Effects of local vibration with blood flow restriction on muscle activation

Abstract

BACKGROUND:

Blood flow restriction (BFR) resistance exercise often elicits additive response of muscle activation. It is not clear whether local vibration (LV)-induced involuntary muscle contraction plus BFR would elicit an additive response of muscle activation.

OBJECTIVE:

To investigate the effects of LV exercise with and without BFR on muscle activation.

METHODS:

Eight physically inactive males were randomly assigned to a sequence of LV (0.45 mm amplitude, 35 Hz sinusoid, lasting 1 min) treatments on unilateral antagonistic muscle groups, including the upper arm (biceps and triceps), the calf (gastrocnemius, GAS; tibialis anterior, TA) and the thigh (rectus femoris, RF; biceps femoris, BF), in a repeated-measures counterbalanced design, with a 5-min interval separating the treatments. The LV treatments on each of the limbs included one bout of LV and one bout of LV $+$ BFR (inflated to 200 mmHg) in a counterbalanced order.

RESULTS:

Only the RF and biceps in the measured muscles showed greater electromyography (EMG) values during LV $+$ BFR than those during LV ( $p<$ 0.05).

CONCLUSIONS:

LV $+$ BFR elicits an increase in EMG amplitude in some muscles, with obviously greater values in the RF for the thigh and in the biceps for the upper limb.

1. Introduction

Performing regular resistance exercise has been demonstrated to elicit multiple health benefits [1]. However, for some populations, engaging in resistance exercise can be difficult. Hence, using portable devices that invoke involuntary muscle contraction as an alternative training modality might be of great significance. Recent studies have reported that individuals exposed to local vibration (LV) regularly can improve their leg extension strength [2, 3], suggesting that LV induces a somewhat similar response as that induced by resistance exercise and may thus be easily used as a training modality. LV is one form of the vibration, also known as direct vibration, in which a vibrator is often placed on the belly or the tendon of a specific muscle. Vibration can contract the users’ muscles involuntarily. This muscle contraction process is termed as tonic vibration reflex, where the muscle spindles work through the stretch reflex, which facilitates the activation of Ia-motoneurons, leading to involuntary muscle contraction [4]. Recent studies have reported that muscle activation was enhanced immediately following either LV with resistance exercise [5] or LV exposure alone [6].

Figure 1.

Schematic timeline of the experimental procedure. LV $+$ BFR $=$ local vibration plus blood flow restriction; LV $=$ local vibration; MVC $=$ maximal voluntary contractions; EMG $=$ electromyography.

Moreover, research conducted in the last few deca-des has indicated that 20%–30% of the one-repetition maximal (1 RM) of low-intensity resistance exercise combined with blood flow restriction (BFR) results in muscle hypertrophy and strength gain in adaptations similar to those of traditional high-intensity resistance training [7, 8, 9], which has been proposed as an alternative to heavy resistance training [8, 9]. This emerging strength training method, in which the proximal portion of the exercising muscles is typically compressed by wrapping an inflated cuff or wraps, is termed as occlusion training or BFR resistance exercise [8]. During BFR resistance exercise, the venous return of blood flow from the muscles is reduced, and even the arterial inflow into the muscles is reduced depending on the cuff pressure applied [10], which might result in an ischaemic/hypoxic environment, subsequently leading to increased metabolites [11, 12]. The increased metabolites due to BFR may increase the perceived effort and muscle activation, showing a greater electromyography (EMG) value during BFR resistance exercise than that during resistance exercise alone [8, 13]. Although the mechanism underlying BFR resistance training induced-muscle adaptation is not completely understood, it has been speculated to be related, in part, to an increase in muscle activation [14, 15].

Table 1

Root mean square (RMS) electromyography (EMG) values (mean $\pm$ SEM) between LV $+$ BFR and LV

Muscles	Treatment
	LV	LV $+$ BFR	$T$	P value
Biceps	5.39 $\pm$ 2.04	6.95 $\pm$ 2.76*	$-$ 2.640	0.033
Triceps	5.28 $\pm$ 2.53	5.31 $\pm$ 2.80	$-$ 0.053	0.959
Tibialis anterior	5.26 $\pm$ 2.22	6.8 $\pm$ 3.61	$-$ 1.493	0.179
Gastrocnemius	9.54 $\pm$ 4.18	9.76 $\pm$ 5.15	$-$ 0.214	0.837
Rectus femoris	4.25 $\pm$ 3.65	10.34 $\pm$ 8.29*	$-$ 2.50	0.041
Biceps femoris	8.42 $\pm$ 4.67	9.33 $\pm$ 5.98	$-$ 0.541	0.605

${}^{*}$ Significant difference between LV $+$ BFR and LV ( $p<$ 0.05). LV $+$ BFR $=$ local vibration plus blood flow restriction; LV $=$ local vibration. Values are presented as a percentage of muscle activity during an isolated MVC (100%).

