Abstract
BACKGROUND:
Extremity strength testing is used to determine the ability of industrial employees to perform a physically demanding occupation safely, evaluate post-injury impairment, and monitor return to function after injury. There is an unmet clinical need for a robust and granular source of normative reference values to serve as a resource for baseline data on upper extremity isometric strength.
OBJECTIVE:
Develop normative reference data for upper extremity strength among industrial employees and investigate associations between strength and physical job demands.
METHODS:
Upper extremity strength data from 107,102 industrial employees were collected post-hire. In this study, isometric tests for pinch; hand grip; wrist pronation and supination; and flexion of the wrist, elbow, and shoulder strength were retrospectively analyzed in relationship to sex, age, and job level. Associations between strength scores and five levels of work, defined the United States Department of Labor’s Dictionary of Occupational Titles by level of physical difficulty (1–5), were determined.
RESULTS:
Higher strength scores were positively associated with more physically demanding job levels (P < 0.001), and there was a progressive increase in strength scores with increased physical job demands for both sexes (P = 0.0002). All strength scores differed significantly by decade of age (P < 0.001). All scores except for pinch strength demonstrated a moderate or high positive correlation with job level (r≥0.50).
CONCLUSIONS:
The normative reference upper extremity strength data collected in this study for industrial employees may be useful for evaluating rehabilitation and recovery following injury or illness. In order to utilize normalized strength data as a post-injury reference, it is important to consider job level in addition to age and sex, as these variables are highly correlated with baseline upper extremity strength.
Introduction
Upper extremity isometric strength testing among employees provides insight into occupational dem-and capacity in relation to age, physical fitness, and general health [1]. The American Medical Asso-ciation‘s Guides to the Evaluation of Permanent Impairment, Sixth Edition identifies upper extremity strength testing as the reference standard for evaluating post-injury impairment and determining optimal rehabilitation goals [2]. Continuous data sets that include isometric upper extremity strength values are useful for documenting changes in fitness and strength related to age, occupational or non-work-related injuries, and medical conditions [3, 4]. Such testing has been used to validate the success of pharmaceutical treatments for cerebrovascular accidents and various autoimmune disorders [3–6].
It is recommended that industrial workers do not exceed 1/3 of their isometric strength on a sustained basis to avoid muscle fatigue and injury [7, 8]. Lifting estimations can be obtained through the National Institute for Occupational Safety and Hea-lth (NIOSH) lifting equation app [9]. With the aging work populations internationally the age and gender differences make the estimation of lifting limits more complex and dependent on individuals’ actual physical abilities [10, 11]. As more physically strenuous jobs have higher potential for work-related injury, the United States Department of Labor’s Dictionary of Occupational Titles (DOT) defines five levels of work according to their level of physical difficulty [12]. Studies have shown that risk of in-jury, including hip fractures and rotator cuff tears, increases when industrial workers exceed their physical capacity, as determined by isometric upper ext-remity strength tests such as grip and shoulder strength [13–15]. In addition, the use of post-job offer upper extremity strength testing among newly hired employees to determine employee-specific physical capacities has been shown to reduce the incidence of work-related injuries [16–18]. However, a worker’s strength capacity most accurately predicts risk of injury when it is carefully equated to physical job demands [19]. Examples of clinically-useful upper extremity strength evaluations include grip and pinch strength tests to determine the normative physical capability of the hand [20]. Isometric strength testing of the wrist, elbow, and shoulder are effective for assessing upper extremity function following surgery, injury, or illness [15, 22]. In assessing the rehabilitation of patients with medical conditions or injuries, the ability to estimate normal baseline physical capacity is useful in developing and evaluating treatment goals [23]. There are a number of other studies that indicate grip, pinch and other upper extremity strength assessments are relevant in the evaluation and treatment of patients with congestive heart failure, sarcopenia, myositis, and various neurological disorders [24–28].
