Movement coordination and postural adjustments during coupled oscillations of the arms in the parasagittal vs. horizontal plane

Objectives: Cyclic adduction-abductions of the arms in the horizontal plane are better coordinated in antidirectional (ANTI) than in isodirectional (ISO) coupling. We recently suggested that the coordination deficit in ISO depends on the interference exerted by the Anticipatory Postural Adjustments (APAs) associated with the arms voluntary movements. Changing the plane of arm motion from horizontal to parasagittal modifies the APAs organization and, in parallel, affects movements coupling so that ISO becomes better coordinated than ANTI. This study analyses arm oscillations in both parasagittal and horizontal planes in one and the same group of subjects, so as to assess if in either case a similar linkage exists between the coupling coordination and the APAs distribution and intensity. Methods: Standing subjects (7) rhythmically oscillated their arms (1.0 to 4.2Hz, 0.4Hz steps), ISO and ANTI coupled, in both horizontal and parasagittal plane. Coordination was evaluated by measuring the absolute mean error, |MΔΦ|, and the variability, SDΔΦ, of the relative phase between arm movements. APAs were evaluated by recording the antero-posterior force, Fy, and the torque about the body vertical axis, Tz, exerted on the ground. Results: |MΔΦ| and SDΔΦ increased (i.e., coordination worsened) when passing from ANTI to ISO horizontal movements and from ISO to ANTI parasagittal movements. The coordination loss (increase of SDΔΦ was larger for horizontal than parasagittal movements. When passing to the opposite coupling modes Tz increased in parallel with SDΔΦ to a larger extent in horizontal movements, well fitting the higher coordination loss in the latter. Conversely, Fy, was null in both ANTI and ISO horizontal movements while it was somewhat larger in ISO than ANTI parasagittal movements, However, the energy cost for increasing Fy in parasagittal ISO was found to be much lower than that for increasing Tz in parasagittal ANTI which thus remains the more disturbed coupling mode. Conclusions: In both parasagittal and horizontal movements an increase of SDΔΦ (loss of coordination stability) occurs in the coupling mode that requires a postural effort for generating Tz, the larger Tz the higher the stability loss.