10.3 Limits of Human Capacity & Risk-Factor Tools: Work Physiology, Strength, Range of Motion, Vision, RULA & Slip Resistance

10.3 Limits of Human Capacity & Risk-Factor Tools

The specification lists limits of human capacity (strength, endurance, metabolic energy, range of motion, vision, hearing, skeletal-joint force analysis, exposure) and methods for quantifying risk factors (NIOSH lifting equation, OSHA noise limits, coefficient of friction, RULA). The NIOSH equation, noise, and heat have their own sections. This section covers the rest.

---

1. Metabolic Energy and Endurance (Work Physiology)

Muscles convert food energy to work at roughly 20% to 25% efficiency, and the rest becomes heat. Energy expenditure is estimated from oxygen uptake:

$\text{Energy (kcal/min)} \approx 5 \times \dot{V}O_2 \text{ (L/min)} \qquad (1 \text{ kcal} = 4.184 \text{ kJ})$

Limits for a full shift. Sustained work over 8 hours is commonly limited to about one third of a person's maximum aerobic capacity ($\dot{V}O_2\text{max}$). A widely used planning value is about 5 kcal/min for men and 4 kcal/min for women. Heart rate rises roughly linearly with workload and is a convenient field measure.

Murrell's rest allowance. When the average work rate $W$ exceeds the standard $S$, rest must be added:

$R = \frac{T(W - S)}{W - 1.5}$

where $R$ is rest time, $T$ is working time, $W$ is the energy cost while working (kcal/min), $S$ is the standard (often 5 kcal/min), and 1.5 kcal/min is approximately the resting rate.

Example: A task costs $W = 6.5$ kcal/min over a 480-minute shift with $S = 5$:

$R = \frac{480(6.5 - 5)}{6.5 - 1.5} = \frac{720}{5} = 144 \text{ min of rest}$

This large allowance points to an engineering fix, such as a lift assist, rather than simply adding rest.

Static work and endurance. Holding a posture or a load is static work. Endurance time falls steeply as force rises above roughly 15% of maximum voluntary contraction (MVC), and contractions near maximum can be held only for seconds. Design jobs so static holding forces stay low and recovery breaks are frequent.

---

2. Strength and Skeletal-Joint Force Analysis

Strength varies widely with person, posture, and direction of force. On average, women's strength is roughly 60% to 70% of men's, with larger differences for upper-body strength. For force requirements, design for the weak end of the user population (for example the 5th percentile female), just as reach is designed for short users.

Static joint analysis applies moment equilibrium. For the elbow held at 90°:

$F_m \cdot d_m = W_{\text{load}} \cdot d_{\text{load}} + W_{\text{forearm}} \cdot d_{\text{forearm}}$

Example: A 50 N load in the hand is 35 cm from the elbow, the forearm and hand weigh 15 N acting 15 cm from the elbow, and the biceps inserts 4 cm from the joint:

$F_m = \frac{50(35) + 15(15)}{4} = \frac{1{,}975}{4} \approx 494 \text{ N}$

The muscle force is about ten times the load because its moment arm is so short. The same leverage explains the large spinal compression in the lifting section. Holding the load closer to the elbow is the most effective fix.

---

3. Range of Motion

Each joint has a range of motion (ROM). Strength and endurance are greatest near the middle of the range and fall off toward the extremes. Design rules:

• Keep the wrists near straight (neutral) and avoid repeated flexion, extension, or deviation.
• Keep the shoulders low (elbows near the body) and avoid frequent reaching above shoulder height or behind the body.
• Keep the neck within a few degrees of neutral, with the line of sight slightly below horizontal.
• Avoid twisting the trunk, which also reduces the NIOSH asymmetry multiplier.

The RULA and REBA tools turn these principles into scores.

---

4. Vision and Hearing

Visual angle decides whether a character or detail can be read:

$\text{Visual angle (arcmin)} \approx 3{,}438 \times \frac{H}{D}$

where $H$ is character height and $D$ is viewing distance in the same units. A 5 mm character viewed from 700 mm subtends about 24.6 arcmin. ANSI/HFES 100-2007 sets 16 arcmin as the minimum character height for computer displays and recommends 22 to 30 arcmin at the design viewing distance (HFES is redeveloping that standard). Legibility also depends on contrast, luminance, and glare, covered in the illumination section.

Hearing limits include the frequency range of speech (most important from about 500 to 4,000 Hz), masking of alarms by background noise, and permanent hearing loss from noise exposure. OSHA limits (90 dBA TWA PEL, 85 dBA action level, 5 dB exchange rate) are covered in the environmental ergonomics section. NIOSH recommends a stricter 85 dBA exposure limit with a 3 dB exchange rate.

Exposure in general means the dose of a stressor over time, such as noise, heat, cold, vibration, or chemicals. Limits are expressed as time-weighted averages, ceiling values, or exposure durations.

---

5. RULA (Rapid Upper Limb Assessment)

McAtamney and Corlett published RULA in 1993 for jobs that load the neck, trunk, and upper limbs, such as assembly, sewing, and computer work. The analyst observes the worst or most frequent posture:

1. Group A: score the upper arm, lower arm, wrist, and wrist twist from posture diagrams, then look up the Group A posture score.
2. Group B: score the neck, trunk, and legs, then look up the Group B posture score.
3. Add a muscle use score (static posture or repetition of 4 or more times per minute) and a force/load score to each group.
4. Combine the two in Table C to get the grand score.

REBA (Rapid Entire Body Assessment) applies the same idea to whole-body tasks such as patient handling.

---

6. Coefficient of Friction and Slip Risk

Same-level slips, trips, and falls are a leading cause of workplace injury. A foot slips when the friction it needs exceeds the friction available:

$\mu_{\text{required}} = \frac{F_{\text{horizontal}}}{F_{\text{vertical}}} \qquad \text{slip likely if } \mu_{\text{required}} > \mu_{\text{available}}$

• Static COF is the friction at the start of motion; dynamic COF is the friction once sliding. Most modern floor tests measure wet dynamic COF.
• Normal level walking needs roughly 0.2. Pushing carts, turning, carrying loads, and walking on ramps need more. On a slope of angle $\theta$, the need rises by about $\tan\theta$; a 1:12 ramp adds about 0.08.
• A static COF of 0.5 is a long-used benchmark from floor-polish testing. OSHA's walking-working surfaces rule (29 CFR 1910.22) requires surfaces to be kept clean, dry, and free of hazards but sets no numeric COF. The tile standard ANSI A326.3 specifies a wet dynamic COF of at least 0.42 for level interior floors expected to be walked on when wet.

Example: A worker pushes a loaded cart with a horizontal force of 180 N while the pushing foot carries 750 N, so $\mu_{\text{required}} = 180/750 = 0.24$. A wet, oily floor with a dynamic COF of 0.20 makes a slip likely. Fixes include degreasing the floor, slip-resistant footwear, lower-resistance casters (which reduce the push force), or a powered tugger.