Since a patient’s pre-injury or pre-illness upper extremity strength status is often unknown, normative strength data stratified by age and sex can be useful to establish rehabilitation goals. Currently available reference standards for upper extremity strength data have notable deficiencies that limit their applicability, such as largely heterogeneous populations or participants who are not employed and/or have pre-existing upper extremity injuries or related medical issues [3, 29]. In addition, many such studies do not include participants from a diverse range of occupational settings [3, 30]. Thus, there is an unmet clinical need for a more robust source of standardized reference data on upper extremity strength. The purpose of this study was to develop and summarize normative reference data for upper extremity strength among healthy, uninjured employees, with stratifications by sex, age, and job level. Post-hire data was retrospectively analyzed for associations between upper extremity strength test scores and DOT job level.
Methods
Study protocol
This study was composed of data from the Occupational Performance Corporation (Salina, KS), obtained from post-offer, pre-placement upper ext-remity strength testing performed between 2005 and 2015 among newly hired industrial workers from fifteen states: Arkansas, Colorado, Florida, Georgia, Iowa, Kansas, Kentucky, Louisiana, Missouri, Nebraska, North Carolina, Oklahoma, Tennessee, Texas, and Wisconsin. Newly hired employees were evaluated for upper extremity strength to determine whether their individual physical capacities met pre-defined criteria for job placement [16, 17]. Demographic (e.g., age, job title) and anthropometric (e.g., height, body mass, body mass index [BMI], body fat percentage) data also were collected either by the test administrator, from the employer, directly from the employee, or by direct measurement. Employees were evaluated on fitness (timed sit-ups and squats), body strength (upper body, lower body, and core), and occupational demands. Strength assessments were conducted in licensed medical facilities, and test adm-inistrators were trained and certified to perform the testing protocols. Employees provided consent for data obtained from their post-job offer testing and hiring processes to be used for research purposes, including publication of the de-identified data.
Employees for whom strength or fitness test re-sults, anthropometric data, or medical information were missing were excluded from the study. Employees’ medical records provided by the employees be-ing tested were reviewed for any history of injury or illness that would affect evaluation of upper extremity strength, and those with any such history were excluded from the study. Employees were excluded if there was evidence of an injury, medical condition, or missing data for test results. All testing was adjusted to body habitus and there were no outliers for height or body mass.
Upper extremity strength testing protocol
All new employees underwent a consistent data collection and testing protocol described previously [16, 17]. All test administrators were trained and certified for strength testing on the equipment used. The testing protocol included seven isometric strength tests, with the postures of the upper extremity segments positioned as described in Table 1, listed in chronological order of the testing sequence. The test protocol is purposely organized in a rigid fashion in order to maximize consistency and minimize any effect of fatigue. For each test, the recorded result was the average score of three trials, with a rest period of 10–30 seconds between trials. Each isometric contraction was held for 5–10 seconds until the subject had demonstrated maximal contracture. Less than 15% variance between trials was required for the test to be considered valid. The isometric tests were unsustained maximal contractures which result in minimal muscle fatigue minimizing the fatigue effect on multiple testing [31]. If the range of the scores from the three trials exceeded 15% variation, the test was repeated until the scores fell into this range. Employee-specific strength data reflect maximum isometric strength. Each strength test was sequenced to minimize any effect of fatigue on test results. Grip and pinch strength were assessed with Jamar dynamometers (Lafayette Instrument Company, Lafayette, IN). Isometric strength values for all other upper extremity tests were obtained using testing equipment and software manufactured by the Occupational Performance Corporation. The entire sequence was completed in less than 30 minutes. Intra-tester, inter-site, and data from multiple tests obtained from some individuals were evaluated, with no statistically significant differences identified (our unpublished data) [27, 32–34]. Occupational physical and strength demands were determined by separate ergonomic testing by each employer for each employee’s job title and each job and then was assigned a job workload level (1 through 5) corresponding as closely as possible to the DOT job levels (Table 2).
Upper extremity strength tests used among 107,102 healthy employees (2005–2015)
Upper extremity strength tests used among 107,102 healthy employees (2005–2015)
*Tests are presented in chronological order of testing.
