## 4 Discussion  
This study was designed as a preliminary study for a long-term biomechanical evaluation of exoskeletons in the workplace. We investigated the inter-day variation for MA and kinematics of trunk and leg, in lifting, carrying, walking, and static bending, with and without a passive back exoskeleton and examined whether the utilization of a passive back exoskeleton may have an impact on the day-to-day results comparability. When biomechanical methods are obtained on different measurement days in a long-term study, we need to be sure that these effects are not due to fundamental measurement variations caused by confounding factors. We therefore based our study design on previous research and considered several factors that can improve the reliability of inter-day biomechanical measurements [10]. Firstly, we emphasized the importance of standardized setup and use of the exoskeleton so it was adapted to the individual requirements of each subject according to the manufacturer's specifications and the optimal, most comfortable support performance. Over the course of two familiarization days, any discomfort in using the exoskeleton was identified and appropriate adjustments were made. This is important because several  
Figure 9: SPM \(t\)-test results and profile plots of kinematic data in walking without load, comparing joint angles (\({}^{\circ}\)) of the right body side in ankle flexion on day 1 (blue resp. red lines) and day 2 (black lines). The blue plot represents the comparison of the measurements with exoskeleton, and the red plot shows the measurements without exoskeleton (n = 19). studies have reported discomfort with the exoskeleton [23; 32], which could affect the exoskeleton-user interaction. We established a high level of standardization in the exoskeleton's assistance performance, which has been a critical issue in previous studies. At the same time, the two training days were designed to counteract any training effects. In addition to the use of validated measurement systems [9; 15], we included standardized sensor placement via procedural instructions [15] as well as sufficient two-day familiarization with the working tasks. As a previous study we conducted suggests that two days of familiarization may provide a high degree of standardization to ensure reliable MVIC normalization for an occupational target group [17], we also included MVIC normalization tasks in the two training days. The results show minor but significant differences between the measurements of day 1 and day 2. Regarding the IMU measurements, we found differences especially in the day-to-day measurements with the exoskeleton. For example, we measure significant unilateral differences during the lifting tasks in the area of knee flexion (\(p=0.038\)), as well as in the carrying and walking task, in the area of initial contact in knee and ankle flexion (\(p=0.037\) & \(p=0.036\)). For the muscle activation, we found significant differences in the measurements without exoskeleton, in the M. biceps femoris, during lifting (\(p=0.001\)), and also during lowering a box (\(p=0.040\)). In order to quantify our resulting significant differences, we also determined the effect size Cohen's d. It ranged from \(-0.0045\leq\) d \(\leq 0.384\) for all significant \(p\)-values. After analyzing relevant publications, we assume that our d values \(<0.3\) can be classified as small effects [33; 34; 35], with exception of the measurement of M. biceps femoris when lifting a box (\(p=0.001\)), where the effect size is d = 0.384. In this context, [PERSON] et al. [34] also discuss the influence of sample size on the effect size. They report that a decrease in the effect size is to be expected as the sample size increases. In our case, this means that the measured effect size of M. biceps femoris \(>0.3\) can also be regarded as low, as our sample is a small number of subjects (n = 20) according to ibid.