# Biomechanical Effects on Lower Extremities in Human-Robot Collaborative Agricultural Tasks
## 4 Discussion
The increased neural activation combined with the accumulated energy while stretching the muscles possibly enhances the participants' effort during the concentric phase of the lifting task at the higher deposit heights (phases 2 and 3). As regards the maximum knee flexion moment, this was noticed during the last phase while the participants placed the crate onto the custom-made shelf. In this phase, the higher reported values took place for the smaller height, namely at 70 cm, while their mean values were approximately 9.89% and 10.99% larger than the corresponding ones for 80 and 90 cm, respectively. This finding implies that knee loading is higher while lifting the crate at the lowest deposit height. Thus, possible knee discomfort or injury may occur, as highlighted by [28]. In the same vein with the knee extensors, the experimental measurements revealed some alterations for different heights regarding ES, the group of muscles extending the vertebral column. Similar to the findings of relevant studies such as [15], which focused mainly on the trunk kinetics and kinematics during symmetric load lifting, as the deposit height increases so does the ES activity (Figure 5c). This was an anticipated result, as the increase of load height causes greater trunk flexion and an additional moment contribution as a result of the upper body mass [14, 69]. Concerning the maximum hip extension moment, it was larger during phase 3 for the case of the smallest load height (Figure 4b). More specifically, it was 8.96% and 17.16% larger than the corresponding moments for the heights of 80 cm and 90 cm, respectively. Finally, no noteworthy alterations were observed for the ankle joint and the remaining biomechanical variables, at least for the three examined heights. Overall, the results of the present study were within the range of the values measured by [PERSON] et al. [71], who examined the differences between squat and stop lifting with three different weights, however, by focusing only on the second phase of the lifting task (as described in Section 2.2). The present experimental findings can provide useful information when a case like the one examined here is designed. Representative agricultural applications are the cases of a UGV functioning as a \"bin dog\" with the aim of carrying apple bins during harvesting [49] and as illustrated in Figure 1 for carrying walnuts. Based on the above results, the deposit height of the crate is a considerable determinant not only of spine loading [14, 72] but also for knee and hip joints, while ankles appear not to be particularly affected. Arguably, there are a few limitations towards the generalizability of the present experimental results that must be stressed. First, the experimental measurements took  
Figure 5: Bar charts showing the values (\(\pm\)mandard deviation) of the statistically important alterations: (**a**) Peak EMG of VM, (**b**) Mean EMG of VM, and (**c**) Peak EMG of ES during phase 1 for the three different deposit heights along with the standard deviation. place in a physiology laboratory so as to avoid visual occlusion problems, based on the available equipment and authors' expertise. Acquiring data from real-time application on the field would definitely have provided more realistic measurements. Second, as described in the second section, the experiments considered a load equal to 20% of each participant's body mass, similar to relevant studies [16,43]. This corresponds to an average load of 15.52 \(\pm\) 1.84 kg, which might be lower than the ones required for some agricultural lifting tasks, thus, underestimating the muscle activation profiles as well as the kinetic and kinematic ones. Thirdly, the definition of an experienced worker is not clear in the literature [69]. Nevertheless, the present incorporated criterion (at least two years of experience) avoids using participants with no skills in manual material handling which can result in misleading conclusions [19].