Comparison of muscle activity of wrist extensors and kinematics of wrist joint during wrist extension in automobile assembly line workers with and without lateral epicondylitis

Abstract

BACKGROUND: Overuse of the extensor carpi radialis (ECR) may play a role in the development of lateral epicondylitis (LE). However, no studies have investigated the muscle activity ratio between the ECR and extensor carpi ulnaris (ECU) associated with the kinematics during wrist extension in workers with LE.

OBJECTIVE: We compared the ratio (ECR/ECU) of muscle activity between the ECR and ECU and the kinematics of the wrist during wrist extension between workers with and without LE.

METHODS: Fifteen automobile assembly line workers with LE and 15 workers without LE participated in this study. The ratio of muscle activity was measured using surface electromyography, and wrist kinematics were measured by a three-dimensional motion analysis system while the workers extended their wrists actively to the maximum range to which they did not feel uncomfortable.

RESULTS: Significantly greater ratios of muscle activity, ranges of radial deviation, and combined motion of radial deviation and extension (CMDE) were shown in workers with LE compared to those without LE. Also, the range of wrist extension was significantly lower in workers with LE than in those without LE.

CONCLUSIONS: Quantifying the ratio of muscle activity with altered kinematics of wrist extension may help researchers to understand why overuse of ECR is occurring and explain LE development in automobile assembly line workers.

Keywords

Electromyography extensor carpi radialis extensor carpi ulnaris workers

1 Introduction

Lateral epicondylitis (LE) is characterized by lateral elbow pain that is commonly associated with resisted wrist or finger extension [1, 2]; the pain is provoked or aggravated by palpation of the lateral side of the elbow [3] and gripping and lifting activities [4]. The pathomechanics of LE seem to be related to the proximal tendons of the extensor carpi radialis (ECR) and extensor digitorum [5] and overuse that results in microscopic or macroscopic tears in the extensor aponeurosis [6]. In terms of conservative methods for treating LE, five interventions are performed to reduce pain and inflammation, facilitate healing, improve strength and flexibility of the muscles, and control force loads [7, 8]. These methods include anti-inflammatory therapy, various physiotherapy techniques, immobilization, orthoses, and surgical operations [9].

One of the most common upper extremity disorders is LE [10]. Its incidence is approximately 1–3% in the general population [11, 12]. Ritz [13] found a prevalence of 14% among employees of gas and waterworks companies, whose jobs demanded repetitive wrist extension. Werner et al. [14] reported a prevalence rate of 16% among auto assembly workers. Other studies have also suggested that LE is most commonly associated with work-related activities, ranging from 35% to 64% of all musculoskeletal disorders documented at industrial health clinics [15, 16]. Thus, LE may result in economic loss for a company secondary to individual sick leave, workers’ compensation claims, transfer to lower-paying jobs, and even early retirement [15]. In the United States, the costs associated with medical care and lost work time among workers with elbow tendonitis is estimated at over $22 billion per year [17].

Tasks requiring wrist extension overuse and repetitive supination and especially pronation are strongly associated with the etiology of LE [18]. Also, the ratio of muscle activity between the extensor carpi radialis (ECR) and extensor carpi ulnaris (ECU) during wrist extension (hereafter, the ECR/ECU ratio) may be important in the etiology of LE. In particular, it has been shown that ECR may become overused due to an imbalance between ECR and ECU [19, 20]. Therefore, workers performing repetitive tasks that involve the wrist extensor muscles becoming either overactive or underused, may be more at risk for tissue pathology [4 , 22].

However, while some previous studies have investigated the ECR/ECU ratio and movement patterns in tennis players [19 , 23–27], no studies have been performed on automobile assembly line workers with LE, whose jobs require forearm pronation to lift, carry, and/or hold the assembly product. Therefore, we compared the ECR/ECU ratio and the kinematics of the wrist during wrist extension between automobile assembly line workers with and withoutLE.

