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Snook MMH Calculator

Liberty Mutual Tables — psychophysical design goals for manual material handling

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Assessment Details

Snook Tables Methodology Guide
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Risk Index (RI) Interpretation:
  • RI ≤ 0.85: Acceptable — task design goal met for ≥75% of women (≈90% of men).
  • RI 0.85–1.0: Advisory caution — not an official Snook threshold, but flagged for proactive review.
  • RI > 1.0: Hazard — exceeds design goal; fewer than 75% of women can perform safely.
  • For push/pull: RI = max(initial ÷ init-goal, sustained ÷ sust-goal). The more limiting force governs.

Formula: RI = Actual Weight (or Force) ÷ Design Goal, where the Design Goal is the Maximum Acceptable Weight/Force for 75% of the female worker population at the specified task conditions.

What the Snook Tables Measure:
  • The Liberty Mutual (Snook & Ciriello, 1991) tables are based on psychophysical research — workers adjusted load weights until the task felt "acceptable" to them for an 8-hour shift. This differs from NIOSH's biomechanical/physiological approach.
  • The tables give the Maximum Acceptable Weight of Lift (MAWL) or Maximum Acceptable Force (MAF) for 75% of women and 90% of men across five task types: Lift, Lower, Push, Pull, and Carry.
  • Unlike NIOSH, Snook tables directly cover push, pull, and carry — making this the preferred tool for those task types.
  • Lowering MAWL ≈ Lifting MAWL (per Bernard USF v2.2), so this calculator applies the lift table to lowering tasks — results match the equivalent lift geometry.
Assessment Best Practice:
  • Evaluate the worst case first: Assess the heaviest load from the most awkward geometry (below knees, above shoulder, maximum reach). Then assess the most common task using its actual frequency.
  • When between table values: If your task values fall between the discrete table options, always select the next more demanding option (e.g. longer distance, higher frequency) for a conservative estimate.
  • Push/pull force measurement: Use a calibrated push-pull dynamometer. Measure initial force (peak to start movement) and sustained force (average while moving) separately — they often differ substantially.
  • Out of Range (OR) cells: Some frequency × distance combinations have no table value because the task is considered impractical or unacceptably fatiguing at that rate. An OR result means the task requires redesign regardless of the actual force or weight.
Task-Type Field Guide:

Lift / Lower

The design goal is highest in the Knuckle-to-Shoulder zone with a close (7") reach — the NIOSH-optimal zone. Goals drop as vertical zone moves away from knuckle height in either direction, or as horizontal reach increases. Short vertical travel (10") gives higher limits than long travel (30") at the same zone and reach. Use the lift table for lowering — results are equivalent per the Bernard reference.

Push / Pull

Two separate forces must be measured: the initial force (peak force to overcome inertia and start the load moving) and the sustained force (maintained while pushing/pulling). Initial force limits are always higher than sustained limits. Middle hand height (~36") typically gives the best design goals. Low hand height (~24") is the most limiting. The risk index is the maximum of the two force ratios — whichever force is closer to its limit governs.

Carry

Carry is evaluated by carry style (waist height with bent elbows vs. below waist with straight arms), distance, and frequency. Below-waist carry (arms straight, load hanging) has higher design goals than waist carry because the load transfers through skeletal rather than muscular support. Short distances with low frequency have higher weight limits. At very high frequencies (every 10s) combined with long distances, the table returns OR — meaning the task must be redesigned.

Snook vs. NIOSH vs. RAPP

Use NIOSH when you need a precise biomechanical lifting analysis with continuous variable inputs (exact hand position, vertical travel distance). Use Snook/Liberty Mutual when assessing push, pull, or carry, or when a fast zone-based lift screening is sufficient. Use RAPP (HSE) for wheeled equipment push/pull in a UK/European regulatory context. The tools complement each other — a task that passes Snook screening may still warrant NIOSH verification if geometry is near the table limits.

Limitations:
  • ▸Psychophysical, not biomechanical: The tables reflect worker perception of acceptable effort, not spinal load or injury risk directly. A task within the design goal may still produce high lumbar disc compression if posture is poor.
  • ▸75th percentile female baseline: Tasks at RI = 1.0 are acceptable for 75% of women. If your workforce is predominantly male, the effective population coverage is higher (~90%). For a mixed or predominantly female workforce, the 75% threshold may still leave a meaningful fraction at risk.
  • ▸Asymmetric lifting not modelled: The Snook tables assume symmetric lifts. Twisting or asymmetric postures are not captured — use NIOSH (asymmetry multiplier) or WISHA (twist factor) alongside for tasks with rotation.
  • ▸Floor surface not modelled for push/pull: Table values assume a flat, dry, firm floor with no gradient. Slopes, wet floors, or carpet significantly increase required push/pull force — measure actual force rather than estimating.
  • ▸Vulnerable workers: Pregnant workers, those returning from MSD injury, or young/older workers may require lower limits than the table values suggest.
Intervention Priority:
  • 1.Optimise lift/lower zone: Keep lifts in the Knuckle-to-Shoulder zone. Lift tables, spring platforms, and conveyors eliminate floor-level and overhead lifts — the two highest-risk zones.
  • 2.Reduce horizontal reach: Tilt containers, use turntables, or reposition loads to allow a 7" close reach. Moving from 15" to 7" can double the design goal.
  • 3.Reduce push/pull forces: Well-maintained swivel castors, pneumatic tyres, and smooth floors dramatically reduce initial and sustained forces. Measure forces after maintenance to confirm improvement.
  • 4.Optimise push/pull hand height: Adjust handle height to middle (~36") for most workers. Low and high handle positions increase required force significantly.
  • 5.Reduce frequency or load: Job rotation, smaller batch sizes, and additional workers sharing the task reduce cumulative exposure even when individual task geometry cannot be changed.
  • 6.Mechanical assist: Vacuum lifters, hoists, powered carts, and counterbalanced manipulators should be considered before administrative controls when redesign is not feasible.
Source: Snook, S. H. & Ciriello, V. M. (1991). The design of manual handling tasks: revised tables of maximum acceptable weights and forces. Ergonomics, 34(9), 1197–1213. Tables adapted by T. E. Bernard (University of South Florida), v2.2 (Oct 2002), with support from the OSHA Salt Lake Technical Center. All 582 cells in this calculator verified against the Bernard v2.2 reference.

