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Effects of Electrostimulation with Blood Flow Restriction on Muscle Size and Strength

NATSUME, TOSHIHARU1; OZAKI, HAYAO1,2,3; SAITO, ANNEYUKO I.4; ABE, TAKASHI5; NAITO, HISASHI1

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Medicine & Science in Sports & Exercise: December 2015 - Volume 47 - Issue 12 - p 2621-2627
doi: 10.1249/MSS.0000000000000722
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Abstract

Application of blood flow restriction (BFR) alone to the lower body in the absence of exercise has been shown to attenuate muscle atrophy after anterior cruciate ligament reconstruction (27) and cast immobilization (14,1514,15). The mechanism of the attenuating effect of BFR alone on disuse atrophy is unclear. However, acute muscle cell swelling caused by BFR may favorably influence net protein balance via activation of the mechanistic target of rapamycin and mitogen-activated protein kinase signaling pathways. This may explain the observed attenuating effects (17). Although BFR alone appears to attenuate muscle atrophy, BFR has not been shown to provide a stimulus adequate to elicit muscle hypertrophy (28). However, low-load exercise training, when combined with BFR, can induce hypertrophy of working muscles (20). BFR accelerates the development of metabolic fatigue, which seems to play a dominant role in inducing muscle hypertrophy, and is considered an alternative method for increasing training efficacy in the absence of high mechanical stress (24). A review article demonstrated that 10% of maximal strength is the minimal exercise intensity required to achieve hypertrophy of limb muscles under BFR, based on previous intervention studies (1).

Given that such a hypertrophic effect is observed even under low-load voluntary training using BFR, it is possible that even involuntary muscle contractions evoked by low-intensity neuromuscular electrical stimulation (NMES) can induce muscle hypertrophy and strength gain when combined with BFR. The significance of the present study is that NMES is commonly used as a rehabilitative technique for preventing muscle atrophy during immobilization periods (8). As the strength-gain effect of NMES is correlated with electrically evoked force, higher-intensity NMES would be expected to be more effective (4). However, the maximal tolerable level of electrically evoked force differs greatly between individuals; in previous studies, the force evoked by NMES ranged from 12% to 95% of maximal strength (5,95,9). Thus, exploring whether low-intensity (ca. 10% of maximal strength) NMES induces strength gain and hypertrophic effect would be useful for developing more effective and better-tolerated exercise methods.

Therefore, the aim of the present study was to investigate the effects of low-intensity NMES combined with BFR (NMES-BFR) on muscle size and strength. Because it is difficult to differentiate the effect of NMES-BFR from those of other rehabilitation programs in studies employing real patients, we enrolled untrained subjects without apparent disease. In addition, based on previous studies that demonstrated a hypertrophic effect after 1–2 wk of low-load BFR training [twice-daily training sessions (10)], we designed a novel training program that could be used even for short-term rehabilitative programs.

METHODS

Subjects

Eight untrained young male participants (mean ± SE; age, 26.2 ± 0.7 yr; height, 1.74 ± 0.02 m; body weight, 71.4 ± 4.8 kg) volunteered to participate in the study. The subjects were recruited through printed advertisements and by word of mouth. None had participated in any regular aerobic or resistance training during the previous year. The subjects were instructed to avoid other physical activities and not to change dietary patterns during the intervention period. All subjects were free of overt chronic disease, as assessed by medical history-taking. Potential candidates who were former or current smokers or who were taking any medication were excluded. All subjects were informed of the methods, procedures, and risks, and signed an informed consent form before participating in the study. This study was conducted according to the Declaration of Helsinki and was approved by the Ethics Committee for Human Experiments of Juntendo University.

