Shoulder vibratory exercises improves shoulder external rotation muscle strength and shoulder function: Randomized comparison trial

Abstract

BACKGROUND:

Based on electromyography measurements, shoulder vibratory exercises efficiently stimulate shoulder muscles activity. Yet very few studies have supported that shoulder vibratory exercises increased shoulder muscles strength and function, and the noninferiority compared with conventional elastic resistance exercises remains unknown.

OBJECTIVE:

This study investigated the effect of vibratory exercises versus conventional elastic resistance exercises with elastic bands on shoulder external rotation muscles strength and functional performance in young adults.

METHODS:

26 young adults (7 males and 19 females, with age 23.89 $\pm$ 3.02) were recruited and randomly allocated to shoulder vibratory exercises with FLEXI-BAR (FLEXI-BAR group) or conventional resistance exercises with elastic band (TheraBand group) for 3 times/week, 4 weeks totally. Shoulder external rotator muscles strength test and Underkofler softball distance throw test (USDTT) were performed before and after the training period.

RESULTS:

After 4 weeks training, shoulder external rotator muscles strength increased 22.25 $\pm$ 15.06 N ( $P=$ 0.004, effect size $=$ 1.48) within FLEXI-BAR group and 22.81 $\pm$ 14.94 N ( $P=$ 0.007, effect size $=$ 1.53) within TheraBand group. There were no statistically significant differences between groups in the three muscle strength tests ( $P>$ 0.65). Regarding shoulder function, FLEXI-BAR exercises increased the throw distance 0.81 $\pm$ 0.92 meters in USDTT ( $P=$ 0.041, effect size $=$ 0.88) while TheraBand exercise did not ( $P=$ 0.284), yet there was no statistically significant between group effects ( $P=$ 0.608).

CONCLUSIONS:

These findings suggest that shoulder vibratory exercises can improve shoulder muscles strength in young adults and can be a useful alternative to the conventional elastic resistance exercises to improve the shoulder muscles strength and function. This provides therapists with more options in terms of choosing training equipment for rehabilitation programs.

Keywords

Vibration FLEXI-BAR TheraBand resistance training rehabilitation

1. Introduction

Shoulder pain is one of the most common musculoskeletal symptoms. It is the third most common reason for musculoskeletal consultations in the primary care setting, affecting up to one-third of the general population [1]. Among competitive swimmers, the shoulder is the most common area for pain or musculoskeletal injuries; as for elite swimmers, 91% suffered shoulder pain on a regular basis [2]. One common cause of shoulder pain has been considered as rotator cuff tendon lesions, accounting for 50% to 85% of shoulder diagnoses [3]. Besides, either laceration or weakness of the glenohumeral external rotators are important causal factors of shoulder pain and injury in handball players, while no association are found between internal rotators strength and shoulder problems [4]. One underlying mechanism is that the week external rotators are not able to effectively dissipate the greatest glenohumeral joint loading including inferior shear forces, compressive forces and adduction torque during the deceleration phase of throwing [5]. Previous study also supported that strengthening of the rotator cuff could prevent shoulder injuries and effectively reduce the incidence of shoulder pain in swimmers and handball players [6, 7]. Moreover, intensive scapular training can improve short-term shoulder pain and function in adults with subacromial pain syndrome (SAPS) [8]. However, complicated training regimens with difficult exercises are often associated with low training adherence. Therefore, simple and effective training methods to improve muscle strength and function is important for the management of shoulder pain.