Select Task Type

L

Lifting Parameters

Total weight of the object including any container. Use the heaviest weight lifted at this task geometry.

lbs
Vertical Lift Zone
Vertical Lift Zone

Height of the hands at the start of the lift. Knuckle-to-Shoulder is the optimal zone — design goals are highest here. Floor-to-Knuckle and Above-Shoulder zones both require more awkward postures and have substantially lower goals. If the lift spans multiple zones (e.g. floor to shoulder), score the worst starting zone.

Vertical Lift Distance
Vertical Lift Distance

Total vertical travel of the hands during the lift — not the starting height. Shorter travel gives higher design goals. If the actual travel falls between options, choose the next longer distance for a conservative estimate.

Horizontal Hand Distance
Horizontal Hand Distance

Distance from the front of the body to the midpoint of the hands grasping the load at the start of the lift. Close (7") = load held tight to the body. Moderate (10") = typical working position. Extended (15") = reaching over a barrier or into a deep container. Moving from 15" to 7" can roughly double the design goal.

Lift Frequency
Lift Frequency

Average lifts per minute (or per shift) during the active task period. Higher frequency substantially lowers the design goal. Count only the upward lift — the set-down is not a separate lift.

L

Lowering Parameters

Per Bernard (USF v2.2), lowering MAWL ≈ lifting MAWL. This calculator applies the lift table to lowering tasks; results equal those for the equivalent lift geometry.

Total weight of the object including any container. Use the heaviest weight lowered at this task geometry.

lbs
Vertical Lower Zone
Vertical Lower Zone

Height range through which the hands travel during the lower. Select the zone that best matches the starting height of the lowering motion.

Vertical Lower Distance
Vertical Lower Distance

Total vertical travel of the hands during the lower. If the actual distance falls between options, choose the next longer distance.

Horizontal Hand Distance
Horizontal Hand Distance

Distance from the front of the body to the midpoint of the hands at the start of the lower. Close (7") = load close to the body; Extended (15") = reaching out.

Lower Frequency
Lower Frequency

How often this lowering task is performed per unit of time.

P

Pushing Parameters

Force measurement required. Use a calibrated push-pull dynamometer. Both initial and sustained forces are evaluated separately — the higher Risk Index governs.

Peak force required to overcome inertia and start the load moving. Typically measured as the maximum reading in the first 1–2 seconds of motion.

lbs

Average force maintained while keeping the load moving. Usually 40–60% of the initial force. Measure over the full push distance.

lbs
Hand Height (Push Point)
Hand Height (Push Point)

Vertical height from the floor to the hands where the pushing force is applied. Middle height (~36") gives the best design goals. Low height (~24") is most limiting — it requires greater trunk lean. High height (~55") reduces mechanical advantage and shoulder leverage.

Push Distance
Push Distance

Total distance the object is pushed per cycle. Longer distance substantially reduces the design goal. If the actual distance falls between options, choose the next longer distance.

Push Frequency
Push Frequency

How often this pushing task is performed. Higher frequencies combined with long distances will produce Out of Range (OR) results — meaning the task cannot be performed safely at that rate regardless of force level.

P

Pulling Parameters

Force measurement required. Use a calibrated push-pull dynamometer. Both initial and sustained forces are evaluated separately — the higher Risk Index governs.

Peak force required to start the load moving. Measure as the maximum reading in the first 1–2 seconds.

lbs

Average force maintained while keeping the load moving. Measure over the full pull distance.

lbs
Hand Height (Pull Point)
Hand Height (Pull Point)

Vertical height from the floor to the hands where the pulling force is applied. Middle height (~36") gives the best design goals. Pulling at low height requires significant trunk lean and produces the most limiting results.

Pull Distance
Pull Distance

Total distance the object is pulled per cycle. If the actual distance falls between options, choose the next longer distance.

Pull Frequency
Pull Frequency

How often this pulling task is performed. High frequencies combined with long distances will return OR — meaning the task requires redesign regardless of force.

C

Carrying Parameters

Total weight of the object including any container. Use the heaviest weight carried at this geometry.

lbs
Carry Style
Carry Style

Posture of the arms and where the load is held while walking. Below-waist carry (arms straight, load hanging at sides) has higher design goals because the load transfers through skeletal support rather than sustained muscle effort. Waist carry (elbows bent, load held to body) is more tiring but more common for bulky items.

Carry Distance
Carry Distance

Total distance carried per cycle. Longer distances substantially reduce design goals. If the actual distance falls between options, choose the next longer distance.

Carry Frequency
Carry Frequency

How often this carrying task is performed. At very high frequencies combined with long distances, the table returns OR — meaning the task must be redesigned regardless of weight.

Snook Risk Index

Lifting

— Risk Index
Acceptable Hazard
0.0 1.0 2.0+
Pending Input

Enter values to calculate risk.

Score Breakdown

Actual Weight— lbs
Design Goal (75% ♀)— lbs
Risk Index—