Guitar Anatomy:

The First Dorsal Interossei

The 1st Dorsal Interossei is the largest of the 4. It is responsible for "abduction" of the index finger, which means moving it away from the middle finger. Where exactly is it? If you look at the  back of your hand, it lies between the thumb and the index finger. If you relax your hand and touch that spot with your finger, and then touch your index to your thumb, you will feel it go from soft to hard as the muscle contracts. The Role Of The 1st Dorsal Interossei In The Left HandIt is interesting that we usually think of the muscle action used to press the strings to the frets as coming from the long flexors in the forearm. But in the case of the index finger, much of that work is done by the 1st Dorsal Interossei as we play. This is especially true in "classical" or acoustic technique, where the left hand takes a more horizontal approach to the neck (as opposed to the rock slanted position, see "The 2 Necessary Left Hand Positions". In The Principles, we learn that the index must lean to the left side of the finger tip as it goes on the string (in most playing situations). This means that the finger is doing a sideways motion to depress the string, which is another way of saying that it is the 1st Dorsal that does the work. Another critical use of the 1st Dorsal Interossei is in the performance of the full bar. This is because another very interesting job of the muscle is to simultaneously flex the big hand knuckle, and extend the middle and tip joints of the finger, as is done in extending the bar. The 1st Dorsal performs this action along with some help from a few other intrinsic hand muscles. The 1st Dorsal Interossei can often be our enemy as well! In many left hand situations it refuses to release, and causes incredible tension throughout the hand and arm, as it keeps the hand nailed to the neck, while the other fingers flail around like dogs staked to the ground on a 2 foot leash! The Role Of The 1st Dorsal Interossei In The Right HandFor Pick style players, the 1st Dorsal will help to hold the pick, as it is involved in the "key pinch" action of  the index and thumb. If you hold the pick with the fingers in a relaxed curl (as many ultra fast players do), you can keep this muscle more relaxed. Also, for pick players, this muscle would be very involved in the technique known as "circle picking". For fingerstyle players, the 1st Dorsal can be a source of trouble, because there is a great tendency to keep it tense while playing, which always shows itself by leaving a space between the index and the middle while playing. I always look for this trouble sign in students, and train them to keep the 1st dorsal relaxed. (In "the Cup" position taught in The Principles, the index touches the middle lightly). The aforementioned ability of the Dorsal Interossei to simultaneously flex and extend the finger joints is critical to the right hand of the classical player. It is used to a great extent in rest stroke (where the finger is more extended), and is important in free stroke as well. Players who rely on only the forearm flexors to execute strokes with the right hand will be at a great disadvantage. This action of the Interossei muscles is done by the action of a pair of them operating together on either side of the hand. In general, the  intrinsic muscles of the hand are very often undeveloped, and this is the cause of countless technical problems in playing. As you can see, these muscles are small and delicate, and yet, in most guitar methods and approaches (except The Principles),  they are called upon to perform, from the beginning, work they are not prepared to do. In the attempt, larger muscles tense in the arm and upper body, and a world of trouble will be waiting the unsuspecting student!So, be careful, and PRACTICE CORRECTLY!

location of 1st Dorsal Interossei

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