Upper Limb Musculoskeletal Disorders and Repetition

Thomas J. Armstrong

The University of Michigan
Ann Arbor, MI 48109-1522

Copyright © 2002, Thomas J. Armstrong

1. Repetition

Medical Concept

  1. Based on the ratio of work time to recovery time
  2. Work-rest cycle
  3. Expressed as:
    • Movements or exertions / time
    • Exertion time

Management Concept

  1. Based on work quantity / time
  2. Expressed as:
    • Number of parts
    • Number of key strokes
    • Number of transactions
    • Nuber of tasks
    • Number of documents
    • Standard time
    • Time on the job

3. Supporting Literature


Table 1: Selected epidemiological studies on repetitive exertions of the hand
Obolenskaja and Goljanitzki (1927) and Kurppa et. al. (1979) Suggested that high rates of work, 7,600 to 12,000 exertions per shift, was a major factor in 189 cases of tenosynovitis of the upper extremities among a group of 700 packers in a tea factory
Hammer (1934) Suggested that human tendons do not tolerate more than 1,500 to 2,000 exertions per hour
Thompson et. al. (1951) Local "strain" either repetitive or single "strain" ... simple repetitive stereotyped movement associated with intensity and speed
Tichauer (1966) Expressed repetitiveness for a ratchet screw driving task as 1,000 screws times 5 exertions per screw or 5,000 exertions per day
Luopajarvi et. al. (1979) Concluded that the prevalence of muscle-tendon syndromes in the hands of assembly-line packers was related to operators keeping up with machine paces of 25,000 cycles per day
Kuorinka and Koskinen (1979) Found that the symptoms of muscle-tendon disorders increased as the number of parts handled per year increased from less than 200,000 to more than 300,000
Cannon et. al. (1981) Performance of repetitive motion tasks associated with carpal tunnel syndrome
Silverstein et. al. (1987) and Armstrong et. al. (1987) Found increases in the risk of carpal tunnel syndrome and tendinitis with fundamental work cycles of less than 30 seconds that were performed for more than 50% of the time (see Table 2)
Latko et al. (1999) Study of 352 industrial workers. Repetitiveness of work was found to be signifcantly associated with prevalence of reported discomfort in the wrist, hand, or Fingers (odds ratio (OR)=1.17 per unit of repetition; OR=2.45 for high vs. low repetition), tendinitis in the distal upper extremity (OR=1.23 per unit of repetition; OR=3.23 for high vs. low repetition), and symptoms consistent with carpal tunnel syndrome (OR=1.16 per unit of repetition; OR=2.32 for high vs. low repetition). An association was also found between repetitiveness of work and carpal tunnel syndrome, indicated by the combination of positive electrodiagnostic results and symptoms consistent with carpal tunnel syndrome (OR=1.22 per unit of repetition; OR=3.11 for high vs. low repetition).

Table 2: Prevalence of hand and wrist tendinitis and carpal tunnel syndrome based on a study of 652 workers at seven industrial plants in the eastern United States (Armstrong et al. 1987; Silverstein et al. 1987
JobsPrevalenceOdds Ratio3 PrevalenceOdds Ratio3
Low Repetitive1 - Low Force2 0.6% 0.6% 
Low Repetitive1 - High Force23.6% 6.11.0%1.8
High Repetitive1 - Low Force2 3.2%3.32.1%1.9
High Repetitive1 -High Force2 10.8%29.4(4)5.6%14.3(4)

1high repetitive jobs (cycle time < 30 seconds & performed > 50% of the work shift); low repetitive jobs (cycle time > 30 seconds or performed < 50% of the work shift)
2average low adjusted force (mean force + variance/mean force) = 3.0+1.6 Kg; average high adjusted force 12.7+8.6 Kg
3odds ratios are with respect to low-force-low-repetition jobs


Table 3: Selected biomechanical studies of repetitive exertions of the hand
Goldstein et al. (1987) Postmortem studies of elastic and viscous deformation in finger flexor tendons subjected to repetitive loading. Only elastic deformation was observed when load:rest cycle =2s:8s; viscous deformation (creep)occurred when load:rest cycle =8s:2s. Viscous deformation after 500 cycles was equal to elastic deformation for one cycle.
Szabo and Chidgey (1989) Intra-carpal canal pressure was measured in 22 carpal tunnel patients and six normal control subjects. Repetitively flexing and extending the wrist at 30 cycles per minute for one minute resulted in significantly elevated pressures in patients with early and intermediate carpal tunnel syndrome for as much as 10 minutes following exercise.


