Postural adjustments during coupled movements of the arms in the horizontal plane

Introduction: It is common experience that when moving two body segments at the same time movement freedom is limited. For instance, rhythmic oscillations of the upper limbs in the horizontal plane are easily coupled when movements are mirror-symmetrical (i.e., anti-directional in the plane of motion) whereas constraints are experienced when movements are iso-directional. Moreover, as the frequency increases beyond a critical value, a spontaneous transition from the iso- to the anti-directional coupling occurs in a short time, the higher the frequency the earlier the transition (see [1] for a review). We recently observed that when a subject performs both unidirectional and cyclic movements of the right arm in the horizontal plane (adduction-abduction) while keeping the left hand in contact with a firm support, excitation of the prime mover muscles of the right arm is accompanied by a significant activation of the contralateral homologous muscles [2]. These contralateral activities are featured as Anticipatory Postural Adjustments (APAs), i.e., they regularly precede the prime movers bursts and exert forces that counterbalance the rotatory action exerted on the trunk by the primary movement. It may thus be argued that, when the two arms are moved together, in the prime mover of each limb the voluntary commands will converge with the APAs associated with movements of the contralateral limb. In particular, the voluntary and postural inputs will have the same sign during anti-directional and the opposite sign during iso-directional movements, in this way favouring the mirror symmetrical with respect to the iso-directional coupling. In the present study we tested this hypothesis by analysing the postural actions associated with coupled movements of the upper limbs in the horizontal plane. Methodology: In standing subjects (n=6), we measured the electromyographic and mechanic postural actions elicited when performing fast unidirectional or cyclic movements, in the horizontal plane, of i) one single arm and ii) both arms, iso- or anti-directionally coupled. Surface electromyograms were recorded from left and right Pectoralis Major (LPM, RPM: arm prime movers), and from left and right Ischiocruralis muscles (LIC, RIC: representative links of the fixation chain connecting trunk to ground). The reaction torque about the body vertical axis discharged to ground (Tz) was measured by a force platform. Arm movements were recorded by optoelectronic tracking of infrared markers. Results: During fast adduction of the right arm in the horizontal plane (prime mover RPM) APAs were recorded in the contralateral LPM as well as in the RIC muscle. This last action was associated with a transient increase of Tz in the direction opposite to arm rotation. Both the APAs in RIC and the Tz changes nearly doubled in size when the arms were coupled iso-directionally (adduction of one arm and abduction of the other) while they vanished when both arms were simultaneously adducted (anti-directional coupling). Conformably, during rhythmic arm oscillations APAs and Tz were cyclically modulated when movements were iso-directional, the modulation amplitude being strongly enhanced by increasing the movement frequency. Instead, when oscillations were anti-directional neither APAs nor Tz changes were observed, even if frequency was incremented. Discussion and Conclusion: During anti-directional movements, APAs in prime movers are synergic with voluntary activation and no postural engagement is requested to leg muscles. Conversely, during iso-directional movements, APAs in prime movers conflict with the voluntary commands, thus APAs should be actively suppressed to perform the movement. Moreover, a strong postural effort is required to leg muscles. Both the neuronal effort required to suppress the arm-arm APAs and the leg muscle effort grow as the frequency increases. It is plausible that this effort might undergo ‘fatigue’ phenomena which lead to the spontaneous transition from iso- to anti-directional coupling. The interference between postural and voluntary commands converging onto the motor pathways of the moving limbs and the necessity to discharge to ground the postural perturbations, both occurring during iso-directional limb movements, may co-operate in determining the preference for anti-directional vs. iso-directional coupling. [1] Swinnen SP. Nature Review Neuroscience 2002; 3: 348-359. [2] Baldissera F, Rota V, Esposti R. Exp Brain Res 2008; 185: 63-74.