Given that an additive effect of BFR resistance exercise is often observed under low-intensity resistance exercise [8, 10, 11, 12, 13], and EMG analysis indicated that the magnitude of muscle activation immediately following LV leads to low intensities of muscle contraction (range: 3.54%–11.55% of maximal voluntary contractions, MVCs) [5, 6], the spurring hypothesis is that even adding BFR during this involuntary muscle contraction process can further increase the muscle activation.

Therefore, the aims of this study were to investigate the effects of LV $+$ BFR on muscle activation in untrained populations and to investigate whether LV $+$ BFR would produce an additive muscle activation response compared to that with LV alone. The major agonist and antagonist muscle groups of the thigh, the calf and the upper arm were selected in this study to investigate whether there is a difference in response to treatment among muscles.

2. Materials and methods

2.1 Subjects

A total of eight healthy males (mean $\pm$ SE, age: 21.75 $\pm$ 1.75 years, height: 170.63 $\pm$ 4.43 cm, body mass: 58.37 $\pm$ 4.65 kg) participated in this study. All of them were determined to be physically inactive through screening with the International Physical Activity Questionnaire-Short Form [16]. No participants reported that they were smokers or alcohol and coffee drinkers. This study was approved by the Institutional Review Board of Kaohsiung Medical University Chung-Ho Memorial Hospital, Kaohsiung, Taiwan, and was conducted in accordance with the ethical standards of the Declaration of Helsinki protocol, and written informed consent was obtained from all subjects.

2.2 Study design

Prior to the experiment, the participants were familiarised with the experimental procedures and devices. In the experimental session, the participants were randomly assigned to a sequence of LV treatments on unilateral antagonistic muscle groups, including the upper arm (biceps and triceps), the calf (gastrocnemius, GAS; tibialis anterior; TA) and the thigh (rectus femoris, RF; biceps femoris, BF) in a repeated-measures design. The treatments were proceeded by two vibrators respectively strapped onto the skin over the belly of the specific antagonistic muscle groups, and then the antagonistic muscles were exposed to LV for 1 min simultaneously. Each of the three limbs was subjected to one bout of LV and another bout of BFR applied on LV in a counterbalanced order, with a 5-min interval separating treatments (Fig. 1). To emphasise the character of passive exercise, during each treatment, the participants were requested to maintain a comfortable and relaxed seated posture, with the knee angle at 90 ${}^{\circ }$ , hip angle at 110 ${}^{\circ }$ , elbow bent at 90 ${}^{\circ }$ and the ankle in a neutral position. To collect EMG signals during each session, the Ag/AgCl circular bipolar surface electrodes (2-cm interelectrode distance) were attached to the designated muscle above the vibrators of the non-dominant limbs before specific muscle exposure to LV began. The procedure began by performing specific muscle isometric MVCs to collect the EMG amplitude [17]. Verbal encouragement was given during MVCs. To normalise the EMG amplitude, muscle activation was recorded during isolated and MVCs. The procedure was repeated until the completion of the six treatments. For the experiments, the participants were instructed to refrain from consuming alcohol, caffeine or nutritional supplements for 24 h before the experiment and to avoid strenuous exercise for 48 h before the sessions.

2.3 LV devices and protocol applied

Briefly, the custom-designed electromechanical vibration device consists of one control unit, two vibrators (0.45 mm amplitude, 35 Hz sinusoid) connected to the control unit and a medically approved power supplier, which results in a peak acceleration of 2.2 g as calculated using the Lorenzen formula [18]. The acceleration is within the range to stimulate the increase in muscle strength when using regularly [3]. Each vibrator has an encoder. The rotating element inside the vibrators is a cylindrical brass component with a length of 30 mm, a diameter of 24 mm and a thickness of 18 mm (weighing about 38.5 g) (Fig. 2). The vibrators were properly fixed to the skin with an elastic belt to prevent from swinging and to avoid movement artefact.

Figure 2.

The local vibrators.

