Disability self-assessment and upper quarter muscle balance between female dental hygienists and non-dental hygienists

Disability self-assessment and upper quarter muscle balance between female dental hygienists and non-dental hygienists

Eric G. Johnson

Introduction

Dental hygienists routinely maintain static postures for extended periods of time, and muscles in the upper quarter are often required to endure high workloads. As a result, dental hygienists suffer from multiple musculoskeletal disorders, including overuse disorders. (1-10) Oberg and Oberg identified the upper quarter as the most frequently injured region in dental hygienists. (9) According to the National Institute for Occupational Safety and Health (NIOSH), there is a strong relationship between upper quarter musculoskeletal disorders and the static muscle contractions and postures maintained by dental hygienists. (10) The sustained-working postures of dental hygienists can lead to musculoskeletal disorders because they contribute to muscle imbalance. A healthy musculoskeletal system is dependent upon the maintenance of proper joint alignment and muscle balance. (11, 12)

Janda defines muscle balance as the relationship between muscle strength and muscle flexibility. (12, 13) Muscle flexibility is described as “the ability of a muscle to lengthen, allowing one joint (or more than one joint in a series) to move through a range of motion.” (14) Muscle balance also is defined by Kendall et al as “state of equilibrium that exists when there is a balance of strength of opposing muscles acting on a joint, providing ideal alignment for movement and optimal stabilization.” (11) Muscle balance, therefore, is a very desirable state to maintain, and imbalances can affect joint position and contribute to musculoskeletal disorders. (11,12)

Physical therapists routinely evaluate muscle balance in patients with a wide variety of musculoskeletal impairments. Procedural interventions for improving muscle balance have been outlined by the American Physical Therapy Association. (15) While dental hygienists are clearly identified in the literature as a group of health care professionals who are pre-disposed to prolonged static postures and disability, little evidence exists as to the relationship between muscle imbalance and disability. (1-10) The purpose of this study was to compare upper quarter muscle balance and self-disability assessment between female dental hygienists and non dental hygienist females. The upper quarter was operationally defined by the authors as the shoulder and neck region, and muscle balance consisted of muscle flexibility and muscle performance. Muscle performance was operationally defined as a combination of muscle strength and muscle endurance.

Methods

Subjects

The primary investigator selected female subjects via a convenience sample from the Southern California area. Dental hygienist subjects currently working at least two days per week in a general dentistry practice (mean = 3.6, SD =.84, range = 2 to 5) and non dental hygiene subjects (currently working in any profession other than the field of dental hygiene, dentistry, or dental assisting) without a history of neck or shoulder pathology were included in the study. In the dental hygienist group, 39 (90.2%) of the subjects were right hand dominant compared to 44 (95.7%) subjects in the non dental hygienist group. Age was limited to 60 years to control for possible variations in muscular torque production capabilities, as research suggests that muscular strength does not begin to decline until the beginning of the sixth decade of life. (16, 17) Exclusion criteria for both groups included multiple sclerosis, post-polio syndrome, or other medical diagnoses that may affect the neuromuscular system.

Prior to participation in this study, all subjects signed an informed consent form approved by the Loma Linda University Institutional Review Board.

Instruments

In this study, muscle flexibility was measured in one of two ways. The first method of measurement was inclinometry. An inclinometer is a plastic device with a fluid-filled center that is placed on the subject’s head for cervical range of motion (ROM) testing. It displays the amount of muscle flexibility in degrees. Reliability of inclinometry was previously reported in the literature (ICC=.92). (18) Inclinometer intratester reliability for this study was established for the left and right levator scapula muscles (ICC=.80 and .86, respectively) and the left and right upper trapezius muscles (1CC=.79 and .80, respectively.)

The second method of muscle flexibility measurement was muscle length testing. Muscle length testing requires a physical therapist to passively lower the subject’s extremity, or joint, to its end ROM and compare the end position to the norm. The test is documented as positive if the extremity, or joint, does not meet the norm, and it is documented as negative if it does meet the norm. This technique and normal end ROM is described by Kendall. (11) Reliability of muscle length testing has not been established in the literature; however, intratester reliability was established prior to data collecting in this study (ICC=1.00).

