Student Friendly Technique to Demonstrate Coordination between Postural (Involuntary) and Voluntary Muscle Contractions

Abstract

Electromyography is a very useful technique for a number of clinical and research applications in physiology and other life science applications. We have adapted this technique as a student exercise to explore important aspects of postural control. With minimal effort and some mathematical calculations this student friendly technique efficiently demonstrates the interaction of anticipatory, or feedforward, mechanisms and feedback correction from sensory input.

Keywords: RMS; electromyography; feedforward; physiology; postural control; second derivative.

Figure 1.

Proper electrode placement to record EMG from biceps brachii and gastrocnemius muscles. Volunteer will react to an auditory (or other) cue and pull him or herself forward, voluntarily activating the biceps.

Figure 2.

Typical EMG signal after response to audio cue (arrow). The green trace is raw bicep signal, pink trace is raw gastrocnemius. Horizontal axis is time in seconds, audio cue (ding) occurs one second after sampling starts, vertical axis is millivolts.

Figure 3.

Onset of muscle contraction, the top (red) trace is the RMS of the bottom raw (pink) EMG signal. Left arrow indicates audio cue (ding) one second after sampling starts. Right arrow is the onset of EMG activity above baseline. Vertical axis is millivolts for EMG and arbitrary units for RMS; horizontal axis is time in seconds.

Figure 4.

The second derivative provides an excellent visual cue for measuring the onset of EMG signal. The top (light red) trace is the second derivative of the dark red RMS signal on the middle channel. The indicated peak represents the point of maximum change (onset of the contraction). The bottom channel (pink) is raw EMG signal. Left arrow indicates audio cue (ding) one second after sampling starts. Vertical axis is millivolts for EMG and arbitrary units for RMS and derivative; horizontal axis is time in seconds.

Figure 5.

Measuring the time between gastrocnemius onset (light red) to biceps onset (light green) is very intuitive using the second derivative peaks with the data traces overlaid. Left arrow indicates audio cue (ding) one second after sampling starts. Vertical axis is arbitrary units for derivative; horizontal axis is time in seconds.

Figure 6.

The second derivative of the RMS signals from biceps (light green) gastrocnemius (light red) and center of gravity (purple). Arrows indicate onset of each event. Vertical axis is arbitrary units for RMS and center of gravity; horizontal axis is time in seconds.