Physical Therapy Korea

Musculoskeletal diseases tend to become chronic when early detection and treatment are delayed [1]. Additionally, these disorders are caused by excessive musculoskeletal tasks. Therefore, employees should work according to proper guidelines to protect them from musculoskeletal disorders. Understanding the complications of inappropriate posture of people due to an inevitable working environment and improving them ergonomically are important in preventing the occurrence of musculoskeletal disorders [2]. Several factors contribute to shoulder pain, including poor neck posture, work involving raising arms above head, constant fatigue of the upper limbs, and vibration exposure [3-5]. However, in most workplaces, regardless of the load, labor that raise the upper limbs upward head or require a poor posture with abnormal neck forwarding are inevitable.

Work-related neck and shoulder pain often occurs at workplaces. Previous articles have reported that repeated motions of the upper extremities above head are associated with muscle fatigue in the shoulder [1,6]. Consistent muscle fatigue around the shoulder can cause excessive muscle activation and movement patterns, leading to shoulder joint dysfunction [7]. Repetitive joint movements with muscle dysfunctions can lead to various musculoskeletal disorders [3]. Ost^ör et al. [2] emphasized that abnormal scapular movements and shoulder muscle activity account for 74% of the causes of shoulder pain. Consequently, the muscles, which are constantly in an excessive activation state, could change the pattern of joint movements that lead to constant stress on the shoulder [8,9]. Therefore, employees who perform repetitive tasks have to minimize inappropriate postures that occur abnormal activation of muscles to prevent musculoskeletal disorders. In the study by Weon et al. [10], neck and shoulder problems could be associated with forward-head posture (FHP). Additionally, previous studies showed that immoderate motions of shoulder joint in healthy subjects are related with increased FHP [11], and that approximately 60% of patients with neck pain have FHP [12]. Nevertheless, the relationship between the repeated use of the upper limbs and inadequate head posture has rarely studied.

Equipment operated by the hands commonly used in tasks involving the upper limbs with vibration exposure. Employee exposed to the vibration could be impacted by the stability and muscle activity in the upper extremity joints [13]. However, previous study has reported that constant vibration exposure could impact improvement of the stability on joints related with muscle recruitment and activation [14]. For this difference reason, we need to verify for the complex study of relationship with repetitive upper limb movements, poor head posture, and constant vibration exposure. In consequence, our study was made to investigate the influence of vibration exposure on the shoulder muscle activity during forward-head and over-head tasks with isometric shoulder flexion. We hypothesize that vibration exposure by hand-held tools might result in excessive muscle activation and that tasks that lead to inadequate head posture could alter muscle activation. Additionally, vibration exposure and tasks that lead to inadequate head posture could be associated with the muscle activity during isometric shoulder flexion.

1. Subjects

Healthy subjects (N = 22, male) were recruited from the Inje University in Gimhae-si, Gyeongsangnam-do, Korea. The demographic characteristics of the subjects are presented in Table 1. All subjects were right-handed. None complained of subjective discomfort or pain. Then, no limitations in the cervical range of motion (CROM) were observed in all participants.

Table 1 . Demographic characteristics of the participants (N = 22).

Variable	Value
Age (y)	24.81 ± 3.29
Height (cm)	173.04 ± 3.83
Weight (kg)	71.09 ± 9.57
Cervical range of motion (°)
Flexion	43.45 ± 2.73
Extension	65.13 ± 3.52
Left lateral flexion	39.54 ± 1.82
Right lateral flexion	40.31 ± 1.91
Left rotation	63.63 ± 3.12
Right rotation	63.79 ± 3.05

Values are presented as mean ± standard deviation..

The exclusion criteria were as follows: 1) individuals with a past or present history of shoulder surgery; 2) those with musculoskeletal, cardiopulmonary, or neurological disease that could interfere with shoulder flexion in the standing; and 3) those receiving physical treatment or forceful fitness for the neck and shoulder within 6 months before the study [10]. Before beginning the process, the primary investigator taught all procedures to the participants in detail. All participants provided informed consent. The study protocol was approved by the Ethics Committee for Human Investigations of the Inje University (IRB no. INJE-2019-04-005-004).

