Using a Back Exoskeleton During Industrial and Functional Tasks—Effects on Muscle Activity, Posture, Performance, Usability, and Wearer Discomfort in a Laboratory Trial

Objective To investigate the effect of using a passive back-support exoskeleton (Laevo V2.56) on muscle activity, posture, heart rate, performance, usability, and wearer comfort during a course of three industrial tasks (COU; exoskeleton worn, turned-on), stair climbing test (SCT; exoskeleton worn, turned-off), timed-up-and-go test (TUG; exoskeleton worn, turned-off) compared to no exoskeleton. Background Back-support exoskeletons have the potential to reduce work-related physical demands. Methods Thirty-six men participated. Activity of erector spinae (ES), biceps femoris (BF), rectus abdominis (RA), vastus lateralis (VL), gastrocnemius medialis (GM), trapezius descendens (TD) was recorded by electromyography; posture by trunk, hip, knee flexion angles; heart rate by electrocardiography; performance by time-to-task accomplishment (s) and perceived task difficulty (100-mm visual analogue scale; VAS); usability by the System Usability Scale (SUS) and all items belonging to domains skepticism and user-friendliness of the Technology Usage Inventory; wearer comfort by the 100-mm VAS. Results During parts of COU, using the exoskeleton decreased ES and BF activity and trunk flexion, and increased RA, GM, and TD activity, knee and hip flexion. Wearing the exoskeleton increased time-to-task accomplishment of SCT, TUG, and COU and perceived difficulty of SCT and TUG. Average SUS was 75.4, skepticism 11.5/28.0, user-friendliness 18.0/21.0, wearer comfort 31.1 mm. Conclusion Using the exoskeleton modified muscle activity and posture depending on the task applied, slightly impaired performance, and was evaluated mildly uncomfortable. Application These outcomes require investigating the effects of this passive back-supporting exoskeleton in longitudinal studies with longer operating times, providing better insights for guiding their application in real work settings.

Moreover, evaluating highly controlled, most of the above-cited studies, does not rep-Additionally, usability and acceptability of exoskeletons is not commonly assessed for tests the exoskeleton's intended function. Tests to be considered to overcome this gap are functional and sitting on a chair (Baltrusch et al., 2018).  (Table 1 had to be excluded from the course because, despite oral feedback, they did not perform the Experimental Procedure subjects practiced all functional tests and indus-lected. The second visit included the experi-malization of muscle activity and a reference posture for correction of joint angles (see Measurements and data analysis). The exper-formed in a predetermined order: stair climbing test (SCT), timed-up-and-go test (TUG), and the course (COU).
For the SCT, subjects ascended seven7 steps of an actual staircase, turned around, and (Bennell et al., 2011). For the TUG test, subjects Dobson et al., 2013). Within the COU, subjects passed three (3) lattice box lifting. During pallet box lifting, subjects had to pick and place eight boxes (9.6 to another pallet. During fastening, subjects fas-ion. During lattice box lifting, subjects picked and placed four boxes (5.9 kg; 20 × 30 × 34 tice box is frequently used in industry and logislattice box) out of the box. In both lifting tasks, on input coming from industry stakeholders to stay as close as possible to real assembly and  During or after all three tests, performance and naire on exoskeleton usability. Passive Back-Support Exoskeleton The Laevo (V2.56, Laevo B.V., Delft, Netherlands) is a passive exoskeleton (2.8 kg) ing of three main components: chest pad; hip belt; leg pads at the anterior side of the thighs ( Figure 2). The left and right torso structures and the left and right torque generating systems tion. The torso structures are semi-rigid bars connecting chest pad and hip belt supporting hip extension. Both bars are exchangeable to erators on either side of the hip have spring-adapted to suit the subjects as good as possible by using one of 2 sets of semi-rigid bars (available in sizes S and L) and adjusting the support angle in the smart joints to avoid any conby a force sensor (8 Hz sampling frequency; 38 × 10 mm; Type KM38-1kN, ME-Meßsysteme GmbH, Henningsdorf, Germany) built into the chest pad.