The FLEXI-BAR ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ Standard is an enjoyable exercise instrument with active shoulder vibration. The patients or athletes use their hands and arms to actively produce oscillations of 4.5 $\sim$ 5 Hz with the FLEXI-BAR, causing the maximum output of resistance up to 151 N and 270 $\sim$ 300 muscle contractions per minute [9]. Using electromyography, many studies have measured shoulder muscles activity when using the FLEXI-BAR. Mechanical vibration with low intensity (below 0.4 g) and low frequency (below 50 Hz) can be effectively transmitted through the body [9], and the shoulder vibratory exercises can even produce greater scapular muscle activity than traditional slow-velocity resistance techniques [10]. Vibratory mechanical stimulation provides strong sensory stimulation, activating muscle spindles [11] and strengthening proprioceptive sense [12]. A previous study did not find strength gains in the shoulder rotator muscles when using a flexible foil in asymptomatic populations [13]. However, another study found increased strength and thickness of the shoulder muscles when combining shoulder vibratory exercises with conventional elastic resistance exercises rather than the shoulder vibratory exercises alone in healthy young adults [14]. Nevertheless, whether shoulder vibratory exercises can directly improve shoulder muscles strength or functional performance, and whether shoulder vibratory exercises are not inferior to conventional elastic resistance exercise remain unknown.

Therefore, investigating the effect of the active shoulder vibratory exercises on the external rotator muscles strength and functional performance of shoulders is necessary for three reasons. First, to investigate whether shoulder vibratory exercises alone can improve muscle strength of the rotator cuff. Second, to investigate differences between shoulder vibratory exercises and elastic resistance exercises on muscle strength and functional performance of shoulder. Third, to contribute with evidence that allows therapists to choose between different types of effective rehabilitation devices.

The purpose of our study was to identify the effect of FLEXI-BAR exercises versus TheraBand exercises on shoulder external rotator muscles strength and Underkofler softball distance throw test (USDTT) among young adults. The hypotheses are that 1) FLEXI-BAR exercises improve the strength of shoulder external rotators and shoulder functional performance, and 2) FLEXI-BAR exercises are not inferior to TheraBand exercises.

2. Methods

2.1 Participants and randomization

The sample size was calculated using the effect size from our pilot study and previous studies [13, 14]. Based on the G*Power 3.1 $t$ test: Means (Difference between two means), we chose a priori: computed required sample size with one tail, effect size $-$ 1.2 (calculated by using Cohen’s d), an alpha of 0.05 and power ( $1-\beta$ ) $=$ 0.8, a minimum of at least 10 participants would be required.

Table 1
The strengthening exercises protocol for FLEXI-BAR and TheraBand

	FLEXI-BAR group	TheraBand group
Movement patterns	Oscillation exercise	Elastic band exercise
Intensity	Start at 5 cm amplitude level; add 5 cm amplitude when Brog CR10 scale is less than 6 scores of after finishing 6 sessions	Start at blue level; change to next deeper color when Brog CR10 scale is less than 6 scores of after finishing 6 sessions
Time	30 s a section, then rest 30 s, the whole 1 minute as a set, totally 6 sets a day	30 s a section, then rest 30 s, the whole 1 minute as a set, totally 6 sets a day
Frequency	3 times a week, at lease 1-day rest between 2 training day, totally for 4 weeks	3 times a week, at lease 1-day rest between 2 training day, totally for 4 weeks
Tools	FLEXI-BAR (standard style) (Togu ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ , Germany)	TheraBand non-latex Clx retentions Loops (TheraBand ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ , USA)
Procedure	At first 3 sets, subjects are in standing position with the dominant shoulder kept neutral position and with the elbow 90 ${}^{\circ}$ flexion; then the dominant hand continuously shakes FLEXI-BAR between left and right direction. At last 3 sets, subjects are in standing position with the dominant shoulder kept the terminally external rotation and with the elbow 90 ${}^{\circ}$ flexion; then the dominant hand continuously shakes FLEXI-BAR between forward and backward direction.	At the beginning, subjects are in standing position with the dominant shoulder kept neutral position and with the elbow 90 ${}^{\circ}$ flexion, hand grasps the TheraBand fixed on an object at the same height with the distance about 0.7 m; at the end, shoulder externally rotated to the end of the range of motion. All the movements were carried out in a slow controlled movement without sudden shaking and acceleration. At each set, the hand elongates the TheraBand to make the shoulder externally rotating to the end in 2 seconds, and then returns to the starting position in 1 second respectively, repeating for 10 times. All the 6 sets are the same.
Attention	During the rest period, the subject maintained static standing position and any other exercise was prohibited.

Participants were recruited at The Sixth Affiliated Hospital, Sun Yat-sen University. The inclusion criteria were: (1) young adults with age from 18 to 40; (2) no history of spine or shoulder surgery, trauma, or history of disease; (3) no participation in any formal or shoulder muscle strength training in the past six months; (4) understanding and voluntary participating in the experiment, and signing the informed consent; (5) being able to maintain FLEXI-BAR movement with amplitude about 5 cm lasting for more than 30 s [15]. The exclusion criteria were: (1) unstable vital signs; (3) any shoulder or spinal symptoms or signs; (2) serious diseases such as heart disease, high blood pressure, or tumor; (3) cognitive dysfunction and inability to cooperate with the experiment. The study fulfilled the ethical standards and was approved by the ethics committee of The Sixth Affiliated Hospital, Sun Yat-sen University (2019zslyec-181).

Randomization was done by a simple random sampling. After informed consent, the subjects were allocated according to Permuted Block Randomization: two pieces of paper were prepared with AB and BA character written, AB represents the enrollment into the TheraBand group, and BA into the FLEXI-BAR group. Numbered, opaque and sealed envelopes containing the exercise allocation were prepared before the trial.

2.2 Study design

The FLEXI-BAR ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ Standard (Togu, Prien am Chiemsee, Germany) was used in the FLEXI-BAR group. All the participant in the FLEXI-BAR group started at a small amplitude (5 cm) level. Before the experiment, all the subjects received FLEXI-BAR skills training for half an hour based on the E-Book Training Manual from www.flexi-bar.co.uk. Subjects learned 1) the basic concept, 2) the General Technique Considerations, 3) Grip Positions, 4) Technique for Shaking and Maintaining, and 5) Frequently Asked Questions. Finally, they were able to maintain at least a small amplitude (5 cm) movement for more than 30 s. Subjects were familiarized with the procedures and methods of the experiment before the program started. As for the TheraBand group, subjects were able to continually complete more than 10 times of external rotation. All the subjects warmed up for 3 minutes, then performed 30 s exercises and rested 30 s as a set, totally 6 sets a day, 3 times a week, at least 1-day rest between 2 training days, totally for 4 weeks [13, 14]. The whole training was supervised by researchers in an exercise room. Previous study has confirmed that 4-week muscle strengthening using vibratory exercises together with elastic resistance exercises was adequate to produce significantly effects on shoulder muscle strength [14]. Shorter than 1 minutes continuous FLEXI-BAR exercises per set have been supported safe for cardiovascular stress and acceptable for participants [15]. Table 1 shows the strengthening exercises protocol for both groups.

Subjects in TheraBand group used TheraBand non-latex Clx retentions Loops (Hygenic Corporation, Akron, USA), starting at the blue level. The beginning position and ending position diagram of TheraBand are shown in Fig. 1. The first 3 sets action diagram of FLEXI-BAR group are shown in Fig. 2a and b. The later 3 sets action diagram is shown in Fig. 2c and d. The FITT was also designed based on the previous literature [13, 14] and were adjusted slightly by experienced physiotherapists.

Figure 1.

Procedure of TheraBand exercise. At the beginning, subjects are in standing position with the dominant shoulder kept neutral position and with the elbow 90 ${}^{\circ}$ flexion, hand grasps the TheraBand fixed on an object at the same height with the distance about 0.7 m (a); at the end, shoulder externally rotated to the end of the range of motion (b).

Figure 2.

Procedure of FLEXI-BAR exercise. At first 3 sets, subjects are in standing position with the dominant shoulder kept neutral position and with the elbow 90 ${}^{\circ}$ flexion (a) (b); then the dominant hand continuously shakes FLEXI-BAR between left and right direction. At last 3 sets, subjects are in standing position with the dominant shoulder kept the terminally external rotation and with the elbow 90 ${}^{\circ}$ flexion (c) (d); then the dominant hand continuously shakes FLEXI-BAR between forward and backward direction.