Perceived exertion v. weight, frequency, and distance for hand transfer task (Krawczyk & Armstrong 1991)

Table 4: Perceived exertion for selected weights, Frequencies and Distances
(Krawczyk S, Armstrong T., Snook S. Preferred weights for hand transfer asks
for an eight hour workday. pp. 152-166. Internanational Scientific Conference
on Prevention of Work-related Musculoskeletal Disorders, PREMUS,
Sweden, May 12-14, 1992.)
WeightRating  FrequencyRating  DistanceRating
Kg(SD=0.5)  (trans/min)(SD=0.5)  cm(SD=0.5)
0.051.6  103.7  25.44.9
1.815.7  204.7  50.85.4
3.637.6  306.6  76.24.7

Also see: Snook S, Vaillancourt D, Ciriello V, Webster B. Psychophysical
studies of repetitive wrist flexion and extension. Ergonomics,
38:1488-1507, 1995.

2. Static exertion


Maintenance of the same position of the body or some part of the body throughout each work cycle or for prolonged periods.

  1. Fatigue Studies
    • It is well established that sustained static exertions in excess of 15%MVC interfere with muscle circulation and produce metabolic imbalances (Monod 1956; Lind 1959; Rhomert 1973; Petrofsky and Lind 1975; Sj¯gaard, Kiens, Jorgensen and Saltin 1986).
    • Sj¯gaard, Savard and Juel (1988) and Sj¯gaard, Kiens, Jorgensen and Saltin (1986) demonstrated significant fatigue using perceived exertion and strength degradation for prolonged static exertions as low as 5%MVC even though circulation appeared to be adequate. Fatigue was attributed to a potassium imbalance.
  2. Epidemiology
    • Herberts et al. (1981) and Herberts et al. (1984) reported increased incidence of shoulder pain in welders. They proposed that the static exertion and resulting supraspinatis tendon ischemia were causes of chronic pain.

Figure 1: Proposed dose-response relationship for work related factors and upper limb disorders.

3. Analysis of repeated and sustained exertions

3.1 Document work job, work equipment, and work environment

3.1 Methods Analysis of Exertion Frequency & Duration

3.2 Ratings and Measurements

3.2.1 Ratings

Repetition or Activity

Repetition is an index of the frequency of movement or exertions, the speed of the motion, and the recovery time. At one extreme the hands are idle with almost continuous recovery time. At the other extreme the hands are moving a rapidly as imaginable with no recovery time.

hands idle most of the time; no regular exertions
consistent, conspicuous long pauses; or very slow motions
slow steady motion/ exertion; frequent brief pauses
Ssteady motion/ exertion; infrequent pauses
rapid steady motion/ exertion; infrequent pauses
rapid steady motion or continuous exertion, difficulty keeping up

Figure 2: Scale for rating hand activity. Either the busiest hand or each hand individually can be rated. (From Latko et al. 1999)

3.2.2 Measurements

4. Reporting Repeated and Static Exertions


4.1 Notebook Packing (see Figure 3)

Job Title:
• Notebook Packing

Production Standard:
• cases/hour

• Inspect and pack notebooks

• 8.5" x 11" notebooks
• cartons of 24 notebooks

Work station:
• Conveyor
• bench
• taping machine/conveyor

• 9 hr. shift
• min. lunch; 2 - 15 min breaks
• 1 hr. overtime frequent

• Inside, 75°-85°F

Get case Erect case Pack case Put case in tapping machine
Right Hand
  • Get, open, & position empty carton
  • Get 12 notebooks &
    put notebooks in carton x 2
  • Close carton, slide into
    taping machine
Left Hand
  • Get, open, & position empty carton
  • Get 12 notebooks &
    put notebooks in carton x 2
  • Close carton, slide into
    taping machine

Figure 3: Notebook packing work station and documentation.


= Production std x Nbr. exertions/ Production unit
= 300 cases/hr x 4 ex/case
= 1,200 exertions/hr

  • How does case packing compare with keyboard work?
  • Exertions per unit time is not an absolute measure of repetition.

    Observe and rate repetition as 7 on a scale of 0 to 10 where: