Clinical efficacy of vibration stimulation therapy to relieve acute exercise fatigue

2.1 Study design

This study used a completely randomized design with three different treatment groups in a randomized controlled study (ratio 1:1:1). Randomization sequences were generated using the SPSS Statistics software. The study groups included vibratory stimulation of acupoints using a fascia gun (acupoint stimulation group), vibratory stimulation of selected muscles using a fascia gun (muscle stimulation group), and sedentary rest treatment (control group).

2.2 Study participants

The participants in this study were all from the dragon boat team of the Shanghai University of Traditional Chinese Medicine. The inclusion criteria were as follows: (1) athletes of the dragon boat team of the Shanghai University of Traditional Chinese Medicine, aged 18–24 years, in good health, with no abnormalities found in the routine physical examination; (2) subjective sensation of fatigue immediately after training, with ratings of perceived exertion (RPE) of ⩾ 18; and (3) extreme fatigue immediately after heavy exercise training. Those who met all the aforementioned criteria were included in the study. The exclusion criteria were as follows: (1) those who had subjective sensation of fatigue immediately after the end of heavy exercise training, with an RPE 17; (2) those whose symptoms failed to reach extreme fatigue after heavy exercise training; and (3) those with recent sports injuries who could not resume normal training, and those who could not complete the treatment according to the experimental requirements. The discontinuation criteria were as follows: (1) poor compliance during the experiment, affecting the evaluation of effectiveness and safety; and (2) withdrawal during the experiment or withdrawal due to various other reasons before the end of the treatment. The diagnostic criteria were as follows: The 6–20 grading (RPE) scale of Swedish scholar Borg [12] was used for assessing fatigue intensity. The scoring criteria were as follows: 7–8 points: extremely light; 9–10 points: very light; 11–12 points: light; 13–14 points: somewhat hard; 15–16 points: hard; 17–18 points: very hard; and 19–20 points: extremely hard. An RPE ⩾ 18 was classified as an acute motor fatigue state [13] (Table 1). The number of participants was 60, including 23 men and 37 women.

This study was conducted in the gymnasium of the Shanghai University of Traditional Chinese Medicine. First, the participants were assessed to determine whether they met the eligibility criteria, and baseline characteristics data were collected. After conducting the assessment, participants were provided a rowing machine to perform a simulated rowing exercise with a prescribed pulp frequency of 2:00–2:15. The rowing resistance with an initial level of 3 was used, starting with increasing 1 frame of resistance after 3 min and increasing 1 frame of resistance every 2 min thereafter, with an upper limit of resistance of 10 frames and an upper limit of rowing time of 30 min to observe the HR reaching 180 + . The pulp frequency exceeded 2:15 and could not return within 30 s. The participant’s immediate RPE score of ⩾ 18 was adjudged exhaustion. The HR, skin conductance level (SCL), heart rate variability (HRV), and blood oxygen saturation (SPO 2 ) were measured using the “Human Fatigue Detection and Assessment System” developed by the Institute of Traditional Chinese Medicine Engineering, Shanghai University of Traditional Chinese Medicine, immediately after the determination of exhaustion. The recording time was 2 min, and the testing site was the Shaochong point. Moreover, the data were collected using the same method after the treatment.

2.4 Intervention

The intervention for the control group was 15 min of sedentary rest.

The intervention for the acupoint stimulation group (acupoint group) was as follows: a ROTAI-G20 fascial gun made by Shanghai Rongtai Health Technology Corporation Limited with a frequency of 30 Hz and an amplitude of 9 mm equipped with a conical head was selected. The acupoints of the upper limbs (shoulder point and Quchi point), lumbar region (large intestine Yu and kidney Yu), and lower limbs (ring jump, Yanglingquan, and foot Sanli) were stimulated for 15 min.

The intervention for the muscle stimulation group was as follows: a ROTAI-G20 fascial gun with a frequency of 30 Hz and an amplitude of 9 mm equipped with a ball head was selected. The muscles of the upper limbs (deltoid and trapezius), lumbar region (erector spinae), and lower limbs (gluteus maximus, rectus femoris, and gastrocnemius) were stimulated for 15 min.

2.5 Observation indexes

Each observation index was collected before and after the intervention. The subjective index included the fatigue subjective perception score using Swedish scholar Borg’s 6–20 grading (RPE) scale, and the physiological indexes included HR, SCL, HRV, and SPO 2 .

2.6 Sample size

The study used a completely randomized design with the F test and one-way analysis of variation (ANOVA) calculations using SPSS Statistics 15.0 software. Based on previously reported HRV (lf/hf) values, this study required 60 participants (20 in each of the three groups) to obtain 90% certainty (power = 1 - β ) at the α = 0.05 (two-sided) level of significance after accounting for a 20% shedding rate to detect statistical differences in clinical outcomes among the three groups.

2.7 Statistical analysis

SPSS Statistics 24.0 software was used to analyze the data, and the measurement data were expressed as mean ± standard deviation. For the measurement data that conformed to a normal distribution with homogeneous variance, one-way ANOVA was used for comparison between groups. The least significant difference test was used for a two-way comparison, while the rank-sum test for comparing multiple independent samples was used for data with nonnormal distribution or nonhomogeneous variance. The Mann-Whitney U test was used for two-way comparison, and 95% confidence interval (CI) of median difference was estimated using the Hodges-Lehmann method. The between-group effect values were expressed as η 2 , with 0.01, 0.06, and 0.14 being small, medium, and large effects, respectively. The two comparison effect values were expressed as r (0.10, 0.30, 0.50) or d (0.20, 0.50, 0.80), with 0.05 indicating a statistically significant difference.

4.1 Effects of different stimulus modalities on RPE scores

The subjective physical sensation scale (RPE) [21] is the most widely used scale in current sports medicine to determine exercise intensity and thus evaluate the degree of exercise-related fatigue, representing proprioceptive exercise intensity. The RPE scale reflects changes in subjective psychology, and changes in RPE are related to physiological variables such as HR and blood lactate concentration [22], reflecting changes in physiological function. It is an objective supplement to some physiological and biochemical indicators, and can accurately reflect the body function of in combination with physiological and biochemical indicators. The test results showed that after the intervention, the differences in RPE scores were significant in all the three groups ( 0.05), and the differences were statistically significant ( 0.05) between the two stimulation groups and the control group, while no differences were found between the two stimulation groups. This indicated that the subjective fatigue of the participants was significantly reduced when the acupoints and muscles were stimulated using the fascial gun, indicating that vibration relaxation is a mechanical, physical stimulation that stimulates the vegetative nervous system and thus causes a relaxation effect on the body, ultimately reducing the subjective fatigue.

4.2 Effects of different stimulation patterns on the electrodermal activity

Numerous studies [23, 24] have shown that the autonomic nervous system, consisting of sympathetic and parasympathetic nerves regulated by brain activity in the brainstem, is involved during muscle contraction. The activation of the autonomic nervous system causes significant effects on the HR, blood pressure, and electrical skin activity. Electrical skin activity refers to changes in sweat gland activity and is mainly controlled by the sympathetic branch of the autonomic nervous system. When the sympathetic nerves are excited, the sweat glands are active, and sweat secretion subsequently increases, resulting in moist skin due to sweat filling, thereby decreasing epidermal electrical resistance and increasing skin conductance. This index can be used to directly assess mental fatigue and stress intensity [25, 26, 27, 28]. As research advanced, the electrical skin activity could also be used to assess muscle fatigue and could be considered an alternative method for effectively monitoring muscle fatigue due to its easy availability from superficial electrodes and lack of invasiveness [24].

The results of this study showed that the differences in all values of SCL between the three groups were statistically significant, with significant differences in SCL (max), SCL (min), and SCL (mean) between the two stimulation groups after the intervention and the control group. No differences were found between the two intervention groups. A significant difference in SCL (v) was found between the acupoint stimulation and the control groups after the intervention. This difference in the acupoint stimulation group may be related to the meridian-visceral theory. The acupoints selected for this study were located in the lower visceral regulatory centers in the spinal nerve segment region, upstream to the visceral regulatory centers in the brain, and integrated to cause the corresponding visceral reflexes via sympathetic and parasympathetic efferents. It leads to changes in autonomic tone, modulates autonomic activity, and inhibits sympathetic activity; therefore, the SCL values in the acupuncture point group were higher than before the intervention. This resulted in higher SCL values in the acupoint stimulation group compared with the muscle stimulation group probably because the 30-Hz vibration frequency could promote the increase in muscle epidermal temperature by stimulating muscles. This, in turn, promoted the increase in blood flow, accelerated the elimination of metabolites such as lactic acid, prompted fatigue recovery, and reflexively inhibited sympathetic excitation.

