WMSDs - Repetition

Upper Limb Musculoskeletal Disorders and Repetition

Thomas J. Armstrong

The University of Michigan
Ann Arbor, MI 48109-1522

1. Repetition

Medical Concept

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

Management Concept

Based on work quantity / time
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

Epidemiology

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 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
Tendinitis CTS

Jobs Prevalence Odds Ratio³ Prevalence Odds Ratio³

Low Repetitive¹ - Low Force² 0.6% 0.6%

Low Repetitive¹ - High Force² 3.6% 6.1 1.0% 1.8

High Repetitive¹ - Low Force² 3.2% 3.3 2.1% 1.9

High Repetitive¹ -High Force² 10.8% 29.4(4) 5.6% 14.3(4)

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
	Tendinitis	CTS
Jobs	Prevalence	Odds Ratio³	Prevalence	Odds Ratio³
Low Repetitive¹ - Low Force²	0.6%		0.6%
Low Repetitive¹ - High Force²	3.6%	6.1	1.0%	1.8
High Repetitive¹ - Low Force²	3.2%	3.3	2.1%	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

Biomechanics

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.

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.

Psychophysics

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

Perceived exertions were recorded using a 10cm visual analog scale
(0=easiest imaginable & 10= hardest imaginable) for 6 female and 6 male subjects
performing a hand transfer task.
Perceived exertion ratings are significantly different (p<0.05) for 0.05, 1.8 and 3.6Kg weights and
for 20 and 30 transfers per minute.
Differences between 10 and 20 transfers per minute and 25, 50 and 76cm are not significant.

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.)

Weight Rating Frequency Rating Distance Rating

Kg (SD=0.5) (trans/min) (SD=0.5) cm (SD=0.5)

0.05 1.6 10 3.7 25.4 4.9

1.81 5.7 20 4.7 50.8 5.4

3.63 7.6 30 6.6 76.2 4.7

Weight	Rating	Frequency	Rating	Distance	Rating
Kg	(SD=0.5)	(trans/min)	(SD=0.5)	cm	(SD=0.5)
0.05	1.6	10	3.7	25.4	4.9
1.81	5.7	20	4.7	50.8	5.4
3.63	7.6	30	6.6	76.2	4.7

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

Definition

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

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; Sjgaard, Kiens, Jorgensen and Saltin 1986).
- Sjgaard, Savard and Juel (1988) and Sjgaard, 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.
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

Identify all tasks performed by worker
Determine time spent performing each task
Determine work unit for each task
Determine production standard for each task
Determine the number of exertions per work unit for each major body part
Determine the number of exertions per hour
Determine the duration of exertions for each major body part
Rate repetition/activity
Perform instrumental measurements

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.

Low Medium High

0
hands idle most of the time; no regular exertions 2
consistent, conspicuous long pauses; or very slow motions 4
slow steady motion/ exertion; frequent brief pauses 6
Ssteady motion/ exertion; infrequent pauses 8
rapid steady motion/ exertion; infrequent pauses 10
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)

Worker ratings
Observer ratings

3.2.2 Measurements

Goniometery
Electormyography

4. Reporting Repeated and Static Exertions

State overall repetitiveness rating for each body part followed by the number of exertions and task.
Mention possible exceptions due to material or equipment problems.
Example: Operating the press requires highly repetitive exertions of the shoulder, arm and hand to get parts, load press, activate the dual palm buttons and unload the press 900 times per hour.
Additional motions may be required to position some batches of parts in the press.
Prolonged exertions are required to stand at the press for eight hours per day.

Examples

4.1 Notebook Packing (see Figure 3)

Job Title:
• Notebook Packing

Production Standard:
• cases/hour

Objective:
• Inspect and pack notebooks

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

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

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

Environment:
• 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.

Repetition

= 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:

0 is idle with infrequent exertions or movements and

10 is continuous exertion with fastest possible movements; difficulty keeping up (see Figure 2).

