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Work 51 (2015) 391–399 DOI 10.3233/WOR-141883 IOS Press
Neck and shoulder muscle activation patterns among dentists during common dental procedures Regina Pope-Forda,∗ and Zongliang Jiangb a
Industrial and Manufacturing Engineering and Technology Department, Bradley University, Peoria, IL, USA The Ergonomics and Biomechanics Laboratory, Department of Industrial and Systems Engineering, North Carolina A&T State University, Greensboro, NC, USA
b
Received 18 May 2013 Accepted 24 November 2013
Abstract. BACKGROUND: Dental practitioners often suffer musculoskeletal disorders (MSDs) in the upper extremity due to the static, repetitive and precise nature of work. Knowledge regarding muscle activation patterns in that region is essential in understanding the mechanism behind the upper extremity MSDs. However, the literature review has revealed few studies with such a focus. OBJECTIVE: This study aims to examine the inter-relationship of the exertion levels of eleven upper extremity muscles during common dental procedures. This provides insights into the interactive utilization patterns of the targeted muscles and their implications in the development of MSDs. METHODS: Twelve dentists were recruited. Electromyography (EMG) signals of sampled muscles were collected for 32 trials, i.e. combinations of independent variables (IVs) (and levels): posture (2), precision (2), line of vision (4), and grip type (2). Multivariate statistical methods were used to analyze the effects of IVs on muscle coactivity patterns. RESULTS: MANOVA showed significant main effects and a 2-way interaction between precision and grip type. Most notably, the upper trapezius exhibited consistently higher utilization than other muscles during a seated posture. CONCLUSIONS: Seated postures, preferred by dentists as a way to relieve back stress, may contribute to the development of shoulder or neck MSDs due to elevated upper trapezius exertions. Keywords: Electromyography, upper trapezius, neck and shoulder pain, myoelectric, musculoskeletal disorders
1. Introduction Musculoskeletal disorders (MSDs) in the neck and shoulder are one of the most important occupational health issues among healthcare workers [1]. This is especially true amongst dental practitioners. In three separate systematic reviews of musculoskeletal disorders (MSDs) in the dental profession, researchers doc∗ Corresponding author: Regina Pope-Ford, 1501 W Bradley Avenue, Peoria, IL 61625, USA. Tel.: +1 309 677 1227; E-mail: [email protected].
umented a high prevalence of musculoskeletal pain experienced by dental workers in the upper body [1–3]. These pain areas are primarily in the back, neck, and shoulders, but also in the hand/wrist sites. Depending on the study, the findings indicated 26–73% and 21–65% of dentists had experienced MSD symptoms in the neck and shoulders respectively [3]. Likewise, 36–50% of dentists and 14–54% of dentists had experienced pain in their back and hand/wrist respectively [1]. Major task-related risk factors for neck and shoulder disorders include fatigue, poor or awkward working posture, repetitive movement, and sustained mus-
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sight into the impact of their daily work on their musculoskeletal system. 2. Methodology 2.1. Participants Four female and eight male dentists were recruited locally to participate in this study. The participants had an average (standard deviation) height of 172.62 (11.25) cm, weight 76 (10) kg, and 18.67 (12.71) years of practice (with a range of 2 to 43 years). All, except one, were right-hand dominant. Dentists who suffered symptoms of MSDs and/or upper extremity injuries in the past were not precluded from participating in this study, as long as they were not currently under a doctor’s care. Experimental procedures were approved by the North Carolina A&T State University Institutional Review Board (IRB). Prior to the start of the experiment, all participants were informed of the requirements of the experiment and written consent was obtained. Fig. 1. View of lab space requirements and mock operatory. (Colours are visible in the online version of the article; http://dx.doi.org/ 10.3233/WOR-141883)
cular contractions [4–8]. As dentistry is one profession where static, often prolonged and awkward, low level exertions are placed on the practitioners, more quantitative data is needed to understand the effects on soft tissue [9]. The objective of this research is to examine the inter-relationship of the exertion levels of the examined upper extremity muscles during common dental procedures. While some researchers have studied the neck and shoulders in the dental profession, these body regions have not been studied extensively [1,7,10–14]. In addition, more quantitative studies are needed. This research makes quantitative measures of muscle activation patterns of highly used muscles of dental practitioners available to the ergonomic, academic and the dental community. Epidemiological benefits are the availability of quantitative data pertaining to identified risk factors leading to upper extremity MSDs in the dental profession. The etiological benefit is the additional insight into the effects of aggregate muscle activity on MSDs. Findings may apply to other healthcare professions and occupations whose work requires the use of some or the same group of muscles. Findings can also influence the design of dental equipment and instruments. In addition, practitioners are given in-
2.2. Apparatus A 3.05 × 3.05 m mock operatory was designed. Participants were seated on a dental stool. To help ensure the environment simulated a dental operatory as close as possible, a Frasaco PK-2 TSE patient simulator, was used (Fig. 1). The unit included a mannequin head, shoulder torso, watertight face mask, and hinged typodont with 32 dentitions. The patient simulator with anatomic limitations of neck movement was strapped into a dental chair. The dental chair had an adjustable range of 44.45–68.58 cm in height. An adjustable range of 44.45–85.09 cm height was achieved by placing the chair legs on individual 16.51 cm risers. The chair back adjustment range was from 90◦ to 180◦. A dental lighting unit was provided. Needed dental instruments – mirror, forceps, high speed handpiece, and rubber dam clamps were used during the experiment. A dental unit containing a high speed handpiece, air, and water hoses were placed by the side of the participant at a convenient distance, determined by the practitioner. Participants could adjust the supplied lighting and chairs as necessary. An air compressor was used to supply power to the handpiece. EMG signals of the sampled muscles were collected using the 16-channel Delsys Bagnoli EMG system (vendor, location) and its DE-2.1 Differential EMG electrodes. EMG signals were recorded at 1000 Hz. They were then pre-amplified (10000x) and filtered.
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Table 1 List of Independent Variables and their Levels vs. Dependent Variables Independent variables General Posture − Sitting/seated (S) − Standing (ST) Precision − Precision (P) − Nonprecision (NP) Line of Vision − Anterior mandibular (ManA) − Posterior mandibular (ManP) − Anterior maxillary (MaxA) − Posterior maxillary (MaxP) Grip type − Pinch grip (PG) − Power overhand/underhand (O/UG)
Dependent variables Upper trapezius Sternocleidomastoid Pectoralis major Supraspinatus Anterior deltoid Lateral deltoid Posterior deltoid Infraspinatus Latissimus dorsi Biceps brachii Triceps
2.3. Experimental design There were four independent variables (IV) for this study; three (general posture, precision, and grip type) at two levels and the fourth (line of vision) at four levels. See Table 1. The precision tasks were defined as cavity preparation and tooth extraction. Nonprecision tasks were the mirror check and dental dam clamp application. The cavity preparation and mirror check both required a pinch grip, while the tooth extraction and dental dam clamp application required an over or underhand grip, depending on whether the task involved a maxillary or mandibular tooth. The dependent variables (DV) were the averaged normalized EMG activity during each task of the sampled muscles. Two neck muscles of interest were the upper trapezius (TRPZ) and sternocleidomastoid (SCM). Seven superficial muscles that cross the shoulder joint were of interest: the pectoralis major (PM), supraspinatus (SPS), anterior deltoid (DA), lateral deltoid (DL), posterior deltoid (DP), infraspinatus (IFS) and the latissimus dorsi (LD). Two arm muscles included were – the long head of biceps brachii (BC), whose tendon helps stabilize the shoulder and the long head of the triceps brachii (TRC), which originates at the infraglenoid tubercle of the scapula. 2.4. Experimental procedures Participants were prepared for the experiment by wiping the area where electrodes were to be placed with alcohol to remove oils and lotions. EMG data was collected on the dominant side only (Fig. 2). Cram’s Introduction to Surface Electromyography was consulted for the placement of surface electrodes [15].