Increasing amplitude during vibration could add exertion to the subjects for the FLEXI-BAR group, while increasing elastic force could add resistance for TheraBand group. To meet the principle of progressive overload, we adjusted the resistance progress according to the Borg CR10 scale rating of perceived exertion [16, 17] (asked after finishing 6 sets exercises for both group), as well as the ability successfully to complete the exercise. If the Borg CR10 scale ranged between 0 and 5 point, the intensity was increased during the subsequent session. The exercise intensity was maintained when subjects scores more than 5. The intensity levels were 5 cm amplitude in the FLEXI-BAR group and next color-level in the TheraBand group.

The experiment would be stopped in case of experiencing any discomfort from the participants. Experiment would be finished before all subjects rested for 30 s and then stretched external rotator muscles for 3 minutes.

Table 2

Number, name, action, and measuring device location of shoulder external rotator muscles strength test

Test name	Action	Measuring device location
External rotation muscle centrifugal test (EC)	Sitting position, the shoulder joint maintains 90 ${}^{\circ}$ abduction, external rotate from 90 ${}^{\circ}$ to 0 ${}^{\circ}$ .	The dorsal part of the distal radioulnar joint
External rotation 0 ${}^{\circ}$ isometric test (0 ${}^{\circ}$ EI)	Sitting position, shoulder joint maintains 90 ${}^{\circ}$ abduction, isometrically contract the external rotator resisted the measuring equipment.	The dorsal part of the distal radioulnar joint
External rotation 90 ${}^{\circ}$ isometric test (90 ${}^{\circ}$ EI)	Sitting position, shoulder joint maintains 90 ${}^{\circ}$ abduction and 90 ${}^{\circ}$ external rotation, isometrically contract the external rotator resisted the measuring equipment.	The dorsal part of the distal radioulnar joint

2.3 Shoulder external rotator muscle strength test

MicroFET 3 Dynamometer and ROM Evaluator (Hoggan Scientific LLC, Salt Lake City, USA) was used to test the centrifugal muscle strength and isometric muscle strength of the shoulder external rotators [18], and the device has been confirmed with very high reliability and validity for measuring the strength of the external and internal rotator of the shoulder [19]. The measurement unit of muscle strength was set as Newton (symbol: N) based on the guideline of HOGGAN ${}^{\@setsize{\scriptsize}{8pt}{\viipt}{\@viipt}\textregistered}$ . A strength change of more than 15% in any position can be considered meaningful [20]. During the measurement, the subjects kept in the sitting position, and the researcher informed the subjects of correct way of exertion, the muscle position of exertion and the avoiding compensation movement. For measuring the strength of external rotators, external rotation muscle centrifugal test (EC), external rotation 0 ${}^{\circ}$ isometric test (0 ${}^{\circ}$ EI) and external rotation 90 ${}^{\circ}$ isometric test (90 ${}^{\circ}$ EI) will be used. The details of each test are described in Table 2. The order of the three tests was randomized in order to control the program learning and fatigue effect. Before the tests, the subjects performed a standardized shoulder warm-up: full range of active range of movement (AROM) in all directions for 3 minutes. All the tests started after an explanation of the movements and a practice test. Each test included three 5-second maximal voluntary isometric contraction (MVIC) with an interval of 30 seconds as a standard measurement. The subjects were unaware of the movement order and test results of the other subjects. The measurements were taken by the same researcher before and after 4 weeks training, and the researchers conducted the post-test 3 days after participants completed their whole program to reduce the fatigue effect induced by post-training.

2.4 Underkofler softball distance throw test (USDTT)

The softball distance throw is regarded as a functional test of the upper extremities [21]. The USDTT has been previously reported with test-retest reliability of intraclass correlation coefficient (ICC) $=$ 0.95 [22]. The subjects tried their best to throw a standard league softball (30.5 cm in circumference, weighing 178 grams) as far as possible using their dominant arms. Subjects were required to perform a maximum 3-minute warm-up (1.5 minutes forward flexion and 1.5 minutes backward extension), followed by 30 seconds of shoulder internal and external rotation and a three-minute flick. At the beginning of the test, the subjects were in a high kneeling position, and the subjects performed four times warm-up exercises of throwing from 25% to 100% strength without stepping out the pre-determined line. The subjects made three maximal effort throwing, all the distances between the position of the softball first landing and the pre-determined line was measured and regarded as the throwing distance (unit set as meter). The thrower had to throw again if crossing the line. The measurements were performed by the same researcher before and after 4 weeks of training, and the researchers conducted the post-test 3 days after participants completed their whole program. The pre- and post-test average distance of the 3 successful throws was calculated and used for statistical analysis.

