4.1.9 Division of labor between biceps, triceps, and forearm

Before undergoing the Conductor’s Jacket experiments, I thought that there would be a tremendous division of labor between the muscles of an arm. This was partly because of my training as a conductor; I had been told to use the different segments of the limb differently, even treating the baton as a related but independent appendage. Sir Adrian Boult, in his well-known treatise on conducting technique, wrote:

"We have seen that the stick should be regarded as an Extra Joint. There will therefore be four pivots in a conductor’s arm: knuckles, wrist, elbow, and shoulder. It is perfectly possible for the rhythm and expression to be clearly shown in a piano or pianissimo passage by means of the fingers alone…for the heaviest fortissimo effects the whole arm will swing from the shoulder…a proportion must always be kept between the movements of the different joints. The point of the stick must travel farther than the fingers and, as it were, round the fingers, the fingers farther than the wrist, the wrist farther than the elbow and the elbow round the shoulder, which can itself never move." The data I collected turns out to partly support Boult’s description, particularly between the different articulated segments of the shoulder, upper arm, forearm, and wrist. Within an articulated segment, however, most muscles yield similar signals; for example, the biceps and triceps muscles tend to generate signals that resemble each other, with small time differentials that can usually be accounted for by the activation of oppositional forces. However, in P3, the only subject for whom I collected forearm data, I got to notice a few occasional examples of different signals between the biceps and forearm muscles. Sometimes the forearm gave different information from the bicep that appeared to represent articulation; this tension pattern in the forearm reflected the use of his wrist. Therefore, one key distinction between the uses of different muscle groups is that the muscles of the upper arm generate beats and amplitudes, while the muscles of the forearm control the articulations and lengths of the individual notes.

For example, in the Love Theme from the movie "Titanic," (the only piece in P3’s program which was uniformly legato in style), there were several places where the amplitude of his right forearm EMG signal was larger than that of his right biceps. Also, there were several places where the forearm gave different information from what the biceps did; this was extremely unusual. At the very beginning of the piece, the biceps signal was almost nonexistent, and the largest signal came from the right forearm, the muscle that most significantly influenced the use of the baton. This example is given below in Figure 36; at the end of this segment, the forearm gives large beats that usher in the main theme of the piece:

Figure 36. P3’s forearm signals eclipse the biceps signals during legato passage

The only apparent reason why the forearm would generate a larger signal than the bicep is that the articulation of the music required it; that is, during a legato, the forearm uses sustained contractions to demonstrate the sustained nature of the music. While the bicep signal gives the beat and overall volume information, it seems as if the forearm demonstrates more about the nature of the articulations and the ‘character’ of the music. This phenomenon is observable visually and palpable viscerally, but not easily quantifiable. From these results, it seems that a forearm EMG measure is more useful than a triceps measurement; the biceps signal, however, is essential. It’s not clear if more than two EMG sensors per arm would yield useful results for larger, conducting-style arm motions.

  Chapter 4.1.10