# Development and Biomechanical Evaluation of a Back Exoskeleton to Support Lifting Tasks
## 5 Discussion
### Influence on Kinematics and Muscle Activity  
The biomechanical evaluation of the exoskeleton focused on its effect on the muscle activity of the m. erector spinae during a lifting task and its impact on hip and knee joint kinematics. The results indicate a 14% reduction in muscle activity when using the exoskeleton compared to the baseline condition without support. This reduction suggests that the exoskeleton effectively offloads the lower back muscles, potentially lowering the risk of muscle fatigue and long-term strain-related injury. Such a reduction is particularly relevant for healthcare workers who frequently perform physically demanding tasks, such as lifting and repositioning patients, which expose them to a high risk of low back pain and musculoskeletal disorders [34]. Other exoskeletons designed for caregiving support have shown similar results [35; 36]. Greater reductions in lumbar muscle activity have also been reported depending on different exoskeleton designs and actuation methods [7; 37]. The combined weight of the participants and the lifted load likely contributed to a lower reduction in musculoskeletal strain than initially calculated in the design requirements, which aimed for 30% relief. The required actuator force was calculated based on the gravitational forces of the individual body segments involved in the lifting motion. However, only 30% of the required force was considered in the exoskeleton design to provide supportive assistance. It is important to note that the applied actuator force does not directly translate into an equivalent reduction in muscle activity, as the biomechanical effects depend not only on the external supportive force but also on the complex interactions between active and passive musculoskeletal structures [38,39]. The discrepancy between the calculated force and measured muscle activity reduction highlights the importance of comprehensive biomechanical evaluations to assess the actual unloading effect and optimize the system performance across different users and task conditions. Interestingly, the analysis of hip and knee flexion in the sagittal plane showed no significant difference between the conditions with and without the exoskeleton. This suggests that the device does not restrict movement patterns and allows the wearer to maintain their typical lifting technique regarding the analyzed degrees of freedom. From a design perspective, this is a positive outcome as it confirms that the exoskeleton aligns with the user's motions without introducing unwanted compensatory movements that could lead to discomfort or other musculoskeletal issues. Other exoskeletons have shown reductions in the knee and hip range of motion, as well as reduced trunk flexion during the lifting phase [7]. However, as this study was conducted with a relatively homogeneous and small sample size of only 10 participants, the generalizability of the findings is limited, particularly in the context of clinical applications. While initial insights into biomechanical effects and user acceptance could be obtained, the sample size restricts the statistical power of the analysis and may not capture the variability present in broader or more diverse populations. This limitation is especially relevant when considering clinical populations, which may differ substantially in terms of physical condition, age, or motor capabilities. To address this, future research should include a larger number of participants across different demographic groups and use cases, including individuals with musculoskeletal or neurological impairments. Such studies would allow subgroup analyses and a better understanding of how user characteristics influence device interaction. Additionally, longitudinal studies could provide valuable information on learning effects, long-term usability, and user adherence over time. Moreover, different lifting conditions, such as higher loads, repetitive lifting tasks, and asymmetric lifting, should be tested to evaluate the exoskeleton's effectiveness across various real-world scenarios.