We hypothesized that workers with LE would have a higher ECR/ECU ratio than those without LE and would demonstrate abnormal kinematics in the wrist during wrist extension.

2 Methods

2.1 Participants

Fifteen male workers with pain on the lateral side of the elbow during resisted wrist extension were recruited from a workplace-based, work-conditioning center in an automobile assembly plant located in Wonju, Korea. Participation in the study was restricted to those workers who met each of the following inclusion criteria for LE: (1) a positive sign on a Mill’s test [28], (2) pain while resisting extension of the wrist or finger [1], (3) local tenderness on palpation over the lateral epicondyle [29], and (4) diagnosis of LE by an orthopedist at an orthopedic clinic outside the assembly plant.

The workers were excluded if they had undergone surgery or had fractures, rheumatoid disease, or neurologic disorders. The healthy group without LE comprised 15 male workers with no history of upper limb musculoskeletal problems who worked in the same automobile assembly plant. Participants were recruited via advertisements posted on a notice board in the plant. All participation was voluntarily. Workers’ body mass, height, and age were recorded. The general characteristics are summarized in Table 1. Prior to the study, the workers were given explanations of the procedures, and each of them signed an informed written consent form. This study was approved by the Yonsei University Wonju institutional review board.

2.2 Instrumentations

A three-dimensional electromagnetic motion tracking system (Polhemus Liberty 240/8 system; Polhemus Incorporated, Colchester, USA) was used to measure the kinematics of the wrist during wrist extension. All kinematic data were collected at 120 Hz using a single electromagnetic sensor (28 mm×22 mm×14 mm) in conjunction with Liberty host software (Version 3.0). The static accuracy of this system is 0.76 mm and 0.15 degrees root mean square error for three dimensions and senor orientation respectively, and the latency is 3.5 ms [30]. To record the kinematics of wrist extension, one electromagnetic sensor was attached proximal to the third metacarpal head on the hand of the symptomatic upper extremity using double-sided tape. A single source (58 mm×52 mm×53 mm) 1 m in front of the worker’s wrist was mounted on top of a rigid wooden frame (height of 23 mm) to avoid metal interference.

Electromyographic (EMG) data were recorded using a wireless telemetry system (TeleMyo 2400T; Noraxon, Scottsdale, AZ, USA) and analyzed using software (MyoResearch Master Edition 1.06 XP; Noraxon). The skin was shaved and washed with alcohol before attaching the electrodes and electromagnetic sensor. Surface electrode pairs were placed at an interelectrode distance of 2 cm, and the reference electrode was placed on the olecranon. EMG data were collected for the ECR (approximately 5 cm distal to the lateral epicondyle of the elbow) and ECU (ulnar side of the forearm, a few centimeters below the elbow) [31].

For normalization, a reference voluntary contraction (RVC) was measured for the extensor carpi muscles while the worker held a 2-kg dumbbell for5 seconds with the wrist in 30 degrees of extension. The mean value of the middle 3 seconds of the 5 second was calculated and used for normalization purposes. Three RVCs were conducted and the mean of the three trials was taken.

The original raw EMG signal of the ECR and ECU was filtered using a bandpass filter of 20 to 500 Hz. The sampling rate was 1024 Hz, and the signal was rectified and determined to a window of 50 ms. The muscles’ activities of ECR and ECU were expressed as a percentage of the RVC.

2.3 Experimental procedure

The worker sat on a chair in front of the treatment table. The worker’s upper extremity was positioned with the shoulder in 90 degrees of flexion, elbow fully extended, forearm pronated with palm-down, and a neutral wrist position (Fig. 1). The forearm position was stabilized with a Velcro^® strap. The worker was asked to maintain the wrist neutral position as the starting position for 5 s. Then the worker was instructed to extend the wrist actively to the maximum range before they felt uncomfortable. The worker was asked to hold the wrist for 5 s at the end of the extension range (Fig. 1). Three trials were performed, and the mean values of the kinematics and EMG data were calculated. The kinematics data were collected while the worker extended the wrist from the neutral position of the wrist, and the EMG data of the ECR and ECU were collected while the workers maintained the extended wrist position for 5 s.

The y-axis of the single electromagnetic sensor was directed laterally along the longitudinal axis of the third metacarpal head and indicated the wrist rotational motion (combined motion of radial deviation and extension, CMDE). The x-axis was directed laterally along the transverse axis of the third metacarpal head and indicated wrist flexion-extension. The Z-axis was directed perpendicular to the y- and x-axis and indicated wrist radial-ulnar deviation (Fig. 1). The data were exported to an ASCII text file and manually analyzed by a secondary investigator. Each angle of wrist extension, deviation, and CMDE was calculated at a neutral position for 5 s and end-range for 5 s. The mean values of the middle 3 s of data during both positions were calculated as the wrist kinematics. The motion of the wrist in three dimensions was described to express the orientation of the end-range relative to the neutral position of the wrist. The kinematic data were collected three times, and the mean value of three trials was used for the data analysis.

2.4 Statistical analysis

All data were calculated as the mean and standard deviation for all measurement variables and analyzed using SPSS version 12.0 software for windows (SPSS Inc., Chicago, USA). Significant differences between the two groups were assessed using an independent t-test with the level of statistical significance set at 0.05.

3 Results

There were statistically significant differences in the ratio of muscle activity and kinematics of wrist motion during wrist extension between the two groups (Table 2, Fig. 2). The mean (SD) ratio of muscle activity in workers with LE [1.2 (1.2)] was significantly greater than in workers without LE (p < 0.05). Also, significantly greater ranges of radial deviation and combined motion of radial deviation and extension (CMDE) [13.0 (4.2) degrees and 2.5 (3.6) degrees, respectively] were shown in workers with LE compared to those without LE (both p < 0.05). Furthermore, in workers with LE, the range of extension was significantly lower by 10.0 (9.3) degrees on average than in those without LE (p < 0.05).

4 Discussion

We compared the ECR/ECU ratio and the kinematics of the wrist during wrist extension between workers with and without LE. The workers with LE had a significantly higher ratio, an increase of wrist radial deviation and CMDE, and a decrease of wrist extension compared to those without LE.

EMG imbalance of the wrist extensor muscles has been reported in patients with LE [19 , 27]. Bauer and Murray [23] reported that workers with LE (injured group, n = 10) showed an earlier, longer, and greater activation of ECR than workers without LE (healthy group, n = 6) under nine simulated tennis playing conditions. They demonstrated that the activity of the ECR in the injured group was more than double that of the healthy group in the tested condition. In addition, Kelley et al. [25] reported that eight tennis players with LE showed significantly increased EMG activity of extensor carpi radialis brevis (ECRB), extensor carpi radialis longus (ECRL), and pronator teres muscles during the ball impact phase of the single-handed backhand technique compared to 14 normal players, while the EMG activity of the extensor digitorum communis and flexor carpi radialis were not significant. Although we studied only two muscles during wrist extension rather than those studied in the above mentioned dynamic activities, similar results were found compared to previous studies in terms of the increased EMG activity of ECR. Our findings that workers with LE had a higher ECR/ECU ratio by a factor of more than double compared to workers without LE [ECR/ECU ratio of 0.9 (0.3)] during wrist extension indicated that there is a difference in muscle activation pattern of the ECR and ECU in the workers with LE. This finding is consistent with previous work [25].

However, unlike the above results, Rojas et al. [27] found that ECU activity was greater than that of the extensor digitorum communis (EDC) at 20%, 50%, and 80% maximal voluntary contraction (MVC) and was also greater than that of the ECR at 20% MVC in patients with LE. Alizadehkhaiyat et al. [19] also found that ECR activity in workers with LE was significantly reduced compared to that in workers without LE during wrist extension at 50% MVC. In the present study, because pain was provoked when workers with LE performed wrist extension at MVC, we used the RVC to normalize the raw EMG signal. If such pain occurs, other muscles may contract to compensate. Thus, we used the RVC to minimize the compensation due to pain. Although it is not possible to directly compare the results of the present study with those of the above mentioned previous studies, the method used to normalize the results (MVC versus RVC), pain severity of workers with LE, or work pattern differences between those with and those without LE may help to explain some of the reported differences between the studies.