Fig. 2. Rear view of electrode placement. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR141883)
Prior to the test trials, maximum voluntary isometric contractions (MVIC) were taken. MVIC, performed for each muscle, was used to isolate the muscle of interest during contraction and to capture the maximum force exerted. These muscle contractions were captured using electromyography (EMG). Participants were asked to reach maximum capacity in three seconds and to hold at that maximum level for five seconds. The MVIC exertion was taken twice for each of the eleven muscle measures and followed by a one minute rest. Informal breaks were given to elucidate the experimental steps to be followed or for adjustments of the chair or computer. Test criteria were taken from the text, Muscles Testing and Function with Posture and Pain [16]. All participants were asked to wear gloves or at least a glove on the dominant hand. Each participant was asked to perform 32 randomized trials. The 32 trials were a combination of four IVs – posture, precision, line of vision, and grip type. Task performed for the trials were a mirror check, application and removal of a dental dam clamp, tooth extraction, and a cavity preparation. Each participant controlled his or her
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R. Pope-Ford and Z. Jiang / Neck and shoulder muscle activation patterns among dentists during common dental procedures Table 2 MANOVA statistics for independent variables Independent Variables Posture Line of Vision Precision*Grip Type Interaction
Wilks’ Lambda 0.80599 0.8769 0.9338
F Statistic F11,323 = 7.07 F11,323 = 1.32 F11,323 = 2.09
p-value < 0.0001 0.1115 0.0212
Number of Times Reported Wrist/Hands Upper Back Thighs Shoulders Neck Lower Legs Lower Back Knees Hips Forearms Elbows Ankles/Feet 0
2
4
6
8
10
12
Fig. 3. Participant report of discomfort by body part (n = 12). (Colours are visible in the online version of the article; http://dx.doi.org/ 10.3233/WOR-141883)
own movement by assuming the posture deemed necessary to perform the task and was free to adjust the equipment to a level that was comfortable for him or her. In addition, each determined the amount of time required to accomplish each task. Due to a computer glitch, several lines of the data (< 5%) were loss. These data points, when evaluated under statistical analysis in SAS 9.2 were treated as missing data. At the end of each trial, data was saved to a file that identified the participant number and the trial activity. All trials were completed in one session. 2.5. Data processing The raw EMG data for both MVIC trials and submax trials were filtered using a 10–500 Hz band-pass filter and notch filters, designed to eliminate 60 Hz and its aliases. Once filtered, these signals were rectified (fullwave). For each MVIC trial, the peak of the fivesecond muscle-specific EMG data was divided into 1/8 s time periods. The average of the values over the 1/8 s period was saved as the muscle-specific MVIC EMG. Of the two MVIC exertions for each muscle, the largest of the values was chosen as the maximum EMG or MVIC. It was used to calculate the denominator of the normalized EMG (NEMG) activity (or %MVIC) during each trial.
Data was collected in a maximum of 20 second increments and automatically restarted, so as not to overload the system. For task requiring more than 20 s, multiple files were combined. Since participants determined the length of time for each task, the duration setting in the filter was adjusted accordingly for each task condition. Outputs of the filter were the peak and average muscle activity levels of each muscle. The average muscle activation levels were selected as the submaximum EMG, as it was believed to represent the muscle activity over the duration of the task. These submaximum EMG are the subMVIC or numerator of the normalized EMG equation. Using the subMVIC and the MVIC calculated previously, the NEMG was determined. The NEMG is an indicator of the percent of muscle force exerted during each trial for each of the 11 muscles. 2.6. Data analysis Data was checked for normality and homogeneity. Because there were two or more DVs, MANOVA was used as a single overall statistical test of the DVs and to check the IV influence. An initial review of residual plots and histograms for each muscle revealed the data was skewed to the right, indicating the normality assumptions were not met. For significant main effects a Tukey test was run to determine for which muscles the
R. Pope-Ford and Z. Jiang / Neck and shoulder muscle activation patterns among dentists during common dental procedures
0.355
anding Sta 0.33 0.255 0.22 0.155 0.1 0.055 0 TRPZ
DA
DL
BC
PM
Fig. 4. Significant muscle exertion levels for posture variable (n = 365). (Colours are visible in the online version of the article; http:// dx.doi.org/10.3233/WOR-141883)
means of IVs were significant. The slicing technique was used to check for the significance of the IV when there was an interaction. Significance was determined by a p-value < 0.05. The analyses were conducted using Statistical Analysis Software (SAS) version 9.2. A correlation test was performed to determine the strength of the inter-relationship between the normalized EMG activities of the neck and shoulder muscles under the different task conditions. The Pearson product-moment (r) correlation coefficients were obtained using the data from all trials for all participants. Significant correlations were determined by p < 0.05. This analysis was conducted using SAS version 9.2.
