Superposition of independent units of coordination during pointing movements involving the trunk with and without visual feedback

Previous studies addressing the problem of the control of multiple degrees of freedom have examined the influence of trunk movement on pointing moveme nts within the arm's reach. Such movements may be controlled by two functio nally independent units of coordination (synergies): one involving only arm joints and producing the hand trajectory to the target (the transport syne rgy), and the other coordinating trunk and arm movements leaving the hand t rajectory unchanged (the compensatory synergy). The question of whether or not this functional subdivision depends on visual Feedback; was addressed i n the present study. We also tested whether or not the motor effects of dif ferent synergies are summated as independent components, a control strategy called "superposition." Finally, we investigated whether or not the relati onship between different degrees of freedom within each synergy could be co nsidered linear resulting in proportional changes in different joins angles . Seated subjects produced fast, uncorrected arm movements to an ipsi- or a contralateral target in the direction of +/-45 degrees to the sagittal mid line of the trunk. Targets could be reached using the arm alone (control tr ials) or by combining the arm motion with a forward or backward trunk motio n produced by hip flexion or extension (test trials), with and without visu al feedback. The shape of the hand trajectory, its direction and tangential velocity, movement precision. joint angles and the sequence of the trunk a nd hand recruitment and de-recruitment were measured. In both visual condit ions, the direction of the hand trajectory observed in control trials was g enerally preserved in rest trials, in terms of sequencing, even in the abse nce of vision, the trunk movement was initiated before the onset of and out lasted the hand shift, indicating that the potential influence of the trunk on the hand movement was compensated by rotations in the elbow and shoulde r joint. The analysis of other variables also implied that the effects of t runk recruitment on the hand trajectory were minor compared to those which could be observed if these these were not compensated by appropriate change s in the arm joint angles, it was concluded that an arm-trunk compensatory synergy is present in pointing movements regardless of visual feedback. Pri ncipal component analysis showed that the relationship between elbow, shoul der and hip joint angles in individual arm and combined arm-trunk movements cannot be considered linear, implying that this relationship is adjusted a ccording to the changing arm geometry. The changes in each arm joint angle (elbow, shoulder) elicited by a forward trunk bending in one block of trial s were compared with those elicited by a backward bending in another block, whereas the hand moved to the same target in both blocks. These changes we re opposite but of similar magnitude, As a result, for each moment of movem ent. the mean joint angle obtained by averaging across two directions of tr unk motion was practically identical to that in control trials in which the trunk was motionless, it is concluded that the transport and arm-trunk com pensatory synergies are combined as independent units, according to the pri nciple of superposition. This principle may simplify the control of the coo rdination of a redundant number of degrees of freedom.