimed to compare the effects of eight weeks of ST and EMS quadriceps muscle training on muscle thickness and strength. We hypothesized that ST would be more effective than EMS in improving the thicknesses of the rectus femoris (RF) and vastus intermedius (VI) muscles, as well as the knee extension strength.

1. Subjects

The sample size was determined a priori using G-power software (ver. 3.1.3; Franz Faul, University of Trier, Trier, Germany) [22] in a pilot study with three participants per group (EMS and ST). The sample size was calculated with a power of 0.95, an alpha level of 0.05, and an effect size of 0.74. The results suggested that more than 10 subjects were required per group. Thirty healthy subjects with no history of cardiovascular disease, neurological disease, musculoskeletal dysfunction affecting the knee, or cardiac pacemakers were recruited and randomly assigned to the EMS or ST group (Table 1). Individuals who were averse to electrical stimulation were excluded from the study. The Yonsei University Mirae Campus Human Studies Committee (IRB no. 1041849-202008-BM-103-02) approved the study procedure and all participants provided written informed consent.

Table 1 . Subject characteristics.

Characteristic	EMS group				ST group			p
	Total (N = 16)	Male (n = 10)	Female (n = 6)		Total (N = 15)	Male (n = 9)	Female (n = 6)
Age (y)	25.3 ± 3.5	24.8 ± 3.9	26.1 ± 2.9		26.1 ± 2.5	25.8 ± 2.9	26.5 ± 1.9	0.12
Body height (cm)	170.5 ± 8.6	175.2 ± 4.6	162.7 ± 8.3		170.4 ± 7.2	175.3 ± 4.2	163 ± 3.3	0.48
Body mass (kg)	74.6 ± 16.7	83.9 ± 12.3	58.9 ± 10.1		76.1 ± 19.1	87.4 ± 15.9	59.1 ± 6.5	0.59
BMI (kg/m2)	25.4 ± 4.3	27.2 ± 3.2	22.4 ± 4.3		25.9 ± 4.8	28.4 ± 4.5	22.2 ± 2.3	0.83

Values are presented as mean ± standard deviation. EMS, electrical muscle stimulation; ST, superimposed technique; BMI, body mass index..

2. Electrical Muscle Stimulation Training

EMS was applied to the quadriceps muscle using an EMS HomT leg belt (Samsung, Seoul, Korea). The EMS device consists of a contoured, flexible, soft silicone belt with electrodes connected to the stimulator without externally visible leads or detachable gel pads. The EMS device delivered biphasic, symmetric pulses of 20–80 Hz, and pulse frequency and duration were controlled using a program that stimulated the quadriceps muscle (mean intensity 29.86 [7.39] mA [range: 17.98–42.15 mA]). The subjects were encouraged to increase the amplitude of the stimulator so that strong contractions of the quadriceps muscle could be elicited (within tolerable limits). The subjects were instructed not to perform voluntary contractions during electrical stimulation and to consistently attach the center of the EMS device over the middle of the femur (Figure 1).

Figure 1. Electrical muscle stimulation attachment location.

3. Superimposed Technique Training

The same EMS device and training protocol as those in the EMS group were used for the subjects in the ST group. In addition, the subjects performed voluntary contractions via two half-squat cycles over a 10-second period (1 cycle = three seconds of tetanic stimulation followed by a 2-second pause). For the half squats, the subjects performed a half range of motion squat (approximately 45° knee flexion). The subjects were asked to hold a half squat for 10 seconds and then return to the starting position. They were asked to perform 10 sets of half squats per the EMS training protocol for four weeks, followed by 15 sets for another four weeks.

4. Assessment of Quadriceps Muscle Thickness

With the participant lying in the supine position, a strap was placed around both feet to prevent external hip rotation. Ultrasonography (Medison Sono Ace X8; Samsung, Seoul, Korea) was used for imaging of the RF and VI muscles. Before taking the measurements, the researcher identified and marked the measurement site, which was 50% of the thigh [23]. A linear transducer (5–12 MHz) was placed perpendicular to the middle of the thigh to capture the ultrasound images of the RF and VI muscles. Images were obtained by a physical therapist trained using real-time ultrasound. The depth of the image was adjusted until the femur was visible in the center of the screen, and the gain was adjusted until the muscle boundaries were visible on the screen. Three images were captured and saved for analysis. The ImageJ software (National Institutes of Health, Bethesda, MD, USA) was used to measure the thicknesses of the RF and VI muscles. The thickness of the VI muscle was defined as the distance between the superficial border of the muscle and the most superficial aspect of the femur. RF muscle thickness was defined as the distance between the superficial border of the muscle and the deep border in the direction of the most superficial aspect of the femur (Figure 2) [23]. The same investigator performed ultrasound imaging capture and measured the thicknesses of the RF and VI muscles and was blinded to participant identity for these measurements.