Grip Type Muscle Exertion Levels (NEMG)
0.4
Sea ated
Over/underhand-P/NP
0.35
Pinch-P/NP 0.3 0.25 0.2 0.15 0.1 0.05 0 SCM
TRPZ
IFS
MANOVA results showed there was no significant 4-way or any 3-way interactions. There was, however, a significant 2-way interaction between the variables precision and grip type (p < 0.05). The MANOVA test criteria for the main effects revealed posture was significant (p < 0.05), but line of vision was not. Corresponding statistics for the main effects and 2-way interaction are provided in Table 2. A Tukey test was run to determine the muscles for which the means of the main effect of posture was significantly different, at α = 0.05. Per the Tukey test, a difference in posture means was significant for, the upper trapezius, anterior and lateral deltoid, biceps and pectoralis major. In each case, a seated posture increased the level of muscle activity (Fig. 4). Trapezius, anterior deltoid, and pectoralis major muscle exertions were 82%, 70.83%, and 45.3% greater when
DL
DP
TRC
BC
0.2 OUG-P
0.18
PG-P
0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 LD
3. Results
DA
Fig. 5. Significant muscle exertion levels for grip type for precision or nonprecision fixed (n = 365). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141883)
Grip Type Muscle Exertion Levels (NEMG)
Posture Muscle Exertion Levels (NEMG)
0.44
395
SPS
Fig. 6. Significant muscle exertion levels for grip type with precision level fixed (n = 192). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141883)
seated than when standing, respectively. Average muscle exertion levels were above 5% for all muscles for all conditions, except for the lateral deltoid with participants in a standing posture. Evaluation of precision-grip type interaction required the use of a statistical slicing technique. With precision fixed, for either the precision or nonprecision level, grip type was significant, for eight muscles – the sternocleidomastoid, upper trapezius, infraspinatus, anterior, lateral and posterior deltoids, triceps and biceps brachii (Fig. 5). The averaged over/underhand grip required higher exertion levels than the pinch grip and in every instance exceeded the suggested 5% static exertion level. In three cases (lateral and posterior deltoid and biceps brachii) average pinch grip muscle ex-
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Precision Muscle Exertion Levels (NEMG)
0.3 OUG- P OUG-NP
0.25 0.2 0.15 0.1 0.05
Table 3 Correlation Coefficients for Independent vs. Dependent Variables Muscle SCM TRPZ LD IFS SPS DA DL DP TRC BC PM
Posture
Precision
Line of vision
−0.339
−0.2859 −0.1215
−0.1616
−0.1834 0.1436 0.1212 0.1484 0.237 0.1352
Grip type −0.1799 −0.2692 −0.1405 −0.2641 −0.1923 −0.3724 −0.2949 −0.3213 −0.2559 −0.4195 0.1393
0 SPS
DA
DL
DP
TRC
BC
Fig. 7. Significant muscle exertion levels for precision with over/ underhand level fixed (n = 192.) (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141883)
ertion levels were less than 5%. For two other muscles, the latissimus dorsi and supraspinatus, with precision fixed, the over/underhand grip type was significant when a precision task was performed. Muscle exertion levels for the over/underhand grip were greater than the exertions levels when a pinch grip was held (Fig. 6). Average exertion levels for the 12 participants were above 5% of the NEMG. When grip type was fixed and precision was varied, the precision levels were significant when an over/underhand grip was held. Increased muscle exertion levels were experienced in the supraspinatus, anterior, lateral and posterior deltoids, triceps and biceps brachii when participants performed a precision task holding the overhand grip (tooth extraction) than when a nonprecision task was performed using the same grip. Applied muscle forces were above 5% when either a precision or nonprecision task was performed. The relationships between each independent variable and each dependent variable, the relationship between dependent variables, and the relation between independent variables were evaluated. The analysis showed there were no significant correlations between the independent variables; eliminating any concerns for collinearity. A statistically significant relationship was found between some of the independent and dependent variables (Table 3). Grip type was correlated with every muscle. The largest correlation coefficient, −0.4195 was between grip type and the biceps muscle; indicating that in some cases as grip strength increased, biceps muscle exertion level decreased. Cases in which a negative correlation could occur were not investigated. All other grip type and dependent variable co-
efficients fell between [−0.3724, 0.1393]. Posture was correlated with the upper trapezius, anterior and lateral deltoid, and pectoralis major. Lateral and posterior deltoids, triceps and biceps were correlated with precision. Line of vision was correlated with the supraspinatus and pectoralis major. There were also significant (p < 0.05) muscle to muscle correlations (Table 4). The neck muscles – sternocleidomastoid and the upper trapezius were not correlated. The anterior deltoid was moderately correlated with the upper trapezius, lateral deltoid and the pectoralis major. The lateral deltoid was also moderately correlated with the posterior deltoid, infraspinatus, and the supraspinatus. The supraspinatus and infraspinatus were moderately correlated, as well. When evaluating the overall muscle exertion for the twelve participants, ranking and frequency count were used to determine which muscle exhibited the highest level of overall exertion for the 32 task conditions. When considering all participants, the upper trapezius was found to be the muscle with the highest muscle exertion (91 times). Interestingly, the muscle activation level of the infraspinatus was highest (71 times). Accounting for the trapezius as the muscle with the most frequent high exertion level, the anterior deltoid was found to have the second highest overall exertion. The muscle with the lowest exertion, over the 32 trials for all participants, was the lateral deltoid. Muscle activity was further scrutinized for muscle activation levels when the upper trapezius was ranked number one. When allowing for only these 91 occurrences, the lateral deltoid maintained the lowest activation level, but the infraspinatus had the second highest overall exertion level. Additional analysis of the data indicated that 69 of the 91 times (76% of the time) when the upper trapezius had the highest exertion, the participant was seated. This revealed a relationship between posture and the upper trapezius. The other independent variables did
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Table 4 Correlation Coefficients for dependent vs. dependent variables Muscle SCM TRPZ LD IFS SPS DA DL DP TRC BC PM
SCM
0.2176 −0.1426 0.1820 0.1314 0.2180
0.3215
TRPZ
0.3963 0.2889 0.4830 0.3413 0.2659 0.2971 0.2140
LD 0.2176
IFS −0.1426 0.3963
SPS 0.2889 0.4332
0.1173
0.2537
0.4332 0.2689 0.4105 0.2925 0.1496 0.3648
0.1899 0.4023 0.1586 0.1897 −0.1660
Table 5 Years of practice vs. number of discomfort areas Participant No.
Yrs. in practice
1 2 3 4 5 6 7 8 9 10 11 12 Mean Std Dev. Median
17 12 29 14 2 38 43 6 16 15 25 7
Number of discomfort areas 1 3 5 8 4 4 9 5 2 1 3 3 4.00 2.49 3.50
not show a difference in the occurrence between levels. Evaluation of the upper trapezius when it was ranked as the muscle with the second highest frequency count did not reveal the same contrast between sitting and standing, as was seen previously. When all levels of the IVs were assessed, the trapezius maintained its position as the muscle with the highest exertion for all IV levels, except for three instances. In these three instances the participants were either standing, looking at the mandibular posterior, or performing a precision task while holding a pinch grip (cavity preparation). In such cases, the infraspinatus had the highest exertion level of all muscles.