Because inclinometry and muscle length testing measure the property of interest (ROM) through direct observation, face validity is easily established for both of these instruments. Portney and Watkins stated that face validity can be attributed to tests of ROM. (19) All inclinometry and muscle length measurements were taken by the primary investigator. Muscle performance (strength and endurance) was performed last and was measured by the time, in seconds, that a subject could hold specific static exercise positions. Seconds were rounded to the nearest whole number.

The Northwick Park Neck Pain Questionnaire (NPNPQ) was used for collecting subjective information about neck pain and activities of daily living from each subject. (20) The NPNPQ is a questionnaire, modeled after the Oswestry Low Back Pain Disability Questionnaire, that is proven to be a valid assessment tool for quantifying neck pain. (20) It consists of nine five-part questions that elicit information that relates neck pain to interference with a person’s activities of daily living. Each question has five possible answers (score values between 0 to 4 points) that range from nil to significant effect of pain on activities of daily living.

Procedures

A brief history was taken from all subjects to determine that they met the inclusion criteria. Subjects then filled out the NPNPQ and were measured for muscle flexibility and muscle performance. Muscle flexibility of the upper trapezius, levator scapula, serratus anterior, rhomboids, middle trapezius, lower trapezius, and pectoralis minor was measured. These muscles are cited by several authors as being important muscular contributors in upper quarter imbalances. (12, 13, 21-23)

Muscle Flexibility

To increase accuracy of all inclinometry measurements, the inclinometer was held by the operator, with fingers spread along the base of the device, while the other hand stabilized the subject’s shoulder of the opposite side to prevent shoulder elevation during testing. (19) Muscle flexibility of the levator scapula was measured by having the subject sit in a mid-back height chair with the operator verbally assisting them into the ideal postural alignment, as stated by Kendall et al. (11) A standard lumbar support roll was placed between the lumbar spine and the back of the chair to ensure an upright trunk posture. A fluid-filled inclinometer was placed on the top of the cranium and set to zero degrees. The subject’s head was passively positioned into 45 degrees of rotation to the contralateral side using a standard goniometer. The head and neck were then passively flexed laterally to the contralateral side while stabilizing the ipsilateral scapula. The passive movement was taken to the terminal position of lateral flexion (defined as the point at which the researcher perceived resistance to stretch), and a measurement in degrees was taken at that point. Both Bandy and Lohman described this passive terminal position in the literature (Figure 1). (24, 25)

[FIGURE 1 OMITTED]

The sitting position and inclinometry techniques for measuring muscle flexibility of the upper trapezius were the same as for the levator scapula except that testing of the upper trapezius flexibility was performed with the subject’s head facing forward.

The head and neck were then passively flexed laterally to the contralateral side while stabilizing the ipsilateral scapula. The passive movement was taken to the terminal position of lateral flexion and a measurement in degrees was taken at that point.

Muscle length testing of the lower fibers of the pectoralis major was tested by having the subject lie supine on a firm table with knees bent and the lumbar spine flat on the table. The subject’s arm was placed in approximately 135 degrees of humeral abduction with the elbow extended and the humerus externally rotated approximately 45 degrees. The subject’s arm was passively lowered toward the table until either the terminal position of the muscle was met or the arm reached the table. The subject was given a positive recording if the arm did not reach the table and a negative recording if the arm did reach the table (Figure 2).

[FIGURE 2 OMITTED]

Muscle length testing of the upper fibers of the pectoralis major was tested by having the subject lie supine on a firm table with knees bent and the lumbar spine flat on the table. The subject’s arm was placed in horizontal abduction with approximately 90 degrees of humeral abduction, the elbow fully extended, and the humerus externally rotated approximately 90 degrees. The subject’s arm was passively lowered toward the table until either the terminal position of the muscle was met or the arm reached the table. The subject was given a positive recording if the arm did not reach the table and a negative recording if the arm did reach the table.

Muscle flexibility of the pectoralis minor was tested by having the subject lie supine on a firm table with knees bent and the lumbar spine flat on the table. The subject’s arms were placed in a neutral position along either side of the subject. The subject’s shoulders were passively lowered toward the table until either the terminal position of either muscle was met or any part of either scapula reached the table. The subject was given a positive recording if the scapula did not reach the table and a negative recording if the scapula did reach the table.

