## 4 Discussion
During retraction, the BSIE caused the highest activity of 0.71 (SD: 0.26) in the RES during asymmetric retraction (21% more than symmetric bending) and increased the activity in the LBF (~8%). The increased activity in the LBF during retraction occurred possibly due to the application of higher forces at the left foot for maintaining balance. Both the switching between bending and retraction, as well as retraction itself, were thus more demanding in the presence of awkward postures. The effects of muscle fatigue were assessed by comparing the effect of time, as the beginning and end portions were performed at the no-fatigue and medium-high-fatigue levels on the Borg CR-10 RPE scale. The presence of muscle fatigue is known to increase the peak amplitude of the EMG signal, and BSIEs have been known to delay muscle fatigue [36; 37]. Increased back muscle activity occurred across all three task portions but increases in all the muscles occurred mostly during the transition portion of the task. This portion involved switching from an ongoing dynamic motion of bending forward to an opposite retraction movement, requiring additional demands to counteract and exceed the inertial forces. The effects of fatigue on the muscular demands were clearly observed from the increased back activity (8-12% (\(p<0.01\))) between the beginning and end trials during bending, with most of the benefits (~7-8%) seen in the LES muscle originating from the reduced demands on the left back musculature during the asymmetric postures. During retraction, higher fatigue effects occurred with the BSIE (LES: 12% increase with time; RES: 11%) vs. without the BSIE (LES: ~10%; RES: 5%) (\(p<0.01\)). Similar increased demands over time were also seen during the transition portions in the LES, RES, and LBF activities (up to 15%), but no effect with time was seen in the RBF activity under either the with- or without-BSIE conditions. This may be because of the asymmetry conditions, as well as the possibility that the study subjects shifted their weight more on the left side when performing the tasks in both the symmetric and asymmetric postures. The use of the BSIE led to consistent reductions across the bending and retraction portions in the movement velocities of the markers located on the upper-back, lower-back, and hip regions. Retraction was performed faster than bending, possibly because the bending movement required more effort to control the fall of the torso, while retraction required pulling the entire torso towards a neutral posture, requiring acceleration phases. Reductions in trunk angular velocities have been reported when performing repetitive-lifting tasks after wearing a BSIE [13]. The upper-back velocities were the highest, as this was the region with the most displacement, followed by the lower-back and hip regions, which were the most stable. Interestingly, most reductions (of \(\sim\)up to 60%) after using assistance occurred in the lower-back region during both the bending and retraction portions. This may have been caused by the structural support provided by the BSIE. On the contrary, the BSIE increased the hip velocity by ~13% during bending with assistance, and no difference was seen during retraction. This may have been the result of the BSIE keeping the trunk and legs straight throughout the bending/retraction cycle, leading to more hip movement. When the subjects were fatigued, they performed faster bending/retraction movements. This was seen from the increase of ~7% in the mean velocity of the upper-back. This movement was the fastest when the BSIE was not worn, while the participants performed the tasks the slowest when using the BSIE and when they were not fatigued. Muscle fatigue in the back and leg regions may have impacted the ability to sustain stable muscle force generation, possibly leading to higher force generation and faster movement. Similar to previous studies [19; 38], we assessed the COP distances and velocities to study the effects on the balance.