No abstract

Spike train coherence is used to characterize common inputs that drive motor unit synchronization. However, data segmentation, overlap, and taper can affect coherence magnitude, thereby influencing the incidence at which significant coherence is detected. Also, the effect of spike train firing rate and common input variability on the detection of significant coherence is unknown. We used a pool of simulated synchronized spike trains with various firing rates (7-19 Hz), coefficients of variation (CV) (0.05-0.50), common input frequencies (10, 20, and 30 Hz, CV: 0.05-0.50), trial durations (30, 60, 90 and 120 sec.), and synchronization strength to explore the effects of segment length (1024 and 2048 1-ms samples), tapering (Hann, Nuttall, and rectangular), and overlap (0, 37.5, 50, 62.5, and 75%). Tapered segments overlapped by at least 50% maximized coherence, regardless of taper type. Coherence for 30-second trials revealed significant coherence for less than half of the motor unit pairs, demonstrating the advantages of longer trails. 2048-sample segments produced similar coherence values with twice the frequency resolution. Increasing the common input variability from 0.15-0.50 reduced coherence incidence by approximately 60%, indicating that synchronized physiological motor unit pairs may fail to show significant coherence if the common input frequency is sufficiently unstable.

No abstract

A reliable stability metric is needed to identify fall susceptibility that better addresses fall prevention and improves gait rehabilitation. One available metric is the margin of stability (MOS), which is calculated using the body’s center of mass (COM) position and velocity along with the location of the maximum center of pressure (COPmax). However, the maximum MOS (MOSmax) has several limitations that may limit stability assessment because of its assumptions of fixed COPmax and constant ground reaction force (GRF), which are not applicable to gait. To address these limitations, a modified MOS equation that includes COP position and velocity and is not dependent on a constant GRF is presented here. The MOS determined by this equation was significantly lower than MOSmax throughout most of single limb support for normal walking gait. This finding indicates the MOSmax metric may lack sensitivity to instability as it may still be positive when the actual MOS indicates existing or impending instability. This comparison also showed that the MOS might offer additional information about walking stability relevant to gait assessment for fall prevention and rehabilitation. However, like other stability metrics, this capability must be established with further investigations of perturbed and pathological gait.