Interactions between interlimb and intralimb coordination during the performance of bimanual multijoint movements.

The simultaneous performance of movements involving different effectors gives rise to neural and biomechanical interactions between and within limbs. The present study addressed the role of interlimb and intralimb constraints during the control of bimanual multijoint movements. Thirteen participants performed eight tasks involving the bilateral elbows and wrists under different coordination conditions. With respect to interlimb coordination, coordination patterns referred to the in-phase and anti-phase coordination modes, involving the simultaneous timing of homologous versus non-homologous muscles, respectively. With respect to inter-segmental (intralimb) coordination, the isodirectional mode referred to simultaneous flexions and extensions in the ipsilateral wrist and elbow joints, whereas the non-isodirectional mode involved simultaneous flexion in one joint together with extension in the other joint, or vice versa. The analysis of the data focused upon measures of relative phasing between proximal and distal joints within a limb as well as between the homologous joints of both limbs. With respect to interlimb coordination, findings revealed that adoption of the in-phase mode resulted in a higher quality of interlimb coordination than the anti-phase mode. However, the mode adopted in the distal joints had a larger impact on the quality of interlimb coordination than the mode adopted in the proximal joints. More specifically, in-phase coordination of the distal joints had a positive, and anti-phase coordination a negative, influence on the global coordinative behavior of the system. Minor effects of intralimb coordination modes on interlimb coordination were observed. With respect to intralimb coordination between the ipsilateral elbow and wrist, the isodirectional mode was performed with higher stability than the non-isodirectional mode. The mode of interlimb coordination also affected the quality of intralimb coordination, such that generating anti-phase coordination patterns in the distal joints had a negative influence on the accuracy and stability of intralimb coordination. Taken together, the present findings suggest a hierarchical structure whereby interlimb coordination constraints have a stronger impact on the global coordinative behavior of the system than intralimb coordination constraints. Moreover, the global coordinative state of the system is more affected by the coordination between the distal than between the proximal joints. Overall, the findings suggest that the mirror-image symmetry constraint has a powerful influence on bimanual multijoint coordination.