NMES training

One week before the beginning of the training period, the subjects participated in a practice session to familiarize themselves with NMES training. Next, they attended two NMES sessions per day (5 d·wk−1) for 2 wk (for a total of 20 sessions) and completed 2 wk of detraining. During all sessions, the subjects were seated on an isokinetic dynamometer (Biodex System 4; Biodex Medical Systems, Shirley, NY) and underwent 23 min of unilateral involuntary muscle contractions of knee extensors (triggered by NMES) at a fixed knee joint angle of 75°. The morning and afternoon sessions were approximately 4–5 h apart. During each session, one leg (determined by randomization) underwent NMES-BFR and the other leg underwent NMES alone (NMES-CON). The dominant limb was randomized into NMES-BFR or NMES-CON. All training sessions were conducted under the direct supervision of persons technically familiar with NMES and BFR training. During all sessions, the participants were instructed to relax their thigh muscles as much as possible.

Quadriceps muscles were stimulated using bipolar electrodes linked to a portable battery-powered neuromuscular electrical stimulator (Compex Sport Energy; Medicompex, Ecublens, Switzerland). Three self-adhesive electrodes (2 mm thick) were placed over each thigh. The negative electrode (10 × 5 cm) was positioned proximally about 11 cm (BFR cuff width) below the inguinal crease, whereas the other two (positive) electrodes (5 × 5 cm) were placed as close as possible to the motor points of the vastus lateralis and medialis muscles. Muscle motor points were identified by stimulating the skin surface with a pen electrode and a large reference electrode placed over the femoral area. The pen electrode was moved slowly over the skin, with the stimulatory current being gradually increased by the operator until a clear muscle twitch was observed. The stimulator discharged biphasic rectangular pulses. The stimulation frequency and duty cycle were approximately 30 Hz and 8 s of stimulation followed by a 3-s pause. The intensity of electrical flow was selected to attain 5%–10% of maximal voluntary contraction (MVC), and the positions of the electrodes were marked. Throughout the intervention period, the electrodes were applied at the same sites, and the intensities of electrical flow volume were held constant. RPE and the Category Ratio 10 scale (CR10) were administered at the end of each training session. CR10 (0, nothing at all; 10, extremely strong) was used to evaluate discomfort induced by NMES-BFR and NMES-CON based on a previous study (19).

Blood flow restriction

A 105-mm-wide nylon cuff (MT-870 Digital Tourniquet; Mizuho, Tokyo, Japan) was applied tightly at the most proximal portion of the BFR leg. Before each session, all subjects were seated on an isokinetic dynamometer (Biodex System 4), and the thigh-mounted cuff was inflated to 100 mm Hg. After 30 s, pressure was released for 10 s and reinflated to a cuff pressure 20 mm Hg higher than the previous one for another 30 s. This process was repeated until the target pressure was attained; the target pressure for each subject was calculated based on midthigh circumference [<50 cm, 140 mm Hg (n = 3); 50–55 cm, 160 mm Hg (n = 4); >60 cm, 200 mm Hg (n = 1)] because arterial occlusion pressure is largely influenced by thigh circumference (18). The subjects received four sets of BFR (each of 5 min) with 1-min rest intervals between sessions. Cuff air pressure was released immediately upon completion of each session.

Muscle thickness

Muscle thickness (MT) was measured via B-mode ultrasound, using a 5-MHz scanning head (SSD-900; Aloka, Tokyo, Japan), at eight sites on the anterior aspect of the thigh (at 30%, 50%, and 70% of thigh length, and at the central, lateral, and medial surfaces, excluding the 30% medial point) and at two sites on the posterior aspect of the thigh (at 50% and 70% of thigh length and at the central surface) 1–2 d before and every week throughout training and detraining [before the training period (PRE), during the training period (MED), immediately after the training period (POST), 1 wk after the training period (POST2), and 2 wk after the training period (POST3)]. Before all scans, the subjects rested quietly in seated position for at least 30 min. To avoid the influence of fluid shift within muscles, we performed the measurements at around the same time. Thigh circumference was also measured at 50% of thigh length, using a tape measure. Thigh length refers to the distance between the lateral condyle of the femur and the greater trochanter. All measurements were performed by the same operator. Measurement sites were marked using a marker pen, as described in a previous study (13). We performed ultrasound measurements of MT with the participants in supine/prone position, ensuring that hip and ankle joint positions and the distance between both legs are the same in all measurements. The scanning head coated with a water-soluble transmission gel was placed on each marked measurement site without depressing the dermal surface. The subcutaneous adipose tissue–muscle and muscle–bone interfaces were identified on ultrasound images, and the distance between the two interfaces was recorded as MT. The mean MT of the eight anterior and two posterior sites were used in data analysis. The posterior MT of NMES-CON and NMES-BFR legs were used to explore the effects of no treatment at all and BFR alone, respectively; NMES was applied to only the anterior aspect of the thigh. Test–retest (intersession) reliabilities of MT measurements were calculated using intraclass correlation coefficient, SEM, and minimal difference, which had been previously determined in 10 young subjects (0.999, 0.21 mm, and 0.58 mm, respectively) in terms of anterior central 50% MT values.

