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RESEARCH: Shoulder posture, hand forces predictive of rotator cuff syndrome

Workers had greater than twice the chance of experiencing a rotator cuff syndrome (RCS) when job duties involved shoulder flexion of 45 degrees or greater combined with high hand forces according to a cross-sectional study of 733 Washington state workers from 12 worksites.  The study also revealed an exposure response relationship between frequency of forceful exertions and RCS (a condition that includes clinically defined subacromial impingement syndrome and rotator cuff tendinitis).

A significant association was found between RCS and:

  • Shoulder flexion ≥ 45° for ≥ 15 percent of the duty cycle time and pinch grip force ≥ 2 lbs. conducted for > 0 percent of the duty cycle time
  • Shoulder flexion ≥ 45° for ≥ 15 percent of the duty cycle time and forceful exertion (power grip, lifting objects or push/pull force ≥ 10 lbs) conducted for ≥ 9 percent of the duty cycle time
  • Low job security
  • High job structural constraints (i.e., pacing, social interaction)

High job security slightly modified the effect of the high force factors.  Age and body mass index demonstrated slight significance.  The combination of upper arm abduction and high force showed no relationship to RCS.

Further, there was no or only marginal association between RCS and:

  • hobbies or sports activities
  • high job demands
  • decision latitude
  • work organization except for structural job constraints
  • diabetes, hypertension, and thyroid disease
  • gender

The Bottom Line – How This Applies To Ergonomists
When shoulder flexion and hand force as defined above coexist in a job, there is a greater chance of shoulder injury.  This study notes that the presence of these key risk factors do not have to occur simultaneously in a task (at the same time) to increase the chance of injury.  RCS can be an expensive disorder easily justifying an intervention (among accepted workers’ compensation claims in the state of Washington for the years 1996 to 2004, this diagnosis produced an average of 334 lost work days and $32,169 in costs).  Personal factors and non-work activities had little impact on the likelihood of a RCS.

Study Design
From 12 manufacturing and health care facilities, 795 employees volunteered for the research project.  A facility was considered for the study if workplace job groups were characterized by three of the following six categories involving hand activity (as per the ACGIH Hand Activity Level tool):

  • Low hand force/low hand activity
  • Low hand force/medium hand activity
  • Low hand force/high hand activity
  • High hand force/low hand activity
  • High hand force/medium hand activity
  • High hand force/high hand activity

After applying other exclusion rules, a final group of 733 subjects was formed.

Data Gathering
A structured questionnaire was conducted with each volunteer to elicit information involving subject demographics, relevant health history, work history, and physical symptoms.

A physical examination was performed to identify those with RCS.  A case definition included a history of:

  • shoulder pain/burning in the past 12 months that repeated three times or occurred more than 1 week
  • shoulder pain/burning in the previous 7 days
  • And no traumatic injury source of the shoulder symptoms

Also necessary for case definition were physical examination findings of:

  • Appropriate tendon pain upon resisted shoulder abduction, external rotation, or internal rotation
  • A “painful arc” upon shoulder abduction (usually between 60° and 120°)

The case definition of RCS was met by 55 subjects (7.5 percent)

Job Assessment
Psychosocial risk factors of general/mental health, social support, job demands, and decision latitude were evaluated with a questionnaire. 

Observation by ergonomists were conducted to assess work organization factors such as gender mix, temperature, humidity, noise, housekeeping, job content, work methods, pacing, worker control, rotation, work hours, and social interaction.

Physical load assessment was determined for each subject through a combination of observation by an ergonomist and evaluation of videotaped tasks.  Force gages were used to measure push/pull forces and objects weights.  For each type of forceful exertion, percent and frequency of duty time was calculated.

In this study, about 95 percent of the subjects performed cyclic task jobs with multiple elements.
Study Limitations
1) It was difficult to obtain a large enough sample of subjects exposed to high force and high frequency.
2) There was the potential that some workers could not have participated in the study because they were off the job due to a work related shoulder injury.
3) Some workers may not have participated in the study out of concern that they would get behind in work production (activities were conducted during work hours).

Article Title: Rotator Cuff Syndrome: Personal, Work-Related Psychosocial and Physical Load Factors

Publication: Journal of Occupational and Environmental Medicine, 50, 1062-1076, 2008

Authors: B A Silverstein, S S Bao, Z J Fan, N Howard, C Smith, P Spielholz, D Bonauto, E Viikari-Juntura

This article originally appeared in The Ergonomics Report™ on 2008-10-08.