2.4 BFR device and pressure applied

In the LV $+$ BFR treatments, all participants wore a custom-made BFR device around the proximal portion of the vibrating muscles. The device is composed of an inflatable nylon cuff, a hand bulb pump with a check valve, a pressure gauge and rubber tubes, similar to a sphygmomanometer. The cuff is connected to the hand bulb by the rubber tubes, through which air passes during pumping. Another tube from the bulb connects the pressure gauge, in which the pressure of the cuffs is obtained. During the LV $+$ BFR treatments, the width of the cuff utilised was 3 cm and the pressure was set at 200 mmHg. Although a previous study had indicated that when using relatively narrow restrictive cuffs (5–6 cm for the lower body or 3–4 cm for the upper body), restrictive cuff pressures between 160–240 mmHg for the lower body and 100–160 mmHg for the upper body may be appropriate for most individuals [19]. However, a pressure of above 200 mmHg for the upper arm has also been used in a previous study [13]. To facilitate the comparison, we utilised uniform cuff width and pressure for all limbs. This is because in the pilot work of our study group, we tested this cuff pressure and width for several times, but the physically inactive individuals did not feel uncomfortable.

2.5 EMG signal analyses

Each participant’s skin was shaved, abraded and cleaned with alcohol to minimise impedance, after which bipolar surface electrodes (Ag/AgCl, 3M Health Care, St. Paul, MN) were applied on the RF, BF, GS, TA, biceps and triceps right below the vibrators, and the ground electrode was placed over the epicondyle of the tibia. The EMG signals were collected using the Noraxon Telemyo DTS EMG system (Noraxon Inc, USA) and were amplified ( $\times$ 1000), bandpass-filtered at 10–500 Hz $\pm$ 2% cut-off (Butterworth/Bessels). The electrodes and the wireless transducer were secured with tape to minimise disruption during exercise. To evaluate the motor unit requirements, the raw EMG signals were processed as the root mean square (RMS) values and were full-wave-rectified and low-pass-filtered (12 Hz) over the given periods. Then, the RMS EMG signals were MVC-normalised according to the isolated MVCs done prior to the treatment. Subsequently, muscle activation was expressed as %MVC for data analysis.

2.6 Statistical analyses

Data are expressed as the mean $\pm$ standard error (SE). A paired $t$ -test was employed to compare the EMG amplitude on each muscle between the LV $+$ BFR and LV treatments. Statistical significance was set at $p<$ 0.05 for all tests.

3. Results

The paired $t$ -test indicated that the RMS values of the RF and biceps during LV $+$ BFR were significantly higher than those during LV alone ( $p<$ 0.05). However, the difference in RMS values of the triceps, TA, GAS and BF between LV $+$ BFR and LV was non-significant ( $p>$ 0.05).

4. Discussion

To our knowledge, this is the first study to examine the effects of a passive LV exercise with BFR on the muscle activation of both the upper and lower limbs. The primary finding of this study was that the external compression applied on the limbs during LV exposure-magnified muscle activation is muscle-specific, which is only pronounced in the RF and biceps, with no further effect on BF, GAS, TA and triceps.

Recent research has indicated that low-intensity resistance exercise combined with BFR enhanced the muscle activation of both the upper and lower limbs, as demonstrated by EMG analysis [13, 20]. A review article on BFR exercise indicated that lower body restrictive cuff pressures ranging from 160 to 240 mmHg are effective in increasing metabolic stress, due to partial but not complete arterial flow restricted to the muscle [19]. It has been reported that the larger circumference of the limb requires a higher cuff pressure to elicit sufficient occlusion effect during BFR exercise [21]. To let the venous outflow of the arm and calf muscles (smaller than the thigh in circumference) were apt to occlude, the cuff pressure on the thigh in the current study was also applied on the upper arm and calf muscles during LV $+$ BFR treatments. However, only less than half of the muscles measured in the present study showed further increase in activation. Our results are not fully consistent with previous studies conducted using resistance exercise plus BFR [13, 20]. It has been speculated that the divergent results could at least, in part, be associated with the muscle contraction intensity among studies. This is because in the current study, only 4.25% $\pm$ 3.65% to 8.42% $\pm$ 4.67% of MVC (EMGrms) was activated during LV exposure in the measured muscles. The intensity of muscle contraction induced by LV is lower than that induced by the common resistance exercise used in BFR resistance exercise (20%–30% 1 RM). Loenneke et al. [22] have shown that the BFR stimulus alone (in the absence of exercise) was not able to elicit changes in muscle activation. Similarly, Yasuda et al. [23] indicated that regular eccentric BFR resistance training (approximately 10% 1 RM) failed to demonstrate expected hypertrophy results. Taken together with the results of our study, it is suggested that the LV treatment might elicit an insufficient stimulus to the muscles, in which most of the muscles were not easily further activated when BFR was applied.

The recommended cuff pressure for the lower limb while applying for the upper arm may make individuals feel tight and theoretically elicit greater levels of BFR than those applied on the lower limb due to a smaller circumference, which may be easier to magnify the muscle activation response when BFR is applied during LV. In general, as expected, greater EMG amplitude was found in the biceps, the part of the upper arm where BFR was applied with LV. Biceps are the antagonist muscles to the triceps; when the elbow flexes, the biceps shorten or contract and the flexion is the opposite action of a triceps extension. In the current study, the participants were required to maintain a sitting posture that approaches the natural resting position, with the elbows bent to 90 ${}^{\circ }$ and the forearms placed on the thigh. In this bending elbow condition, the biceps may slightly contract while the triceps may remain relax. The findings of the experiment suggest that the posture of the arm when shortened may tend to magnify the muscle activation as an external compression was applied during LV.

When the calf muscle was exposed to LV with or without BFR sessions, the participants maintained an anatomically natural sitting posture, with the knee bent to 90 ${}^{\circ }$ and the foot maintained 0. In this condition, the GAS and TA muscles were nearly not contracted or shortened. Furthermore, the greater circumference of the calf relative to that of the upper arm may elicit a lower level of BFR when the same cuff pressure was applied, which might in turn make the calf activation to be unaffected by BFR during LV.

With regard to the thigh muscles, during the LV $+$ BFR, the muscle activation is enhanced only in RF, but not in BF, as compared with LV. A negative relationship has been previously reported between adipose tissue thickness and pressure with BFR in the brachial artery [24], suggesting that adipose tissue thickness may partly affect the degree of intramuscular pressure by BFR. Although not measured in the current study, it has been consistently reported elsewhere that adipose tissue percentage in BF is on average higher than reported here for the RF [25, 26], suggesting that when BFR is applied around the thigh, a greater compressive force may be put on the RF than that on the BF. These findings could lead to the assumption that the increased compressive force may attain the level to elicit greater muscle activation when BFR is combined with LV. On the other hand, it should be noted that the EMG amplitude in response to this LV manipulation by BFR between the thigh and the calf appears to contradict the perspective of limb circumference factor, since the circumference of the thigh is proportionately larger than that of the calf. Though no other study to date has investigated the effectiveness of BFR on EMG amplitude when combined with LV, other factors that affect the level of BFR have been described, which may include structural features of the muscle (muscle or fat) [27], leg position and its haemodynamics [28]. The factor of structural features of the muscle is illustrated by our data comparing the antagonistic muscular groups in the thigh as mentioned above. Hence, the possible explanations for a part of the thigh muscle to be easily activated by BFR under this LV manipulation could be due to the leg position and its haemodynamics. A previous study has reported that the occluded thigh displays its highest volume expansion rate values in the horizontal position relative to elevated and down positions, and the calf muscle displays its lowest volume expansion rate values in the down position relative to the horizontal and elevated positions [28], which may suggest that the thigh muscle tends to be activated. Altogether, the findings in our study using a similar position in which the participants’ thigh was horizontal to the ground, and the calf was vertical to the ground, might lead to the assumption that a part of the thigh tends to respond to BFR compared with the calf when being locally vibrated. However, this assumption needs to be clarified, since local vessel characteristics among muscles might, at least in part, affect the level of BFR [23].

Regarding the results obtained in the present study, some limiting factors become relevant and warrant emphasis. First, to facilitate the comparison among different limbs, we utilised uniform cuff width and pressure for all limbs, which might have led to a disproportionate increase in BFR levels even when the participants were physically inactive. Second, since the vibrators were applied on the mid part of the target muscles, the EMG electrodes attached on the belly of the RF approaching the vibrators may not fully reflect the maximal muscle activity when exposed to LV, despite normalisation of the data. In spite of these limitations, the findings provide information on the target muscle activation with LV $+$ BFR and LV exercise.

In conclusion, this study provided some evidence that the combination of LV and BFR promotes an increase in muscle activation, with obviously higher values in the RF for the lower limb and in the biceps for the upper limb. The greater muscle activation of the RF and the biceps during LV $+$ BFR compared to that with LV may somewhat be related to a position-specific effect and the soft tissue surrounding the muscles.