Job workload levels defined by the United States Department of Labor’s Dictionary of Occupational Titles (DOT)
Data analyses were performed using SYSTAT 13 (Systat Software, Inc., San Jose, CA). Means and 95% confidence intervals (95% CI) for anthropometric data and isometric strength test results were stratified by sex, age, and job level. Normal distributions were noted for all studied isometric strength values as indicated by the symmetrical bell-shaped curve determined using SYSTAT. Kolmogorov–Smirnov test results for normality for all strength tests separated for males and females demonstrated a two-tail p-value of <0.0001.
In order to evaluate the consistency of the collected data, the 10-year data was divided into 2 separate subgroups representing the first 5 years and the second 5 years of data collection. We then compared the first five years and the second 5 years of data collection to the full test subject dataset in order to assure statistical similarity in the data collection for the 10 year time frame of the study. To assure a homogeneous group for equivalency testing, all male test subjects were derived from DOT level 4 and females from DOT level 3, because they make up the largest group of test subjects for each sex. Each of the 7 isometric tests were separated for right side, left side, and sex, and compared for the first and second 5 years to the full male DOT level 4 (for males) or full female level 3 DOT (for females), resulting in a total of 56 separate 2-sample t-tests.
An unpaired two-sample t-test was used to identify significant differences by sex, age group, and job level. Statistical significance was accepted when P < 0.05. Associations between upper extremity str-ength test scores and job level were analyzed using Pearson’s correlation coefficients. Previously published guidelines on the use of correlation coefficients in medical research propose the following definitions of strength of association based on correlation coefficients: very high positive, 0.90–1.00; high positive, 0.70–0.90; moderate high positive, 0.50–0.70; and low positive, 0.30–0.50 [19], and thus the strength of associations between upper extremity strength scores and job workload level was categorized as follows: no correlation (r = 0.0 to < 0.30), low correlation (r = 0.30 to < 0.50), moderate correlation (r = 0.50 to < 0.70), and high correlation (r = 0.70 to < 0.90) [19].
Results
Subject characteristics
Of 196,043 employees who met eligibility criteria for the study, the final dataset comprised 107,102 (54.6%) non-injured, healthy subjects after excluding those with an injury, medical condition, or mis-sing data. Anthropometric data are presented by age category for males and females in Tables 3 and 4, respectively. Of the 107,102 employees, 38,037 (35.5%) were female. Mean age was 32±11.5 for males and 32±11.9 for females. Mean height was 178.1±8.0 cm for males and 164.5±7.3 cm for females. Mean body mass was 90.6±20.8 kg for males and 76.4±20.5 kg for females. Mean BMI was 28.4±6.1 kg/m2 for males and 28.2±7.2 kg/m2 for females. Mean body fat percentage was 25.5±7.0% for males and 35.6±6.8% for females. Of the 107,102 employees, 1,525 (1.4%) were tested for a DOT level 1 job, 7,313 (6.8%) for a level 2 job, 54,952 (51.3%) for a level 3 job, 38,775 (36.2%) for level 4 job, and 4,488 (4.2%) for a level 5 job.
Anthropometric data among 69,065 healthy male employees by age group (2005–2015)
Anthropometric data among 69,065 healthy male employees by age group (2005–2015)
BMI = body mass index. Data are reported as mean (standard deviation).
Anthropometric data among 38,037 healthy female employees by age group (2005–2015)
BMI = body mass index. Data are reported as mean (standard deviation).
When the first vs. second 5-year data collection for DOT level 4 males and DOT level 3 females was compared to the full male DOT level 4 and female DOT level 3 DOT, respectively, all but 7 of the 56 comparisons demonstrated P≥0.05 (Table 5). The seven comparisons with P < 0.05 were male first 5 years left protonation (P = 0.0499), male second 5 years right protonation (P = 0.0197) and right grip (P = 0.0.0495), female first 5 years data left grip (P = 0.0386), right grip (P = 0.0470), and left protonation (P = 0.0300), and females second 5 years shoulder flexion (P = 0.0317). The remaining 49 equivalency tests of all right and left isometric tests yielded P≥0.05, demonstrating sufficient equivalency of the 10-year data collection.