Muscle Performance

Muscle performance (strength and endurance) was measured by recording the time, in seconds, that a subject could hold specific static exercise positions. Subjects were given a one page written instruction sheet explaining the exact testing procedure. They were then shown pictures of each exercise and given an opportunity to try the exercise prior to testing. Once the subject clearly understood the exercise, she began by saying “go.” The position was held until the subject could no longer maintain the gravity-resisted position without compensating, or until the subject said “stop.” The duration of the hold was timed in seconds with a digital stopwatch.

The scapular stabilizers were tested in biased positions for muscle performance against gravity. (22, 23) A standard high-low table, a standard height chair, and a digital stopwatch were used for testing purposes. The serratus anterior was tested by having the subject lie prone on a table. On the subject’s command of “go,” the subject would perform a push-up with a “plus.” The “plus” refers to abducting and protracting the scapula to maximize the contraction of the serratus anterior (Figure 3).

[FIGURE 3 OMITTED]

The middle trapezius and rhomboids were tested by having the subject lie prone on a table with the trunk supported on a stable platform with one pillow between the trunk and platform for comfort. The beginning position was with the arms in horizontal abduction, humerus abducted approximately 90 degrees, and elbows fully extended. On the subject’s command of “go,” the subject would maximally retract the scapula and lift her arms off of the table.

The upper trapezius and levator scapula were tested by having the subject lie prone on a table with the trunk supported on a stable platform with one pillow for comfort. The beginning position was arms along the trunk and elbows flexed approximately 90 degrees. On the subject’s command of “go,” the subject would maximally elevate the scapula.

The lower trapezius and pectoralis minor were tested by having the patient sit on the floor (legs straight out in front of the body with hips flexed approximately 90 degrees and knees fully extended). The hands were placed on stable platforms along either side of the subject. On the subject’s command of “go,” the subject would push through the hands to lift her bottom off the floor. Subjects performed an additional push at the end of the range to maximally contract the lower trapezius and pectoralis minor muscles.

Data Analysis

Means and standard deviations were calculated for each flexibility measurement by group. Using independent two-tailed t-tests, flexibility measures of the upper trapezius, levator scapula, and pain measures were compared for the two groups. To determine if age was a significant factor between groups, analysis of covariance (ANCOVA) was used with age as the covariate. Chi square tests for homogeneity were used to compare muscle length tests for the pectoralis major and pectoralis minor. Significance was set at p=0.05 for all tests.

Results

A convenience sample of 87 subjects between the ages of 20, and 60 years was enrolled in the study and all subjects completed the study. Forty-one dental hygienist subjects were in the study group (age range=22 to 60, mean age=38.0 years, SD=10.3), and 46 subjects were in the control group (age range=20 to 54, mean age=29.3 years, SD=8.2). Age was significantly different between groups (p<0.001). ANCOVA, with age as the covariate, was used in all quantitative statistical analyses to determine if age influenced any statistical differences found.

Muscle Flexibility

Mean differences were significant when comparing the left upper trapezius and levator scapula (p=0.007, p=0.01, respectively). The dental hygienist group had a mean difference of 5.9 degrees less flexibility for the left upper trapezius compared to the non dental hygienist group, and a mean difference of 5.4 degrees less flexibility for the left levator scapula compared to the non dental hygienist group (Table 1).

Significant differences in the proportion of positive test results were found for the left pectoralis minor (p=0.001). The left pectoralis minor was positive in 70.7% of subjects tested in the dental hygienist group, as compared to 30.4% of the non dental hygienist group (Table 2).

Significant differences were found between groups in muscle length testing of the lower fibers of the pectoralis major on the right side (p=0.030). The dental hygienist group had 9.8% positive findings compared to no positive findings in the non dental hygienist group (Table 3).

Muscle Performance

Muscle performance was measured for the following four groups: serratus anterior, rhomboids/middle trapezius, upper trapezius/levator scapula, and lower trapezius/pectoralis minor. Due to high variability in the subject’s holding times for the four positions, statistically significant differences were not found between groups (Table 4). Significant differences were seen, however, within the dental hygienist group when the number of days worked per week were considered. Dental hygienists working four days per week or greater had significantly shorter holding times for position 2 when compared to dental hygienists working less than four days per week (mean=116.7 (52.7), mean=180.9 (85.0), respectively with p=0.006). Dental hygienists working four days per week or greater also had significantly shorter holding times for position 3 when compared to dental hygienists working less than four days per week (mean=89.3 (50.2), mean=148.9 (76.6), respectively with p=0.005).

