Strain Index: How to Rate Hand and Wrist Jobs with the Moore-Garg Method

What is the Strain Index in ergonomics?

The Strain Index, often called the Moore-Garg Strain Index after its authors, is a job analysis method for estimating whether a task puts workers at increased risk of disorders of the elbow, forearm, wrist and hand, known as distal upper extremity disorders. J. Steven Moore and Arun Garg published it in the American Industrial Hygiene Association Journal in 1995.

It is semi-quantitative. You measure or estimate six task variables, give each an ordinal rating, convert each rating to a multiplier, and multiply them together. The result is a single score you can compare across jobs and before and after a change.

Unlike posture tools such as RULA, the Strain Index puts force, duration and repetition at the center. If you also want a posture score for the same job, the free RULA calculator covers the arm, wrist, neck and trunk side.

This post is general information, not medical advice. Anyone with persistent hand, wrist or elbow pain should see a clinician.

What are the six Strain Index variables?

From the original paper:

1. Intensity of exertion (IE): how hard the effort is, as a share of maximal strength.
2. Duration of exertion (DE): the percentage of the work cycle spent exerting force.
3. Efforts per minute (EM): how often exertions occur.
4. Hand/wrist posture (HWP): how far the hand or wrist is from neutral.
5. Speed of work (SW): how fast the person is working.
6. Duration of task per day (DD): hours spent on the task.

Two of them are calculated from observation:

• % duration of exertion = 100 x (total duration of all exertions / total observation time)
• Efforts per minute = number of exertions / total observation time in minutes

How do you rate each variable?

The rating criteria below come from Cornell University's reproduction of Moore and Garg's tables.

Intensity of exertion

Rating	% of maximal strength	Borg CR-10	Perceived effort
Light	Under 10%	2 or less	Barely noticeable or relaxed effort
Somewhat hard	10-29%	3	Noticeable or definite effort
Hard	30-49%	4-5	Obvious effort; unchanged facial expression
Very hard	50-79%	6-7	Substantial effort; changes facial expression
Near maximal	80% or more	Over 7	Uses shoulder or trunk to generate force

Hand/wrist posture

Rating	Wrist extension	Wrist flexion	Ulnar deviation	Perceived posture
Very good	0-10°	0-5°	0-10°	Perfectly neutral
Good	11-25°	6-15°	11-15°	Near neutral
Fair	26-40°	16-30°	16-20°	Non-neutral
Bad	41-55°	31-50°	21-25°	Marked deviation
Very bad	Over 60°	Over 50°	Over 25°	Near extreme

Speed of work

Rating	Compared with MTM-1 predicted pace	Perceived speed
Very slow	80% or less	Extremely relaxed pace
Slow	81-90%	Taking one's own time
Fair	91-100%	Normal speed of motion
Fast	101-115%	Rushed, but able to keep up
Very fast	Over 115%	Rushed and barely or unable to keep up

How is the Strain Index calculated?

Each rating maps to a multiplier. The Strain Index calculation is simply the product of the six multipliers:

SI = IE x DE x EM x HWP x SW x DD

Rating	Intensity	Duration of exertion	Efforts/min	Hand/wrist posture	Speed	Hours/day
1	Light: 1	Under 10%: 0.5	Under 4: 0.5	Very good: 1.0	Very slow: 1.0	Under 1: 0.25
2	Somewhat hard: 3	10-29%: 1.0	4-8: 1.0	Good: 1.0	Slow: 1.0	1-2: 0.50
3	Hard: 6	30-49%: 1.5	9-14: 1.5	Fair: 1.5	Fair: 1.0	2-4: 0.75
4	Very hard: 9	50-79%: 2.0	15-19: 2.0	Bad: 2.0	Fast: 1.5	4-8: 1.00
5	Near maximal: 13	80% or more: 3.0	20 or more: 3.0	Very bad: 3.0	Very fast: 2.0	8 or more: 1.50

Notice how much weight intensity carries. Going from "somewhat hard" to "hard" doubles the score on its own.

