Revisiting the Sternocleidomastoids: Accessory Muscles of Respiration

We know that sternocleidomastoids (strap muscles of the neck), scalene muscles and alae nasi are considered accessory muscles of breathing, although some controversy exists. If you saw an asthmatic individual in his desperate attempts at breathing or an agitated person or a person exercising vigorously, you could watch these muscles in action. However, that wasn't exactly what I had in mind while I was doing this experiment. I was really thrilled to chance upon it. Kind of a serendipitous discovery in its own right. You too can figure this out easily. The placement of the surface leads were as shown on the right. Red dots were for the reference and recording electrodes whereas the blue dot represents the ground (as usual). 

The neck muscles of each side were tested one at a time. For example, This wav file (open it Audacity or BYB neuron recorder) was obtained from EMG recording from the right sternocleidomastoid; the leads were placed on the right side and the ground lead on the manubrium. The head was at mid-line (neutral position) to start with. Next, it was turned to the right, then midline again, then to the left (without any external resistance applied), then against resistance applied by the left hand. The muscle of the right side moves the head to the left. Finally, the head was again restored in its neutral position. An improvised notation could be N to R to N to L to L+ to N (Legend: N=neutral, R = right, L=left, L+ = left against resistance). 

Similarly, the left side was tested in a likewise manner (N to L to N to R to R+ to N). And this is the waveform obtained. The surprise awaited me, I was in for an ambush!

While the leads were still on the strap muscles of the left side of my neck, I observed the EMG waveform in real-time. I noticed that as I was taking a deep breath, the EMG activity increased significantly. I didn't have to turn to my head to the right anymore! I then maintained this position by holding my breath. The activity continued. Here's the recorded .wav file. The associated camera recording will speak for it (see below: Youtube). Yes, it proved that it was indeed an accessory muscle called in to address forceful inhalation. I then did a forceful exhalation, but no increase in EMG activity was observed, buttressing my observation. 



As you may have noticed that I have used the terms inhalation and exhalation, in lieu of, inspiration and expiration. Well, inspiration, it definitely is! Expiration? No way!
Further analysis still awaits.
Spikerbox recordings may also possibly illustrate simultaneous EMG activities in protagonist and antagonist muscles. More of these later.

To be continued............

This work by Amiya Sarkar is licensed under a Creative Commons Attribution 4.0 International License.

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