Coordination of coupled arm oscillations in the parasagittal plane

The present study evaluates the coordination of iso- and antirectionally coupled arm oscillations in the parasagittal plane (P-ISO and P-ANTI). Standing subjects (7) were asked to rhythmically oscillate the arms in each coupling modality at 1.0 to 4.2 Hz (0.4Hz steps) in random order. Coordination was quantified by the absolute mean error of the relative phase between arm movements (|MΔΦ|), an index of accuracy, and the corresponding variability (SDΔΦ), an index of coupling stability. It was found that both |MΔΦ| and SDΔΦ were much higher in P-ANTI than in P-ISO, indicating that coordination is better and easier in the latter modality. The preference for ISO coupling is opposite to the preference for ANTI which is observed for cyclic arm adduction-abduction in the horizontal plane. Moreover, the difference in stability (Δ SDΔΦ) between the preferred and non-preferred coupling is larger for horizontal than for parasagittal movements and grows as the frequency increases. Further, the maximal frequency at which a correct coupling can be maintained is similar for P-ISO and P-ANTI (3.85 vs 3.64 Hz), while it is quite different between H-ANTI and H-ISO (4.23 vs 2.52 Hz). The large difference in stability between H-ANTI and H-ISO is linked to a proportionally high difference in the related postural effort. Preliminary measurements indicate that, correspondingly, the small difference in stability between P-ISO and P-ANTI is linked to a decreased difference in postural effort.