Summary:

Notebook packing is a highly repetitive job, 7 on a scale of 0 to 10, requiring 1,200 exertions per hour of the shoulders, arms and hands to get, inspect and pack notebooks for eight and frequently nine hours per day. The hands are in constant motion, particularly when cases are not fully formed and additional motions are required to open them, and when the tape machine malfunctions and cartons must be re-fed.

4.2: Bone Turkey Thighs

Line speed: 1,080
Bones one thigh of every other turkey
Knife hand - 4 cuts per thigh
Non-knife hand - holds thigh

4.3 Keyboard Jobs

a. Case A: 10,000 alpha numeric keys/hour: =10,000 / 5 char/word / 60 min/hr = 33 wpm); Sustained static exertions of the torso, neck, shoulders and elbows

b. Case B: 10 checks/minute = 30 numeric digits/check Sustained static exertions of the torso, neck, shoulders and elbows
c. Case C: Newspaper reporter with variable work routine

Table 6.5: Work Sample for a typical day.
Time Task Pace Phone

11:15 writing low hand

11:25 writing med hand

11:35 reading low

11:45 absent

11:55 keying med

12:05 reading low

12:20 reading low

12:30 keying med head

12:40 reading low

12:50 reading low

1:05 keying med neck

1:30 absent

1:40 absent

1:50 absent

2:00 writing low

2:10 writing low

2:20 keying med

2:35 keying med

2:50 writing med head

3:05 talking low neck

3:15 reading low

3:25 reading low

3:35 reading low

3:45 discuss low

3:55 reading low

4:05 talking low head

4:15 reading low

4:30 absent

4:45 absent

4:55 stapling low

5:10 talking< low head

5:25 absent

5:40 writing low head

5:55 talking low head

Keying 5%

Writing 17%

Phone

hand held 6%

head-neck 9%

headset 17%

Talking 3%

Reading 29%

Absent 20%

Note: There may be prolonged keying for two or three days once or twice a month

4.4 Assembly Line

Repeated exertions:

Line speed: 6 assemblies/mi

Fasteners per unit: 4 screws per assembly

Exertions per fastener 1 to get screw + 1 to drive screw

Sustained static exertions: back, neck, shoulder hand to hold tool

5. Factors for Evaluation and Control
These factors should be considered in the evaluation of Repetition as well as the control.

Standards
Motion economy
Worker rotations
Work enlargement

Mechanical aids
Processes
Quality control

Low		Medium		High
0 hands idle most of the time; no regular exertions	2 consistent, conspicuous long pauses; or very slow motions	4 slow steady motion/ exertion; frequent brief pauses	6 Ssteady motion/ exertion; infrequent pauses	8 rapid steady motion/ exertion; infrequent pauses	10 rapid steady motion or continuous exertion, difficulty keeping up


Get case	Erect case	Pack case	Put case in tapping machine

Time	Task	Pace	Phone
11:15	writing	low	hand
11:25	writing	med	hand
11:35	reading	low
11:45	absent
11:55	keying	med
12:05	reading	low
12:20	reading	low
12:30	keying	med	head
12:40	reading	low
12:50	reading	low
1:05	keying	med	neck
1:30	absent
1:40	absent
1:50	absent
2:00	writing	low
2:10	writing	low
2:20	keying	med
2:35	keying	med
2:50	writing	med	head
3:05	talking	low	neck
3:15	reading	low
3:25	reading	low
3:35	reading	low
3:45	discuss	low
3:55	reading	low
4:05	talking	low	head
4:15	reading	low
4:30	absent
4:45	absent
4:55	stapling	low
5:10	talking<	low	head
5:25	absent
5:40	writing	low	head
5:55	talking	low	head

Keying	5%
Writing	17%
Phone
	hand held	6%
	head-neck	9%
	headset	17%
Talking	3%
Reading	29%
Absent	20%

Line speed:	6 assemblies/mi
Fasteners per unit:	4 screws per assembly
Exertions per fastener	1 to get screw + 1 to drive screw
Sustained static exertions:	back, neck, shoulder hand to hold tool