Finsen et al. [20] found that intramuscular EMG levels in the ECR were higher in the pronated position [637 (304) μV] than in the semipronated position [487 (411) μV] during 5% MVC of wrist extension. The ECU showed no significant difference between the two positions. They also reported that intramuscular EMG levels for the ECR were higher than those for the ECU during 5% MVC in the pronated position. Similarly, workers who participated in the present study worked at an auto assembly plant that required forearm pronation to lift, carry, and/or hold the assembly product [32]. Therefore, it is hypothesized that the forearm pronation that was needed by workers with LE in the present study may contribute to the increased ECR/ECU ratio during active extension of the unloaded wrist.

The LE workers also showed altered wrist extension (42.4° wrist extension, 27.5° radial deviation, and 16.2° CMDE) while extending the wrist compared to workers without LE (52.3°, 14.5°, and 13.6°, respectively). In the wrist joint, the osteokinematics of the wrist are limited to two degrees of freedom: flexion-extension and ulnar-radial deviation. The ECRL and ECRB are wrist extensor and radial deviators, and the ECU works along with the ECRL during wrist extension [33]. Normally, the wrist extends with slight radial deviation during functional wrist motion [34]. If the ECRL co-contracts with the ECU, the wrist is extended while maintaining neutrality. However, the dominant contraction of the ECRL and ECRB result in wrist extension and increased radial deviation when the forearm is pronated [4]. In the present study, LE workers demonstrated higher activity of the ECR than the ECU during wrist extension. The higher activity of the ECR may have induced greater radial deviation during wrist extension compared to workers without LE. The actual sagittal plane (wrist extension) motion was significantly lower in workers with LE than in those without LE. The higher wrist CMDE and radial deviation in LE workers seemed to compensate for the decreased extension. In addition, their angle of wrist CMDE was greater than that of workers without LE. This seemed to occur due to the increased EMG ratio between the ECR and ECU in LE workers. Excessive and repeated ECR contraction during wrist extension may contribute to overuse syndrome of the tendon of the ECR and thus result in LE.

The wrist kinematics and ECR/ECU ratio results indicated that workers with LE dominantly contracted the ECR more than the ECU to extend the wrist. However, whether these altered movement patterns caused LE or LE caused the altered movement patterns is unclear. Further studies are needed to clarify the cause and effect relationship between altered movement patterns and LE.

This study has some limitations. First, we did not measure the EMG activities of the ECRL and ECRB separately. This was because the space between these two muscles is small and the surface electrodes relatively large in comparison. Therefore, cross-talk between the muscles would likely have been an issue [35]. Also, it was impossible to completely exclude the cross-talk from other wrist extensor muscles while recording the muscle activity. A fine-wire EMG study is needed to selectively measure the activity of each muscle and minimize the cross-talk during wrist extension. Third, we recruited only men from an automobile assembly plant. Further studies should include workers of both sexes and different work environments in order to improve the generalizability of the results. Finally, this study measured wrist kinematics only at the end position of wrist extension. Therefore, in order to identify a more accurate cause and effect relationship between altered movement patterns and LE, further study is needed to analyze the movement patterns during wrist extension in workers with LE.

5 Conclusion

Workers with LE in the present study had a significantly higher ratio of muscle activity between the ECR and ECU, increased radial deviation and CMDE of the wrist, and decreased wrist extension while actively extending the wrist compared to workers without LE. These differences may help researchers to understand why overuse of ECR is occurring and explain LE development in automobile assembly line workers.

Conflict of interest

The authors have no conflict of interest to report.

Footnotes

Acknowledgments

This work was supported by the Yonsei University Research Fund of 2014.

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