2.5 Statistical analysis

Data analysis was performed using SPSS ver.20 (SPSS Inc., Chicag, USA). The Studentized Residuals and Box plot in SPSS ver.20 were used to check for the presence of abnormal values. We used the independent $t$ -test and Chi-square test to determine the homogeneity of means of baseline variables between the two groups, used the independent $t$ -test to compare the change values of muscle strength and USDTT variables between groups, and used the paired $t$ -test to compare the change value within each group. If tests of normality were not satisfied, the non-parameter method Mann-Whitney U test and Wilcoxon sign rank test were used. For all tests, we set $P\leqslant$ 0.05 as the significant level. The effect size calculated by Cohen’s d is used to show the difference within/between groups. The value of Cohen’s $d>$ 0.01 indicates very small effect size, $>$ 0.2 indicates small effect size, $>$ 0.5 indicates medium effect size, $>$ 0.8 indicates large effect size, $>$ 1 indicates very large effect size, $>$ 2 indicates huge effect size.

3. Results

As shown in the flowchart (Fig. 3), 33 subjects were recruited of which 26 were eligible for participation (7 males and 19 females, with age 23.89 $\pm$ 3.02) and randomization. The overall characteristics of the 26 participants are summarized in Table 3. There were no statistically significant differences in characteristic values between the FLEXI-BAR group and the TheraBand group ( $P>$ 0.47). Pre-test values of all outcomes between groups were also not statistically significant. No adverse events and complaint were found in the FLEXI-BAR exercises and TheraBand exercises.

Table 3
Characteristics of the included participants

	FLEX-BAR group		TheraBand group		$P$
$n$ (male/female)	13 (2/11)		13 (5/8)		N/A
Age (years)	24.31	$\pm$ 3.97	23.5	$\pm$ 1.83	0.499
Height (cm)	164.18	$\pm$ 8.15	161.86	$\pm$ 8.25	0.470
Weight (kg)	56.43	$\pm$ 9.64	55.32	$\pm$ 10.65	0.780
BMI (kg/m ${}^{2}$ )	20.95	$\pm$ 3.31	20.99	$\pm$ 27.2	0.973
Dominant side (L/R)	1/12		2/11		0.357
EC (N)	82.1	$\pm$ 19.3	89.96	$\pm$ 19.92	0.317
0 ${}^{\circ}$ EI (N)	64.74	$\pm$ 21.65	74.78	$\pm$ 16.6	0.197
90 ${}^{\circ}$ EI (N)	76.73	$\pm$ 18.67	83.26	$\pm$ 20.06	0.399
USDTT (m)	10.06	$\pm$ 2.14	11.81	$\pm$ 3.97	0.178

EC $=$ centrifugal test; 0 ${}^{\circ}$ EI $=$ 0 ${}^{\circ}$ isometric test; 90 ${}^{\circ}$ EI $=$ 90 ${}^{\circ}$ isometric test; USDTT $=$ Underkofler softball distance throw test; the unit N is Newton, while the unit m is meter; the value is mean $\pm$ SD, $P<$ 0.05 is set as statistical significance.

Figure 3.

The flow chart.

The results of the external rotator muscles tests within each group and between groups are shown in Table 4. After 4-weeks training, EC increased from 84.57 $\pm$ 21.76 N to 102.99 $\pm$ 29.46 N in FLEXI-BAR group and the change value was about 21.8% improvement, while EC increased from 76.70 $\pm$ 7.35 N to 89.50 $\pm$ 12.09 N in TheraBand group and the change value was about 16.7%, but the change after training for each group was not statistically significant. No significant difference was found between groups ( $P=$ 0.65, effect size $=$ 0.25), although the change value in FLEXI-BAR group (18.41 $\pm$ 30.33) seemed a little better improvement than in TheraBand group (12.80 $\pm$ 7.64).

