## 4 Discussion
We observed enhanced _TMCf_ of these muscles with the risk level in the one-person team lifting and a strong mitigation of this effect in the two-person team lifting. Co-activation widening may potentially be a compensatory technique, as prolonging co-contractions has been shown to stabilize joints [6; 22; 32; 33]. Regarding synthetic indexes computed over the co-activation maps, we considered the mean and the maximum value of the lifting cycles. The _CI_ is the mean of the co-activation function over the lifting cycle, and it has been chosen because it is connected with the average level of _TMCf_ during the lifting cycle, hence it provides information about the overall task execution. The Max over the lifting cycle is a timely index that indicates the maximum value of antagonist muscle activation while lifting. It has been proven that, in terms of the trunk, it relates to peak loads that can produce severe spinal injuries, resulting in degeneration and pain [59]. In the obtained results we can observe that, in the global full leg and extensors approach there is a significant increase in _CI_s as risk levels increase in both one-person and two-person team lifting. Regarding the _Max_, the same results emerge in the approach that takes into consideration only the extensor muscles. Considering the flexors there are no statistically significant differences in terms of _CI_ and _Max_. This is understandable, as the flexor muscles play a less significant role within the task under consideration, unlike the role of the extensor muscles which generate the necessary moment concentrically in lifting and counteract the external moment of lowering by contracting eccentrically. In fact, the full leg and extensor approach shows a significant reduction of _CI_ at all risk levels and of _Max_ at LI = 3 for the Max in two-person compared to the one-person team lifting. Moving on to the rostro-caudal approach, results between LI pairs are observed in muscles innervated at the L3 level, while the co-activation increases significantly again proceeding towards L4 At L5 co-activation is significantly reduced in two-person compared to the one-person team lifting, up to levels S1 and S2 in which there is a similar behavior to that observed in co-activation with a global approach. These results are in relation to what we have already found for the trunk both in the case of lifting performed individually and in teams [6; 22; 31]. Furthermore, our findings are consistent with the necessity for the CNS for greater co-activation, and therefore the rigidity of the lower limb, to cope with greater efforts and gain stability. Finally, the fact that in teams the co-activation at the same level of risk is almost always lower than that which occurs in single lifts, shows that the need to coordinate between subjects does not affect the ability of individual coordination. Our findings indicate that the CNS streamlines motor regulation of lifting by adjusting whole-limb stiffness based on risk level and lifting type. Our findings indicate that the CNS reduces motor control of lifting by adjusting whole-limb stiffness based on risk level and lifting type. The first limitation of this study is that the electromyographic activity of only one of the two subjects of the team was investigated and, in the future, it will be necessary to investigate both the involved subjects; then, the study is still based on a small number of participants, so another need is to increase the sample size; together with the expansion of the examined sample, it will be possible to analyze the data differently by gender, as in this case, for the few subjects available, we have mixed males and females with different anthropometric characteristics, as well as leg extensor muscle and back extensor muscle strength levels, which is an additional limitation of the study. Furthermore, it will also be necessary to evaluate the case of asymmetric lifting in which the rotation of the trunk must be taken into consideration.