Figure 2. Thickness of rectus femoris and vastus intermedius.

5. Measurement of Quadriceps Muscle Strength

Maximal isometric muscle strength of knee extension was measured using a Smart KEMA tension sensor (KOREATECH Co., Ltd., Seoul, Korea). The tension sensor contained a load cell with a measurement range of 0–1,960 N and an accuracy of ± 4.9 N. The tension sensor data were transmitted at a sampling frequency of 10 Hz to a recording Android tablet running the Smart KEMA software, and the average of the data for the middle 3-second was computed for analysis. In previous studies, a tension sensor was used to measure isometric strength, and it showed high interrater reliability (85–90) [24,25]. The tension sensor had two rings. One side was fixed to the therapeutic table using an orthopedic belt, and the other side was fixed to the area above the ankle joint using a Smart KEMA tension strap (KOREATECH Co., Ltd.).

To measure the isometric strength of the quadriceps muscles, the length of the restraining belt was adjusted such that the subjects reached a knee extension of 45°. The subjects performed knee extensions against a strap anchored by a therapeutic table to measure maximal voluntary isometric contraction. An ankle strap was then placed above the ankle joint. The subjects were instructed on how to stabilize themselves by holding on to the side of the table with their hands while sitting upright. To prevent compensation, such as the posterior aspect of the pelvis, the examiner fixed the pelvis (Figure 3). The strength was measured thrice and averaged for data analysis.

Figure 3. Knee extension strength.

6. Procedures

During the preliminary session, all participants underwent baseline measurement of the thicknesses of the RF and VI muscles and quadriceps strength. EMS and ST training were performed twice daily for 23 minutes for eight weeks. Adherence to this schedule was confirmed by telephone every day, and the participants were encouraged to perform EMS or ST training at least five days weekly [26]. The subjects were instructed not to perform additional fitness training or lower-extremity exercises during the study period. For all participants, the instrument settings were the same as those used during the preliminary session.

7. Statistical Analysis

Statistical analyses were performed using the SPSS for Windows (ver. 25.0 software; IBM Co., Armonk, NY, USA). The Kolmogorov-Smirnov test was used to verify the normality of the data distribution. An independent t-test was conducted to compare the general characteristics of the subjects between the groups. Analysis of covariance (ANCOVA) was conducted to identify differences in the post-intervention values of the dependent variables between the two groups. The pre-values of the both groups were used as covariates. A paired t-test was used to compare the pre- and post-intervention values of the dependent variables in each group. The significance level was set at p < 0.05 for all analyses.

After training, the quadriceps muscle strength and the thicknesses of the RF and VI muscles significantly increased in both groups (p < 0.05) (Table 2). RF thickness was significantly higher in the ST group than in the EMS group (F = 4.294, p = 0.048). The changes in VI thickness (F = 0.234, p = 0.632) and quadriceps muscle strength (F = 0.775, p = 0.386) showed no significant differences between the groups (Table 3).

Table 2 . Comparison of between pre- and post-values.

Variable	Group	Pre	Post	t	p
Thickness of RF	EMS	2.11 ± 0.42	2.36 ± 0.53	–3.481	0.003*
	ST	2.17 ± 0.42	2.64 ± 0.58	–6.551	0.000*
Thickness of VI	EMS	1.98 ± 0.59	2.24 ± 0.73	–2.947	0.010*
	ST	2.20 ± 0.43	2.40 ± 0.48	–2.353	0.034*
Knee extension strength	EMS	39.56 ± 17.31	49.80 ± 13.96	–6.030	0.000*
	ST	44.49 ± 27.93	58.72 ± 28.76	–3.219	0.006*

Values are presented as mean ± standard deviation. RF, rectus femoris; VI, vastus intermedius; EMS, electrical muscle stimulation; ST, superimposed technique. *p < 0.05..

Table 3 . Comparison of post-values between groups.