After 4-weeks training, 0 ${}^{\circ}$ EI increased 18.86 $\pm$ 16.63 N in FLEXI-BAR group ( $P=$ 0.015, effect size $=$ 1.13) and 0 ${}^{\circ}$ EI increased 15.46 $\pm$ 11.01 N in TheraBand group ( $P=$ 0.01, effect size $=$ 1.4), but there was no significant difference found between groups. What is more, 90 ${}^{\circ}$ EI increased 22.25 $\pm$ 15.06 N approximated 31.2% improvement in FLEXI-BAR group ( $P=$ 0.004, effect size $=$ 1.48), and 90 ${}^{\circ}$ EI increased 22.81 $\pm$ 14.94 N approximated 39% improvement in TheraBand group ( $P=$ 0.007, effect size $=$ 1.53) which seemed better than the former, but there was also no significant difference between groups ( $P=$ 0.94, effect size $=$ 0.04).

As to the result of USDTT, FLEXI-BAR exercises increased the distance of softball throw from 9.66 $\pm$ 1.55 m to 10.47 $\pm$ 1.30 m which approximated 8.4% improvement with significant difference ( $P=$ 0.041, effect size $=$ 0.81), yet TheraBand exercises increased 0.53 $\pm$ 1.18 m which approximated 5.6% without significant difference ( $P=$ 0.284, effect size $=$ 0.45). After the training, there was no significant difference between groups ( $P=$ 0.608, effect size $=$ 0.26), these results of USDTT shows that both FLEXI-BAR exercises and TheraBand exercises exerted the same effect on shoulder function.

4. Discussion

The randomized study compared the effect of vibratory exercise with conventional elastic resistance exercises on shoulder muscles strength and functional performance in young adults. The two study hypotheses were supported by the results, i.e. the FLEXI-BAR exercises improved shoulder external rotator strength and functional performance and were not inferior to the TheraBand exercises.

These findings suggest that both active shoulder vibratory exercises and elastic resistance exercises can improve the shoulder external rotator muscles strength without significant difference between two types of exercises. Furthermore, the mean changes in the two groups were very close. It is there plausible that both types of training exhibit similar strengthening effects for the shoulder muscles. The present findings also support previous study which concluded that both Body-blade (FLEXI-BAR-like) plus TheraBand exercise and TheraBand exercise are able to increase the muscle strength after 4 weeks exercise [23]. In that study, the muscle thickness improvement in the Body-blade plus TheraBand training group were better than TheraBand

Table 4
The results of the external rotator muscles tests and Underkofler softball distance throw test (USDTT) within and between groups

	FLEXI-BAR group					TheraBand group					Between groups
	Pre-test	Post-test	Change	$P$	Cohen’s d	Pre-test	Post-test	Change	$P$	Cohen’s d	$P$	Cohen’s d
EC (N)	84.57 $\pm$ 21.76	102.99 $\pm$ 29.46	18.41 $\pm$ 30.33	0.13	0.61	76.70 $\pm$ 7.35	89.50 $\pm$ 12.09	12.80 $\pm$ 7.64	0.054	1.68	0.65	0.25
0 ${}^{\circ}$ EI (N)	76.34 $\pm$ 19.69	95.20 $\pm$ 17.88	18.86 $\pm$ 16.63	0.015	1.13	72.15 $\pm$ 10.7	87.61 $\pm$ 13.87	15.46 $\pm$ 11.01	0.01	1.4	0.65	0.24
90 ${}^{\circ}$ EI (N)	70.13 $\pm$ 15.43	92.38 $\pm$ 19.27	22.25 $\pm$ 15.06	0.004	1.48	58.44 $\pm$ 6.45	81.25 $\pm$ 12.67	22.81 $\pm$ 14.94	0.007	1.53	0.94	0.04
USDTT (m)	9.66 $\pm$ 1.55	10.47 $\pm$ 1.3	0.81 $\pm$ 0.92	0.041 ${}^{\text{a}}$	0.88	9.39 $\pm$ 1.67	9.92 $\pm$ 2.13	0.53 $\pm$ 1.18	0.284 ${}^{\text{a}}$	0.45	0.608 ${}^{\text{b}}$	0.26