Because intensity is rated against perceived effort, it helps to know how the Borg CR10 ratings in the first table work. Our guide to how the Borg perceived exertion scales work explains the 0-10 category-ratio scale and how to give an honest rating.

How do you interpret a Strain Index score?

Cornell's summary of the original paper gives three reference points:

Strain Index	Interpretation
3 or less	Probably safe
Above 5	Range where jobs associated with distal upper extremity disorders scored in preliminary testing
7 or more	Probably hazardous

Scores between 3 and 7 are a gray zone that calls for judgment and a closer look at the highest multipliers.

Later field studies supported the method. In Rucker and Moore (2002), investigators blinded to health outcomes classified 28 single-task manufacturing jobs as hazardous or safe with the Strain Index, then checked OSHA 200 injury logs. For jobs, they reported sensitivity of 1.00 and specificity of 0.91, while noting the jobs involved single tasks only.

What does a worked Strain Index example look like?

This example is from the Cornell teaching slides. A worker performs a task rated:

Variable	Exposure	Rating	Multiplier
Intensity of exertion	Somewhat hard	2	3.0
Duration of exertion	60% of cycle	4	2.0
Efforts per minute	12	3	1.5
Hand/wrist posture	Fair	3	1.5
Speed of work	Fair	3	1.0
Duration per day	4-8 hours	4	1.0

SI = 3.0 x 2.0 x 1.5 x 1.5 x 1.0 x 1.0 = 13.5

That is well above 7, so the job is probably hazardous. The breakdown shows where to act. Reducing the exertion duration to under 50% of the cycle would drop that multiplier from 2.0 to 1.5, and improving wrist posture to "good" would drop another from 1.5 to 1.0. Together those two changes bring the score to 6.75, which is still in the gray zone. The next lever is force: if a better tool made the effort "light" (multiplier 1), the score would fall to 2.25.

What changed in the Revised Strain Index?

In 2017, Garg, Moore and Kapellusch published the Revised Strain Index (RSI). The main changes:

• Continuous multipliers instead of five categories, so small changes in a variable move the score smoothly.
• Duration per exertion replaces duty cycle (percentage duration of exertion).
• Speed of work is dropped, leaving five variables: intensity, frequency, duration per exertion, hand/wrist posture and duration per day.

In a simulation of 13,944 tasks, the two models agreed well when the 1995 score was 3 or less (safe) or above 13.5 (hazardous). In between, they disagreed markedly, with the RSI giving much sharper separation between safe and hazardous tasks. The authors also say the RSI applies to very low force, very high repetition tasks such as keyboard use.

When should you use the Strain Index?

Use it when the risk is concentrated at the hand and wrist and the job is a single, repetitive task: trimming, gripping tools, fastening, crimping, cutting, or meat and poultry processing.

Combine it with other tools as needed:

• RULA for shoulder, neck and trunk posture on the same job.
• Our side-by-side look at RULA and REBA, for tasks where posture matters more than hand force.
• HSE's Assessment of Repetitive Tasks (ART) tool, which assesses common risk factors in repetitive upper limb work.
• Our comparison of ergonomic assessment methods such as REBA, RULA and OWAS, which shows where the Strain Index fits.

For background on the conditions these tools aim to prevent, see our post on repetitive strain injuries.

How can video help with Strain Index ratings?

Two of the six variables, duration of exertion and efforts per minute, are timing measurements. Video makes them much more accurate because you can step through frames and count. Wrist posture is also easier to rate from a paused frame. ViveLab's video ergonomic assessment estimates joint angles from video on your device and currently outputs REBA and RULA, not the Strain Index. You can still use the same footage to time exertions by hand, and a trained assessor should rate intensity, which cannot be read from video alone.

Next steps

1. Film several cycles of a hand-intensive task and count exertions and their duration.
2. Rate all six variables, multiply, and note the two largest multipliers.
3. Fix those first, re-rate, and score shoulder and neck posture for the same job in the RULA score calculator.

For more on high-repetition jobs on the line, read our guide to ergonomics in manufacturing.