4. Discussion Results revealed a consistent pattern of an unsafe level of muscle exertion among participants; muscle exertion levels above 5% for persons holding static postures [8]. All except seven trials (out of 365 trials in total across all participants) had an averaged EMG ex-
DA 0.1820 0.4830 0.2689 0.1899 0.5048 0.3640 0.1974 0.3811 0.4830
DL 0.1314 0.3413 0.1173 0.4105 0.4023 0.5048 0.4017 0.2160 0.3863 0.2466
DP 0.2180 0.2659
TRC
0.2925
0.1496 0.1586 0.1974 0.2160 0.1894
0.3640 0.4017 0.1894 0.4068 0.1931
BC 0.2971 0.3648 0.1897 0.3811 0.3863 0.4068 0.3483
PM 0.3215 0.2140 −0.1660 0.4830 0.2466 0.1931
0.3483
ertion level exceeding 5%. The seven instances of exception included twice for the latissimus dorsi, twice for the lateral deltoid, and once each for the posterior deltoid, triceps, and biceps brachii. There were six additional instances when participants exceeded 5% MVIC only once. This occurred once for each the latissimus dorsi, posterior deltoid, and biceps brachii, and on three occasions for the triceps brachii. Participants repeatedly exceeded the 8% MVIC levels. This is of concern, due to the likelihood of a cumulative effect of the postures held over the course of an eight hour work day [8]. Six of the twelve participants reported being in good health (defined as minimal health issues, with some or no medication), while the other six reported being in excellent health (no health problems or medication). All indicated they exercise at least once a week, with eight indicating they exercise three or more times/week. With the exception of one participant, each person reported experiencing at some time in the last twelve months some musculoskeletal pain thought to be related to work. Without exception, all participants identified areas of discomfort in at least one body region. The average number of identified areas of body pain was four, with a maximum number of areas identified as nine (Table 5). Statistical analysis showed no correlation between years of experience and the number of pain areas. The neck was the highest reported pain area; eleven of twelve (92%) reported neck pain. Eight dentists (67%) reported they experienced lower back pain. This was followed by seven reporting pain in the shoulders and upper back. Wrist/hand and hip pain followed with the next highest levels of stated discomfort. See Fig. 3 for comparison of body part pain. Participants expressed the reported discomfort was felt occasionally (63.3% of the time) or often (36.7% of the time). The discomfort was described as aching (dull persistent pain), pain (acute), tingling, stiffness, burning, numbness, or cramping. This discomfort for some was
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found as early as dental school. Despite the occasional or often felt discomfort, no dentist reported work time lost due to the problems experienced. Despite the recorded high muscle activity for all the muscles under study and the reported pain symptoms in the neck, shoulder and back, the dentists’ selfreported being in good to excellent health. They reported often compensating for the discomfort/pain by modifying their work load, seeking chiropractic care, massages, and taking more breaks. In a seated posture the muscle activity in the upper trapezius, anterior and lateral deltoids, biceps brachii, and the pectoralis major were significantly higher than when the participants stood. When participants held an over/underhand grip and performed a precision task, muscle activity increased significantly in muscles that cross or stabilize the shoulder – supraspinatus, lateral and posterior deltoids, triceps and biceps brachii than did nonprecision tasks (mirror checks and application/removal of dental dam clamps). The over/underhand grip was statistically significant for all muscles, in comparison with the pinch grip, excepting the pectoralis major; although, it too had a higher exertion level when participants held an over/under hand grip versus a pinch grip. For some muscles, such as the infraspinatus, anterior, lateral, and posterior deltoids, and the biceps brachii, the over/underhand grip exertion levels were 100% more than the pinch grip exertion levels. The findings from this research could provide insight into potential improvement that can be made to the design of dental equipment and instruments for a reduction in the risk of upper extremity MSDs in the dental profession. A good design should not only bring dentists closer to their workstation and patients, but more importantly allow dentists to conveniently adjust the workspace settings accordingly to optimize their comfort level. The relatively small sample size, especially of female subjects, could limit the generalizability of the findings of this research. Future study may increase the percentage of women included to investigate the gender effect, as some researchers have suggested the gender as a risk factor for neck disorders [17]. Future research also warrants a closer look at the non-dominant side of participants as a number of participants were observed to transfer the mirror to the non-dominant hand when performing a task. Dentists are often observed to guide or use the non-dominant hand to minimize patient movement. Further, all tasks were performed without loupes, which are worn by
some dentists. A study which incorporates loupes could also be used to determine their effect on muscle exertion minimization and hence the ergonomic benefit. Finally, these biomechanical findings should be combined with personal and organizational factors to develop a comprehensive system to identify an individual’s level of risk for developing MSDs. In summary, this research investigated the exertion levels of eleven muscles in the neck and shoulders of dentists during common dental procedures. The experimental protocol allowed participants to perform tasks using their preferred techniques and for the length of time they deemed necessary. Such an approach promoted external validity although it also increased the variability in data. The quantitative evidence from this study provides strong support for the need of administrative and engineering controls among dental practitioners. Recommended administrative controls include adequate rest breaks, flexible scheduling for lengthy procedures, exercise, and other stress reduction techniques. For effective engineering controls, designers and makers of dental equipment and instruments must continue to be part of the solutions to minimizing the risk of MSDs.
References [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Hayes MJ, Cockrell D, Smith DR. A systematic review of musculoskeletal disorders among dental professionals. Int J Dent Hyg. 2009; 7(3): 159-65. Leggat PA, Kedjarune U, Smith DR. Occupational health problems in modern dentistry: A review. Ind Health. 2007; 45(5): 611-21. Morse T, Bruneau H, Dussetschleger J. Musculoskeletal disorders of the neck and shoulder in the dental professions. Work-a Journal of Prevention Assessment & Rehabilitation. 2010; 35(4): 419-29. Davidson AW, Rice CL. Effect of Shoulder Angle on the Activation Pattern of the Elbow Extensors During a Submaximal Isometric Fatiguing Contraction Muscle & Nerve. 2010; 42(4): 514-21. Wiker SF, Chaffin DB, Langolf GD. Shoulder Posture and Localized Muscle Fatigue and Discomfort Ergonomics. 1989; 32(2): 211-37. Marras WS. Biomechanical Basis for Ergonomics. In: Marras WS, Karwowski W, editors. Fundamentals and Assessment Tools for Occupational Ergonomics. Boca Raton, FL: CRC Press Taylor & Francis Group, 2006, pp. 11-1–43. Akesson I, Johnsson B, Rylander L, Moritz U, Skerfving S. Musculoskeletal disorders among female dental personnel – clinical examination and a 5-year follow-up study of symptoms. International Archives of Occupational and Environmental Health. 1999; 72(6): 395-403. Sjogaard G, Jensen BR. Low-Level Static Exertions. In: Marras WS, Karwowski W, editors. The Occupational Ergonomics Handbook: Fundamentals and Assessment Tools
R. Pope-Ford and Z. Jiang / Neck and shoulder muscle activation patterns among dentists during common dental procedures for Occupational Ergonomics. Boca Raton, FL: CRC Press Taylor & Francis Group, 2006, pp. 14-1–3. [9] Jonsson B. Measurement and Evaluation of Local Muscular Strain in the Shoulder During Constrained Work Journal of Human Ergology. 1982; 11(1): 73-88. [10] Akesson I, Hansson GA, Balogh I, Moritz U, Skerfving S. Quantifying work load in neck, shoulders and wrists in female dentists. International Archives of Occupational and Environmental Health. 1997; 69(6): 461-74. [11] Åkesson I, Lundborg G, Horstmann V, Skerfving S. Neuropathy in Female Dental Personnel Exposed to High Frequency Vibrations. Occupational and Environmental Medicine. 1995; 52(2): 116-23. [12] Brookham RL, Middlebrook EE, Grewal T-j, Dickerson CR. The utility of an empirically derived co-activation ratio for muscle force prediction through optimization. Journal of Biomechanics. 2011; 44(8): 1582-7. [13] Kierklo A, Kobus A, Jaworska M, Botulinski B. Work-
[14]
[15]
[16]
[17]
399
Related Musculoskeletal Disorders Among Dentists – A Questionnaire Survey. Annals of Agricultural and Environmental Medicine. 2011; 18(1): 79-84. Lalumandier JA, McPhee SD, Parrott CB, Vendemia M. Musculoskeletal pain: prevalence, prevention, and differences among dental office personnel. General Dentistry: Journal Of The Academy Of General Dentistry 2001; 49(2): 160-6. Criswell E, editor. Cram’s Introduction to Surface Electromyography. 2nd ed. Massachusetts: Jones and Bartlett Publishers, 2011. Kendall FP, McCreary EK, Provance PG, Rodgers MM, Romani WA. Muscles: Testing and Function with Posture and Pain. 5th ed. Baltimore, MD: Lippincott Williams & Wilkins, 2005, p. 480. Chiu TTW, Ku WY, Lee MH, Sum WK, Wan MP, Wong CY, et al. A study on the prevalence of and risk factors for neck pain among university academic staff in Hong Kong. Journal of Occupational Rehabilitation. 2002; 12(2): 77-91.