Comparison of first vs. second 5 years of collection for male DOT level 4 and female DOT level 3 data
Data are p values resulting from two-sample t-tests comparing against full male DOT level 4 data (for males) and full female DOT level 3 data (for females).
All distribution plots for hand grip, elbow flexion, shoulder flexion, and pinch strength scores among male employees demonstrated normal bell-shaped distributions. Bilateral isometric strength scores for all upper extremity strength tests are summarized by age group for males and females in Tables 6 and 7, respectively. On average, mean strength values for males were higher than females in all categories relating to upper extremity strength. Utilizing a two-sample t-test, there was a significant difference between sexes for all isometric strength tests (P < 0.001). For both sexes, upper extremity strength decreased progressively beginning at age 50 years, and all test results differed significantly by decade of age (P <0.001). Isometric strength scores for both sexes in-creased by decade of age up to age 30 to 39 years, remained constant during the 40s, 50s, and 60s, and then decreased at age 70 years or older.
Bilateral isometric strength test scores among 69,065 healthy male employees by age group and test
Bilateral isometric strength test scores among 69,065 healthy male employees by age group and test
Data are reported as mean (95% confidence interval).
Bilateral isometric strength mean test scores among 38,037 healthy female employees by age group and test
Data are reported as mean (95% confidence interval).
Mean values for age, anthropometric data, and isometric strength test scores are presented by DOT job level for males and females in Tables 8 and 9, respectively. Both male and female results based on job level demonstrated Normal distributions. Utilizing an unpaired two-sample t-test, there was a significant increase in all tested isometric strength values correlating with an increase in the physical job demands for males and females (P = 0.002). Height and body mass increased the least with physical demand, with height showing a 1.2% increase from DOT job level 1 to job level 2 (P = 0.19 for males, P = 0.27 for females). Neither height nor body mass from males and females differed significantly between job level groups 1 and 2 by age or job levels (P > 0.05).
Age, anthropometric data, and isometric strength test scores among 69,065 healthy male employees by job workload level
Data are reported as mean (standard deviation).
Age, anthropometric data, and isometric strength test scores among 38,037 healthy female employees by job workload level
Data are reported as mean (standard deviation).
Pearson’s correlation coefficients for associations between each of seven isometric strength test scores (both right- and left-sided data) and job level groups are presented for males and females in Tables 10 13. Each isometric strength test demonstrated a notable (low, moderate, or high) positive correlation with job level. Only pinch strength demonstrated a low strength of association with job level (males, r = 0.49 for right and left pinch; females, r = 0.47 for right pinch, r = 0.48 for left pinch). Left and right strength tests for grip, wrist supination, and wrist pronation for males and females were moderately associated with job workload level, with r values ranging from 0.58 (for female left wrist supination) to 0.68 (for male right wrist pronation). Left and right strength tests for wrist, elbow, and shoulder flexion were highly associated with job workload level, with r values ranging from to 0.70 (for female right wrist flexion) to 0.80 (for male left elbow flexion).
Correlations between right side isometric strength test scores and job workload levels among 69,065 healthy male employees
Correlations between right side isometric strength test scores and job workload levels among 69,065 healthy male employees
Data presented are Pearson’s correlation coefficients (r values) between employees’ (right-sided) isometric strength test scores and their job workload levels, as categorized according to the U.S. Department of Labor’s Dictionary of Occupational Titles.
Correlations between left side isometric strength test scores and job workload levels among 69,065 healthy male employees
Data presented are Pearson’s correlation coefficients (r values) between employees’ (left-sided) isometric strength test scores and their job workload levels, as categorized according to the U.S. Department of Labor’s Dictionary of Occupational Titles.
