# Lifting Activities Assessment Using Lumbosacral Compression and Shear Forces
## 4 Discussion
In contrast, in the tasks with LI = 3, the vertical position of the load is further from the subject's trunk, being close to the floor (V = 30 cm, see Figures 1 and 2). In this case, the subjects had to move closer to the load using the strategy they deemed appropriate, either bending the legs or the trunk (the task execution strategy, in fact, was self-selected). Therefore, when normalized forces are considered with respect to the value before performing lifting, these are found to reduce the effect of posture, and the values increase with LI (Figure 3c). The effect of the posture in estimating forces was shown by [PERSON] and colleagues [29]: they have shown that the compression and shear forces at the L5-S1 joint in the symmetric lifting task range from 568 N to 5474 N and 221 N to 1924 N depending on trunk flexion angle, the lumbar/pelvis ratio, the load weight and the load position [29]. The results of our study show that the range could be a good parameter to classify the risk. Indeed, it increases with LI in all considered forces, also reducing the effect of the posture present in the other parameters for \(CF_{L5-S1}\). Another important result of this study is the similar results obtained with these two different models. In particular, the results obtained with the upper model could be useful for evaluating these forces in work environments as well. In fact, while the lower model requires the use of platforms to evaluate the forces to assess GRF, the upper model does not require such forces. Therefore, these forces could be estimated only from kinematic data measured, for example, by IMU in the workspace. Indeed, the promising use of these wearable sensors and algorithms for an instrumental biomechanical risk assessment was shown [18]. Future studies could involve the use of risk assessment tools directly in real work life, including indices based on compression and shear forces on L5-S1 that can be measured directly in the field by using only IMUs. Furthermore, these indices could be used to enrich the existing trained artificial intelligence algorithms, allowing biomechanical risk assessment in lifting tasks. Therefore, the findings confirm our hypotheses that the shear and compression forces and the kinetic-based indices extracted from them are related to risk conditions: there are statistically significant differences among LI pairs. These findings could be used to enrich the indices used in quantitative tools and to improve machine-learning approaches [6; 15], which can also be used in the workspace and to assess biomechanical risk conditions during lifting tasks. The limitation of this study could be the dimensionality of the investigated sample. Indeed, even if the current sample size had been chosen to achieve adequate statistical power (the statistical power for this sample was 0.83) [30], increasing the sample could strengthen the results obtained. Furthermore, another experiment with a larger sample would allow the recruitment of female subjects, which would also allow an analysis of gender differences. Indeed, it has been shown that there is a different risk of developing LBD between men and women due to various factors [31; 32; 33; 34]. It will be interesting to compare these forces with those estimated through neuromuscular models [35; 36; 37]. Another limitation of this study is related to the fact that the participants only performed one task for each LI category, and this did not allow the assessment of force variations associated with any changes in the tasks with the same LI. Therefore, future development will be to investigate compressive and shear forces during the execution of different tasks with the same LI. **Author Contributions:** Conceptualization, T.V., G.C., M.S. and A.R. ; methodology, T.V., G.C. and A.R. ; software, T.V. ; validation, T.V., G.C., M.S. and A.R. ; formal analysis, T.V. ; investigation, T.V. and G.C. ; resources, T.V., G.C., M.S. and A.R. data curation, T.V. ; writing--original draft preparation, T.V., G.C. and A.R. ; writing--review and editing, T.V., G.C., M.S.