Variable	Covariate	ST group	EMS group	Partial η2	F	p
Thickness of RF	2.14	2.64 ± 0.58	2.36 ± 0.53	0.133	4.294	0.048*
Thickness of VI	2.08	2.40 ± 0.48	2.24 ± 0.73	0.008	0.234	0.632
Knee extension strength	41.94	58.72 ± 28.76	49.80 ± 13.96	0.027	0.775	0.386

Values are presented as mean ± standard deviation. ST, superimposed technique; EMS, electrical muscle stimulation; RF, rectus femoris; VI, vastus intermedius. *p < 0.05..

Quadriceps muscle training has been highly emphasized for knee joint stabilization and improvement of physical performance in the clinical, sports, and fitness fields [21]. The results of this study indicate that eight weeks of EMS training of the quadriceps muscle induced an increase in the quadriceps muscle strength and the thicknesses of the RF and VI muscles. Therefore, both EMS and ST training are useful options for improving quadriceps strength and thickness.

Regarding muscle thickness, the results of this study indicate that eight weeks of ST training of the quadriceps muscle induced a greater increase in RF thickness than EMS training alone. ST induces more pronounced neuromuscular adaptation for hypertrophy of the RF muscle than EMS training alone [14]. EMS and voluntary contraction could lead to greater physiological adaptation than either type of contraction alone. Therefore, ST training is a good option for improving RF thickness. However, no significant difference in VI muscle thickness was noted between groups in this study post-intervention. There are several possible reasons for this result. First, the VI muscle is located deeper than the RF muscle, and since EMS provides electrical stimulation through the skin, the VI muscle may not receive sufficient electrical stimulation. Moreover, the RF muscle would be thicker owing to contraction during squatting, and it would have been difficult for the VI muscle to receive electrical stimulation. In addition, ST was in the form of a squat exercise combined with EMS in the present study. In a previous study on muscle activation during squats using a musculoskeletal model, the activation of the RF muscle was greater than that of the VI muscle at 50% of the squat cycle [27]. Because half squats were used in this study for the ST method, the advantage of ST would be less for VI muscle activation than for RF muscle activation. In the present study, there was no significant difference in the thickness of the VI muscle between the groups; therefore, we could not confirm whether ST was superior to EMS in increasing the thickness of the VI muscle.

No significant difference in quadriceps muscle strength was noted between the groups in this study. In a previous study, the integration of ST into training programs did not yield significant benefits over the programs using EMS alone with respect to the strength or power of the quadriceps muscle [28,29]. The reason for this result is probably that the free-weight squat is a low-load exercise. Load is arguably the most important exercise-related variable for the stimulation of muscle growth [30]. Usui et al. [31] reported that low-load squat training does not increase isometric quadriceps muscle strength in healthy young men. For another reason, as a well-known fact, a training-induced strength gain is most clearly observed in the task performed in the training regimen, and this is called “task-specificity&quot;. For example, Sale et al. [32] reported that a 19-week leg press training significantly increased one-repetition maximum (1-RM) leg press as well as the muscle size of the quadriceps muscle, but no change was found in maximal isometric quadriceps muscle strength. Further studies are needed to clarify whether ST with 1-RM squat exercise can improve muscle strength more than EMS alone.

Our study had some limitations. First, it is difficult to generalize our results to various age groups because the current study focused on young men and women. Second, the subjects were healthy individuals without pain or disease. Therefore, further studies should be performed on patients of various ages with musculoskeletal diseases.

Both EMS and ST training for eight weeks were effective in improving quadriceps muscle strength and RF and VI muscle thickness. There were no significant differences in quadriceps muscle strength and VI muscle thickness between groups. Therefore, EMS can be used to improve quadriceps muscle strength and RF and VI muscle thickness in individuals with difficulty with voluntary quadriceps contractions due to surgery or immobility. The present study demonstrated that improvements in RF thickness were greater in the ST training group than in the EMS training group. Therefore, ST can be used to improve RF thickness in the context of athletic training and fitness.

None.

This study was supported by the “Brain Korea 21 FOUR Project” and the Korean Research Foundation for the Department of Physical Therapy at the Graduate School of Yonsei University.

No potential conflict of interest relevant to this article was reported.

Conceptualization: YW, GG, OK. Data curation: YW, DL. Formal analysis: YW, GG, DL. Funding acquisition: YW, OK. Investigation: YW, DL. Methodology: YW, JK, GG, OK. Project administration: DL. Resources: YW, JK, GG, OK. Supervision: YW, GG, OK. Validation: YW, JK, OK. Visualization: JK, DL. Writing - original draft: YW. Writing- review & editing: YW, JK, OK.