Maximal isometric and isokinetic strengths

The maximal voluntary isometric and isokinetic strengths of knee extensors and flexors were determined using a Biodex System 4 dynamometer 1–2 d before and every week throughout the training and detraining periods (PRE, MED, POST, POST2, and POST3). Three days or 4 d before baseline strength testing, participants were familiarized with the strength-testing protocol. During testing, each participant was seated on a chair with the hip joint angle positioned at 85° of flexion (full hip extension, 0°). The center of rotation of the knee joint was visually aligned with the axis of the dynamometer lever arm, and the ankle was firmly strapped to the distal pad of the lever arm. A knee joint angle of 0° corresponded to full knee extension. Several warm-up contractions (four to five submaximal contractions and one to two near-maximal contractions at 180°·s−1) were performed before testing. Participants were instructed to perform maximal isometric knee extension for about 5 s at a fixed knee joint angle of 75°, preceded by maximal isokinetic knee extension from 0° to 90° at 90°·s−1 and 180°·s−1. Next, they performed maximal isometric knee flexion for about 5 s at a fixed knee joint angle of 30°. Two maximal efforts for each isometric measurement and three maximal efforts for each isokinetic measurement were performed, and each peak torque was used in data analysis. Maximal isometric knee flexions of NMES-CON and NMES-BFR legs were recorded to explore the effects of no treatment and BFR alone, respectively. Test–retest (intersession) reliabilities of strength measurements calculated using intraclass correlation coefficient, SEM, and minimal difference had been previously determined in 10 young subjects (0.988, 5.20 N·m, and 14.41 N·m, respectively) performing maximal isometric knee extension.

Statistical analyses

All results are expressed as mean ± SE. Statistical analysis featured two-way ANOVA with repeated measures [condition (with and without BFR) × time (PRE, MED, POST, POST2, and POST3)]. All baseline values for NMES-CON and NMES-BFR and measured knee flexor data variables were compared using paired t-test. Statistical significance was set at P < 0.05. Effect size (ES) was calculated as [(POST mean − PRE mean)/PRE standard deviation] (25) (<0.20, trivial; 0.20–0.49, small; 0.50–0.79, moderate; ≥0.80, large) (2).

RESULTS

NMES training and BFR application did not give rise to any relevant side effects such as subcutaneous hemorrhage, numbness, and cerebral anemia. All subjects tolerated training well; the adherence rate was 100% under both training conditions. No significant difference in baseline values of MT or muscle strength was evident when the two training conditions were compared. No significant change in body mass or body mass index was noted throughout training and detraining (Table 1).

TABLE 1
TABLE 1:
Physical characteristics of subjects.

Figure 1 illustrates changes in the MT of knee extensors throughout the training and detraining periods. Two-way repeated-measures ANOVA showed that condition–time interaction (P < 0.001) was significant. Under NMES-BFR conditions, MT increased after 2 wk of training (+3.9%) and decreased after 2 wk of detraining (−3.0%), whereas no notable change was observed under NMES-CON conditions. The ES for NMES-BFR and NMES-CON conditions was 0.18 and 0.03, respectively. The MT of knee flexors did not change under either NMES-BFR or NMES-CON.

FIGURE 1
FIGURE 1:
Changes in thigh MT under NMES-CON (□) and NMES-BFR (▪) conditions: PRE, MED, POST, POST2, and POST3. Data are presented as mean ± SE.