Pain Questionnaire

The NPNPQ was compared between groups. Significant differences (p<0.05) between groups were found for five of the nine sections. The five sections were pain intensity level, numbness, pain duration, working, and driving (Table 5). No significant correlations (p<0.05) were found between self-disability assessment and muscle balance. When the number of days worked per week were examined in the dental hygienist group, pain intensity while sleeping was significantly higher in dental hygienists working four or more days per week compared to dental hygienists working less than four days per week (mean=.59 (.50) and mean=.17 (.38), respectively with p=.004).

Discussion

This study identified statistically significant differences in muscle flexibility in the upper quarter between female dental hygienists and female non dental hygienists. The results suggest that dental hygienists are prone to develop tightness of the upper trapezius and levator scapula of the non dominant side. In the dental hygienist group, 90.2% of the subjects were right hand dominant compared to 95.7% in the non dental hygienist group. While the dental hygienist group had decreased passive muscle flexibility in the upper trapezius and levator scapula of both the dominant and non dominant sides, only the non dominant side was statistically significant (p<0.05). Dental hygienists typically use the dominant upper quarter and extremity for scaling and hand-intensive instrumentation activities. The non dominant, or assisting upper quarter, is frequently maintained in static positions that elevate the scapula. The upper trapezius and levator scapula are primary scapular elevators. These findings support previous studies that identified dental hygienists as being prone to musculoskeletal disorders related to muscle imbalances, prolonged static postures, and high muscle workloads in the upper quarter. (1-9)

Muscle length tests of the pectoralis minor were positive in the majority of the dental hygienist subjects. Pectoralis minor tightness of the non dominant side was statistically significant (p<0.05) and found in 70.7% of the dental hygienist group as compared to 30.4% of the non dental hygienist group. Although a significant difference for the pectoralis minor was not found in the dominant side, the dental hygienist group had 18.5% more positive findings than the non dental hygienist group (70.7%, 52.2% respectively). These results suggest that work-related static positions of the non dominant upper quarter contribute to the muscular imbalances found in the dental hygienist group. Similar conclusions were made by Milerad et al who used electromyography to analyze the workload of muscles in the upper quarter of dental hygienists. (7)

The muscle length tests of the lower fibers of the pectoralis major on the dominant side revealed statistically significant (p=<0.05) positive tests in 9.8% of the dental hygienist group, as compared to no positive tests in the non dental hygienist group. The positive findings in the dental hygienist group were most likely due to the amount of time that the dominant humerus is internally rotated during the workday. The muscle length tests of the lower fibers of the left pectoralis major revealed positive tests in 14.6% of the dental hygienist group compared to 4.3% positive tests in the non dental hygienist group.

The results of active muscle performance revealed high variability in the length of time individual subjects were able to hold each of the four positions. This was consistent with the results of another study that measured isometric muscular strength and endurance in normal subjects. (16) Because of this high variability, statistical significance between groups was not achieved for any of the four testing positions. Despite the lack of statistical significance, several clinically important trends were noted. Position 1 required the subject to maintain an isometric contraction of the serratus anterior. The mean holding time for the dental hygienist group was 17.9 seconds less than the mean holding time for the non dental hygienist group (97.1 and 115.0 seconds, respectively). This is clinically relevant because the serratus anterior is an important scapular stabilizer and is part of the scapular force couple that allows for normal scapular upward rotation during humeral flexion. Humeral flexion in the absence of scapular upward rotation can lead to sub-acromial impingement. (26-27)

Position 2 required the subject to maintain an isometric contraction of the rhomboids and middle trapezius. The mean holding time of the dental hygienist group was 47.9 seconds more than the mean holding time of the non dental hygienist group (144.1 and 116.2 seconds, respectively). Position 3 required the subject to maintain an isometric contraction of the levator scapula and the upper trapezius. The mean holding time of the dental hygienist group was slightly higher (6 seconds higher) than the mean holding time of the non dental hygienist group (115.0 and 109.0 seconds, respectively). These are clinically relevant findings because both position 2 and position 3 required scapular adductors as part of the testing position and other studies have identified all four of the muscles involved with positions 2 and 3 as being primary scapular adductors. (22) Since the upper trapezius and the levator scapula were less flexible in the dental hygienist group, this supports popular muscle balance theory that shortened muscles remain strong while lengthened muscles become weak. (11-13)

Significant differences (p<0.05) were seen within the dental hygienist group when the number of days worked per week were considered. The dental hygienists who were working four or more days per week had significantly shorter holding times for positions 2 and 3 when compared to the dental hygienist working fewer than four days per week. This suggests that muscle imbalance can be increased by working more than three days per week.

