Moore-Garg Strain Index
Distal upper extremity (DUE) RSI risk assessment for the hand, wrist, forearm, and elbow
Assessment Details
Strain Index Methodology Guide
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- SI < 3: Safe. Task is probably safe for most workers.
- 3 ≤ SI < 5: Uncertain. Risk level is uncertain; monitor the task and consider improvements.
- 5 ≤ SI ≤ 7: Some Risk. May place the worker at increased risk for DUE disorders.
- SI > 7: Hazardous. Redesign the task; the job is associated with distal upper extremity morbidity.
Formula: SI = IE × DE × EM × HWP × SW × DD (Intensity × Duration of Exertion × Efforts/Min × Hand-Wrist Posture × Speed × Duration per Day). The score is a product of all six multipliers. A single high-risk factor can drive the total score even when the others are low.
Note: Moore & Garg's original 1995 paper used a single threshold of SI = 5 (safe vs. hazardous). The 4-tier version (3 / 5 / 7) is the USF/Bernard worksheet extension and is the most widely used clinical interpretation.
- The SI evaluates risk of developing distal upper extremity (DUE) disorders such as carpal tunnel syndrome, tendinitis, DeQuervain's, and lateral epicondylitis, from repetitive hand-intensive tasks. It covers the hand, wrist, forearm, and elbow.
- It was validated against morbidity data from food processing plants: positive (hazardous) job categories had a mean SI of 29 (range 4.5 to 81); negative (safe) categories had a mean of 2.3 (range 0.5 to 4.5). The difference was statistically significant (p < 0.01).
- Three variables are quantitative and measured directly: DE (timed), EM (counted), and DD (task time). Three are qualitative and require the assessor's judgment: IE (Borg scale), HWP (observed posture), and SW (observed pace).
- The SI is designed for single mono-task jobs, meaning one repeated force exertion. It should not be applied directly to multi-task jobs; use the Composite SI approach (Drinkaus et al., 2003) for those.
- Observe during normal production: Score the worker at their actual sustained work pace, not during a demonstration. IE and SW are especially sensitive to observation effects, since workers often slow down when watched.
- Assess each hand separately: Left and right hands may have very different postures or exertion levels depending on tool design and fixture layout. Score both if there is any meaningful difference.
- Time DE over multiple cycles: Use a stopwatch over at least 5 to 10 cycles to get a representative average. A single cycle may be unrepresentative if pacing varies.
- When in doubt on qualitative variables, score conservatively: If IE appears to be between "Somewhat Hard" and "Hard," score Hard. For HWP, if the wrist is simultaneously in extension and lateral deviation, score the worst applicable combination.
- Identify the limiting factor: After scoring, look at which multiplier is highest. That variable is the primary driver and the first target for intervention.
Intensity of Exertion (IE)
The single most powerful multiplier, ranging from 1 to 13. Estimate the strength required to perform the exertion once, referenced against the Borg CR-10 scale and percentage of maximal strength (%MS). Slight = <10%MS / Borg ≤2. A bit hard = 10 to 30%MS / Borg 3. Hard = 30 to 50%MS / Borg 4 to 5, obvious effort with no facial change. Very hard = 50 to 80%MS / Borg 6 to 7, major effort with facial changes. Near maximal = ≥80%MS / Borg >7, requires shoulder or trunk involvement. Even a step from "Light" to "Somewhat Hard" triples the SI.
Duration of Exertion (DE)
Percentage of the work cycle during which muscle exertion is sustained. Time all individual exertions within the observation period (usually 5 to 10 cycles), sum them, and divide by total observation time × 100. A worker who grips continuously for 60% of each cycle scores 2.0 (50 to 79%); one who uses brief pinch grips that total only 8% of the cycle scores 0.5. This captures the difference between sustained tension (e.g. sustained power grip) and intermittent effort (e.g. momentary trigger pulls).
Efforts per Minute (EM)
Count the number of discrete exertions over the full observation period and divide by time in minutes. A "discrete exertion" is one complete force event: one trigger pull, one pinch, or one torque application. Count over at least 3 to 5 minutes for a stable rate. Assembly tasks with short cycle times often score 15 to 20+ per minute. Compared to DE, EM captures how frequently the muscle is loaded rather than for how long each time. Both matter independently. A task can score high on one while low on the other.
