Tri-dimensional and triphasic muscle organization of whole-body pointing movements.

Previous kinematic and kinetic studies revealed that, when accomplishing a whole-body pointing task beyond arm's length, a modular and flexible organization could represent a robust solution to control simultaneously target pointing and equilibrium maintenance. Here, we investigated the underlying mechanisms that produce such a coordinative kinematic structure. We monitored the activity of a large number of muscles spread throughout subjects' bodies while they performed pointing movements beyond arm's length, either with or without imposition of postural or pointing constraints. Analyses revealed that muscle signals lied on a tri-dimensional hyper-plane and were temporally organized according to a triphasic pattern (three components, each one exhibiting one single peak of activation and the peaks being consecutive in time). Such a functional muscle synergy was found to be robust across conditions. Also the activities of the separate groups of muscles acting at each body joint resulted tri-dimensional. In particular, those associated with the muscles of the lower-body joints (ankle, knee and hip) always presented the three sequences in all conditions. However, a slightly different organization was found for the muscle activities of the upper-limb, suggesting a moderate level of flexibility of the activity of such muscles to movement constraints. The present findings link together, in a hierarchical view of motor control, the joint coordination characterizing whole-body pointing movements with a basic muscle synergistic organization, namely a triphasic pattern.