Figure 2 shows changes in isometric and isokinetic knee extension strengths throughout training and detraining. Two-way repeated-measures ANOVA showed that condition– time interaction (P < 0.05 isometrically, P < 0.01 at 90°·s−1, and P < 0.01 at 180°·s−1) was significant for all angle velocities. The NMES-BFR condition showed maximal voluntary strength improvements under isometric (+14.2%) and isokinetic (+7.0% at 90°·s−1 and +8.3% at 180°·s−1) conditions after the 2-wk training had been completed. In addition, after 2 wk of detraining, isometric maximal strength (Fig. 2A) decreased (−6.8%), but no large decreases (−1.9% at 90°·s−1 and −0.6% at 180°·s−1) in isokinetic maximal strength (Fig. 2B and C) were observed. Under the NMES-CON condition, no noticeable change was observed, except for a negligible effect on isometric strength. ES was calculated under the NMES-BFR condition (0.64 isometrically, 0.31 at 90°·s−1, and 0.35 at 180°·s−1) and the NMES-CON condition (0.20 isometrically, 0.03 at 90°·s−1, and 0.05 at 180°·s−1). Neither isometric nor isokinetic knee flexion strength changed under either the NMES-CON or NMES-BFR condition.

FIGURE 2
FIGURE 2:
Changes in maximal isometric (A) and isokinetic (B and C) knee extension under NMES-CON (□) and NMES-BFR (▪) conditions: PRE, MED, POST, POST2, and POST3. Data are presented as mean ± SE.

Figure 3 shows changes in RPE and CR10 after each training session. For RPE, the main effects of condition and training session were significant (P < 0.001). The interaction between condition and training session was significant for CR10 (P < 0.01). RPE after NMES-CON and NMES-BFR treatments fell to similar extents as training advanced. CR10 fell more rapidly under NMES-BFR condition than under NMES-CON condition early in the training period but fell similarly under either condition thereafter.

FIGURE 3
FIGURE 3:
Changes in RPE (A) and CR10 (B) scores after each training session under NMES-CON (□) and NMES-BFR (▪) conditions. Data are presented as mean ± SE.

DISCUSSION

The major finding of our present study was that low-intensity NMES training, when combined with BFR, induced muscular hypertrophy and concomitant increases in isometric and isokinetic strength.

During the past decade, many peer-reviewed studies have found that low-load (10%–30% of maximal strength) voluntary exercise training of working muscles, combined with BFR, can induce muscle hypertrophy and strength gain (20). The mechanisms underlying such hypertrophy are not completely understood, but metabolic stress resulting from the accumulation of metabolic by-products such as H+ and Pi seems to play a dominant role in the creation of hypertrophic effect under low-load resistance training with BFR, although mechanical stress also plays a part (24). Metabolic stress triggers secondary reactions, including recruitment of additional motor units to compensate for loss of force (32), enhanced acute muscle cell swelling (34), and production of reactive oxygen species (11). Such events may increase the rate of muscle protein synthesis through activation of anabolic signaling pathways, inhibition of catabolic signaling pathways (3,7,16,213,7,16,213,7,16,213,7,16,21), and proliferation of satellite cells (23), triggering hypertrophy. Furthermore, muscle hypertrophy and strength gain during low-load BFR training are observable even if training periods are short (1–2 wk of twice-daily sessions) (10). Thus, it is not surprising that 2 wk of NMES training at 5%–10% of MVC, combined with BFR, induced muscle hypertrophy and strength gain. However, the magnitude of such improvements induced by NMES-BFR appears to be lower than those attainable using other training modalities.

Previous studies have found that the ES of isometric strength gain and muscle hypertrophy were 1.08 and 0.41 for low-load BFR training (20) but 1.25 (25) and 0.35–1.23 (12,3112,31) for high-load resistance training, respectively. Compared to the latter type of training, the strength-gain effects noted were about half upon NMES-BFR training (0.31–0.64) and the hypertrophic effects were less than half upon NMES-BFR training (0.18). The small ES of NMES-BFR training may be attributable to the short intervention period, in addition to differences among exercise types.