Work 51 (2015) 391–399 DOI 10.3233/WOR-141883 IOS Press
Neck and shoulder muscle activation patterns among dentists during common dental procedures Regina Pope-Forda,∗ and Zongliang Jiangb a
Industrial and Manufacturing Engineering and Technology Department, Bradley University, Peoria, IL, USA The Ergonomics and Biomechanics Laboratory, Department of Industrial and Systems Engineering, North Carolina A&T State University, Greensboro, NC, USA
b
Received 18 May 2013 Accepted 24 November 2013
Abstract. BACKGROUND: Dental practitioners often suffer musculoskeletal disorders (MSDs) in the upper extremity due to the static, repetitive and precise nature of work. Knowledge regarding muscle activation patterns in that region is essential in understanding the mechanism behind the upper extremity MSDs. However, the literature review has revealed few studies with such a focus. OBJECTIVE: This study aims to examine the inter-relationship of the exertion levels of eleven upper extremity muscles during common dental procedures. This provides insights into the interactive utilization patterns of the targeted muscles and their implications in the development of MSDs. METHODS: Twelve dentists were recruited. Electromyography (EMG) signals of sampled muscles were collected for 32 trials, i.e. combinations of independent variables (IVs) (and levels): posture (2), precision (2), line of vision (4), and grip type (2). Multivariate statistical methods were used to analyze the effects of IVs on muscle coactivity patterns. RESULTS: MANOVA showed significant main effects and a 2-way interaction between precision and grip type. Most notably, the upper trapezius exhibited consistently higher utilization than other muscles during a seated posture. CONCLUSIONS: Seated postures, preferred by dentists as a way to relieve back stress, may contribute to the development of shoulder or neck MSDs due to elevated upper trapezius exertions. Keywords: Electromyography, upper trapezius, neck and shoulder pain, myoelectric, musculoskeletal disorders
1. Introduction Musculoskeletal disorders (MSDs) in the neck and shoulder are one of the most important occupational health issues among healthcare workers [1]. This is especially true amongst dental practitioners. In three separate systematic reviews of musculoskeletal disorders (MSDs) in the dental profession, researchers doc∗ Corresponding author: Regina Pope-Ford, 1501 W Bradley Avenue, Peoria, IL 61625, USA. Tel.: +1 309 677 1227; E-mail: [email protected].
umented a high prevalence of musculoskeletal pain experienced by dental workers in the upper body [1–3]. These pain areas are primarily in the back, neck, and shoulders, but also in the hand/wrist sites. Depending on the study, the findings indicated 26–73% and 21–65% of dentists had experienced MSD symptoms in the neck and shoulders respectively [3]. Likewise, 36–50% of dentists and 14–54% of dentists had experienced pain in their back and hand/wrist respectively [1]. Major task-related risk factors for neck and shoulder disorders include fatigue, poor or awkward working posture, repetitive movement, and sustained mus-
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sight into the impact of their daily work on their musculoskeletal system. 2. Methodology 2.1. Participants Four female and eight male dentists were recruited locally to participate in this study. The participants had an average (standard deviation) height of 172.62 (11.25) cm, weight 76 (10) kg, and 18.67 (12.71) years of practice (with a range of 2 to 43 years). All, except one, were right-hand dominant. Dentists who suffered symptoms of MSDs and/or upper extremity injuries in the past were not precluded from participating in this study, as long as they were not currently under a doctor’s care. Experimental procedures were approved by the North Carolina A&T State University Institutional Review Board (IRB). Prior to the start of the experiment, all participants were informed of the requirements of the experiment and written consent was obtained. Fig. 1. View of lab space requirements and mock operatory. (Colours are visible in the online version of the article; http://dx.doi.org/ 10.3233/WOR-141883)
cular contractions [4–8]. As dentistry is one profession where static, often prolonged and awkward, low level exertions are placed on the practitioners, more quantitative data is needed to understand the effects on soft tissue [9]. The objective of this research is to examine the inter-relationship of the exertion levels of the examined upper extremity muscles during common dental procedures. While some researchers have studied the neck and shoulders in the dental profession, these body regions have not been studied extensively [1,7,10–14]. In addition, more quantitative studies are needed. This research makes quantitative measures of muscle activation patterns of highly used muscles of dental practitioners available to the ergonomic, academic and the dental community. Epidemiological benefits are the availability of quantitative data pertaining to identified risk factors leading to upper extremity MSDs in the dental profession. The etiological benefit is the additional insight into the effects of aggregate muscle activity on MSDs. Findings may apply to other healthcare professions and occupations whose work requires the use of some or the same group of muscles. Findings can also influence the design of dental equipment and instruments. In addition, practitioners are given in-
2.2. Apparatus A 3.05 × 3.05 m mock operatory was designed. Participants were seated on a dental stool. To help ensure the environment simulated a dental operatory as close as possible, a Frasaco PK-2 TSE patient simulator, was used (Fig. 1). The unit included a mannequin head, shoulder torso, watertight face mask, and hinged typodont with 32 dentitions. The patient simulator with anatomic limitations of neck movement was strapped into a dental chair. The dental chair had an adjustable range of 44.45–68.58 cm in height. An adjustable range of 44.45–85.09 cm height was achieved by placing the chair legs on individual 16.51 cm risers. The chair back adjustment range was from 90◦ to 180◦. A dental lighting unit was provided. Needed dental instruments – mirror, forceps, high speed handpiece, and rubber dam clamps were used during the experiment. A dental unit containing a high speed handpiece, air, and water hoses were placed by the side of the participant at a convenient distance, determined by the practitioner. Participants could adjust the supplied lighting and chairs as necessary. An air compressor was used to supply power to the handpiece. EMG signals of the sampled muscles were collected using the 16-channel Delsys Bagnoli EMG system (vendor, location) and its DE-2.1 Differential EMG electrodes. EMG signals were recorded at 1000 Hz. They were then pre-amplified (10000x) and filtered.
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Table 1 List of Independent Variables and their Levels vs. Dependent Variables Independent variables General Posture − Sitting/seated (S) − Standing (ST) Precision − Precision (P) − Nonprecision (NP) Line of Vision − Anterior mandibular (ManA) − Posterior mandibular (ManP) − Anterior maxillary (MaxA) − Posterior maxillary (MaxP) Grip type − Pinch grip (PG) − Power overhand/underhand (O/UG)
Dependent variables Upper trapezius Sternocleidomastoid Pectoralis major Supraspinatus Anterior deltoid Lateral deltoid Posterior deltoid Infraspinatus Latissimus dorsi Biceps brachii Triceps
2.3. Experimental design There were four independent variables (IV) for this study; three (general posture, precision, and grip type) at two levels and the fourth (line of vision) at four levels. See Table 1. The precision tasks were defined as cavity preparation and tooth extraction. Nonprecision tasks were the mirror check and dental dam clamp application. The cavity preparation and mirror check both required a pinch grip, while the tooth extraction and dental dam clamp application required an over or underhand grip, depending on whether the task involved a maxillary or mandibular tooth. The dependent variables (DV) were the averaged normalized EMG activity during each task of the sampled muscles. Two neck muscles of interest were the upper trapezius (TRPZ) and sternocleidomastoid (SCM). Seven superficial muscles that cross the shoulder joint were of interest: the pectoralis major (PM), supraspinatus (SPS), anterior deltoid (DA), lateral deltoid (DL), posterior deltoid (DP), infraspinatus (IFS) and the latissimus dorsi (LD). Two arm muscles included were – the long head of biceps brachii (BC), whose tendon helps stabilize the shoulder and the long head of the triceps brachii (TRC), which originates at the infraglenoid tubercle of the scapula. 2.4. Experimental procedures Participants were prepared for the experiment by wiping the area where electrodes were to be placed with alcohol to remove oils and lotions. EMG data was collected on the dominant side only (Fig. 2). Cram’s Introduction to Surface Electromyography was consulted for the placement of surface electrodes [15].