Measurements and Data Analysis
Muscle activity. Using sEMG, activity of cles belonging to the target area supported by the exoskeleton and those belonging to the nontarget areas not supported by the exoskeleton. the COU, 5 s maximal reference contractions (MVC), and 10 s submaximal reference contractions (RVC). After data processing of sEMG (see supplemental Appendix A for details), the malized to MVC (1) and of BF, RA, VL, GM, and TD to RVC (2; Mathiassen et al., 1995) and calculated for each exoskeleton condition and RVC (Steinhilber & Rieger, 2013; for a detailed explanation, see supplemental Appendix A).

per minute (bpm) for all exoskeleton conditions
Performance Time-to-task-accomplishment. Performance -analyses.
After SCT, TUG, Table 4). Usability. The questionnaire (see supplemental Appendix B) on usability of the Usability Scale (SUS; Brooke, 1996), and 7 logical skepticism and user-friendliness derived from the Technology Usage Inventory (TUI; Kothgassner et al., 2013 calculated: one for the self-developed questions, technological skepticism and user-friendliness (Table 4).
Wearer comfort. After SCT, TUG, and exoskeleton (Table 4 or in the turned-on state for COU.

Statistical Analysis
We checked normal distributions of the outcomes by visually inspecting histograms and Kim, 2012Kim, , 2013 r for Wilcoxon signed-rank tests (using z-score and number of total observations; Field, 2018); Cohen's d for both the paired-samples t-tests (using

Usability
The average sum-score on the four general  (Table 7).

DISCUSSION
In light of the primary aim (target region), ES and RMS BF pallet box lifting, fastening, and lattice box lifting (COU). In addition, for all three tasks of KNEE HIP increased secondary aim (nontarget region), the results GM for fastening and lattice box lifting (COU) and increased RMS RA and RMS TD for fastening but not for pallet box Time-to-task-accomplishment of SCT, TUG,   Note. *Significant p-value, α = .05; †medium effect size, r ≥ .3. TUI = technology usability inventory; SC = skepticism; UF = user-friendliness; SUS = system usability scale; SD = standard deviation; IQR = inter-quartile range. Koopman, Näf, et al., 2020;Madinei et al., 2020aMadinei et al., , 2020b, most studies reported a greater reduction in trunk extensor activity up to 38% (Alemi et al., 2020;for example, Bosch et al., 2016;Koopman et al., 2019). These variable reductions in trunk the reported outcome parameter (average vs. peak), task content, task duration, and support characteristics of the back-supporting exoskeleobliquus) are often included in evaluations of back-supporting exoskeleton because these are co-activators of trunk extensors. In static no changes (Graham et al., 2009), increases ( ), or decreases (Bosch et al., 2016 either increased (e.g., Alemi et al., 2019) or did eton (e.g., Baltrusch et al., 2020). The current of the abdominal muscles, raising the question skeleton are relevant and can be expected to have long-term health consequences.
by the biceps femoris. In the current study, -be the result of the leg pads pressing against the upper leg and the gastrocnemius muscle acting against this pressure for preventing an over-extended knee position (Bosch et al., 2016). Knee extensor activity (here: vastus - Frost et al., 2009;von Glinski et al., 2019), but in contrast to other Alemi et al., 2019 characteristics of the assistive devices. It is non-target regions of the exoskeleton can be neglected or may have long-term detrimental places seems to be important.

Heart Rate
that evaluated a 45 min repetitive lifting task ( -(Lotz et al., 2009) and 7% increase (Marino, 2019). Based on these contrasting results from ences cardiovascular strain.

Performance
All three tests (SCT, TUG, COU) lasted sig-- Baltrusch et al. (2018), although they evaluated the exoskeleton in turned-on mode. For the lifting tasks as parts of the COU, time-to-task-ings suggest that the exoskeleton supports diftime-to-task-accomplishment. For both the SCT and the COU, depending on the magnitude of time-to-task-accomplishment may have an impact on the production process. Usability a good usability (Bangor et al., 2009). Baltrusch et al. (2018 assessed that usability in terms of Baltrusch et al. (2018) may be the result of the cuted task. Furthermore, usability may provide insight into people's consideration of using the evaluated exoskeleton for appropriate tasks in (Abdoli-Eramaki et al., 2006;Madinei et al., 2020a;von Glinski et al., 2019). A similar trend ings are based on laboratory studies assessing lack longitudinal data from longer-term studies, the positive result from subjective evaluations Hensel and Keil (2019).