Position 4 required the subject to maintain an isometric contraction of the scapular depressors (lower trapezius and pectoralis minor). The mean holding time for the non dental hygienist group was 98.0 seconds compared to the dental hygienist group mean holding time of 75.1 seconds. This is clinically relevant because the opposing muscle group, the scapular elevators (upper trapezius and levator scapula), was less flexible in the dental hygienist group (statistically significant for the non dominant side only). The trend of markedly weaker holding times in the muscles opposing the tight muscles by the dental hygienist group (22.9 seconds) again supports popular muscle balance theory. (11-13)

The NPNPQ revealed statistically significant differences between groups in five of the nine categories. The dental hygienist group reported consistently higher frequencies of pain experiences throughout all applicable categories of the questionnaire. This suggests that female dental hygienists are more likely to develop some degree of upper quarter pain that may impact activities of daily living than female non dental hygienists. Additionally, significant differences (p<0.05) were seen within the dental hygienist group when the number of days worked per week were considered. The dental hygienists who were working four or more days per week had significantly higher intensity levels associated with sleeping when compared to the dental hygienist working fewer than 4 days per week. This suggests that disability self-assessment can be increased by working more than three days per week.

No correlations were found between upper quarter muscle imbalance and disability self-assessment. This may be due to the fact the subjects in the dental hygienist group were working between two and five days per week and that variability in the disability level ranged widely.

This reduction of upper quarter muscle flexibility coupled with abnormal muscle performance of the scapular stabilizers sheds light on the role that muscle imbalance plays in upper quarter musculoskeletal disorders commonly seen in female dental hygienists. The muscles identified in this study as lacking flexibility or strength and endurance should be included as part of a general screening examination by health care professionals who treat dental hygienists with upper quarter musculoskeletal disorders. These results may also serve as an important component of the dental hygiene education process. Teaching students preventive strategies aimed at maintaining normal muscle balance in the upper quarter could reduce their risks of developing upper quarter musculoskeletal disorders.

There were several limitations in this study. A convenience sample versus random sampling was used in the pilot study, thus, generalizations cannot be assumed. Also, the number of days worked per week and type of work were not controlled in the non dental hygienist group, and age was not controlled between groups. An ANCOVA was used to adjust for the mean age difference between groups. Also, only dental hygienists working a minimum of two days per week were subjects in this study. Positive correlations might be found between disability self-assessment and upper quarter muscle imbalances if the inclusion criteria did not require a minimum number of days worked per week. It is likely that many dental hygienists work less than two days per week, or not at all, because of work-related disability. Another limitation was that lifestyle habits in either group (weight training, gardening, etc …) were not considered and may affect a person’s muscle balance.

Further research is needed to determine if upper quarter strengthening and flexibility exercises performed by dental hygienists reduces disability self-assessment. Further research should also be conducted on dental hygienists who are working fewer than two days per week to study the possible relationship between disability self-assessment and muscle imbalance in the upper quarter in a potentially more disabled group. Also, the affect of muscle imbalance on disability self-assessment in groups of people working in other occupations requiring repetitious upper extremity movement should be studied.

Conclusion

The results of this study suggest that muscle imbalances in the upper quarter exist in the female dental hygienist population and contribute to upper quarter pain and musculoskeletal disorders. Statistically significant muscle tightness was found in the dominant pectoralis major (lower fibers) and non dominant upper trapezius, levator scapula, and pectoralis minor of the dental hygienist group. This suggests that the non dominant upper extremity is performing activities that either statically elevate the scapula and/or side bend the head and neck toward the non dominant shoulder. It also suggests that the dominant upper extremity performs activities that require extensive humeral internal rotation, flexion, and/or adduction. It is the primary author’s opinion that education relative to the preventive maintenance of upper quarter muscle balance should be part of the dental hygiene curriculum to help minimize musculoskeletal health risks associated with the practice of dental hygiene.

