###### Abstract  
Exoskeletons potentially reduce physical strain on workers. However, studies investigating the long-term effects of exoskeletons in the workplace are rare, not least because demonstrating physical long-term impacts faces several challenges, including the collection of reliable biomechanical data with the exoskeleton. By examining the potential impact of using an exoskeleton on inter-day measurements, we can provide valuable insights into the suitability of long-term studies. Therefore, this study aims to investigate the inter-day variation in muscle activity (MA) and kinematics of the trunk and legs during lifting, carrying, walking, and static bending with and without a passive back exoskeleton. The majority of results show no significant differences in inter-day variation. However, we found minor significant unilateral variation in knee and ankle kinematics when using the BSE during the lifting, carrying, and walking tasks, as well as in MA of. biceps femoris when measuring without the BSE during the lifting tasks. [PERSON]'s d showed small effect sizes, ranging from \(-0.0045\leq\mathrm{d}\leq 0.384\) for all significant \(p\)-values. While we classify the observed significant differences as minor, it is still crucial to consider day-to-day variations in long-term studies. However, by implementing high levels of standardization in study designs, including precise exoskeleton fitting, consistent assistance level, familiarization with measurement systems, and standardized working tasks, the impact of the exoskeleton on inter-day measurements can be minimized. Additional field studies are necessary to validate our findings in real work conditions. occupational health; reproducibility of results; statistical parametric mapping; muscle activity; kinematics +
Footnote †: journal:  
The most important prerequisite for conducting a biomechanical long-term study is the use of measurement parameters and systems, which can provide comparable results at different points in time. In addition to others, the measurement of MA using surface electromyography (sEMG) and the assessment of movement patterns using wearable sensors, such as inertial measurement units (IMUs), are suitable for this purpose [10]. Both measurement systems enable the comparison of workloads and movement patterns between different individuals and days [11; 12] and are reported as being reliable during static [9; 13; 14] and dynamic conditions [9; 15; 16]. In order to ensure inter-day reliability of measurement with the described systems, a high degree of execution standardization is required, e.g., consistent fitting and support of the exoskeleton, familiarization with the work task, as well as the normalization task of sEMG measurements [17], and accurate placement of sEMG and IMU sensors [18]. However, even with high standardization, we cannot exclude the exoskeleton-user interaction as a factor influencing the measurements reliability. An exoskeleton, as a rigid mechanical system, has the same effect on the user's body at all times during standardized use. However, we cannot rule out the possibility that the interaction between the user and the exoskeleton may vary on inter-day measurement due to intrinsic factors, which would skew the results. Results of biomechanical studies using exoskeletons vary widely even when using similar or the same exoskeletons [19], which can also be explained by individual user characteristics [10]. Several investigations have already demonstrated significant biomechanical inter-day variation in dynamic and static work tasks without an exoskeleton [16; 20; 21; 22] using methods such as interclass correlation coefficient (ICC), coefficient of variation (CV) and standard error of measurement (SEM). However, these methods do not enable such a detailed analysis, as required for clinical inference, or the determination of reliability at different stages of a work movement, because they are pooling discrete time points (e.g., peaks) or the entire time series (e.g., root mean square or mean frequency).