A new method to determine reflex latency induced by high rate stimulation of the nervous system

Abstract

High rate stimulations of the neuromuscular system, such as continuous whole body vibration, tonic vibration reflex and high frequency electrical stimulation, are used in the physiological research with an increasing interest. In these studies, the neuronal circuitries underlying the reflex responses remain unclear due to the problem of determining the exact reflex latencies. We present a novel "cumulated average method" to determine the reflex latency during high rate stimulation of the nervous system which was proven to be significantly more accurate than the classical method. The classical method, cumulant density analysis, reveals the relationship between the two synchronously recorded signals as a function of the lag between the signals. The comparison of new method with the classical technique and their relative accuracy was tested using a computer simulation. In the simulated signals the EMG response latency was constructed to be exactly 40 ms. The new method accurately indicated the value of the simulated reflex latency (40 ms). However, the classical method showed that the lag time between the simulated triggers and the simulated signals was 49 ms. Simulation results illustrated that the cumulated average method is a reliable and more accurate method compared with the classical method. We therefore suggest that the new cumulated average method is able to determine the high rate stimulation induced reflex latencies more accurately than the classical method.

Keywords: EMG; averaging; cumulated density; human; reflex latency determination.

Figure 1

Determination of the position of the effective stimulus in the force trace for one participant. (A) Averaged force data for each of the six different vibration frequencies. The vertical arrow shows the peak of MUAP that was used as the trigger for averaging the force data. (B) Mean (thick line) and standard error (SE) (dotted lines around the mean trace) for the cumulated average force trace. Thick continuous line shows the mean of the cumulated average force with its minimum value indicated using the white arrowhead. Thick dashed line is the SE for the cumulated average force trace with its minimum value indicated using the black arrowhead.

Figure 2

Cumulated average MMU and SEMG where the local minimum of the force trace was used as the trigger. Reflex latency was indicated by the lowest SE on the cumulative averaged data and shown with an empty circle. (A) Indicates the MMU data and (B) shows the SEMG data. Reflex latency for the MMU data was 38.0 ms and 40.0 ms for the SEMG data.

Figure 3

Reflex latency measurement using cumulated average method. The arrow and open circle shows the position of the effective stimulus point (the lowest value of the SE of force trace). Solid circle shows the onset of EMG response (the lowest value of the SE of MMU trace).

Figure 4

The reflex latencies for different frequencies of WBV and the logic underlying the cumulated average method. The line 1 represents the position of the effective stimulus point and the line 2 represents the reflex latency of the system to that stimulus. Note that the reflex responses are lined up even though the WBV frequencies were different.

Figure 5

The cumulant density analysis for Force-MMU data. (A) Spectral analysis of SEMG data, (B) Spectral analysis of force (vibration stimuli), (C) The coherence analysis shows a significant correlation between SEMG and vibration signals, (D) Phase analysis and (E) Lag time between SEMG and vibration stimuli.

Figure 6

Reflex latency measurement using cumulated average method in simulation data. Open circle shows the lowest value of the SE of force trace (effective stimulus point). Solid circle shows the lowest value of the SE of EMG trace (onset of EMG response).

Figure 7

Lag time between vibration stimuli and EMG response. This figure shows the lag time calculated by using cumulant density analysis for six vibration frequency. The second vertical line corresponds to the lag between the local minima of the force trace and the peaks in the rectified MUAP. Note that the lag time between local minima in force trace and the related MUAP peaks are lined up even though the WBV frequencies were different. The confidence limits are marked with horizontal lines in the graphs.