What methods are available to evaluate occasionally performed tasks in which a two person lift team is used to pick up, carry a short distance and then place compact, stable but heavy loads?
There are a number of models such as the Revised NIOSH model which can estimate the demands of the lift and the lowering element for a single person making the lift or lower but what about for both persons? Can the NIOSH model be used to estimate the loads for one person in this case and if so, how?
And, then, how can you integrate these results to create a usable metric?
based on what you’ve described, with the usual caveat that I’d need to see the job and know more details, I’d suggest using a biomechanical model and possibly the NIOSH Lifting Equation (and possibly the Snook and Ciriello / Liberty Mutaul Tables).
The first step is estimating (or better yet, measuring, if you have the equipment to do so) the actual loading at the hands during this lift. A rule of thumb for two person lifts is to estimate that each person, at one point or another during the lift, will bear up to 75% of the weight of the item. Under perfectly coordinated lifting, under perfect lifting conditions, they each might only bear 50% of the load, but under real conditions, estimate at least 75% for each.
With a biomechanical model, you can now model the posture at the start of the lift and at the end of the lower, typically the most extreme points for a lifting task. Such methods will give you estimates of low back compressive force (at the L5/S1 location of the spine) and strength requirements at each major joint, comparing the calculated values to population norms.
You would probably be pushing, if not stepping beyond, the validity boundaries of the NIOSH and Snook/Ciriello tables. However, as long as you know this when you apply and interpret them, you might still learn something, or reinforce the results you attain from the biomechanical analysis.
I hope this is helpful.
Good response from Peter B. I would have thought that a 2 person lift would definitely be beyond the validity boundaries of NIOSH and Snook. I suppose it’s all in the interpretation of the results.
I have a question for Peter B. You indicate that during a 2 person lift under real conditions each person could bear up to 75% of the weight of the object at some point during the lift. This is a much higher percentage than I’ve seen published. Is there a source for this number? (I assume we are not talking about extreme 2 person lift scenarios such as moving furniture up or down stairs)
Peter Goyert CCPE
Good questions. I initially thought the NIOSH Equation wouldn’t be valid for a two-person lift, but I quickly referenced the user manual before publishing that last post and didn’t find that team or two-person lifting was specifically excluded (though some of the other specific limitations might apply to this task). The Snook and Ciriello / Liberty Mutual Tables are intended for loads with handles located in the middle width (depth from body) dimension of the load. Since it’s not clear what type of hand holds, if any, are available on the load Steve is dealing with, I didn’t specifically exclude the Snook and Ciriello data, but left it to him to judge it’s applicability.
And professional judgment is at the heart of estimating hazards related to tasks like this. Since we don’t have fully validated methods that apply specifically to every conceivable task, we often find ourselves making professional judgments based on the information available. Sometimes it’s useful to apply various methods (e.g., biomechanical analysis, NIOSH Equation, Snook and Ciriello data, in this case), even when none of the evaluation methods are a perfect fit. As long as we don’t misinterprate or missapply the results, the information we gain can still be very helpful in understanding and controlling potential hazards.
Since we have little information about the particulars of Steve’s tasks, my advice can only be general in nature. I’ve used the 75% of the total load weight as a conservative estimate for two person lifts for a long time. I don’t have a specific reference at hand, but am comfortable offering it as a conservative ‘rule of thumb’, accounting for unforeseen task conditions, especially when I know little about the specific lifting conditions. As mentioned in my original post, I’d prefer to see actual measurements from this task, and would need to know much more detail before giving specific recommendations.
What load estimates do others use for two-person or team lifting? Is 75% for a two-person lift too high an estimate?
Your comments on using good professional judgement while estimating hazards are true.
Two-person or team lifting weight limits seem to vary depending on what literature you look at. I have a guidance document on two person lifting that opens with…”Most studies show that the lifting capacity is lower than the sum of the individual member’s capacity by 10-20%”. Although the studies cited are not referenced, I read that to mean that two people can lift twice as much as one, minus 10-20% of the total load. This goes on to say that “the lifting capacity of team lifting increases when the workers are height matched”. Increase in relation to what and by how much is not addressed. There is no information on the particulars under which these lifts were measured or what kind of sample was used.
I’d also like to hear what others have to say on the topic of two-person lifting.
The Mital tables (1993) briefly discuss how to use their data to assess two person lifting (page 71 of my reference). They acknowledge the limitation on the literature and it looks like there is a still a big requirement for professional judgement.
The European Norm (EN1005-2) and the UK’s MAC both give procedures for evaluating a two person lift. The EN basically uses a NIOSH calculation, but with differing load constants depending on the user population (e.g. gender, age). If a team lift is involved, it adds an additional multiplier (0.85) to determine the weight for an individual.
The UK system uses color codes; in order of increasing severity they are green, amber, red, purple. For 2 person lifts, the corresponding masses are Green, , 35 kg; Amber 35 < 50 kg; Red, 50 < 85 kg, Purple > 85. The MAC provides information regarding 3 and 4 person lifts as well.
I’m not sure of the derivation of the purple color code; perhaps to indicate that you’re royally hosed?
On the basis of the research done in the eighties by Karwowski and Mital I would have thought that the lifting power of a two-person lifting team would be closer to 90% of the sum of the sum of their individual lifting capacities provided these were fairly well matched and the load is not an awkward one (i.e. it is easy to grip and it is not so bulky that the workers have to lean over too much etc). I have attached a summary of their work that dates back to our last discussion on hospital lift teams,
David McFarlane MAppSc (Ergonomics)
Ergonomist, WorkCover NSW
Any recommendation concerning the use or representation of a particular brand of product in this document or any mention of them whatsoever (whether this appears in the text, illustrations, photographs or in any other form) is not to be taken to imply that WorkCover NSW approves or endorses the product or the brand.
Details on how we arrived at the boundaries for the colour bands for team handling in the MAC are available in a report on its development: Monnington, S.C., Pinder, A.D.J. and Quarrie, C. (2002a). Development of an inspection tool for manual handling risk assessment., Report No HSL/2002/30. http://www.hse.gov.uk/research/hsl_pdf/2002/hsl02-30.pdf . In places it involved using an 85% multiplier taken from CEN.
The purple zones in the MAC are meant to indicate load/frequency combinations that would indicate a very high level of risk of injury due to them being acceptable to very few (<10% of males) industrial workers
To be honest, the evidence about the effect of team size is contradictory. A large psychophysical study (Meyer, J.P., Macquet, J., Briand, D., Deyris, H., Joguet, M. and Salaun, J.Y. (1988), D
I just came across this post as I was searching for information re a 2-person lift max. As reported in Kodak’s Ergonomic Design for People at work, Lee and Lee (2001) report, to determine the safe lifting weight for a team, one should add the maximum acceptable weights of each person and then reduce the total by 12 to 15 percent. The books goes on to say that, A general design guideline would be to restrict the weight of the loads handled by two people to 40kb (85lb) so most workers can make them safely.
Obviously just one source but thought I would post this regardless. I’d love to find additional information on this and will continue my search through the literature but wonder if anyone else has additional information since the last post on this topic.
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