Wearer Discomfort
-tics and assembly departments and in three labpostures and repetitive lifting, discomfort in the Baltrusch et al., 2018;Bosch et al., 2016;Hensel & Keil, 2019;Madinei et al., 2020a particular the chest, seem to have increased dis- Baltrusch et al., 2018;Hensel & Keil, 2019). These aspects of discomfort are important to bear in mind for the further development of exoskeletons because negative sequently, acceptance should be avoided.

Study Limitations
This study is accompanied by some limitations. First, the sample consisted of young, therefore, generalizability may be limited.
individual-dependent. Since the exchangeable set of semi-rigid bars of the exoskeleton ing against the subject's chest in upright. This skeleton should be used as suggested by the manufacturer (User manual, 2018). We conto be minimal, because support characteristics User manual, 2018). side of the body and averaged RMS of each muscle across participants. This may have regard to potential asymmetric muscle acti-pared to asymmetric lifting (Abdoli-Eramaki & Stevenson, 2008;Abdoli-Eramaki et al., 2006;Madinei et al., 2020a). Therefore, future studies may include more detailed information on provide additional information, because high peak loadings at the lumbar spine are associated culoskeletal disorders (Norman et al., 1998).
forming the tasks (Luger et al., 2019 familiarization in the current study may not have been extensive enough since the subjects got continuous assistance during donning/adjustmin. According to Moyon et al. (2019), these aspects are required to reach at least familiarization level four (out of seven) and become a night sleeps are required for ensuring that motor skill learning related to using an exoskeleton is enhanced (Luger et al., 2019;Walker et al., 2003 aging, it should be taken into account that an adaptation (Vandevoorde & Orban de Xivry, 2019).
Fifth, the current study tried to provide an exoskeleton by including a combination of physiological, performance and subjective --and joint angles and a limited simulation and more realistic circumstances, localized muscle fatigue may be very interesting to evaluate in developing musculoskeletal disorders (Rashedi & Nussbaum, 2015). Previous results are prom-eton (PLAD) delays the onset of muscle fatigue in the back in both males and females during 45 min lifting session ( ;Lotz et al., 2009). Yet, for future exoskeleton selection and implementation, subjective outespecially in light of usability and acceptability (Moyon et al., 2019).

CONCLUSION
The results of this study demonstrated taskposture. In most of the tasks, activity of the biceps femoris and, to a lesser extent, of the the gastrocnemius medialis increased as a result of using the exoskeleton. Based on the current positive long-term changes in musculoskeletal health due to using the exoskeleton. Based on exoskeleton on musculoskeletal complaints in possibly longer time-to-task-accomplishment that using the exoskeleton entails, should be further investigated in long-term applications to see if a longer familiarization period may counteract or even completely remove this negative

ACKNOWLEDGMENTS
Caputo, Pia Rimmele, Sylvia Weymann, and Stefanie Lorenz for their assistance in the data BMW AG, Daimler AG, Iturri GmbH, BASF SE, Deutsche Post DHL Group, MTU Aero Engines AG, and DACHSER SE cial support and their practical input in developing the simulated industrial tasks investigated in of Occupational and Social Medicine and Health an unrestricted grant of the employers' association of the metal and electrical industry Badenfunders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.

KEY POINTS
Using the exoskeleton induced task-dependent or negative for musculoskeletal health on basis target areas of future research activities.
of the task performed. Using the exoskeleton may increase time-totask accomplishment, although this could be dependent on the type and duration of the task performed.
task accomplishment is reduced to a minimum. situations.

SUPPLEMENTAL MATERIAL
The online supplemental material is avail-HF