I think there are two kinds of endurance on the trombone, each having it’s own separate characteristics in terms of the nature of muscular duress and failure: short-term, and medium-term endurance. (Really, there is a third category of endurance, long-term endurance. But, as is pointed out in the Seven Habits of Effective Trombonists article, that is more about having endurance as a person and taking care of one’s body into middle and old age through a holistic approach to mental and physical health, than it is about a given set of techniques, as with the issues of short and medium-term playing endurance…)
1. Short-Term Endurance
The first kind, short-term endurance, is that which is at issue in a given piece, or a high-intensity portion of a piece, wherein the set threatens to break down. By break down, I mean a generic state of muscular failure within which the risks run from strained pitch and tone, to not being able to maintain notes, to complete failure – not being able to make a sound at all.
I believe the primary way of improving the short-term endurance is through practicing high-intensity playing, that is the precise kind of playing that has a tendency to put us in a muscular breakdown situation.
There is that which automatically happens during such practice: the facial tissue develops strength as a result of the physical duress. However, this is but a small part of what improves as a result of practicing high-intensity material.
The most valuable thing that happens during this variety of practicing is when the body learns to make physiometric adjustments to control the airstream and position the parts of the blowing apparatus to make the playing part most efficient. I feel like I’ve already hammered home how important it is to adjust the size of the airstream during high-intensity playing a dozen times over, in articles such as Developing the Upper Register.
Practicing high-intensity material (that is, music that poses physical challenges to us in terms of range, volume, articulation and weight of sound) is the ultimate laboratory within which to experiment with modulating the size and speed of the airstream. And the effectiveness and efficiency of that modulation is key to taking the heavy lifting away from the comparitively weak muscles of the lips, which is most critical in terms of developing short-term endurance.
2. Medium-Term Endurance
The second kind of endurance is medium-term endurance, that is the ability to play for hours and hours, or the ability to play multiple gigs on a given day. The nature of stress or failure with this kind of breakdown is likely not as catastrophic as the short-term kind, it is more likely to manifest as a general feeling of fatigue, or numbness, or a feeling that the sound is spread, or an inability to maintain sound quality or intonation. Of course, a state of medium-term endurance stress can leave one much more susceptible to a short-term endurance problems, as we have all likely experienced from time to time.
And if you can believe it, the skill of solid medium-term endurance is even less based on physical strength than is short-term endurance.
Medium-term endurance is almost solely based on how well a player has learned to base his or her fundamental, physical approach to the trombone on a strong and efficient air stream. Again, the muscles that drive the airstream in the lower torso are comparatively large and strong when compared to the minute muscles of the face. Additionally, the force from the “gut” muscles is not fighting against the force of adjacent muscles; conversely, the complex small muscles in the face are often exerting opposing forces on one another.
The fatigue that results largely from asking the facial muscles to do too much, combined with the tendency of the small muscles to unconsciously loose coordination and begin opposing one another, is the cause of the overall facial fatigue that many players identify as a medium or long-term endurance issue. When in reality it is based on incorrect technique and reliance on the wrong muscles.