Can back exosuits simultaneously increase lifting endurance and reduce musculoskeletal disorder risk?

Abstract

The objectives of this case series study were to test whether an elastic back exosuit could increase a wearer's endurance when lifting heavy objects and to assess whether lifting more cancels out the exosuit's risk reduction benefits. We found that 88% of participants increased their lifting repetitions while wearing an exosuit, with endurance increases ranging from 28 to 75%. We then used these empirical data with an ergonomic assessment model based on fatigue failure principles to estimate the effects on cumulative back damage (an indicator of low back disorder risk) when an exosuit is worn and more lifts are performed. Participants exhibited 27-93% lower cumulative back damage when wearing an exosuit. These results confirmed that wearing an exosuit increased participants' lifting capacity without canceling out injury risk reduction benefits. Back exosuits may make it possible to simultaneously boost productivity and reduce musculoskeletal disorder risks, which is relevant to workers in civilian and defense sectors.

Keywords: exoskeletons; lifting; low back disorder; military; performance augmentation.

Figure 1.

(A) Overview of lifting endurance tests. In case series 1, participants performed an AB test in which they lifted 45 kg repeatedly until failure without the exosuit and then performed this task while wearing the exosuit. In case series 2, participants performed an ABA test in which they lifted 55 kgs repeatedly until failure without the exosuit, then with the exosuit, and then again without the exosuit. (B) The photos show representative participants wearing the exosuit during case series 1 (left) and 2 (right), as well as the operationally relevant objects they lifted.

Figure 2

(A, B) Bar plots show the percent change in lifting repetitions when wearing the exosuit relative to not wearing one. Across case series 1 and 2, seven (S1, S3–S8) out of the eight participants increased the number of lifting repetitions they performed while wearing the exosuit. (C, D) Bar plots show the percent change in cumulative damage when wearing the exosuit relative to not wearing one. For example, a value of −40 in this plot represents a 40% reduction in damage when wearing the exosuit relative to not wearing one. Across case series 1 and 2, all eight participants (S1–S8) exhibited lower cumulative damage while wearing the exosuit, despite seven of eight increasing the number of lifting repetitions they performed. In subplots (B) and (D), the bar plots depict results from the exosuit (set B in Figure 1) relative to the average of the two control sets (A₁, A₂), while the circle and triangle symbols show comparisons relative to each individual control set (B versus A₁, and B versus A₂).

Figure 3.

Contour plots show the model-predicted relationships between exo moment, lifting repetitions, object weight, and cumulative damage. Along the x axis, a 0% increase represents the nominal lifting scenario, and a 100% increase represents double the weight or repetitions. Negative values shown in shades of blue indicate conditions where there is a decrease in the cumulative damage when wearing an exo relative to the nominal lifting condition without an exo. These negative regions depict when an exo (with an associated exo moment, y-axis) could simultaneously increase performance (lifting repetitions or object weight) and reduce risk (cumulative damage). In contrast, yellow regions (0+) of each contour plot indicate conditions when the injury risk benefits of the exo are fully canceled out. For these conditions, there is an increase in the cumulative damage when wearing an exo and increasing lifting repetitions or weight, relative to the nominal lifting condition. Therefore, to receive dual benefits from a back exo (performance enhancement and risk reduction), you want to avoid the yellow regions. (A) The relationship between exo moment, cumulative damage, and lifting repetitions is shown for any constant object weight. Increasing lift repetitions generally does not cancel out reductions in cumulative damage provided by the exo. Next, the relationship between exo moment, cumulative damage, and object weight is shown for nominal object weights of (B) 5 kg, (C) 23 kg, and (D) 45 kg, respectively. The higher the nominal object weight, the smaller the increase (percentage-wise) in the weight needed to cancel out the reduction in cumulative damage provided by the exo.

Figure 4.

Breakeven contours are shown for the four-parameter sweeps depicted in Figure 3. Each curve represents the conditions at which the cumulative damage reduction benefits of an exo with a given exo moment (y axis, in Nm) are perfectly canceled out by an increase in performance (x axis). In other words, these curves show conditions of 0% change in cumulative damage relative to the nominal lifting condition without an exo. The curve depends on whether performance increases come from increasing lifting repetitions (purple dotted line) or from increasing object weight from a nominal weight (blue, red, and gold solid lines). Model results indicate that it is preferable to increase performance by increasing the lifting repetitions, not object weight. Exo moment must increase as object weight and lifting repetitions increase to achieve zero change in cumulative damage. For example, a 20% increase in object weight requires 6, 27, and 53 Nm of exo assistance for 5, 23, and 45 kg objects, respectively. However, an increase in lifting repetitions of 20% only needs 5 Nm of exo assistance to maintain constant cumulative damage, regardless of nominal object weight.

Figure 5.

Model results demonstrate why it is preferable to increase lifting repetitions, not object weight, when wearing a back exo. Here are four modeled scenarios: (A) a nominal lifting task: 50 lbs (22.7 kg) lifted 1000 times causes a certain amount of cumulative damage to the back. (B) Completing the same lifting task (i.e., same productivity) with the exo decreased cumulative damage to the back by 68%. The next two modeled tasks involve 20% more productivity (more total weight lifted) than the nominal task. (C) Wearing an exo while also increasing repetitions by 20% leads to a 62% reduction in cumulative damage relative to the nominal task. This is an example of simultaneously increasing productivity and decreasing low back disorder risk. (D) Wearing the exo and increasing object weight by 20% leads to a 3% increase in cumulative damage relative to the nominal task. These modeled scenarios used a 70 cm horizontal distance between the object and spine and a 30 Nm exo moment.