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Abstract:
The shifts in relative phase that are observed when rhythmically coordinated limbs are submitted to asymmetric mass perturbations have typically been attributed to the induced eigenfrequency difference (DLoM) between the limbs. Modeling the moving limbs as forced linear oscillators, however, reveals that asymmetric mass perturbations may induce a difference not only in eigenfrequency (i.e., DLoM 0) but also in the covarying low-frequency control gains (i.e., DLk 0). Because the inverse of the lowfrequency control gain (k) reflects the level of muscular torque (input) required for a particular displacement from equilibrium (output), asymmetric mass perturbations may result in an imbalance in the muscular torques required for task performance (related to DLk 0). Thus, it is possible that the effects attributed to DLoM were in fact mediated by DLk. In 2 experiments, the authors manipulated DLk and DLoM separately by applying mass perturbations to the lower legs of 9 participants. The relative phasing between the legs was not affected by DLk, but manipulation of DLoM (while DLk remained approximately 0) induced systematic relative phase shifts that were more pronounced for antiphase than for in-phase coordination. That indication that the coordination dynamics is indeed influenced by an imbalance in eigenfrequency is discussed vis-à-vis the question of how such a merely peripheral property may affect the underlying coordination process.
