How Do Exoskeletons Affect Back and Leg Muscular Demands During Repetitive Trunk Bending?

Abstract

Back Support Industrial Exoskeletons (BSIEs) offer support to the wearer’s torso during trunk flexion tasks thereby alleviating low-back muscle strain and risk of injury. We conducted a controlled laboratory experiment to bridge the gap between laboratory and field assessments by simulating realistic working scenarios. Twelve participants performed 30 repetitions of trunk flexion tasks with/without BSIE and with/without ~45° left asymmetry at no fatigue and then at medium-high fatigue after performing intermittent posture maintenance tasks. Findings show reduction of 0% to 1.8% with BSIE in erector spinae and 10% to 18% in the biceps femoris muscle activity using Electromyography (EMG). Demands in low-back were higher during retraction portion ~7%, which increased even more to ~12% at medium-high fatigue when not using BSIE. General findings can contribute to the development of guidelines for design and implementation of BSIEs and may be beneficial in paving way for optimized designs that are tailored to complex industrial activities.

Prevalence of strain, discomfort, and injuries resulting from repetitive industrial manual tasks highlight a need for effective interventions. Despite efforts to mitigate these risks through traditional ergonomic controls (like workforce training, equipment, and safety protocols), workers continue to face significant demands due to ever-increasing consumer demands (Bureau of Labor Statistics, 2023). Wearable assistive devices, particularly Industrial Exoskeletons, have emerged as promising solutions to augment human capabilities and reduce the risk of injury (Kuber & Rashedi, 2020). Among the body regions most vulnerable to injury includes the lumbar region of the lower-back. Back Support Industrial Exoskeletons (BSIEs) offer support to the wearer’s torso during tasks that require trunk flexion, thereby alleviating low-back muscle strain, and potentially reducing the risk of injury (Ali et al., 2021).

BSIEs require thorough evaluation as their effects on the musculature are still very task dependent. Thus, BSIEs undergo testing in multiple phases of validation, laboratory, and field evaluation (Kuber et al., 2022). Laboratory studies on BSIEs have mainly considered static posture maintenance and dynamic lifting tasks, wherein the devices demonstrated significant reductions in low-back muscle activity (Kermavnar et al., 2021). On the other hand, their effectiveness in real-world environments remains mixed, possibly due to lack of control over body movements (Schwerha et al., 2022). Furthermore, the dynamic nature of tasks involving repetitive trunk bending, combined with varying postures and movements, presents unique challenges in evaluating benefits of BSIEs.

To understand the effects of using BSIEs during fatiguing trunk bending tasks, we conducted a controlled laboratory experiment to evaluate the impacts of wearing BSIEs on muscular demands in the low-back and leg regions during trunk flexion tasks. For simulating real-world scenarios in a laboratory environment, we considered factors such as awkward postures, and fatigue induced by performing intermittent task sequences. We expected that assistance provided by BSIE may be beneficial in reducing overall muscle activation and fatigue in the low-back region, and that the benefits would be higher for lower-back compared to leg region.

Twelve male university students performed trunk bending/retraction tasks. Tasks were performed with/without Assistance from a BSIE (SuitX BackX Model AC), with/without ~45° asymmetry in transverse plane toward the left, and with/without a medium-high level of back fatigue. The task consisted of 30 repetitions of unloaded trunk bending to grasp a wiring setup placed on a portable adjustable stand. Measures of interest in this study included muscle demands in both low-back and leg regions, selected based on perceived fatigue ratings during pilot testing. An electromyography (EMG) system (Trigno Delsys, Massachusetts, 1,200 Hz) was used to record muscle activity while perceived fatigue level, obtained using the Borg RPE CR-10 scale, was used to distinguish no fatigue (RPE level:0) and medium-high fatigue (RPE level: 7) in the back region (Williams, 2017).

To achieve a fatigued state, participants performed intermittent sustained bending sequences. Tasks were performed in symmetric and asymmetric postures (with a 15-min rest break) on the same day with a 48-hr. break between with/without assistance conditions. Assessed measures included peak activity in left/right erector spinae (LES/RES) and left/right biceps femoris (LBF/RBF) muscles over the following three portions of each bending/retraction cycle: (a) trunk bending movement, (b) switching between trunk bending to retraction, and (c) trunk retraction movement.

Overall results showed that assistance from the BSIE was minimally helpful for the back muscles (0%–1.8%). The evaluated BSIE was most helpful for the low-back when the wearer was bending forward where activity in LES and RES reduced by 4% and 9%. On the other hand, BSIE was not helpful during retraction movement of the trunk. The device increased demands during retraction in both back muscles by ~3.5% to 5% (p < .01). Meanwhile, no effects were observed during the transition portions. Reductions of 10% to 18% in peak muscle activity were clear in the leg regions and occurred during all three portions of the task (a) bending (~9%–22%), (b) retraction (~22%), (c) transition (~2.8%–15%). Considering the differences between symmetric and asymmetric postures, we found that asymmetry reduced activity in LES by ~18% but increased activity in RES (~10%) and both leg muscles by up to 30%. Lastly, performing the 30 cycles of repetitive bending when having medium-high back fatigue led to higher muscle activity, which was most prominent in the back region. During retraction, BSIE led to a higher increase in back muscle activity at medium-high fatigue level of ~12% versus ~7% increase when not using the assistive device.

Our study demonstrated benefits and limitations of BSIEs during repetitive bending tasks through an in-depth controlled laboratory evaluation. We focused on evaluating variations in muscle demands across different phases of repetitive trunk flexion cycles. Contrary to their conventional designation, our findings revealed minimal benefits in the back region but substantial advantages in the leg region during repetitive bending tasks. Specifically, BSIEs significantly reduced muscle demands in the legs compared to the back. Higher demands during retraction portions were seen possibly due to the added weight of the device. This insight highlights their potential benefits in reducing leg muscle fatigue during repetitive bending tasks. Our next steps will include evaluating effects on body movement, and balance to assess changes in risk of fall when using BSIEs. Findings from this study can contribute to the development of guidelines for design and implementation of back assist devices.