Hand/Wrist Posture (HWP)
Observe the wrist position during the exertion phase, not at rest. Score the worst sustained posture across both flexion-extension and lateral (ulnar/radial) deviation. If both apply simultaneously, use the worse score. Very good = extension 0 to 10°, flexion 0 to 5°, deviation 0 to 10°. Good = extension 11 to 25°, flexion 6 to 15°, deviation 11 to 15°. Fair = extension 26 to 40°, flexion 16 to 30°, deviation 16 to 20°. Bad = extension 41 to 55°, flexion 31 to 50°, deviation 21 to 25°. Very bad = extension >55°, flexion >50°, deviation >25°. Inline-handle tools, angled jigs, and repositioning the workpiece are the most effective fixes.
Speed of Work (SW)
A qualitative estimate of the worker's pace compared to a "normal" standard (referenced to MTM-1 time standards). Very slow = ≤80% of normal, extremely relaxed. Slow = 80 to 90%, unhurried. Fair = 90 to 100%, normal sustainable pace. Fast = 100 to 115%, rushed but able to keep up. Very fast = ≥115%, impetuous and barely sustainable. In practice, "Fair" is the most common rating for production workers at steady state. Score "Fast" when the worker visibly hurries between cycles or skips recovery micro-pauses. This multiplier is often underestimated. A paced line that demands 110% of normal effort doubles the effective exposure compared to a self-paced task.
Duration per Day (DD)
Total hours per day the worker performs this specific task, not the full shift length. Measure directly or confirm with production records. A worker who performs this task for 6 hours interspersed with other duties scores 1.0 (4 to 8 hours); one who performs it for only 45 minutes scores 0.25, which halves the SI compared to a full-day exposure. Job rotation is a direct intervention against DD. Rotating to a different task before the cumulative exposure threshold is reached can keep the effective SI below the hazardous zone even when the per-task SI is high.
- ▸Mono-task only: The SI was validated for single repeated exertions. Jobs with multiple different force exertions (e.g. grip + pinch + push in the same cycle) require the Composite SI approach and should not be scored with a single SI calculation.
- ▸Vibration and impact not captured: The SI does not account for hand-arm vibration, impact forces, or contact stress, all established DUE risk factors. A task that scores "Safe" on the SI may still be hazardous if vibrating tools or sharp edges are involved.
- ▸Subjective variables require training: IE and SW rely on the assessor's judgment. Untrained assessors consistently underestimate IE in familiar tasks and overestimate SW in tasks they perceive as "rushed." Inter-rater reliability is moderate; calibration between assessors is strongly recommended.
- ▸Proximal arm not modelled: The SI focuses on the distal upper extremity (hand to elbow). Shoulder and neck disorders from the same task are not assessed, so combine with RULA or REBA for a full upper-limb evaluation.
- ▸Original validation was food processing: The morbidity data underpinning the SI thresholds came primarily from pork and turkey processing. Predictive validity in other industries (electronics assembly, healthcare, automotive) has been confirmed in later studies but with somewhat different sensitivity/specificity characteristics.
Identify which multiplier contributes most to the SI score and target it first. The biggest return comes from reducing IE or HWP, which have the widest multiplier range.
- 1.Reduce IE (Intensity): Power tools instead of manual tools, torque-limiting devices, better coupling (handles, grip texture, gloves), reduced fastener torque specifications, or mechanisation of the force-intensive step. Moving from IE=6 (Hard) to IE=3 (Somewhat Hard) halves the SI.
- 2.Improve HWP (Posture): Inline-handle tools for neutral wrist, angled fixtures, adjustable work surfaces, and repositioning the workpiece. Moving from HWP=2.0 (Bad) to HWP=1.0 (Good) halves the SI.
- 3.Reduce EM (Frequency): Larger batch sizes, auto-feed mechanisms, or combining micro-tasks to reduce the number of discrete exertions per minute. Particularly effective when EM ≥ 3 (15 to 19/min).
- 4.Reduce DE (Duration of exertion): Tool triggers that release on their own, spring-loaded fixtures, and workholding devices that eliminate the need to grip while repositioning or inspecting.
- 5.Reduce DD (Duration per day): Job rotation to a lower-SI task before cumulative exposure builds. The rotation interval should be shorter than the time it takes for symptoms to develop, often 1 to 2 hours for high-SI tasks.
- 6.Reduce SW (Speed): Self-paced work rather than machine-paced, buffer stock between stations, or reduced line speed. Address the root cause of time pressure rather than coaching workers to slow down, which is rarely effective.
Task Multiplier Variables
Strain Index Score
IE × DE × EM × HWP × SW × DD
Task is probably safe (SI < 3).