4.3 Effects of different stimulation modes on the electrocardiogram activity

HRV has recently been used to quantify the autonomic activity of the heart after exercise because it can reflect sympathetic and parasympathetic changes through the frequency domain indexes hf, lf, and hf/l of HRV and can be used in combination with electrical skin activity to assess exercise fatigue and monitor exercise status [30]. Related studies [29, 30, 31] have shown that when the body is fatigued, sympathetic nerves are significantly active and parasympathetic nerve activity is reduced, resulting in an increase in the HR, a decrease in hf, and an increase in lf/hf. When the hf value increases, it represents an increase in vagal efferent activity, and when the lf/hf ratio decreases, a shift occurs from sympathetic to parasympathetic activity, which is consistent with the present study. The results showed that after the intervention, the differences in HR and HRV (hf) were statistically significant in all three groups, with statistically significant differences in HR in the two stimulation groups compared with the control group. The muscle stimulation group had a greater effect on the HR than the acupoint stimulation group and had a faster heart recovery rate. The differences in HRV (hf) in the acupoint stimulation group were statistically significant compared with the muscle stimulation group, and no differences were found between the two intervention groups compared with the control group. Although the values of lf and lf/hf in the acupoint and muscle stimulation groups were not statistically significant, the values before and after the intervention showed that lf in the two stimulation groups was still elevated after the intervention. The elevated lf reflected the enhanced sympathetic efferent activity, which might be because the fascial gun technique belonged to the vibration stimulation therapy, and the after-effect of the vibration stimulation therapy made the proprioceptive receptors remain in an excited state after a period of vibration [9]. The decrease in the lf/hf ratio represented an enhanced parasympathetic activity; the lf/hf value of the acupoint stimulation group decreased, while that of the muscle stimulation group remained unchanged. Further, the effect of the muscle stimulation and the control groups decreased after the intervention, which might be due to the effect of vibration stimulation on the muscle stimulation group alone and faster HR recovery after the exercise, thus delaying the parasympathetic magnitude of the decrease in sympathetic activity. Combined with the SCL index, it was therefore observed that the two stimulation groups were able to relieve sympathetic excitability, and the acupoint stimulation group was able to effectively relieve sympathetic excitability and enhance parasympathetic activity, while the muscle stimulation group did not have a significant modulation of parasympathetic activity possibly due to its ability to rapidly recover HR.

4.4 Effects of different stimulation modes on SPO 2

Oxygen saturation can reflect the stress level of the body’s exercise load, and high-intensity exercise can lead to a significant decrease in the body’s oxygen saturation. A study revealed that the different levels of fatigue caused by different exercise intensities correspondingly triggered changes in the body’s physiological indicators, manifesting in the cardiovascular aspects of the recovery rate of oxygen saturation [32]. This was not consistent with the present study, which showed that the differences in the values of oxygen saturation in the three groups were not statistically significant. The post-intervention values were significantly lower than the pre-intervention values in this study, which might be because the participants were often trained in rowing sports and the differences in the physical quality of the participants, and their tolerance to fatigue varied.

Based on the results of the aforementioned physiological indicators, it was found that both the stimulation groups could affect RPE, HR, SCL (max), and SCL (mean), and the differences between the two groups were not significant. The acupoint stimulation group had significant effects on SCL (v) and HRV (hf), and the muscle stimulation group had significant effects on SCL (min). Therefore, it was concluded that using the fascial gun technique to stimulate either acupoints or muscles could alleviate acute exercise fatigue. The two groups were able to relieve acute motor fatigue using different mechanisms due to the different stimulation modes. Based on the physiological indices, it was concluded that the mechanism of relieving fatigue in the acupoint stimulation group involved its ability to relieve sympathetic excitability and enhance parasympathetic activity, while the mechanism of relieving fatigue in the muscle stimulation group involved its ability to regulate sympathetic activity.

Although this study provided a new research perspective and means to relieve acute exercise fatigue, it had some shortcomings. The HRV (hf), HRV (lf/hf), and SPO 2 values were not statistically significant, which might be due to individual differences. During the experiment, we ignored the biomechanical effect of the pressure applied by the fascial gun on the treatment site of the body and the changes in electrical resistance of different acupuncture points. These deficiencies need further investigations in future studies.