Question 2 of 4: NIOSH Lifting Equation — Apple-Sack Loading Task

Question 2: NIOSH Lifting Equation — Apple-Sack Loading Task (40 marks: a–10, b–10, c–10, d–10)

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.

Part (a).

Given. Load $L=20$ kg; horizontal hand distance $H=30$ cm (constant); vertical hand height at the conveyor (origin) $V_1=60$ cm and at the chute (destination) $V_2=100$ cm; angle of asymmetry $A=45^\circ$; frequency $F=5$ lifts/min; work pattern two 1.5-hour lifting bouts per 8-hour day.

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

Approach. Apply the revised NIOSH lifting equation $RWL=LC\times HM\times VM\times DM\times AM\times FM\times CM$ once at the origin and once at the destination (multipliers read directly off the exam's own attached tables), then take $LI=L/RWL$ at each point and report the governing (higher-$LI$) value.

1. Multipliers common to both points. $LC=23$ kg. $H=30$ cm $\Rightarrow HM=0.83$ (Table 1). $D=|100-60|=40$ cm $\Rightarrow DM=0.93$ (Table 3). $A=45^\circ \Rightarrow AM=0.86$ (Table 4). $F=5$ lifts/min, $>1$–$2$ h band $\Rightarrow FM=0.60$ (Table 5, same for both $V<75$cm and $V\ge75$cm columns at $F=5$).
2. Vertical and coupling multipliers at the origin ($V_1=60$ cm $<75$ cm): $VM_1=0.96$ (Table 2); Poor coupling at $V<75$cm $\Rightarrow CM_1=0.90$ (Table 7). $$RWL_1 = 23\times0.83\times0.96\times0.93\times0.86\times0.60\times0.90=\boxed{7.92\ \text{kg}}$$ $$LI_1=\frac{L}{RWL_1}=\frac{20}{7.92}=\boxed{2.53}$$
3. Vertical and coupling multipliers at the destination ($V_2=100$ cm $\ge75$ cm): $VM_2=0.93$ (Table 2); Poor coupling at $V\ge75$cm $\Rightarrow CM_2=0.90$ (Table 7, Poor is 0.90 in both columns). $$RWL_2 = 23\times0.83\times0.93\times0.93\times0.86\times0.60\times0.90=\boxed{7.67\ \text{kg}}$$ $$LI_2=\frac{L}{RWL_2}=\frac{20}{7.67}=\boxed{2.61}$$
4. Governing value. $LI_2=2.61 > LI_1=2.53$, so the destination (placing the bag on the chute) is the more restrictive point: $RWL_{gov}=7.67$ kg, $LI_{gov}=2.61$.

Part (b) — Safety issues and affected structures. With $LI\approx2.5$–$2.6$ at both control points, the worker is lifting roughly two-and-a-half times the recommended weight limit for this posture, frequency and coupling — NIOSH classifies this range as a substantially elevated risk of low-back injury for a large fraction of the working population, not a marginal exceedance. Contributing risk factors, read directly from the multipliers that most depress $RWL$: the 45° twist ($AM=0.86$) combined with a moderately fast, sustained frequency ($FM=0.60$) and a poor grip on an unwieldy sack ($CM=0.90$) compound with the vertical travel between two different heights ($DM=0.93$). The primary anatomical structures at risk are the lumbar spine (intervertebral discs L4/L5, L5/S1, and the erector spinae/paraspinal muscles) from the combined compressive and torsional (twisting) loading of lifting while rotating toward the chute; secondary risk to the shoulders and forearm/wrist flexors from repeatedly gripping and controlling a loose, shifting 20 kg sack without a purpose-built handle, particularly at the destination where the load must be lowered and released with control rather than dropped.

Part (c) — Recommended resolutions. (1) Eliminate the twist: reposition the chute directly in line with the conveyor (or add a rotating conveyor segment) so $A\to0^\circ$, raising $AM$ from 0.86 to 1.00 — appropriate because the 45° asymmetry is a pure layout choice, not an inherent part of the task, and removing it costs nothing per lift once installed. (2) Improve the coupling: transfer apples into rigid, handled crates or use a scoop/cradle tool instead of a loose sack, raising $CM$ toward 1.00 (Good) — appropriate because the "handle carefully" requirement is driven by the apples being loose and fragile inside a soft sack, and a rigid container both protects the fruit and gives the worker a secure grip. (3) Reduce the vertical reach range: raise the conveyor or lower the chute (or add an intermediate powered lift) so $V_1$ and $V_2$ move closer to the 75 cm "neutral" height where $VM$ peaks near 1.00, and reduce $D$ — appropriate because both $VM$ terms are currently below their achievable maximum. (4) Reduce frequency/mechanize: introduce a short powered conveyor or gravity chute that carries sacks the last stretch instead of a hand lift-and-place, which would remove the manual lift from the task entirely for the highest-value fix. Each of these targets a specific multiplier the calculation shows is depressing $RWL$, rather than a generic "lift with your legs" instruction that does not change the underlying $RWL$ at all.

Part (d) — Job rotation between lifting shifts. The worker should be rotated to a task that uses different muscle groups and postures than the sack-lifting task — for example, seated quality-inspection/sorting work, light machine-tending, or administrative/paperwork duties — rather than another manual-handling task of similar demand (e.g. box stacking), which would not give the lumbar spine and shoulders genuine recovery time. From a human factors perspective this is justified because MSD risk accumulates with cumulative loading and insufficient recovery time between bouts of the same stressor; alternating to a dissimilar task lets the loaded tissues recover while keeping the worker productively occupied for the full 8-hour day, and it is also the condition (adequate recovery between bouts) that this solution's own frequency-multiplier assumption in part (a) depends on — if the "other work" were itself another heavy manual-handling task, the ">1–2 hour" duration band used above would understate the true cumulative exposure and a re-evaluation using the ">2–8 hour" column ($FM=0.35$, giving an even lower $RWL$) would be the more conservative and defensible choice.