2. Instrumentation

Surface electromyography (EMG) data were collected using a BIOPAC system (MP150 acquisition system unit; Acqknowledge TM software and surface EMG electrodes; Biopac Systems Inc., Goleta, CA, USA). EMG signals were amplified and band-pass-filtered through 20–450 Hz, and an additional band stop was set at 60 Hz. Then, 1,000 Hz of sampled data was calculated as the root mean square with a window length of 0.25 [15]. Skin preparation of the electrode sites involved cleaning and shaving with rubbing alcohol, as suggested by Cram et al. [16]. Upper trapezius electrodes were attached on 2 cm lateral to the midpoint of a line between the C7 spinous process and the posterolateral acromion. Then, lower trapezius electrodes were placed superior and inferior to a point 5 cm inferomedial from the root of the spine of the scapula. The electrodes of the serratus anterior were placed vertically along the mid-axillary line at rib levels 6–8. Lastly, muscle activity of the erector spinae of the lumbar vertebra was recorded at the L3–L4 level laterally 2 cm from the spinous process (Figure 1) [17,18]. Disposable Ag-AgCl surface electrodes (Biopac Systems Inc.) were positioned at an inter-electrode distance of 2 cm. The reference electrode was attached to the 7th spinous process of the cervical vertebra.

Figure 1. Place of the electrodes. (A) Backward and (B) side view.

All CROM measurements were recorded using a CROM instrument (Performance Attainment Associates, St. Paul, MN, USA) [19]. Sagittal and coronal inclinometers were used to measure flexion-extension and lateral flexion, respectively. The horizontal inclinometer is a compass goniometer that requires the subjects to put a magnetic yoke on the neck to measure axial rotation. Before the test, CROM was measured three times in the standing position for all participants.

A hand-held tool (L212 model; ES Sanjeon Co., Cheongju, Korea) that can maximally vibrate up to 20 Hz was used. Furthermore, the weight of the device was 2 kg. This stool is a drill with operating vibration (ON) and non-operating vibration (OFF) buttons commonly used in the workplace. During the isometric shoulder flexion, the subjects were instructed to continuously press the ON button while the effects of vibration exposure were recorded. In contrast, the isometric shoulder flexion of the subjects while they were gripping the tool when the tool was OFF was assessed.

3. Procedure

Before performing the test, the subjects were uniformly instructed by an investigator on the standardized position for the three tasks and how to do each task, they had a warm-up exercise with a 2 kg dumbbell for 10 minutes. Each task was repeated three times, we allow subjects to rest for 2 minuites before starting each tasks in order to stop muscle fatigue. The three tasks were as follows: forward-head task (FHT), neutral-head task (NHT), and over-head task (OHT) with and without vibration exposure. While standing in front of an arthropometer, the subjects were instructed to raise the right upper limb to 60° in both the FHT and NHT and then to 120° in the OHT in sagittal plane. The surface EMG data was recorded from the serratus anterior, upper trapezius, lower trapezius and lumbar erector spinae muscles. In the NHT, each subject’s tragus, acromion, and greater trochanter were aligned, making them vertical to the floor level. In the FHT, the subjects were instructed to posture their head forward in a horizontal plane allowing the external auditory meatus to be position to the line, which was located 5 cm forward from the NHT’s line. In the OHT, each subject was instructed to raise up the end of the nose until 40° cervical extension using a CROM device. An inclinometer was used to measure shoulder flexion at 60° and 120°. The transverse bar of the arthropometer was placed on the right wrist of the subjects, and they were instructed to hold a power drill using their right hand until the styloid process of the radius contacted the horizontal bar without elbow flexion for 5 seconds. The surface EMG signal was collected while the subjects were in the holding position for 5 seconds (Figures 2-4).

Figure 2. Isometric shoulder flexion 60° in neutral-head task with and without vibration exposure.

Figure 3. Isometric shoulder flexion 60° in forward-head task with and without vibration exposure.

Figure 4. Isometric shoulder flexion 120° in over-head task with and without vibration exposure.

As normalization for the EMG data of all muscles, the reference voluntary isometric contraction (RVIC) data were recorded. To collect RVIC data, the participants were instructed to grip a 3-kg dumbbell using right hand and to raise upper extremity in the scaption until the shoulder joint was flexed at a 90° in a quadra-ped posture. The subjects maintained this posture for 5 seconds. The middle 3 seconds of the 5-second holding was used for data analysis [20].

4. Statistical Analysis

The surface EMG data were analyzed using a 2-way repeated analysis of variance (ANOVA) to determine the effects of vibration exposure on tasks. The alpha level for statistical significance was chosen as 0.05. All statistical analyses were performed using PASW Statistics 18.0 (IBM Co., Armonk, NY, USA).

1. Demographic Characteristics of Participants

In this study, 22 subjects are participated (age: 24.81 ± 3.29 years, height: 173.04 ± 3.83 cm, and weight: 71.09 ± 9.57 kg, respectively). CROMs are: flexion: 43.45° ± 2.73°, extension: 65.13° ± 3.52°, left lateral flexion: 39.54° ± 1.82°, right lateral flexion: 40.31° ± 1.91°, left rotation: 63.63° ± 3.12°, and right rotation: 63.79° ± 3.05°. No CROM limitations were observed in all subjects (Table 1).

2. Comparisons of the Vibration Exposure OFF and ON on Each Task

The EMG data of the muscle activity in the NHT, FHT, and OHT with isometric shoulder flexion was compared with and without vibration exposure (Figure 5). In all tasks, the EMG data of the upper trapezius and serratus anterior significantly increased on isometric shoulder flexion with vibration exposure. Furthermore, the EMG data of the lumbar erector spinae significantly increased with vibration exposure in the NHT and FHT. However, the EMG muscle activity of the lower trapezius significantly decreased with vibration exposure in the FHT only (Table 2).

Table 2 . Comparisons of the electromyography date on tasks with/without vibration exposure (N = 22).

Tasks on muscle	Vibration OFF	Vibration ON	t	p-value
Upper trapezius
NHT	15.51 ± 7.82	18.51 ± 8.73	–4.94	< 0.001
FHT	22.90 ± 8.86	32.19 ± 10.50	–5.93	< 0.001
OHT	55.13 ± 23.19	67.31 ± 28.63	–5.85	< 0.001
Lower trapezius
NHT	27.66 ± 15.76	28.09 ± 18.72	–0.36	0.72
FHT	33.51 ± 20.26	31.03 ± 20.67	2.34	0.02*
OHT	25.77 ± 14.14	24.64 ± 12.40	1.06	0.30
Serratus anterior
NHT	27.36 ± 16.18	31.95 ± 17.23	–5.64	< 0.001
FHT	26.44 ± 16.77	36.33 ± 14.53	–10.04	< 0.001
OHT	73.60 ± 28.84	86.01 ± 30.61	–9.61	< 0.001
Lumbar erector spinae
NHT	54.26 ± 21.50	58.04 ± 22.64	–3.07	0.006
FHT	61.43 ± 25.13	67.74 ± 24.85	–2.69	0.01*
OHT	57.75 ± 23.42	60.30 ± 22.79	–1.09	0.28

Values are presented as mean ± standard deviation. OFF, non-operating vibration; ON, operating vibration; NHT, neutral-head task; FHT, forward-head task; OHT, over-head task. *p < 0.05..

Figure 5. Summary of the comparisons of the electromyography date on tasks with/without vibration exposure. RVIC, reference voluntary isometric contraction; OFF, non-operating vibration; ON, operating vibration; NHT, neutral-head task; FHT, forward-head task; OHT, over-head task. *p < 0.05.

3. The Main Effect and Interactions Between the Tasks and Vibration Exposure

The interactions between the tasks and vibration exposure are presented in Table 3. The main effect between each task and vibration exposure in the upper trapezius and serratus anterior was significant. However, all interaction effects did not reach statistical significance between the tasks and vibration exposure. The results of analysis revealed that the upper trapezius showed significant differences in activity among all tasks and that the serratus anterior showed significant differences in activity in the OHT compared with those in the NHT and FHT (Figure 5).

Table 3 . Two-way repeated analysis of variance results for electromyography data (N = 22).

Effect on muscle	F	p-value
Upper trapezius
Task	83.82	< 0.001
Vibration	5.01	0.006
Task × Vibration	0.89	0.42
Lower trapezius
Task	1.87	0.15
Vibration	0.12	0.72
Task × Vibration	0.07	0.92
Serratus anterior
Task	75.85	< 0.001
Vibration	5.63	0.01*
Task × Vibration	0.37	0.68
Lumbar erector spinae
Task	1.47	0.23
Vibration	1.06	0.30
Task × Vibration	0.07	0.92

*p < 0.05..

Pain in the shoulder and neck was the usual causes of shoulder disorder, which leads to an inability to work in the workplace [4]. Upper extremity discomfort and postural neck pain are related to static and high loading of the cervical spine and shoulder complex; furthermore, these disorders might be caused by faulty posture and incorrect movements [21,22]. In a working environment, static and high loading is usually caused by common factors, including vibration exposure. Many studies have analyzed the relationships between FHP and scapular stability muscles and between FHP and neck/shoulder pain [23,24]. However, we studied the influence of vibration exposure on the EMG data of the shoulder muscles during the FHT and OHT. As a result, the upper trapezius showed significant differences in activity among all tasks. Additionally, the serratus anterior showed significant differences in the EMG activity in the OHT compared with those in the NHT and FHT. In contrast, the EMG activity of the lower trapezius significantly decreased with vibration exposure in the FHT compared with that without vibration exposure.

Several mechanisms can explain our results. The postures during the tasks, such as forward-head and over-head postures, could modify the tension and length of the levator scapula during shoulder flexion. A study has reported that the EMG activity of the levator scapulae and upper trapezius significantly increased during FHP and that of the upper trapezius significantly increased on head extension postures [17,25]. Additionally, the excessive activation of the levator scapulae should modify shoulder kinematics by contributing to the downward rotation of the scapula during arm elevation [26]. In contrast, the levator scapulae, upper trapezius, and serratus anterior are agonist muscles for upward rotation of the scapula. Therefore, the excessive activation of the levator scapulae on altered head position may contribute to humeroscapular rhythm dysfunction through the imbalanced musculature.

It has been commonly reported that vibration in the working environment causes stress and strain, which are factors for the development of musculoskeletal disorders [4]. Several studies have presented that using hand tools, which produce vibration, in the workplace could occur musculoskeletal disorder on neck and shoulder [27,28]. Electric tools used by hand in public utilities, manufacturing, and construction expose workers to vibration, which is mainly transmitted to the hands, arms, and shoulders [5]. Furthermore, vibration exposure has been reported to be associated with several peripheral circulatory disturbances in the nerves, muscles, bones, and joints [29]. Therefore, local vibration exposure or shocks could not only lead to musculoskeletal disorders in joint structures but also affect stability in joints and muscle activity. Bogaerts et al. [30] suggested that constant stimulation of vibration may increase proprioception and stimulate muscle spindles. Vibration stimulation could activate the α-motor neurons and cause reflective contraction of the muscle belly or tendon, which in turn increases myotonic reflection. Additionally, the increased proprioceptive sense of the muscles may increase muscular activity [30,31]. Lee [32] reported that the EMG date of the serratus anterior and upper trapezius significantly increased after exposure to vibration of more than 3.5 Hz. Furthermore, vibration may be used during stability exercises, which may increase the recruitment of the serratus anterior and upper and lower trapezius. However, in the workplace, vibration exposure from hand-held tools along with several factors could have harmful effects, causing overactivation and fatigue of muscles because of static load, continuous repetition, and inadequate posture [15,33]. Furthermore, the EMG muscle activity of the lumbar erector spinae significantly increased in the NHT and FHT with vibration exposure compared with that without vibration exposure. Previous studies have provided important evidence of the interactions between postural control and activation of muscle. Stability of proximal structures could affect positioning and movements of distal structures. In addition, instability caused by external factors may cause movement dysfunction that could cause muscle imbalance [34,35]. In a previous study, Chen et al. [36] reported that the EMG muscle activity of the lumbar erector spinae significantly increased in each of the four exercises with vibration exposure (15 Hz) on the entire body. However, another study suggested that entire-body vibration exposure has harmful effects, such as lumbar repositioning error, in patients with chronic back pain [37]. As a result, vibration exposure from hand-held tools in the workplace might have harmful effects on the shoulder and also vertebra joints of lumbar. Therefore, we suggest that vibration exposure along with inappropriate postures in the workplace should be avoided.