Correlations between right side isometric strength test scores and job workload levels among 38,037 healthy female employees
Data presented are Pearson’s correlation coefficients (r values) between employees’ (right-sided) isometric strength test scores and their job workload levels, as categorized according to the U.S. Department of Labor’s Dictionary of Occupational Titles.
Correlations between left side isometric strength test scores and job workload levels among 38,037 healthy female employees
Data presented are Pearson’s correlation coefficients (r values) between employees’ (left-sided) isometric strength test scores and their job workload levels, as categorized according to the U.S. Department of Labor’s Dictionary of Occupational Titles.
In this study, normative data from upper extremity strength testing among 107,102 healthy, uninjured, newly hired employees were collected, described, and analyzed, with stratification by age, sex, and job workload level. Given the large sample size and standardized protocol, this study provides a clearly defined baseline data set for normative reference that may be clinically useful to assess risk of injury and recovery after injury or treatment for medical disorders such as autoimmune or neurological diseases [3–6].
A 2018 meta-analysis of published literature reported reference values for isometric strength of 14 muscle groups from a total of 46 published studies, 32 of which reported data for the upper extremities in a total of 9,449 subjects [35]. The data was separated for dominant/non-dominant sides, and measured with dynamometers/myometers, for males and females, and in adults and the elderly. The reported meta-analytical values agree with the data from the present study, with values declining with increase age; however, that study only reported data for a maximum of n = 66 per value per sex per decade age group. Average elbow flexion values for 136 males aged 50–79 ranged from 234.20–289.78 N (24.8–29.5 kg), considering both the dominant and non-dominant side. For 138 women in the same age range, average elbow flexion values ranged from 134.62–161.50 N (13.7–16.5 kg) [35–37]. Grip strength was reported for males and females aged 65–74 for both the right and left side. Average grip strength values from 91 men ranged from 79.1–91.3 lb (35.9–41.4 kg) on the right and 71.8–81.5 lb (32.6–37.0 kg) on the left. For 127 women, average grip strength values ranged from 51.2–52.5 lb (23.2–23.8 kg) on the right and 45.3–46.7 lb (20.5–21.2 kg) on the left [35, 39]. Values for pulp-to-pulp pinch were also reported for subjects age 65–74. For 91 men, average pinch strength was 14.3–16.5 lb (6.5–7.5 kg) on the right and 13.6–15.4 lb (6.2–7.0 kg) on the left, and for 124 women, ranged from 10.0–10.3 lb (4.5–4.7 kg) on the right and 9.5–10.1 lb (4.3–4.6 kg) on the left. The study did not provide meta-analytical reference values for pronation/supination or for shoulder flexion. A 2015 study measuring upper limb strength in healthy normal subjects reported values for shoulder flexion ranging from 18.4–23.9 kg for 90 adult males and 9.2–12.1 kg for 90 adult females [20].
Few previous studies incorporate multiple upper extremity strength testing modalities, and data on grip and pinch strength are particularly lacking [4, 5]. The few studies that evaluated hand grip strength either do not include data on pinch strength specifically [4, 5], or combine pinch and grip strength together in the context of testing hand strength alone [21, 40–42]. In addition, many studies separate hand grip strength from other upper extremity tests and focus primarily on the arms and shoulders [20, 37]. The UK Biobank dataset reported isometric strength testing results from 502,713 people in the United Kingdom [3]. This study, however, did not control for confounding factors such as health-related issues such as carpal tunnel syndrome and pain experienced during strength assessment. Significant differences in isometric strength have been demonstrated among subjects with carpal tunnel syndrome [21, 43]. The current study excluded employees with carpal tunnel syndrome and other injuries in order to provide more narrowly defined, normalized reference data set applicable to the general working population.
The usefulness of isometric strength determinations in the upper extremities has been reported by a number of studies to be a valid assessment for the treatment of patients with sarcopenia, critical medical issues, shoulder and upper extremity injuries [22–26, 28]. The use of well-defined normative data also helps in the follow-up of fractures and work-related issues [10, 33].