Fig. 2. Rear view of electrode placement. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR141883)
Prior to the test trials, maximum voluntary isometric contractions (MVIC) were taken. MVIC, performed for each muscle, was used to isolate the muscle of interest during contraction and to capture the maximum force exerted. These muscle contractions were captured using electromyography (EMG). Participants were asked to reach maximum capacity in three seconds and to hold at that maximum level for five seconds. The MVIC exertion was taken twice for each of the eleven muscle measures and followed by a one minute rest. Informal breaks were given to elucidate the experimental steps to be followed or for adjustments of the chair or computer. Test criteria were taken from the text, Muscles Testing and Function with Posture and Pain [16]. All participants were asked to wear gloves or at least a glove on the dominant hand. Each participant was asked to perform 32 randomized trials. The 32 trials were a combination of four IVs – posture, precision, line of vision, and grip type. Task performed for the trials were a mirror check, application and removal of a dental dam clamp, tooth extraction, and a cavity preparation. Each participant controlled his or her
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R. Pope-Ford and Z. Jiang / Neck and shoulder muscle activation patterns among dentists during common dental procedures Table 2 MANOVA statistics for independent variables Independent Variables Posture Line of Vision Precision*Grip Type Interaction
Wilks’ Lambda 0.80599 0.8769 0.9338
F Statistic F11,323 = 7.07 F11,323 = 1.32 F11,323 = 2.09
p-value < 0.0001 0.1115 0.0212
Number of Times Reported Wrist/Hands Upper Back Thighs Shoulders Neck Lower Legs Lower Back Knees Hips Forearms Elbows Ankles/Feet 0
2
4
6
8
10
12
Fig. 3. Participant report of discomfort by body part (n = 12). (Colours are visible in the online version of the article; http://dx.doi.org/ 10.3233/WOR-141883)
own movement by assuming the posture deemed necessary to perform the task and was free to adjust the equipment to a level that was comfortable for him or her. In addition, each determined the amount of time required to accomplish each task. Due to a computer glitch, several lines of the data (< 5%) were loss. These data points, when evaluated under statistical analysis in SAS 9.2 were treated as missing data. At the end of each trial, data was saved to a file that identified the participant number and the trial activity. All trials were completed in one session. 2.5. Data processing The raw EMG data for both MVIC trials and submax trials were filtered using a 10–500 Hz band-pass filter and notch filters, designed to eliminate 60 Hz and its aliases. Once filtered, these signals were rectified (fullwave). For each MVIC trial, the peak of the fivesecond muscle-specific EMG data was divided into 1/8 s time periods. The average of the values over the 1/8 s period was saved as the muscle-specific MVIC EMG. Of the two MVIC exertions for each muscle, the largest of the values was chosen as the maximum EMG or MVIC. It was used to calculate the denominator of the normalized EMG (NEMG) activity (or %MVIC) during each trial.
Data was collected in a maximum of 20 second increments and automatically restarted, so as not to overload the system. For task requiring more than 20 s, multiple files were combined. Since participants determined the length of time for each task, the duration setting in the filter was adjusted accordingly for each task condition. Outputs of the filter were the peak and average muscle activity levels of each muscle. The average muscle activation levels were selected as the submaximum EMG, as it was believed to represent the muscle activity over the duration of the task. These submaximum EMG are the subMVIC or numerator of the normalized EMG equation. Using the subMVIC and the MVIC calculated previously, the NEMG was determined. The NEMG is an indicator of the percent of muscle force exerted during each trial for each of the 11 muscles. 2.6. Data analysis Data was checked for normality and homogeneity. Because there were two or more DVs, MANOVA was used as a single overall statistical test of the DVs and to check the IV influence. An initial review of residual plots and histograms for each muscle revealed the data was skewed to the right, indicating the normality assumptions were not met. For significant main effects a Tukey test was run to determine for which muscles the
R. Pope-Ford and Z. Jiang / Neck and shoulder muscle activation patterns among dentists during common dental procedures
0.355
anding Sta 0.33 0.255 0.22 0.155 0.1 0.055 0 TRPZ
DA
DL
BC
PM
Fig. 4. Significant muscle exertion levels for posture variable (n = 365). (Colours are visible in the online version of the article; http:// dx.doi.org/10.3233/WOR-141883)
means of IVs were significant. The slicing technique was used to check for the significance of the IV when there was an interaction. Significance was determined by a p-value < 0.05. The analyses were conducted using Statistical Analysis Software (SAS) version 9.2. A correlation test was performed to determine the strength of the inter-relationship between the normalized EMG activities of the neck and shoulder muscles under the different task conditions. The Pearson product-moment (r) correlation coefficients were obtained using the data from all trials for all participants. Significant correlations were determined by p < 0.05. This analysis was conducted using SAS version 9.2.
