###### Abstract  
Back-Support Industrial Exoskeletons (BSIEs) are designed to reduce muscle effort during repetitive tasks that involve trunk bending. We recruited twelve participants to perform 30 cycles of 45\({}^{\circ}\) trunk bending with/without the assistance of BSIEs and with/without postural asymmetry, first without any back fatigue, and then at the medium-high level of perceived back fatigue. To study the benefits of BSIEs, the effects of being in a fatigued state were assessed by comparing the muscle demands, kinematics, and stability measures during bending, retraction, and their transition portions per cycle across the study conditions. Overall, the BSIEs caused a minimal decrease in the lower-back activity (0-1.8%), caused by the increased demands during the retraction portion. A substantial decrease in leg activity was observed (10-18%). Asymmetry increased the right-lower-back and leg demands. Medium-high fatigue caused an increase in the lower-back activity (8-12%) during bending and retraction. The BSIEs caused slower movements and improved the stability by lowering the maximum distance of the Center of Pressure (COP) during the transition portion, as well as by lowering the mean velocity of the COP during the bending/retraction portions. This controlled study demonstrated the use of a cyclic trunk flexion-extension paradigm to study the effects of BSIEs, and the outcomes can help with understanding the temporal effects of using BSIEs on physiological measures, ultimately benefiting their proper implementation. erognomics; industrial exoskeleton; human muscle fatigue; muscle activity; motion analysis; stability; trunk bending +
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our prior work [15]. Conducting in-depth evaluations can provide key insights, which may be beneficial to improving their design. Furthermore, the outcomes from evaluations can be beneficial to developing guidelines for their proper use and implementation in the real world. Temporal considerations during biomechanical evaluations are valuable for determining the realistic effects of an intervention. For instance, fatigue is often the result of the repetitive activation of the same muscle group, as observed commonly during tasks involving the picking up/placing of objects at the lower-body levels. Besides suffering from muscle strain/sprain, when fatigued, a worker may be more vulnerable to injury due to the affected neuromuscularuloskeletal systems. High demands on the body compared to the body's capacity to generate vital forces can lead to failure, poor work quality, and performance errors, and they can cause injury [16]. Therefore, estimating the impacts of fatigue on the human body during the performance of tasks can be beneficial. Global fatigue has been traditionally measured using subjective scales by recording the ratings of perceived exertion (RPEs) on the Borg scale [17], while the impacts of fatigue on specific body regions have been measured by recording changes in the muscle force generation capacity. Impacts of fatigue are also commonly observed in measures of muscle activity, body movement, and whole-body stability. Muscle fatigue has been defined in the literature as \"the inability of muscles to sustain force generation over time\" [18]. Moreover, performing activities in a fatigued state may also cause detrimental effects, such as a lack of balance and proper control over body movement, increasing the risk of falls [19; 20]. While BSIEs may provide benefits in reducing the rate of muscle fatigue, they may expose users to a higher fall risk [21], especially with additional weight (2.2-4.5 kg), and assistive torque could affect the wearer's stability [19]. Conditions may worsen during the performance of dynamic tasks with increased inertial forces, as well as during awkward/asymmetric postures [14; 22; 23; 24]. Considering these aspects in task simulation can provide valuable insights into improving BSIE designs.