Time color map and histogram of electromyography (EMG) sample amplitudes: possible tools for global electromyogram analysis by images

Abstract

To present a method for surface electromyogram (EMG) analysis by images. 12 healthy sedentary males were studied. Surface EMG (from lateral deltoid) was recorded (1000 samples/s) and analysed during self-selected angular speed shoulder abduction/adduction actions, made at 40% of 1 revolution maximum, up to exhaustion. EMG was rectified and normalized to its maximum (100%). Histogram of sample amplitudes: the 0–100% amplitude range was divided in 20 levels (step 5%). Each sample was assigned to its bin for the histogram generation 2D time color map: the amplitude range was divided in five colored levels: blue (0–20%), green (21–40%), yellow (41–60%), orange (61–80%), red (81–100%). Along the time bar each ms was colored according to the sample relative amplitude. The whole colored bar was divided into 100 ms slices. These short color bars were graphically stacked one on top of each other and interpolated on the x/y plane using a standard contour plotting procedure. The spike shape analysis (SSA) was performed and the average rectified value (ARV) was calculated. From the first to the last action the histogram median amplitude increased from 34.33 ± 12.36 to 46.74 ± 14.04%. On the 2D map the areas of activity hues enlarged in both directions showing more regular recurrence of longer electrical events. As expected, the ARV increased and the SSA reflected the spikes changes at fatigue. Histogram of the EMG sample amplitudes and 2D time color map provide data, at a glance, about the regularity of the EMG spikes occurrence and their amplitude distribution. This is a different, adjunct information with respect to the one provided by traditional EMG time domain analysis measurements.