Grip Type Muscle Exertion Levels (NEMG)
0.4
Sea ated
Over/underhand-P/NP
0.35
Pinch-P/NP 0.3 0.25 0.2 0.15 0.1 0.05 0 SCM
TRPZ
IFS
MANOVA results showed there was no significant 4-way or any 3-way interactions. There was, however, a significant 2-way interaction between the variables precision and grip type (p < 0.05). The MANOVA test criteria for the main effects revealed posture was significant (p < 0.05), but line of vision was not. Corresponding statistics for the main effects and 2-way interaction are provided in Table 2. A Tukey test was run to determine the muscles for which the means of the main effect of posture was significantly different, at α = 0.05. Per the Tukey test, a difference in posture means was significant for, the upper trapezius, anterior and lateral deltoid, biceps and pectoralis major. In each case, a seated posture increased the level of muscle activity (Fig. 4). Trapezius, anterior deltoid, and pectoralis major muscle exertions were 82%, 70.83%, and 45.3% greater when
DL
DP
TRC
BC
0.2 OUG-P
0.18
PG-P
0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 LD
3. Results
DA
Fig. 5. Significant muscle exertion levels for grip type for precision or nonprecision fixed (n = 365). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141883)
Grip Type Muscle Exertion Levels (NEMG)
Posture Muscle Exertion Levels (NEMG)
0.44
395
SPS
Fig. 6. Significant muscle exertion levels for grip type with precision level fixed (n = 192). (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141883)
seated than when standing, respectively. Average muscle exertion levels were above 5% for all muscles for all conditions, except for the lateral deltoid with participants in a standing posture. Evaluation of precision-grip type interaction required the use of a statistical slicing technique. With precision fixed, for either the precision or nonprecision level, grip type was significant, for eight muscles – the sternocleidomastoid, upper trapezius, infraspinatus, anterior, lateral and posterior deltoids, triceps and biceps brachii (Fig. 5). The averaged over/underhand grip required higher exertion levels than the pinch grip and in every instance exceeded the suggested 5% static exertion level. In three cases (lateral and posterior deltoid and biceps brachii) average pinch grip muscle ex-
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Precision Muscle Exertion Levels (NEMG)
0.3 OUG- P OUG-NP
0.25 0.2 0.15 0.1 0.05
Table 3 Correlation Coefficients for Independent vs. Dependent Variables Muscle SCM TRPZ LD IFS SPS DA DL DP TRC BC PM
Posture
Precision
Line of vision
−0.339
−0.2859 −0.1215
−0.1616
−0.1834 0.1436 0.1212 0.1484 0.237 0.1352
Grip type −0.1799 −0.2692 −0.1405 −0.2641 −0.1923 −0.3724 −0.2949 −0.3213 −0.2559 −0.4195 0.1393
0 SPS
DA
DL
DP
TRC
BC
Fig. 7. Significant muscle exertion levels for precision with over/ underhand level fixed (n = 192.) (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/WOR-141883)
ertion levels were less than 5%. For two other muscles, the latissimus dorsi and supraspinatus, with precision fixed, the over/underhand grip type was significant when a precision task was performed. Muscle exertion levels for the over/underhand grip were greater than the exertions levels when a pinch grip was held (Fig. 6). Average exertion levels for the 12 participants were above 5% of the NEMG. When grip type was fixed and precision was varied, the precision levels were significant when an over/underhand grip was held. Increased muscle exertion levels were experienced in the supraspinatus, anterior, lateral and posterior deltoids, triceps and biceps brachii when participants performed a precision task holding the overhand grip (tooth extraction) than when a nonprecision task was performed using the same grip. Applied muscle forces were above 5% when either a precision or nonprecision task was performed. The relationships between each independent variable and each dependent variable, the relationship between dependent variables, and the relation between independent variables were evaluated. The analysis showed there were no significant correlations between the independent variables; eliminating any concerns for collinearity. A statistically significant relationship was found between some of the independent and dependent variables (Table 3). Grip type was correlated with every muscle. The largest correlation coefficient, −0.4195 was between grip type and the biceps muscle; indicating that in some cases as grip strength increased, biceps muscle exertion level decreased. Cases in which a negative correlation could occur were not investigated. All other grip type and dependent variable co-
efficients fell between [−0.3724, 0.1393]. Posture was correlated with the upper trapezius, anterior and lateral deltoid, and pectoralis major. Lateral and posterior deltoids, triceps and biceps were correlated with precision. Line of vision was correlated with the supraspinatus and pectoralis major. There were also significant (p < 0.05) muscle to muscle correlations (Table 4). The neck muscles – sternocleidomastoid and the upper trapezius were not correlated. The anterior deltoid was moderately correlated with the upper trapezius, lateral deltoid and the pectoralis major. The lateral deltoid was also moderately correlated with the posterior deltoid, infraspinatus, and the supraspinatus. The supraspinatus and infraspinatus were moderately correlated, as well. When evaluating the overall muscle exertion for the twelve participants, ranking and frequency count were used to determine which muscle exhibited the highest level of overall exertion for the 32 task conditions. When considering all participants, the upper trapezius was found to be the muscle with the highest muscle exertion (91 times). Interestingly, the muscle activation level of the infraspinatus was highest (71 times). Accounting for the trapezius as the muscle with the most frequent high exertion level, the anterior deltoid was found to have the second highest overall exertion. The muscle with the lowest exertion, over the 32 trials for all participants, was the lateral deltoid. Muscle activity was further scrutinized for muscle activation levels when the upper trapezius was ranked number one. When allowing for only these 91 occurrences, the lateral deltoid maintained the lowest activation level, but the infraspinatus had the second highest overall exertion level. Additional analysis of the data indicated that 69 of the 91 times (76% of the time) when the upper trapezius had the highest exertion, the participant was seated. This revealed a relationship between posture and the upper trapezius. The other independent variables did
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Table 4 Correlation Coefficients for dependent vs. dependent variables Muscle SCM TRPZ LD IFS SPS DA DL DP TRC BC PM
SCM
0.2176 −0.1426 0.1820 0.1314 0.2180
0.3215
TRPZ
0.3963 0.2889 0.4830 0.3413 0.2659 0.2971 0.2140
LD 0.2176
IFS −0.1426 0.3963
SPS 0.2889 0.4332
0.1173
0.2537
0.4332 0.2689 0.4105 0.2925 0.1496 0.3648
0.1899 0.4023 0.1586 0.1897 −0.1660
Table 5 Years of practice vs. number of discomfort areas Participant No.
Yrs. in practice
1 2 3 4 5 6 7 8 9 10 11 12 Mean Std Dev. Median
17 12 29 14 2 38 43 6 16 15 25 7
Number of discomfort areas 1 3 5 8 4 4 9 5 2 1 3 3 4.00 2.49 3.50
not show a difference in the occurrence between levels. Evaluation of the upper trapezius when it was ranked as the muscle with the second highest frequency count did not reveal the same contrast between sitting and standing, as was seen previously. When all levels of the IVs were assessed, the trapezius maintained its position as the muscle with the highest exertion for all IV levels, except for three instances. In these three instances the participants were either standing, looking at the mandibular posterior, or performing a precision task while holding a pinch grip (cavity preparation). In such cases, the infraspinatus had the highest exertion level of all muscles.
