Question 2 of 5: NIOSH Lifting Analysis of a Repetitive Tray-Unloading Task

Question 2: NIOSH Lifting Analysis of a Repetitive Tray-Unloading Task (20 marks: a–5, b–5, c–15)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) Recommended Weight Limit and Lifting Index

Given. Horizontal distance $H=50\ \text{cm}$; hand height $V=60\ \text{cm}$; vertical travel distance $D=60\ \text{cm}$; asymmetric twist $A=45^{\circ}$ (turn to place on the conveyor); frequency $F=4\ \text{lifts/min}$ over an 8 h shift ($>2$ but $\le 8$ h duration band); load $L=12\ \text{kg}$; load constant $LC=23\ \text{kg}$.

Find. $RWL$ and $LI=L/RWL$ for the task as measured.

Approach. Apply the revised NIOSH lifting equation $RWL=LC\times HM\times VM\times DM\times AM\times FM\times CM$ with the table-derived multipliers above, then compare the 12 kg load against $RWL$ via $LI=L/RWL$.

1. Recommended Weight Limit. $$RWL = 23 \times 0.50 \times 0.96 \times 0.85 \times 0.86 \times 0.45 \times 0.95 = \boxed{3.45\ \text{kg}}$$
2. Lifting Index. $$LI=\frac{L}{RWL}=\frac{12}{3.45}=\boxed{3.48}$$

(b) Is the Task Safe? Risk Factors and Potential Injuries

The task is not safe. A Lifting Index of 3.48 means the worker is lifting nearly three and a half times the recommended weight limit for the measured posture, frequency and duration; NIOSH treats $LI>1$ as elevated risk, and a value above 3 falls in the range associated with substantially increased low-back injury risk for most of the working population — well past the "acceptable to nearly everyone" zone the RWL is calibrated to.

The multiplier table shows exactly where that risk comes from: a large asymmetric twist ($AM=0.86$, from turning 45° under load rather than reorienting the feet or the workstation), a sustained high frequency held for a full 8-hour shift ($FM=0.45$, the single most punishing multiplier here), a moderate horizontal reach ($HM=0.50$), and only fair coupling. These multipliers compound, which is why the RWL falls to under 15% of the 23 kg load constant. The worker repeats this twisted lift roughly 1,920 times per shift (4/min × 60 × 8), a classic cumulative-loading profile. Likely injuries include low-back strain/sprain, lumbar disc injury (torsion combined with compressive loading is more damaging to the disc annulus than pure compression, which is why twisting-under-load lifts are particularly implicated in disc herniation), and progressive musculoskeletal fatigue that can develop into a chronic low-back disorder if the exposure is not reduced.

(c) Solutions to Reduce Risk

Three independent redesigns are evaluated below, each targeting a different multiplier in the RWL equation.

1. Solution 1 – eliminate the twisting motion. Reorient the workstation (angle the oven's outfeed to align with the conveyor, or add a small turntable) so the worker faces the conveyor directly and no longer twists to place the tray, taking $A:45^{\circ}\to 0^{\circ}$ ($AM:0.86\to 1.00$). $$RWL_1 = 23\times0.50\times0.96\times0.85\times1.00\times0.45\times0.95=\boxed{4.01\ \text{kg}},\quad LI_1=\frac{12}{4.01}=2.99$$ Advantages: removes the single mechanism most strongly linked to disc injury (torsion under load); low capital cost if only a turntable or minor layout shift is needed. Disadvantages: may need floor space or oven relocation that is not available in an existing kitchen footprint; LI is still well above 1, so this alone does not make the task safe.
2. Solution 2 – reduce the horizontal reach. Move the conveyor and the tray pickup point closer together so the worker's horizontal reach falls to $H\le 25\ \text{cm}$, the table's saturation point ($HM:0.50\to 1.00$). $$RWL_2 = 23\times1.00\times0.96\times0.85\times0.86\times0.45\times0.95=\boxed{6.90\ \text{kg}},\quad LI_2=\frac{12}{6.90}=1.74$$ Advantages: the largest single-change improvement of the three (RWL roughly doubles); also shortens the moment arm on the spine, which reduces compressive as well as shear loading beyond what the RWL formula alone credits. Disadvantages: the oven and conveyor geometry may not allow the pickup point to be brought that close without a mechanical redesign (e.g., a slide-out oven rack); LI is improved but still above 1.
3. Solution 3 – reduce lifting frequency. Batch two trays at a time onto an intermediate cart, halving the effective lift rate to $F=2/\text{min}$ ($FM:0.45\to 0.65$). $$RWL_3 = 23\times0.50\times0.96\times0.85\times0.86\times0.65\times0.95=\boxed{4.98\ \text{kg}},\quad LI_3=\frac{12}{4.98}=2.41$$ Advantages: requires no change to the workstation geometry, only a work-pacing/scheduling change; also reduces total cumulative exposure time, not just per-lift risk. Disadvantages: may not be operationally possible if trays must be unloaded as they emerge to avoid oven congestion; smallest single-factor improvement of the three.

No single solution alone brings $LI$ to 1; combining Solutions 1 and 2 (eliminate twist and reduce reach together) gives $RWL_{1+2}=23\times1.00\times0.96\times0.85\times1.00\times0.45\times0.95=8.02\ \text{kg}$, $LI_{1+2}=12/8.02=1.50$ — still elevated but a substantial improvement, and the practical recommendation is to combine at least two of the three changes rather than rely on any one alone.