Ergonomics: anthropometry and workplace design

Why it matters

A workstation that is too high, a control that is out of reach or a lift that is too heavy costs output every cycle and, over months, causes back and shoulder injuries. Ergonomics designs the job to fit the worker, and anthropometry supplies the body dimensions that turn that idea into numbers for bench heights, reach distances, seat sizes and clearances.

Key ideas

Ergonomics (human factors engineering) is the study of people at work and the design of tasks, equipment and environments to suit human capabilities and limitations, so that work is safe, comfortable and efficient. Its first principle: fit the task to the worker, not the worker to the task. It covers physical ergonomics (posture, reach, lifting), cognitive ergonomics (displays, controls, mental load) and the work environment (lighting, noise, heat; see the next topic).

Anthropometry is the measurement of human body dimensions.

• Structural (static) dimensions are measured in fixed standard postures: stature, sitting height, eye height, elbow height, popliteal height, buttock–popliteal length, hip breadth.
• Functional (dynamic) dimensions are measured during movement: forward reach, overhead reach, grip reach envelope.
• Data differ with sex, age, region and occupation, so use data for the actual user population (Indian data where available), and add allowances for footwear and clothing.

Percentiles. Most body dimensions are close to normally distributed. The p-th percentile is the value below which p % of the population lies: X_p = μ + z·σ. The 5th and 95th percentiles (z = ∓1.645) are the usual design limits; the 50th percentile is the mean. Percentiles are not additive: a person at the 95th percentile in stature is not at the 95th percentile in every body segment.

Three design principles

1. Design for extremes.
   • Clearance dimensions (door height, leg room, hatch width, chair width) use a large user, typically the 95th percentile male, so that nearly everyone fits.
   • Reach dimensions (distance to a control, height of a top shelf, a pedal) use a small user, typically the 5th percentile female, so that nearly everyone can reach.
   • Strength limits for operating forces use a weak user.
2. Design for an adjustable range. Seats, work surfaces, monitor arms and steering wheels adjust, typically from the 5th percentile female to the 95th percentile male. This is the preferred approach when it is affordable.
3. Design for the average. Used only when neither extreme nor adjustment applies and the cost of misfit is low (e.g. a shop counter). No one is average in every dimension, so this is the last resort.

Workstation design rules (typical guidelines; confirm with your text or code)

• Work-surface height relates to elbow height and the nature of work: precision work slightly above elbow height, light assembly roughly 10–15 cm below it, heavy work with downward force roughly 15–40 cm below it.
• Keep frequently used items within the normal working area (forearm sweep) and all items within the maximum working area (full arm sweep).
• Seat height about equal to popliteal height (plus shoe allowance); seat depth set by a small user's buttock–popliteal length so that the backrest can be used; footrest for short users at fixed-height benches.
• Monitors at or slightly below eye level; wrists straight; avoid twisting, stooping and overhead work.

Manual lifting: the revised NIOSH lifting equation gives a recommended weight limit (RWL) that nearly all healthy workers can lift repeatedly without increased risk of low-back injury. The lifting index LI = load/RWL; LI > 1 indicates increased risk, and the job should be redesigned.

Formulas

X_p = μ + z·σ

• μ = mean dimension (cm); σ = standard deviation (cm); z = −1.645 (5th), 0 (50th), +1.645 (95th), ±2.326 (1st, 99th).

RWL = LC × HM × VM × DM × AM × FM × CM (kg)

• LC = load constant 23 kg; HM = 25/H, H = horizontal distance of hands from mid-point between ankles (cm, 25–63); VM = 1 − 0.003|V − 75|, V = vertical height of hands at origin (cm); DM = 0.82 + 4.5/D, D = vertical travel distance (cm, ≥ 25); AM = 1 − 0.0032A, A = asymmetry angle (degrees); FM = frequency multiplier and CM = coupling multiplier, both read from the NIOSH tables.

LI = L / RWL

• L = actual load (kg).

Worked examples

Example 1 (standard). For a user population, male stature has μ = 168 cm, σ = 7 cm; female forward reach has μ = 68 cm, σ = 4 cm. Find (a) the minimum clearance height of a doorway to pass 95 % of men (ignore footwear and headgear) and (b) the maximum forward distance of a control so that 95 % of women can reach it.

1. (a) Clearance uses the large user: X_95 = 168 + 1.645 × 7 = 168 + 11.5 = 179.5 cm.
2. (b) Reach uses the small user: X_5 = 68 − 1.645 × 4 = 68 − 6.6 = 61.4 cm.

In practice, add footwear and a safety margin to (a).

Example 2 (GATE level). A worker lifts a 12 kg box with hands 40 cm in front of the ankles, from 50 cm to 120 cm above the floor, twisting 30°. From the NIOSH tables FM = 0.75 and CM = 0.95 (given). Find the RWL and the lifting index.

1. HM = 25/40 = 0.625.
2. VM = 1 − 0.003 × |50 − 75| = 1 − 0.075 = 0.925.
3. D = 120 − 50 = 70 cm; DM = 0.82 + 4.5/70 = 0.884.
4. AM = 1 − 0.0032 × 30 = 0.904.
5. RWL = 23 × 0.625 × 0.925 × 0.884 × 0.904 × 0.75 × 0.95 = 7.57 kg.
6. LI = 12 / 7.57 = 1.58, greater than 1, so the lift carries increased risk. Bring the box closer (smaller H), raise the origin to about 75 cm and remove the twist.

Common mistakes

• Designing a reach dimension for the 95th percentile (small people then cannot reach) or a clearance for the 5th percentile (large people then do not fit).
• Assuming one person is at the same percentile for every body dimension, or adding percentiles of body segments.
• Using foreign anthropometric data for an Indian workforce without checking.
• Using 2σ for the 95th percentile: one-sided 95 % is 1.645σ (2σ is about the 97.7th percentile).
• In the NIOSH equation, using metres instead of centimetres, or omitting the asymmetry multiplier.

For GATE PI

Expect percentile calculations with the normal distribution, choice of the design percentile for a clearance or reach, one-mark questions on design principles and work heights, and occasionally a lifting-equation numerical with the multipliers given. Practise identifying whether a dimension is a clearance or a reach before calculating.

Quick check

1. Which percentile governs the height of an emergency stop button that must be reachable?
2. Stature μ = 160 cm, σ = 6 cm. Find the 95th percentile.
3. Name the three anthropometric design principles.
4. A lift has RWL = 10 kg and the load is 8 kg. Is the lifting index acceptable?

Answers: 1. A small user, typically the 5th percentile (female) reach; 2. 160 + 1.645 × 6 = 169.9 cm; 3. Design for extremes, design for an adjustable range, design for the average; 4. LI = 0.8, below 1, so acceptable.