Equivalent Weight: Connecting Exoskeleton Effectiveness with Ergonomic Risk during Manual Material Handling

Abstract

Occupational exoskeletons are becoming a concrete solution to mitigate work-related musculoskeletal disorders associated with manual material handling activities. The rationale behind this study is to search for common ground for exoskeleton evaluators to engage in dialogue with corporate Health & Safety professionals while integrating exoskeletons with their workers. This study suggests an innovative interpretation of the effect of a lower-back assistive exoskeleton and related performances that are built on the benefit delivered through reduced activation of the erector spinae musculature. We introduce the concept of “equivalent weight” as the weight perceived by the wearer, and use this to explore the apparent reduced effort needed when assisted by the exoskeleton. Therefore, thanks to this assistance, the muscles experience a lower load. The results of the experimental testing on 12 subjects suggest a beneficial effect for the back that corresponds to an apparent reduction of the lifted weight by a factor of 37.5% (the perceived weight of the handled objects is reduced by over a third). Finally, this analytical method introduces an innovative approach to quantify the ergonomic benefit introduced by the exoskeletons’ assistance. This aims to assess the ergonomic risk to support the adoption of exoskeletons in the workplace.

An ergonomic assessment tool for evaluating the effect of back exoskeletons on injury risk

“…For instance, Lamers et al (2020) evaluated assistance benefits of a soft, passive-elastic back exo using two separate analyses -EMG, and a physics-based moment balance -and found the magnitude of back offloading estimated by each analysis to be similar. Likewise, a study on a rigid robotic back exo (Di Natali et al, 2021) found EMG and physics-based analyses yielded similar results in terms of reductions in back loading during lifting. In both studies, the physics-based analysis predicted slightly less back offloading than the EMG-based analysis, suggesting it may provide slightly more conservative estimates of exo benefits.…”

“…LiFFT is based on fatigue failure processes, which underly how microdamage within materials (including biological tissues) accumulates during repeated loading cycles. Fatigue failure underlies both the etiology of overexertion injuries (Edwards, 2018;Gallagher and Heberger, 2013;, and the biomechanical rationale for why/how exos are expected to reduce injury risks (Abdoli-Eramaki et al, 2007;Di Natali et al, 2021;Lamers et al, 2018), which is why this tool has the potential to unify ergonomic assessment and exo assistance. Here we describe how LiFFT can be adapted to assess injury risk when wearing back exos and share examples of various uses.…”

“…One alternative way to express exo assistance is in terms of "equivalent weight" or "equivalent effort" metrics (Di Natali et al, 2021;Lamers et al, 2020). For instance, instead of reporting Cumulative Damage or LBD Risk reduction, Example 1 could be summarized by stating that the 30 Nm back exo makes lifting a 15 kg box feel like lifting a 6.4 kg box from the standpoint of loading experienced by the back.…”

“…But we caution against over-reliance on EMG for assessing ergonomic effects of back exos due to the time, cost, and complexity burden it places on safety professionals and companies. To date, much of the back exo EMG research boils down to a confirmation that the lumbar moment borne by the musculoskeletal system is reduced by the magnitude of moment provided by the exo (e.g., Di Natali et al, 2021;Lamers et al, 2020), and that for passive exos inserting a stiffer spring generates a larger exo moment (e.g., Frost et al, 2009). We recommend the use of physics-based (lumbar moment) analysis for assessing back exos, particularly for devices or assistance levels where EMG studies have already been conducted that confirm consistency with moment-based estimates, and where evidence exists indicating there is not substantial co-contraction of the abdominal muscles or major changes to lifting kinematics.…”

“…But we caution against over-reliance on EMG for assessing ergonomic effects of back exos due to the time, cost, and complexity burden it places on safety professionals and companies. To date, much of the back exo EMG research boils down to a confirmation that the lumbar moment borne by the musculoskeletal system is reduced by the magnitude of moment provided by the exo (e.g., Di Natali et al, 2021;Lamers et al, 2020), and that for passive exos inserting a stiffer spring generates more force and thus a larger exo moment (e.g., Frost et al, 2009).…”