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 (Δuω) 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., Δω ≠ 0) but also in the covarying low-frequency control gains (i.e., Δk ≠ 0). Because the inverse of the low-frequency 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 Δk ≠ 0). Thus, it is possible that the effects attributed to Δω were in fact mediated by Δk. In 2 experiments, the authors manipulated Δk and Δω separately by applying mass perturbations to the lower legs of 9 participants. The relative phasing between the legs was not affected by Δk, but manipulation of Δω (while Δk 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.
Peper, C. E., Nooij, S. A. E., & van Soest, A. J. (2004). Mass perturbation of a body segment: 2. Effects on interlimb coordination. Journal of Motor Behavior, 36, 425-441. https://doi.org/10.3200/JMBR.36.4.425-441