${}^{\text{a}}$ : Wilcoxon sign rank test is used; ${}^{\text{b}}$ : Mann-Whitney U test is used; EC $=$ centrifugal test; 0 ${}^{\circ}$ EI $=$ 0 ${}^{\circ}$ isometric test; 90 ${}^{\circ}$ EI $=$ 90 ${}^{\circ}$ isometric test; USDTT $=$ Underkofler softball distance throw test; the unit N is Newton, while the unit m is meter; the value is mean $\pm$ SD, $P<$ 0.05 is set as statistical significance; the value of Cohen’s d: $>$ 0.01 (very small effect size), $>$ 0.2 (small effect size), $>$ 0.5 (medium effect size), $>$ 0.8 (large effect size), $>$ 1 (very large effect size), $>$ 2 (huge effect size).

training group. However, the pattern of FLEXI-BAR movement and TheraBand movement is not similar. Using electromyography (EMG), we know that FLEXI-BAR assist the muscle group to contract continually and rapidly, while the shoulder muscles contract intermittently and more slowly with the TheraBand [9, 24]. Nevertheless, both devices are able to induce a high training intensity in the muscles of interest.

The gains in muscle strength for both groups after 4 weeks resistance training may be partially attributed to the neural activation. Previous researches have already suggested that elastic resistance exercise can improve muscle strength of shoulder external rotators [17, 25], and its strength improvement is similar to the improvement of the training with conventional resistance [26]. A systematic review and meta-analysis compared muscle activation between exercises with elastic resistance and isokinetic resistance, and the authors pointed out that there were no significant differences between groups [27]. They performed to analyze EMG of different muscle activation between the two type of trainings, and the results showed that the muscle activation assessed by surface EMG is not directly associated to muscle strength development, the initial gains in muscle strength following resistance training are primarily attributed to a greater neural activation followed by subsequent strength gains associated with muscle hypertrophy. FLEXI-BAR exercises are also able to or even have an advantage over any other type of strengthening exercise to activate the abdominal and shoulder muscles in a short time [9, 14, 28, 29]. Moreover, a previous study showed that after 3 weeks FLEXI-BAR training, the percentage of reference voluntary contraction (%RVC) of infraspinatus changed markedly, which means the muscle activation had been attributed to a greater neural activation [30]. Another study also found that vibration exercises improved muscles performance through neural modulations which at least happen in the level of the supra-spinal [31]. Therefore, active vibratory exercises may have potential to provide strong neural adaptations.

Our study is the first to document improved shoulder functional performance from shoulder vibratory exercise. As highlighted by Freeston et al., there was a connection between muscle strength and throwing velocity in players, and they emphasized the urgency of muscle strengthening because the throwing velocity associated with shoulder muscles strength [32]. Likewise, the throw distance in our study was probably related to the force yielded by the shoulder muscles. FLEXI-BAR exercises improved the strength of the external rotators and increased the distance of softball throw by 8.4%. On one hand, this may be because that FLEXI-BAR exercises stimulated both internal rotators and external rotators of the involved shoulder. Moreover, performing FLEXI-BAR exercises need the subjects contiguously to grasp FLEXI-BAR, thus the stimulation of hand muscles and forearm muscles may contribute to the better performance of softball throw after 4 weeks resistance training, which is in good agreement with the results of the previous study [33, 34]. On the other hand, neuromuscular training has been reported to improve the throwing speed [35], and vibratory exercise is similar to the rhythmic stabilization drills in neuromuscular training, so the active vibratory exercise may have the potential to improve the functional performance. Nevertheless, the present data to support the effect of shoulder vibratory exercises on the shoulder functional performance was week, and more studies are needed to focus on shoulder function.

