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Introduction: In recent years, the number of commercially available industrial exoskeletons has increased substantially. Most previous studies have only collected EMG data and focused on passive BSEs [1].

The objective of the present study was therefore to include multiple exoskeletons (one active, A1, and two passives, P1 and P2) in the investigation and to assess their impact on L5/S1 joint moments and erector spinae (ES) activity during lifting and holding of 10 kg. It was hypothesized that a BSE-specific but activity-independent support effect could be observed for passive systems. Conversely, a more task-specific support effect was expected for the active system.

Methods: As part of a laboratory study 12 subjects (6 m, 6 f; height: 1.77 ± 0.08 m; weight: 70.0 ± 11.4 kg; age: 25 ± 2 years) performed dynamic lifting (5 repetitions of freestyle lifting in front of the body) and static holding (20 s in 45° torso forward bend with legs extended) of a 10 kg load weight. Activities were performed in a randomized fashion under NoExo, A1, P1 and P2 conditions.

Full body 3D-motion capture (12 Cam, Vicon Nexus) of the subjects and the BSE was conducted at 100 Hz and used to calculate L5/S1 extension moments via inverse dynamics in accordance with the Plug-in Gait Standard. A 4-channel sEMG of the ES was recorded at vertebrae levels T11/12 and L2/3 at 1000 Hz.

For lifting and holding the mean and peak values for the lumbar extension moments and muscle activity were analyzed with separate 2-way mixed analysis of variance (ANOVA) with the factors subject (random effects factor) and exoskeleton (fixed effects factor). The level of significance for all tests of p < 0.05 was used.

Results: During the dynamic lifting task, the mean peak L5/S1 extension moments for NoExo were 1.76 ± 0.16 Nm/kg. The analysis of the EMG data revealed peaks in muscular activity of 39.9 ± 9.5 %MVC. The application of exoskeletons A1 and P1 resulted in an average reduction of the maximum L5/S1 extension moments of 15% (p < 0.01) and 11% (p < 0.01), respectively. A reduction of 22% (p < 0.01) was observed for P2. Maximum ES muscle activity during lifting was also significantly reduced with all systems (A1: 32%; P1: 13%; P2: 17%, p < 0.01).

During the holding task, average L5/S1 extension moments were 1.41 ± 0.15 BW, and the average ES activity was 20.9 ± 4.9 %MVC for NoExo. Comparable reductions in L5/S1 extension moments were observed for both passive systems, with a 12% reduction (p < 0.01) for P1 and a 20% reduction (p < 0.01) for P2. A greater reduction of 41% (p < 0.01) was observed for A1. The muscle activation data exhibited a comparable pattern, with a reduction in mean ES EMG amplitude for P1 of 23% (p < 0.01) and P2 of 16% (p < 0.01), and a greater reduction by A1 of 54% (p < 0.01).

Discussion: The results confirmed our hypothesis. The passive exoskeletons showed a support effect that was largely independent of activity. The active system, on the other hand, showed greater musculoskeletal support for the static task.

Conclusion: When planning the implementation of exoskeletons in a workplace, detailed information about the worker's movement patterns and the dynamic characteristics of the different types of exoskeletons is crucial.

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