4. Discussion Results revealed a consistent pattern of an unsafe level of muscle exertion among participants; muscle exertion levels above 5% for persons holding static postures [8]. All except seven trials (out of 365 trials in total across all participants) had an averaged EMG ex-
DA 0.1820 0.4830 0.2689 0.1899 0.5048 0.3640 0.1974 0.3811 0.4830
DL 0.1314 0.3413 0.1173 0.4105 0.4023 0.5048 0.4017 0.2160 0.3863 0.2466
DP 0.2180 0.2659
TRC
0.2925
0.1496 0.1586 0.1974 0.2160 0.1894
0.3640 0.4017 0.1894 0.4068 0.1931
BC 0.2971 0.3648 0.1897 0.3811 0.3863 0.4068 0.3483
PM 0.3215 0.2140 −0.1660 0.4830 0.2466 0.1931
0.3483
ertion level exceeding 5%. The seven instances of exception included twice for the latissimus dorsi, twice for the lateral deltoid, and once each for the posterior deltoid, triceps, and biceps brachii. There were six additional instances when participants exceeded 5% MVIC only once. This occurred once for each the latissimus dorsi, posterior deltoid, and biceps brachii, and on three occasions for the triceps brachii. Participants repeatedly exceeded the 8% MVIC levels. This is of concern, due to the likelihood of a cumulative effect of the postures held over the course of an eight hour work day [8]. Six of the twelve participants reported being in good health (defined as minimal health issues, with some or no medication), while the other six reported being in excellent health (no health problems or medication). All indicated they exercise at least once a week, with eight indicating they exercise three or more times/week. With the exception of one participant, each person reported experiencing at some time in the last twelve months some musculoskeletal pain thought to be related to work. Without exception, all participants identified areas of discomfort in at least one body region. The average number of identified areas of body pain was four, with a maximum number of areas identified as nine (Table 5). Statistical analysis showed no correlation between years of experience and the number of pain areas. The neck was the highest reported pain area; eleven of twelve (92%) reported neck pain. Eight dentists (67%) reported they experienced lower back pain. This was followed by seven reporting pain in the shoulders and upper back. Wrist/hand and hip pain followed with the next highest levels of stated discomfort. See Fig. 3 for comparison of body part pain. Participants expressed the reported discomfort was felt occasionally (63.3% of the time) or often (36.7% of the time). The discomfort was described as aching (dull persistent pain), pain (acute), tingling, stiffness, burning, numbness, or cramping. This discomfort for some was
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found as early as dental school. Despite the occasional or often felt discomfort, no dentist reported work time lost due to the problems experienced. Despite the recorded high muscle activity for all the muscles under study and the reported pain symptoms in the neck, shoulder and back, the dentists’ selfreported being in good to excellent health. They reported often compensating for the discomfort/pain by modifying their work load, seeking chiropractic care, massages, and taking more breaks. In a seated posture the muscle activity in the upper trapezius, anterior and lateral deltoids, biceps brachii, and the pectoralis major were significantly higher than when the participants stood. When participants held an over/underhand grip and performed a precision task, muscle activity increased significantly in muscles that cross or stabilize the shoulder – supraspinatus, lateral and posterior deltoids, triceps and biceps brachii than did nonprecision tasks (mirror checks and application/removal of dental dam clamps). The over/underhand grip was statistically significant for all muscles, in comparison with the pinch grip, excepting the pectoralis major; although, it too had a higher exertion level when participants held an over/under hand grip versus a pinch grip. For some muscles, such as the infraspinatus, anterior, lateral, and posterior deltoids, and the biceps brachii, the over/underhand grip exertion levels were 100% more than the pinch grip exertion levels. The findings from this research could provide insight into potential improvement that can be made to the design of dental equipment and instruments for a reduction in the risk of upper extremity MSDs in the dental profession. A good design should not only bring dentists closer to their workstation and patients, but more importantly allow dentists to conveniently adjust the workspace settings accordingly to optimize their comfort level. The relatively small sample size, especially of female subjects, could limit the generalizability of the findings of this research. Future study may increase the percentage of women included to investigate the gender effect, as some researchers have suggested the gender as a risk factor for neck disorders [17]. Future research also warrants a closer look at the non-dominant side of participants as a number of participants were observed to transfer the mirror to the non-dominant hand when performing a task. Dentists are often observed to guide or use the non-dominant hand to minimize patient movement. Further, all tasks were performed without loupes, which are worn by
some dentists. A study which incorporates loupes could also be used to determine their effect on muscle exertion minimization and hence the ergonomic benefit. Finally, these biomechanical findings should be combined with personal and organizational factors to develop a comprehensive system to identify an individual’s level of risk for developing MSDs. In summary, this research investigated the exertion levels of eleven muscles in the neck and shoulders of dentists during common dental procedures. The experimental protocol allowed participants to perform tasks using their preferred techniques and for the length of time they deemed necessary. Such an approach promoted external validity although it also increased the variability in data. The quantitative evidence from this study provides strong support for the need of administrative and engineering controls among dental practitioners. Recommended administrative controls include adequate rest breaks, flexible scheduling for lengthy procedures, exercise, and other stress reduction techniques. For effective engineering controls, designers and makers of dental equipment and instruments must continue to be part of the solutions to minimizing the risk of MSDs.
References [1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
Hayes MJ, Cockrell D, Smith DR. A systematic review of musculoskeletal disorders among dental professionals. Int J Dent Hyg. 2009; 7(3): 159-65. Leggat PA, Kedjarune U, Smith DR. Occupational health problems in modern dentistry: A review. Ind Health. 2007; 45(5): 611-21. Morse T, Bruneau H, Dussetschleger J. Musculoskeletal disorders of the neck and shoulder in the dental professions. Work-a Journal of Prevention Assessment & Rehabilitation. 2010; 35(4): 419-29. Davidson AW, Rice CL. Effect of Shoulder Angle on the Activation Pattern of the Elbow Extensors During a Submaximal Isometric Fatiguing Contraction Muscle & Nerve. 2010; 42(4): 514-21. Wiker SF, Chaffin DB, Langolf GD. Shoulder Posture and Localized Muscle Fatigue and Discomfort Ergonomics. 1989; 32(2): 211-37. Marras WS. Biomechanical Basis for Ergonomics. In: Marras WS, Karwowski W, editors. Fundamentals and Assessment Tools for Occupational Ergonomics. Boca Raton, FL: CRC Press Taylor & Francis Group, 2006, pp. 11-1–43. Akesson I, Johnsson B, Rylander L, Moritz U, Skerfving S. Musculoskeletal disorders among female dental personnel – clinical examination and a 5-year follow-up study of symptoms. International Archives of Occupational and Environmental Health. 1999; 72(6): 395-403. Sjogaard G, Jensen BR. Low-Level Static Exertions. In: Marras WS, Karwowski W, editors. The Occupational Ergonomics Handbook: Fundamentals and Assessment Tools
R. Pope-Ford and Z. Jiang / Neck and shoulder muscle activation patterns among dentists during common dental procedures for Occupational Ergonomics. Boca Raton, FL: CRC Press Taylor & Francis Group, 2006, pp. 14-1–3. [9] Jonsson B. Measurement and Evaluation of Local Muscular Strain in the Shoulder During Constrained Work Journal of Human Ergology. 1982; 11(1): 73-88. [10] Akesson I, Hansson GA, Balogh I, Moritz U, Skerfving S. Quantifying work load in neck, shoulders and wrists in female dentists. International Archives of Occupational and Environmental Health. 1997; 69(6): 461-74. [11] Åkesson I, Lundborg G, Horstmann V, Skerfving S. Neuropathy in Female Dental Personnel Exposed to High Frequency Vibrations. Occupational and Environmental Medicine. 1995; 52(2): 116-23. [12] Brookham RL, Middlebrook EE, Grewal T-j, Dickerson CR. The utility of an empirically derived co-activation ratio for muscle force prediction through optimization. Journal of Biomechanics. 2011; 44(8): 1582-7. [13] Kierklo A, Kobus A, Jaworska M, Botulinski B. Work-
[14]
[15]
[16]
[17]
399
Related Musculoskeletal Disorders Among Dentists – A Questionnaire Survey. Annals of Agricultural and Environmental Medicine. 2011; 18(1): 79-84. Lalumandier JA, McPhee SD, Parrott CB, Vendemia M. Musculoskeletal pain: prevalence, prevention, and differences among dental office personnel. General Dentistry: Journal Of The Academy Of General Dentistry 2001; 49(2): 160-6. Criswell E, editor. Cram’s Introduction to Surface Electromyography. 2nd ed. Massachusetts: Jones and Bartlett Publishers, 2011. Kendall FP, McCreary EK, Provance PG, Rodgers MM, Romani WA. Muscles: Testing and Function with Posture and Pain. 5th ed. Baltimore, MD: Lippincott Williams & Wilkins, 2005, p. 480. Chiu TTW, Ku WY, Lee MH, Sum WK, Wan MP, Wong CY, et al. A study on the prevalence of and risk factors for neck pain among university academic staff in Hong Kong. Journal of Occupational Rehabilitation. 2002; 12(2): 77-91.
