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Exo-LiFFT Calculator A Practical Tool For Evaluating Back Support Exoskeletons

Exo-LiFFT Calculator A Practical Tool For Evaluating Back Support Exoskeletons

Verifying the application and usefulness of exoskeletons at a job site can be costly and time-consuming. Wouldn’t it be easier and more economical to evaluate the potential use of specific back-support exoskeletons without having to resort to costly EMG testing? Exo-LiFFT is an online calculator that aims to do just that for back support exoskeletons. The Exo-LiFFT is an ergonomics assessment tool designed to estimate the effect of back-assist exoskeletons and exosuits on low back disorder (LBD) risk during occupational lifting tasks, without the need for complicated test equipment.

As the name implies, the Exo-LiFFT equation is derived from the Lifting Fatigue Failure Tool (LiFFT); an easy-to-use risk assessment tool for cumulative low back loading originally published in 2017 by Gallgher et al.

How does the Exo-LiFFT work?

The calculator comes in two variants, one for a single task and a version for multiple work tasks. The exoskeleton evaluator inputs the task parameters: weight of the objects and the lever arm (peak horizontal distance from the lumbar spine to object during a lift). This is followed by the number of repetitions for the work shift. Finally, the exoskeleton moment of the back assist exoskeleton is added. The exo moment is defined as the “assistive lumbar moment provided by exo; obtained from exo manufacturer or empirical testing of the device for the specified task. Must be less than Peak Load Moment for Exo-LiFFT assessment.”

The Exo-LiFFT equation then calculates the Low Back Disorder (LBD) risk with and without the exoskeleton. LBD Risk is defined as the probability of a job being a high-risk job. High-risk jobs are defined as 12+ injuries per 200,000 hours worked. Reductions in LBD Risk are correlated with reductions in actual low back injury incidence in the workplace, based on a multi-year prospective study.


Professor Zelik’s lab, which currently hosts the Exo-LiFFT equations (link) also has a posted example of the calculator in use:

“Imagine a safety professional has already implemented good ergonomic practices within the hierarchy of controls, to the extent practical. However, back discomfort and injuries persist amongst workers. They decide to evaluate the potential benefits of a commercially-available back exo that provides 30 Nm of torque about the back during a typical lift. In practice, this 30 Nm exo moment comes from mapping the lifting postures of workers (e.g., bend angles during lifting, as assessed by the safety professional) onto the mechanical assistance provided by an exo (e.g., back extension moment vs. joint angle curve provided by the exo manufacturer, or found from empirical testing of the device). If needed, contact the exo manufacturer to request exo moment values for relevant work tasks.

In this example, let’s say the safety professional already knows (from their previous ergonomic assessments, reviewing videos of work tasks, or from operational/organizational data) that their workers perform about 2,000 lifts per day, the average object lifted is 15 kg, and average horizontal distance from the spine to the object is about 50 cm. For this precursory analysis these values can be ballpark estimates, but note that the average peak exo moment (at or near the time of peak load moment) expected for a given job or task should be used, not simply the maximum moment an exo can generate.

Given this simple information, the safety professional can now use Exo-LiFFT to estimate the Cumulative Damage and LBD (Low Back Disorder) Risk to the workers with vs. without the exo, to quantify the expected effects of augmenting workers with exos. The safety professional would simply input into the Exo-LiFFT Calculator:

  • Repetitions: 2000
  • Lever Arm: 50 cm
  • Object Weight: 15 kg
  • Exo Moment: 30 Nm

The Exo-LiFFT Calculator will compute the reductions in Cumulative Damage and LBD Risk with vs. without the exo.”

In this case, the calculator (Beta Version 0.9) shows that the exoskeleton will reduce LBD Risk by 21.3%, and the cumulative damage by 68.0%

Discussion and limitations:

The Exo-LiFFT equation has been well received by the professional community. However, one temptation is to take the equation and plug it in directly into an exoskeleton ROI calculation. This can be done by approximating the cost to the employer of on-the-job injuries vs. the cost of purchasing and maintaining wearable devices. This type of calculation will ignore other benefits of using occupational exoskeletons.

Another limitation of the equation is that it rewards wearables with a high exo moment. For passive exoskeletons, that could be achieved using stiffer springs at the cost of making it harder for the user to move and lean forward. In the same line of thinking, powered lower back exoskeletons can have more powerful motors, but they would be heavier and harder to move with. Larger and bulkier exoskeletons could also change the moment arm of the lift.

Finally, the Exo_LiFFT is only designed for back support exoskeletons that are meant to help workers who are performing repetitive upper-body tasks. It therefore can’t be applied to all exoskeletons.

The equation is not meant to replace a professional evaluation of an exoskeleton at a job site, but to give an additional practical tool in the hands of the professionals.


Examples of Lower Back Exoskeletons Catalog 2021
Examples of Lower Back Exoskeletons: see ExR Catalog

Even with automation, lower back pain and injuries continue to dominate bodily work-related incidents. Occupation (industrial) exoskeletons are being deployed around the world in greater numbers (Innophys alone has shipped over 20,000 units). If the correct device (or solution) is matched to a high-risk work task workers can look forward to retiring with fewer musculoskeletal injuries.

“We’ve been exploring exoskeletons at Boeing for the last few years, with encouraging results to date,” said Christopher Reid, Associate Technical Fellow of Human Factors and Ergonomics at Boeing. “It’s incredibly important and encouraging to see academia and industry coming together to develop practical risk assessment tools that can help identify and leverage the benefits of emerging safety technologies like exoskeletons.”

“At Toyota, we have relied heavily on new ergonomic assessment tools to support our teams in identifying processes on our manufacturing lines that would benefit from shoulder exoskeletons being deployed as personal protective equipment,” said Aaron Sparks, safety project engineer at Toyota North America. “As we begin to investigate and deploy back exoskeletons, it’s incredibly exciting, and a major relief, to see similar tools being developed to support with the identification and deployment.”


  • New study reveals breakthrough tool to show how much exoskeletons reduce back injury risk, Vanderbilt News, Nov 30, 2021, link to Vanderbilt News
  • Exo-LiFFT Calculators, Zelik Lab for Biomechanics & Assistive Technology
  • An ergonomic assessment tool for evaluating the effect of back exoskeletons on injury risk
    Karl E. Zelik, Cameron A. Nurse, Mark C. Schall Jr, Richard F. Sesek, Matthew C.Marino, Sean Gallagher
  • Sean Gallagher, Richard F. Sesek, Mark C. Schall, Rong Huangfu, Development and validation of an easy-to-use risk assessment tool for cumulative low back loading: The Lifting Fatigue Failure Tool (LiFFT), Applied Ergonomics, Volume 63, 2017, link

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