Coordination of iso- and antidirectionally coupled arm oscillations in the parasagittal plane

At variance with other types of coupled cyclic limb movements, in which all subjects show a clear-cut preference for one coupling mode (e.g., antidirectional coupling for arm adduction-abduction in the horizontal plane), in cyclic flexion-extension movements of the arms in the parasagittal plane, no striking subjective difference is felt when coordinating movements in iso- vs. antidirectional coupling (ISO vs. ANTI). To quantitatively substantiate this phenomenon, we evaluated coordination of parasagittal arms oscillation in both ISO and ANTI coupling by measuring: 1) two well established indexes of coupling accuracy and stability, i.e., respectively, the absolute mean error, |MΔΦ|, and the variability, SDΔΦ, of the relative phase between arm movements; and, 2) the maximal frequency, Fmax, at which the required coupling can be maintained for at least 10 s. Standing subjects (7) were asked to rhythmically oscillate their arms in each coupling modality at 1.0 to 4.2 Hz (0.4Hz steps) in random order. It was found that both |MΔΦ| and SDΔΦ were significantly higher in ANTI than in ISO coupling, indicating that coordination was better in the latter modality. Instead, no significant difference was found for Fmax between the two coupling modes. Comparing the above results with those obtained in horizontal arm oscillation, it is worth noting that: 1) the more coordinated (preferred) mode is ISO for parasagittal movements while it is ANTI for horizontal movements; 2) the loss in stability when passing from the preferred to the non-preferred mode is significantly lower for parasagittal movements (small increase of SDΔΦ and no change of Fmax) than for horizontal movements (large increase of SDΔΦ and large decrease of Fmax). These differences might explain why passing from the preferred to the non-preferred coupling mode is accompanied by a clear cut perception of difficulty in horizontal but not in parasagittal oscillations.