skip to main content

In the Updates & Extensions to Manual Materials Handling Assessment Tools Session of the 2021 VelocityEHS CEU Conference, Blake McGowan, CPE and Director of Ergonomics Research with VelocityEHS, shared some shocking information on the impact of musculoskeletal disorders (MSDs) worldwide. He also explained equations used to calculate measurements in ergonomics, as well as recent updates made to those equations.

MSDs, low back pain and neck pain fall in line right behind HIV as the highest ailments of concern for the entire world.

It’s an issue that is often overlooked, despite the prevalence of those who experience MSDs:

  • 31% of people in the United States.
  • 50% of people in Canada.
  • 39% of people in Europe.

And MSDs are expensive! Just in the United States, roughly 5% of our gross domestic product is spent on MSD medical treatment:

  • ~$30k (average cost per case) is spent on hand/wrist/shoulder injuries.
  • ~$50k (average cost per case) is spent on low back injuries.

The leading causes of MSDs are lifting, lowering, pulling and carrying—actions which are all part of manual materials handling.

What contributes most to the development of an MSD is high force, more than high repetition. If an employee is performing a task repetitively, but isn’t exerting much force to complete it, they have a much lower risk of developing an MSD. However, if an employee is performing a task repetitively and is exerting a high amount of force, they are at a much, much higher risk of developing an MSD.

Thankfully, there are many equations to help ergonomists determine MSD risk levels for specific types of tasks, the actions it takes to complete them, and the person performing the task.

In 1979-1980, the National Institute for Occupational Safety & Health (NIOSH) developed an equation to calculate the risks for manual materials handling tasks.

  • An equation to determine the acceptability of a lifting/lowering task, based on 5 risk factor variables: AL = LC x HM x VM x DM x FM
    • (Action Limit (Acceptable Limit) = Low Constant x Horizontal Multiplier x Vertical Multiplier x Distance Multiplier x Frequency Multiplier)

In 1991, NIOSH published the Scientific Support Document, which added the concepts of the Recommended Weight Limit (RWL) and the Lifting Index (LI).

  • RWL was included to calculate the amount of weight an employees can safely lift, given a specific job geometry, frequency and duration.
    • RWL = Load Constant x Work Task Variables
      • Load constant is the estimated safest amount of weight someone can lift in perfect conditions at one time (lbs).
      • Work task variables are the same multipliers from above. They range from 0-1; 1 being the best conditions, where tasks can be completed close to the body and 0 being the worst conditions.
  • LI was included to calculate the amount of weight that can be safely lifted by an employee (that suits 75% of women and 99% of men).
    • LI = Object Weight / Recommended Weight Limit
      • If your LI is above 3 – High Risk! Engineering controls are highly recommended.
      • If your LI is between 1-3 – Moderate Risk. Administrative or engineering controls are recommended.
      • If your LI is below 1 – Normal Risk. Workplace is generally safe for the healthy working population.
    • Extensions were added to the equation to account for continuous lifting hours and frequency multipliers (lifts per minute).

In the mid-90s through 2016, various indexes were added to create a multi-task lifting index, such as the:

  • Composite Lifting Index (CLI) – Measuring the same weight in different geometries.
  • Sequential Lifting Index (SLI) – Counting rotation between unique tasks.
  • Variable Lifting Index (VLI) – Measuring different weights in different geometries.
  • Cumulative Lifting Index (CULI) – Counting rotation between unique tasks, but using continuous lifting hours and update frequency multipliers.

Outside of NIOSH, Snook & Ciriello from the Liberty Mutual Research Institute for Safety developed the Psychophysical Tables for Manual Materials Handling Assessments.

To determine and control the costs associated with MMH, the nation’s greatest research institute developed capability tables for lifting, lowering, pushing, pulling and carrying, based on the psychophysical method (oxygen consumption, heart rate and anthropometric measures).

They gave test subjects the control of how much weight they were using, and monitored their feeling of exertion or fatigue. Using a table to find the maximal acceptable weights and forces (for the average person), they included:

  • The percent of the population that is capable
  • Sex
  • Hand height
  • Push distance
  • Frequency of push or pull

Later on, they developed an extension on their guide which included details such as:

  • Maximum force (kg)
  • Distance (ranging from 2.1-6.1m)
  • Frequency of exertions per minute (-0.002-10/min.)
  • Handle height (in meters; ranging from 57-135 cm)

And then there’s the Bureau of Worker’s Compensation/Ohio State University (BWC/OSU) Biomechanical Push/Pull Guidelines.

  • These measured hand force limits based on biomechanical assessment of the forces on the lumbar spine in both one and two-handed tasks, as well as straight and turning.

And the Lifting Fatigue Failure Tool (LiFFT).

  • A lower back exposure assessment tool which estimates a “daily dose” of cumulative loading on the lower back using fatigue failure principles.
  • Using a table to determine the cumulative load associated with a task, the factors included are:
    • Weight of the load,
    • Maximum horizontal distance from the spine to the load,
    • And the number of repetitions for tasks performed during the workday.

After McGowan’s presentation, audience members entered questions into the session’s chat to get specific insight into their concerns. Their questions and McGowan’s answers are below.

“Regarding the lift tool and job rotation, if the job rotation lowers the frequency of lifts per day, could rotation still benefit the worker?”

  • Rotating a person into a high-risk job causes more high risk for your population. We tend not to rotate people through low-risk jobs, we rotate people through high-risk jobs. Subjecting more people to the high-risk job doesn’t lower the risk for each person; it subjects the population to more risk. Exposing more people to higher risk. You should aim to get those high-risk jobs down to a moderate risk level to utilize the rotation program and not significantly increase the risk to the whole population.

“Do you have a tool for people who lift weights while sitting? Employees use their elbow on their thighs and force their arm to lift the load.”

  • There are some tools that are available, albeit they are obscure. The simple answer is that when you’re sitting down, you’re going to be lifting less. Whatever you think you should be doing while standing, if you’re sitting down and lifting, you’re going to do less.

“How will these updates / changes / new analyses be incorporated into the VelocityEHS ergonomics assessment process?”

  • Most of them already are; people just haven’t seen it because they’re all on the back end. The most obvious one that we currently don’t have at the moment is the 5 different levels of threshold, but that’s on the roadmap. Many extensions are behind the scenes, and people don’t really know they’re there.

“I have used stair climbers where I do not have an elevating device for loads over 45 lbs. What about aids?”

  • There are all types of aids and solutions out there for specific needs and situations, and that could be a whole other discussion; this session was just focused on quantifying the risk.

With this list of equations and considerations in their back pockets, ergonomists can positively impact the global health crisis of musculoskeletal disorders, one detail at a time—and so can you! Watch the session for yourself and dive into this plethora of information from a certified professional ergonomist. While you’re there, check out the other sessions as well!