Abstract
Theoretical and empirical work indicates that the central nervous system is able to stabilize motor performance by selectively suppressing task-relevant variability (TRV), while allowing task-equivalent variability (TEV) to occur. During unperturbed bipedal standing, it has previously been observed that, for task variables such as the whole-body center of mass (CoM), TEV exceeds TRV in amplitude. However, selective control (and correction) of TRV should also lead to different temporal characteristics, with TEV exhibiting higher temporal persistence compared to TRV. The present study was specifically designed to test this prediction. Kinematics of prolonged quiet standing (5 minutes) was measured in fourteen healthy young participants, with eyes closed. Using the uncontrolled manifold analysis, postural variability in six sagittal joint angles was decomposed into TEV and TRV with respect to four task variables: (1) center of mass (CoM) position, (2) head position, (3) trunk orientation and (4) head orientation. Persistence of fluctuations within the two variability components was quantified by the time-lagged auto-correlation, with eight time lags between 1 and 128 seconds. The pattern of results differed between task variables. For three of the four task variables (CoM position, head position, trunk orientation), TEV significantly exceeded TRV over the entire 300 s-period.The autocorrelation analysis confirmed our main hypothesis for CoM position and head position: at intermediate and longer time delays, TEV exhibited higher persistence than TRV. Trunk orientation showed a similar trend, while head orientation did not show a systematic difference between TEV and TRV persistence. The combination of temporal and task-equivalent analyses in the present study allow a refined characterization of the dynamic control processes underlying the stabilization of upright standing. The results confirm the prediction, derived from computational motor control, that task-equivalent fluctuations for specific task variables show higher temporal persistence compared to task-relevant fluctuations.
Highlights
Both theoretical and empirical evidence indicates that the central nervous system is able to exploit motor equivalence in order to stabilize motor performance [1,2,3,4]
The present study aims at clarifying the relation between motorequivalent and temporal structure of postural fluctuations during prolonged bipedal quiet standing without visual feedback
Based the uncontrolled manifold (UCM) [3] and the covariation by randomization method (COV) [21] method, the motor-equivalent structure of postural fluctuations was characterized with respect to four hypothesized task variables previously described in the literature [8,20]: anterior-posterior center of mass (CoM) position, head position, trunk orientation and head orientation
Summary
Both theoretical and empirical evidence indicates that the central nervous system is able to exploit motor equivalence (the abundance of biomechanical degrees of freedom, DOF, over task variables) in order to stabilize motor performance [1,2,3,4] This could be achieved by a biological control scheme selectively suppressing task-relevant variability (TRV) while allowing taskequivalent variability (TEV) to occur. Selective control of task-relevant deviations, as proposed by dynamic models of multi-DOF coordination [4,6,7], should lead to different temporal structures of TEV and TRV As such a control scheme exerts tight control on TRV while allowing TEV to accumulate (within functional constraints), TEV should exhibit higher temporal persistence compared to TRV. The present study was designed to test this prediction for the sensorimotor task of unperturbed bipedal standing
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