Table I. Positive Range of Motion Results of Muscular

Flexibility Using Inclinometry

NDH DH

(n = 46) (n = 41)

Muscle Mean (SD) Mean (SD) P-value *

Upper Trapezius

Right 41.2[degrees] (6.4) 35.2[degrees] (9.5) 0.06

Upper Trapezius

Left 35.3[degrees] (5.7) 29.4[degrees] (7.0) 0.007

Levator Scapula

Right 37.5[degrees] (7.1) 34.0[degrees] (8.1) 0.08

Levator Scapula

Left 30.4[degrees] (8.0) 25.0[degrees] (7.0) 0.01

* ANCOVA was used to determine significance with age as the covariate.

Significance levels were set at p=0.05 for all tests.

Table IV. Results of Passive Muscle Length Testing for

Pectoralis Minor

NDH DH

(n = 46) (n = 41)

Muscle % % P-value

Pectoralis Minor Right 52.2 * 70.7 .08

Pectoralis Minor Left 30.4 70.7 .001

* Percentages represent positive test result. Significance

levels were set at p=0.05 for all tests.

Table III. Results of Muscle Length Testing

for Pectoralis Major

NDH DH

(n = 46) (n = 41)

Muscle % % P-value

Pectoralis Major Upper Fibers (L) 2.4 * 0.0 * 0.287

Pectoralis Major Upper Fibers (R) 0.0 0.0

Pectoralis Major Lower Fibers (L) 4.3 14.6 0.097

Pectoralis Major Lower Fibers (R) 0.0 9.8 0.030

* Percentages represent positive test result. Significance

levels were set at p=0.05 for all tests.

Table IV. Results of Active Muscle Performance Measured in Seconds

NDH DH

(n = 46) (n = 41)

Muscle(s) Mean (SD) Mean (SD) P-value **

Position 1 * 115.0 (84.0) 97.0 (49.0) 0.22

Position 2 116.0 (62.0) 144.0 (75.0) 0.06

Position 3 109.0 (55.0) 115.0 (69.0) 0.67

Position 4 98.0 (67.0) 75.0 (42.0) 0.07

* Position 1 is serratus anterior, position 2 is rhomboids and middle

trapezius, position 3 is upper trapezius and levator scapula, and

position 4 is lower trapezius and pectoralis minor. Means and standard

deviations were rounded to the nearest whole number. Significance

levels were set at p=0.05 for all tests.

** ANCOVA was used to determine significance with age as the covariate.

Table V. Results of Northwick Park Neck Pain Questionnaire

NDH DH

(n = 46) (n = 41)

Section % * % P-value

Current Pain Intensity 15.2 34.1 0.04

Pain with Sleeping 21.8 39.0 0.08

Numbness at Night 28.3 58.6 0.005

Duration of Symptoms 28.2 65.0 0.001

Pain with Carrying 15.2 21.9 0.42

Pain with Reading/T.V. 34.8 41.5 0.43

Pain with Work 2.0 31.7 0.001

Pain with Social Activities 8.7 14.6 0.34

Pain with Driving 4.3 24.4 0.006

* Percent represents the number of subjects who reported that pain

or numbness affected their activities of daily living to some degree.

Significance levels were set at p=0.05 for all tests.

References

(1.) Oberg T, Karsznia A, Sandsjo L, Kadefors R: Workload, fatigue, and pause patterns in clinical dental hygiene. J Dent Hyg 1995;69:223-229.

(2.) Liss GM, Jesin E, Kusiak RA, White P: Musculoskeletal problems among Ontario dental hygienists. Am J Ind Med 1995;28:521-540.

(3.) Stentz TL, Riley MW, Harn SD, Sposato RC, Stockstill JM, Harn JA: Upper extremity altered sensations in dental hygienists. Int J Ind Ergon 1994;1:107-112.

(4.) Michalak-Turcotte C, Atwood-Sanders M: Ergonomics strategies for the dental hygienist, Part 1. JPH 2000;39-42.

(5.) Michalak-Turcotte C, Atwood-Sanders M: Ergonomics strategies for the dental hygienist, Part 2. JPH 2000;35-38.

(6.) Stitik TP, Conte M, Foye PM, Schoen D, Marini JS: An analysis of cumulative trauma disorders in dental hygienists. JPH 2000;19-25.