In addition, we found that 2 wk of detraining reduced MT (rate of change, −3.0%; −0.2% d−1) to basal levels. Yasuda et al. (33) recently investigated the effects of short-term (3 wk) detraining after low-load BFR training on muscle size and found that muscle size returned to basal levels after detraining. Furthermore, Gondin et al. (6) showed that cessation of NMES training (at 68% of MVC) for 4 wk induced a significant decrease in muscle cross-sectional area. Such results are consistent with our present data, suggesting that muscle size returned to basal levels when relatively short-term detraining followed NMES and BFR training. We found that isometric strength decreased throughout detraining (rate of change, −6.8%; −0.5% d−1), but no large decrease in isokinetic knee extension strength (−1.9% at 90°·s−1 and −0.6% at 180°·s−1) was evident. Marqueste et al. (22) showed that the increase (14% from the pretraining level) in the concentric maximal strength of knee extensors after 6 wk of NMES training was preserved (19% above the pretraining level) after 6 wk of detraining. In contrast, another study found a gradual decrease in isometric strength after cessation of NMES training (6). Changes in muscle strength during detraining may depend on the type of muscle contraction measured (i.e., dynamic vs static strength). However, no other studies have investigated the effects of detraining after NMES and BFR training on increases in muscle size and strength. The topic warrants further work.

In general, BFR induction with a tourniquet may suppress the clearance of metabolites, creating pain (30). We found that the NMES-BFR condition was associated with higher CR10 and RPE scores than the NMES-CON condition. One previous study found that when subjects performed resistance exercise at moderate intensity (45%–60% of one repetition maximum), the range of RPE scores indicating perceived exhaustion was 13.0–17.0 (29), similar to the RPE scores noted under the NMES-BFR condition. In addition, CR10 and RPE scores under the NMES-BFR condition were lower than those recorded during knee extension exercise at 20% of one repetition maximum with BFR (cuff width, 135 mm) (26). These results suggest that the NMES-BFR training protocol used in the present study is generally well-tolerated.

NMES-CON had no effect on MT or isokinetic strength and had only a negligible effect on isometric strength in the present study. Even when the training intensity is high (i.e., 68% of MVC), a training period of 1–2 months appears to be necessary to achieve both muscle hypertrophy and strength gain via NMES (5). Therefore, it is possible that a short training period (2 wk) featuring low-intensity electrical current (5%–10% of MVC) did not greatly affect skeletal muscle size or strength in the present study.

A limitation of our current study was that the device inflating the nylon cuffs did not allow for an initial compressive force to be set, although the cuffs were tightly wrapped around the upper thigh. Thus, we have no data on the relationship between inflated cuff pressure and compressive force on the skin under the cuff. Furthermore, some variables of electrical stimulation were not recorded, although the extent of strength development during training was similar (5%–10% of MVC) for each subject. Additional research is needed to address these issues.

In conclusion, the present study is the first study to show that low-intensity NMES training, when combined with BFR, induces muscular hypertrophy and concomitant increases in isometric and isokinetic strengths in stimulated muscles. Our results indicate that addition of BFR to current NMES protocols affords potential benefits that are clinically relevant and thus warrants further investigation among patients who are immobilized. Further work is needed to define the stimulation conditions that maximize muscle hypertrophy when electrical stimulation is combined with BFR.

We express our sincere appreciation to Mr. Keisuke Watanabe for his technical assistance during the course of this study. We also gratefully acknowledge the cooperation of all subjects who volunteered.

This work was supported by the Japan Society for the Promotion of Science Grant in Aid for Scientific Research (Grant No. 24500870). The Juntendo University Institute of Health and Sports Science & Medicine also supported our research.

No commercial company or manufacturer has any professional relationship with any of the authors involved in this work. The results of this work will not confer any commercial benefits on any of the authors.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

MUSCLE ADAPTATIONS; TRAINING; ELECTRICALLY EVOKED FORCE; OCCLUSION; KNEE EXTENSORS

© 2015 American College of Sports Medicine