Evaluation of two upper-limb exoskeletons during overhead work: influence of exoskeleton design and load on muscular adaptations and balance regulation.

Purpose: Overhead works (OHW) are identified as a major risk factor for shoulder musculoskeletal disorders. The use of upper-limb exoskeletons (EXO) is emerging to address these challenges. This research tested the influence of EXO design and load on the upper-limb and postural muscles activity, and on the balance control, during OHW.

Methods: This study compared two passive EXO, notably differing by the level of assistive torque delivered. Both EXO was examined in two load conditions (2 vs. 8 kg). Twenty-nine volunteers performed a static OHW for each condition.

Results: Both EXO led to similar reductions in shoulder flexor muscle activity (12.3 ± 7.8% of RMS), compared to without equipment (29.0 ± 14.2% RMS). Both EXO resulted in a reduction in the activity of shoulder (3.6 ± 3.2% RMS) and wrist (2.4 ± 1.7% RMS) extensor muscles (4.9 ± 3.9 and 5.9 ± 6.1% RMS, respectively). The use of EXO led to reductions in back muscle activity, depending on the exoskeleton design (in % RMS, 12.9 ± 9.4 for EXO1, 22.8 ± 12.6 for EXO2 and 32.0 ± 18.4 without equipment). Wearing EXO induced changes in balance regulation, depending on both exoskeleton design and load condition.

Conclusion: The increase of assistive torque was not associated with an increase in EXO performance. However, the exoskeleton design (mass, balance, and assistive torque) has to be suitable for the load handled during static OHW to optimize the effects of using an EXO on the postural muscles.