(7.) Milerad E, Ericson MO, Nisell R, Kilborn A: An electromyographic study of dental work. Ergonomics 1991;34:953-962.

(8.) Barry RM, Woodall WR, Mahan JM: Postural changes in dental hygienists; Four-year longitudinal study. J Dent Hyg 1992;3:147-150.

(9.) Oberg, T, Oberg U: Musculoskeletal complaints in dental hygiene; a survey study from a Swedish county. J Dent Hyg 1993;67:257-261.

(10.) NIOSH, 2nd Printing;1997. Musculoskeletal disorders and workplace factors: A critical review of epidemiological evidence for work-related musculoskeletal disorders of the neck, upper extremity, and low back. Cincinnati, OH: Department of Health and Human Service, Center for Disease Control and Prevention, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication Number 97-141.

(11.) Kendall FP, McCreary EK, Provance PG: Muscles Testing and Function, 4th ed. Baltimore, Williams & Wilkins, 1993, p.3.

(12.) Janda V: Muscles and cervicogenic pain syndromes. Physical Therapy of the Cervical and Thoracic Spine. Grant R, ed. New York, Churchill Livingstone, 1988, p. 153-166.

(13.) Janda V: Muscles and motor control in low back pain, assessment and management. Physical Therapy of the Low Back. Twomey L ed. New York, Churchill Livingstone, 1987, p.253-278.

(14.) Zachezewski J: Physical Therapy. Scully RM, Barnes RM, eds. Philadelphia, JB Lippincott Co, 1989, p.698-699.

(15.) American Physical Therapy Association: Guide to physical therapist practice, Phys Ther, 2001;81:145-160.

(16.) Graves JE, Pollock ML, Carroll JF: Exercise, age, and skeletal muscle function, South Med J, 1994;87:17-22.

(17.) Backman E et al: Isometric muscle strength and muscular endurance in normal persons aged between 17 and 70 years, Scand J Rehab Med, 1995;27:109-117.

(18.) Bush KW, Collins N, Portman L, Tillett N: Validity and intertester reliability of cervical range of motion using inclinometer measurements, J Manual Manipulative Ther, 2000;8:52-61.

(19.) Portney L, Watkins M: Foundations of Clinical Research, 2nd ed. New Jersey, Prentice Hall, 2000, p.82.

(20.) Leak A et al: The Northwick Park Neck Pain Questionnaire, devised to measure neck pain and disability, Br J Rheumatol, 1994;33:469-474.

(21.) Godges J, Yakura J: Manual Therapy and Movement, Manual Therapy for Flexibility and Motor Coordination Impairments. Course manual at Loma Linda University Department of Physical Therapy, 2001.

(22.) Paine RM, Voight ML: The role of the scapula, Journal of Sports Physical Therapy, 1993;18:386-391.

(23.) Mosely JB et al: EMG analysis of the scapular muscles during a shoulder rehabilitation program, Am J Sports Med, 1992;20:128-134.

(24.) Bandy WD, Irion JM, Briggler M: The effect of static stretch and dynamic range of motion training on the flexibility of the hamstring muscles, Phys Ther, 1998;27:295-300.

(25.) Lohman EB: Orthopedic Physical Therapy Clinics of North America, Godges J, Deyle G eds. Philadelphia, W.B. Saunders Company, 1998;367-396.

(26.) Sahrmann SA: Diagnosis and Treatment of Movement Impairment Syndromes. St. Louis, Mosby, 2002, pp.195-196.

(27.) Donatelli RA, Wooden MJ: Orthopedic Physical Therapy, 3rd ed., New York, Churchill Livingstone, 2001, pp.155-156.

Eric G. Johnson, PT, DPTSc is an associate professor and clinical science coordinator in the Department of Physical Therapy; Joseph J. Godges, DPT, OCS is an assistant professor in the Department of Physical Therapy; Everett B. Lohman, PT, DPTSc, OCS is an associate professor and program director in the Department of Physical Therapy; Joni A. Stephens, EdS, MS, RDH is a professor and director of research in the Department of Dental Hygiene; and Grenith J. Zimmerman, PhD is a professor and director of research in the Department of Physical Therapy, all at Loma Linda University. Sharon P. Anderson, PT, DrPH is a private consultant.

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