In this study, the EMG of the lower trapezius significantly decreased in the FHT with vibration exposure compared with that without vibration exposure. It may be associated with several reasons. First, the decreased EMG could be caused by the hyperactivation of the upper trapezius, which can in turn alter normal scapular kinematics [26]. Continuous activation and excessive elevation of the scapular posture could cause tightness of the levator scapulae [38]. Therefore, lack of activity in the lower trapezius, which is caused by muscle imbalance in the upper trapezius and levator scapulae, could contribute to the changes in position of the scapula. Cools et al. [39] reported that the muscle activity of the lower trapezius significantly decreased in the impingement group during isokinetic retraction, although the upper trapezius showed no significant differences in muscle activity. Second, modified scapular kinematics could cause problems in the recruitment timing of the trapezius muscles. Upward rotation of the scapula makes up the motion of the trapezius and serratus anterior muscles. However, excessive external load, such as continuous vibration and unexpected shaking, might delay the muscle recruitment of the upward rotation muscles [26]. Previous study reported that the activation timing of the middle and lower trapezius was delayed in over-head athletes with shoulder impingement on an unexpected drop of the arm compared to that in the control group [40]. For such a reason, we suggest that vibration stimulation is associated with decreased muscle activity of the lower trapezius, which could cause shoulder dysfunction.

There are several limitations. First, we investigated surface EMG data indicating the activity of each upward rotator muscle. However, the surface EMG signal analysis may have involved cross-talks around the muscles because of the movements of the shoulder joint contributing to the scapular position. Second, the results in this study could not be generalized to worker population, because all participants in this study were young and healthy. Additionally, we did not conduct the experiments on subjects who have pathological postures in order to perform tasks related with the pathological postures. Consequently, to verify the influence of vibration exposure on shoulder joint during variety tasks observed in this study, further studies involving subjects who have pathological postures should be conducted.

We showed that the sustained tasks using a hand-held tool changed the EMG muscle activity with and without vibration exposure. The results of this study indicated that the EMG data of the upper trapezius and serratus anterior significantly increased in all tasks with vibration exposure compared with those without vibration exposure during isometric shoulder flexion. Therefore, we suggest that continuous vibration exposure during the use of hand-held tools in the FHT and OHT with isometric shoulder flexion could be associated with harmful effects in the workplace. Lastly, we clinically need to examine the guidelines regarding the optimal posture and vibration exposure.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: CP, DA, JO, WY. Data curation: CP, DA. Formal analysis: CP, DA. Investigation: CP. Methodology: CP, DA, JO, WY. Project administration: CP, DA. Resources: CP, DA, JO, WY. Software: CP. Supervision: CP, DA. Validation: CP, DA, JO, WY. Visualization: CP, DA, JO, WY. Writing - original draft: CP. Writing - review & editing: CP, DA, JO, WY.