## 4 Discussion  
With this work, we investigated the behavior of global muscle co-activation, the one calculated with the rostro-caudal approach and then separating flexors and extensors, during lifting activities under different risk conditions and performed by a single person and in a team. Team lifting is one of the ergonomic strategies suggested in ISO 11228-1 [12] to decrease the exposure of workers to biomechanical risk and could influence lower limb co-activation. As already carried out for the analysis of the trunk [6], to better understand how a two-person team lifting strategy might influence the biomechanical risk, intended as a mechanical risk due to ergonomic risk factors, such as aspects of the job that post a mechanical stress to the employee (i.e., forceful exertion, repetition, awkward or static postures...) and that can cause ergonomic injuries and/or illnesses (e.g., injuries and illnesses of the muscles, nerves, tendons, ligaments, joints, cartilage and spinal discs), we evaluated the effect of two factors: the lifting type (i.e., one- vs. two-person team lifting) and risk level (low LI = 1, medium LI = 2, and high LI = 3). Moreover, we decided to investigate the _TMCf_, because it is already known for the trunk that it is related to the force acting at the lumbosacral level and is sensitive enough to be able to discriminate between the different levels of risk [6, 22, 30]. More in detail, regarding the _TMCf_ maps we found that the activity profiles of the co-activation of the muscles innervated at the level of the sacral segments widens considerably as the risk level increases in one-person lifting, while in team lifting it remains contained. In single lifting, spinal maps demonstrated a propensity toward a greater spread level of the _TMCf_ during most parts of the lifting cycle, initially affecting the sacrum and lower lumbar regions while the risk level increases, while this does not happen in team lifting. Such a pattern highlights how team lifting is an ergonomic tool also effective in reducing co-activation of the lower limb and how this tool contributes to reducing the concurrent activation of the muscles innervated by the distinct spinal levels, both lumbar and sacral. Interestingly, our results are in accordance with what has already been published on myotomal charts [55, 58], which showed that muscles with larger activations include tibialis anterior, peroneus longus, soleus, gastrocnemius medialis and lateralis, biceps femoris and semitendinos. They are innervated from the spinal cord's more distal segments (L4-S2) and have a greater range of activity, mostly involving the sacral segments and then, in more severe neurological patients, the lumbar segments. This type of behavior can be explained in two ways. Voluntary control is pyramidal and is therefore significantly expressed in distal districts. Furthermore, in heavy lifting activities, the kinematic chain remains open for the upper limbs and is closed for the lower limbs. This indicates the necessity to control  
Figure 5: Violin plots of the maximum of the _TMCf_ function over the lifting cycle (Max) over all the subjects for each risk level (LI = 1 green, LI = 2 yellow, LI = 3 red) in one-person and two-person team lifting for each muscle co-activation investigated: global (Max\({}_{glob}\)), full leg (Max\({}_{fill,l,l,gg}\)), extensor (Max\({}_{ext}\)), flexor (Max\({}_{flex}\)), and rostro-caudal organization (from L3 to S2: Max\({}_{L3}\), Max\({}_{L4}\), Max\({}_{LS}\), Max\({}_{S1}\) and Max\({}_{S2}\)). The dotted black lines correspond to the mean of each Max value over all the subjects. An asterisk (\(\blacktriangledown\)) indicates significant differences. the ground reaction force that acts distally on the lower limbs. For the reasons listed above, co-activation increases mainly distally due to the need to stabilize the entire system by responding to the stresses determined by the reaction force.