Furthermore, published articles on isometric pro-nation, supination, and elbow and shoulder strength are few and include small numbers of participants, which limits the statistical power of their findings [35–37, 44]. The large size of the study population presented here allowed the data to be stratified by important variables such as age, sex, and workload demands. Sex-specific differences in upper extremity strength have been reported in numerous studies [3, 45–49]. Previous studies also have reported significant differences in isometric strength by age [1, 50], which emphasizes the importance of including a broad range of ages and controlling for age-related effects on strength results.
There were no major deviations concerning the upper extremity isometric testing over the 10-year data collection period. There were minor deviations in equivalency with 4 of 48 comparisons measuring isometric strength correlation for the time frame of the study. Prior studies have indicated difference in upper extremity strength correlations to gender, height, weight, age, BMI and the strength demands of the job [29, 45]. It could be anticipated that changes of specific job ergonomic demands within various industrial/manufacturing could change the work force ratio of various industrial/manufacturing jobs therefore causing changes in the overall strength within the work force. As the ergonomics of specific jobs changes the physical requirements and physical demands the ratio of and the manor of use between right and left side may affect the use and physical requirement between right and left physical demand therefore affecting the actual strength of various employees.
The inclusion criteria yielded a largely homogenous group of healthy, uninjured employees. The study population excluded employees with injuries and/or ongoing worker’s compensation claims, and data were collected in post-job offer testing of newly hired employees; therefore, biases related to physical and/or psycho-social factors that influence an employee’s effort in work-related strength testing likely were minimized. The data provided in this study could be used to define pre-injury strength when assessing impairment resulting from a work-related injury. Indeed, the American Medical Association’s Guides to the Evaluation of Permanent Impairment, Sixth Edition encourages the use of objective tests, such as isometric strength evaluations, to assess a patient’s recovery [2].
A potential limitation of our study is that among the total 196,043 employees who were identified in the study period, 88,941 (45.4%) were excluded due to a medical history indicative of a condition that might affect isometric strength testing results. This finding, which suggests that close to half of industrial employees may have a confounding condition, may explain the large variability seen among the results of many prior studies reporting upper extremity strength. Additionally, the data in this study were collected from multiple sources over a 10-year period, during which the collection sites increased from 9 to 17 states, creating the potential for variability due to site-specific or operator-specific differences. However, the clinicians were trained specifically on strength testing and the equipment used to measure it. There may have also been differences in the types of industries/manufacturing tested that could affect the testing database. Some studies have reported that increased level of work intensity can result in increased population reported isometric strength results [49]. A further limitation of this study is the lack of information on subject nationality/ethnicity, socioeconomic status, which has been shown to affect individual isometric strength [51, 52]; however, we felt that height, weight, and BMI were most relevant to the strength testing protocols used in the study. We have described additional limitations with respect to the data collection previously [16, 17]. Finally, caution should be taken when using isometric strength testing to predict dynamic performance; while many studies have measured isometric strength to predict work performance and injuries, others have shown that isometric strength tests may not correlate to dynamic performance [53].
Conclusions
In this study, we provide a large and well-defined data set with upper extremity strength testing results from 107,102 healthy, uninjured, industrial employees in the U.S., with stratifications by age, sex, and job workload category. These data fulfill a previously unmet clinical need for a robust source of normative reference data on upper extremity isometric strength. This information can serve as a useful resource for baseline data on upper extremity strength, which may be helpful to assess risk of injury in newly hired employees, and to evaluate subjects’ return to function following an injury or medical condition. The narrow confidence intervals and significant differences for isometric strength values between age groups, sex and work levels suggests that if isometric upper extremity strength values are used as a valued reference for medical comparisons, these factors must be considered.
Conflict of interest
None to report.
Footnotes
Acknowledgments
Thanks to John Olson of Occupational Performance Corporation for obtaining such an immense amount of data. Thanks to Christian Leyh for his help in arranging the massive data set and literature searches during his summer college vacation. The authors also acknowledge Superior Medical Experts for editing assistance.