As for the effect of between-groups, there was no statistical difference in the functional performance tests of shoulder joints, although some tendencies existed. Three possible reasons may explain this phenomenon. First, it is difficult to accurately quantify the functional performance of shoulder joint, and there is no gold standard for the functional measurement of the shoulder joint. Most of the targeted methods are questionnaires, such as the arm, shoulder and hand questionnaires (DASH), shoulder pain and disability index (SPADI), simple shoulder test (SST), Oxford shoulder score (OSS) and the shoulder disability questionnaire (SDQ) [36]. Although these questionnaires have good reliability and validity, most of them are targeted at the symptomatic population, while the recognition ability for the asymptomatic and normal population is low. Some studies used ball throwing velocity or arm speed [37, 38] to measure the functional change, but these need to use specific instruments and cannot be convenient in the daily clinical use. Thus, we chose to use the UDSTT to assess the function in the study, which is an objective measurement tools fulfilling the normal population and daily clinical application. However, it may not sensitively reflect the minimal changes of shoulder function between groups. Second, the participants in our study were young adults of the general population. On the contrary, majority of studies recruited athletes to participate [17, 32], and ball throwing was considered a skillful movement which may make the common population feel demanding, possibly influencing their functional performance in our study, so the study of FLEXI-BAR exercises on the shoulder function of athletes are also urgent. Third, strengthening only the external rotators may not be sufficient to improve shoulder function especial for those subjects from TheraBand group. In order to improve sports performance, some studies are designed to strengthen different muscle groups in various positions [22, 38], thus further studies should focus on more types or positions of exercises so that to differentiate the functional change for both groups. All in all, the present data supported that the FLEXI-BAR exercises showed noninferior to the TheraBand exercises in the shoulder functional performance as there was no between-groups significant difference.

Limitations as well as strengths exist in our study. First, a limitation is the difficulty in finding indicators for assessing shoulder function for common population without shoulder symptom. Since most functional assessment methods are in the form of questionnaire, and the differentiation is not obvious to the normal population, it is difficult to measure the degree of functional improvement. A solution could be to recruit patients with mild pain and capable of performing softball throw test without causing any aggravation. Second, small sample size and the 4 weeks intervention time might be not enough for both groups, possibly causing the difficulty to detect the between-group difference in the functional evaluation. To resolve this problem, a larger sample size, a longer intervention protocol such as 3 months training and longer term follow up for both groups should be consider. Third, the study only focused on the three group muscles and one shoulder function, resulting in its generalizability is limited. Finally, satisfaction of participants and adherence of exercise were not monitored, which may affect between-group results.

5. Conclusions

The study shows that shoulder vibratory exercises effectively improve shoulder muscles strength. Shoulder vibratory exercises were not inferior to TheraBand exercises and can be a useful alternative to conventional elastic resistance training to strengthening the shoulder muscles and function. This provides therapists with more options in terms of choosing training equipment for rehabilitation programs, but the cost effectiveness is needed to consider. However, more studies investigating the effect of shoulder vibratory exercise on shoulder functional performance are needed.

Ethical approval

The study fulfilled the ethical standards and was approved by the ethics committee of The Sixth Affiliated Hospital, Sun Yat-sen University (2019zslyec-181).

Funding

This work was funded by the Guangdong Hopson-Pearl River Education Development Foundation (No. H20190116202012724).

Informed consent

All the participants voluntary participated the experiment and signed the informed consent.

Author contributions

WJ Lin and WM Wang conceived the research idea and developed the methodology. WJ Lin organized throughout the project, performed the data analysis and wrote the main parts of the manuscript. YP Sun and LF You contributed to co-write the manuscript. YL Li, SP Wu and LX Liao helped to conduct the experiment and data analysis. YL Wang and LL Andersen involved in drafting and revising the manuscript. All authors read and approve the final manuscript.

Footnotes

Acknowledgments

Not applicable.

Conflict of interest

Not applicable.

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