I've been singing 15 years, have decent technique but have recently experienced a sudden disappearance of voice. This last year was quiet for me, singing-wise, but recently, after three (very) hard singing sessions in the one week, it hasn't recovered, even after 2 months. I've seen a specialist and ruled out nodules/cysts. He suspected reflux and I've since ruled that out. All I can assume is that there'd been some level of muscle atrophy and I over-strained the voice. I wonder if you'd be inspired to have an article on voice stamina and building strength after technique?
Since you've ruled out medical issues, we have to assume your vocal fatigue is due to something you're doing. I hear your concern about getting back into intense singing after a quiet spell, but I don't think you're looking at anything as dramatic as muscle atrophy. Without actually hearing you sing, I can only speculate as to the reason, but if you're like most singers I've encountered with similar problems, your fatigue is most likely due to your approach to breath management.
To address the question of how to avoid fatigue and improve vocal stamina, we must first take a look at the physiology of how fatigue and stamina are created.
What is muscular fatigue? The article Fatigue in Skeletal Muscle offers this definition: "An individual’s capacity to perform during high-intensity exercise depends upon his/her ability to generate and maintain a high power output…The inability to maintain the desired power output defines fatigue, and mechanisms that attempt to explain fatigue have been generally classified as product accumulation or substrate depletion." Read the article if you want the full explanation, but the main point is that when you experience muscular fatigue, you're essentially running out of fuel faster than you can replace it.
If you want to increase muscular stamina, you must therefore improve your ability to consume and replace fuel. Resistance strength training programs achieve this by working muscles to the point of fatigue, which "leads to trauma or injury of the cellular proteins in muscle. This prompts cell-signaling messages to activate satellite cells to begin a cascade of events leading to muscle repair and growth." (Here's the article I'm quoting about how muscles grow.) When you want a muscle to get stronger, exercise it to the point of fatigue and then allow it to recover. It will respond to injury in its cellular proteins by building itself back bigger and stronger than it was before.
While resistance training may work like a charm for your pecs and your quads, it is not an effective means of strengthening the vocalis or other intrinsic laryngeal muscles. Like your pecs and quads, they're composed of skeletal muscle fibers. But relatively speaking, there are so few of these fibers that pumping them up won't have that great an impact. Even if it did, there isn't enough room in there for them to get all that much bigger.
Strengthening the vocal folds and other laryngeal muscles is not the cure for vocal fatigue. If you want to build vocal stamina and avoid fatigue, you need to build coordination throughout your instrument and strengthen the supporting skeletal muscles that are designed to get bigger and stronger with exercise – for our purposes, the muscles used in skillful breath management.
One excellent general guideline for all physical (and musical) endeavors is:
Use the Big Muscles for the Heavy Lifting
We're constantly admonished to "lift with your legs, not your back."
While all agree that this is the safer, more efficient strategy, we need this reminder because it's counterintuitive, as yoga instructor Willow Ryan explains here. We're still too recently evolved from primates for whom it was more efficient to lift with their backs; our instincts haven't yet caught up with our musculature.
Pianists are taught to rely as much as possible on the big muscles. It's more intuitive for them to focus on their fingers, but it's vital that they develop a technique that makes good use of the bigger muscles of the chest, back and arms so that their fingers are free to do the intricate work that only they can accomplish. You can read a simple, straightforward description of the physiology here.
Playing the piano well is no simple matter, but the physiology of it is easier to comprehend than singing because the job of pianists is to evoke sounds from an instrument that is separate from their bodies. Unlike a piano, your instrument is seamlessly integrated into your body. Most of its components are not visible and many of them are difficult to directly sense or move.
If a pianist's finger muscles fatigue excessively, they soon learn that it's not because their fingers weren't strong enough but rather that they weren't letting the big muscles do their proper share of the work. When your voice fatigues, it's a similar scenario – the small, intricate muscles inside your larynx have been overly taxed because the larger muscles comprising your instrument aren't doing their share of the work.
Skill in singing rests on your ability to optimize and regulate your subglottal air pressure. The more concentrated and "compressed" the air in your lungs, the greater its potential power to vibrate your vocal folds and contribute to range and dynamic variation.
As I pointed out in my post on breath management, there are multiple ways to achieve an increase in subglottal air pressure. Let's examine two of them.
Hook & Push
The most intuitive and expedient way to increase subglottal air pressure is to tighten up your throat and then override that tightness by driving the breath against it. The tightness can be created by over-adducting your vocal folds, pushing the base of your tongue back and down, and/or tightening up your constrictor muscles. The breath can then be driven by pushing with your abdominal muscles, compressing your sternum down, and/or pulling in your rib cage.
I realize it sounds pretty awful when I describe it that way. Nevertheless, it's a very effective and easy way to increase subglottal air pressure, and that's why so many singers do it. Some are even able to achieve a very impressive sound with this strategy. However, even they would be able to achieve a vastly more impressive and flexible sound (not to mention a less fatigue-inducing one) with a better breath management strategy.
If your approach to breath management uses your laryngeal musculature to increase subglottal air pressure, then the musculature is no longer entirely available to do what you need it to do. Your vocal folds are not free to simply vibrate while creating pitches and dynamics, because they're also responsible for creating resistance for the breath to push against. Your larynx is not free to remain in a settled, low position because when you drive breath pressure at it, it will consequently ascend (which means that if you want a low larynx you have to hold it down, taxing the musculature even further).
I realize that no caring, responsible voice teacher is going to advise you to tighten up your throat and then drive a lot of air through it. Everyone wants you to have a loose throat and a low larynx. Everyone will tell you not to "push". But there simply aren't two ways about it. If your breathing strategies include mobilizing your abdominal muscles to support your breath, you have, to some extent, built a technique around creating resistance in your throat and overriding it with breath pressure.
There can be no distinction between this kind of "support" and pushing. It's just a matter of scale.
To whatever extent you are pushing your breath out, you're also tightening the muscles in your throat. They will give out at some point. They are small muscles, and they are ill-suited for heavy lifting. It's true that when you engage your abs, you are also using some big muscles, but consider what that means. If you're creating compression in your lungs by tightening your throat and using those big muscles to drive air through it, you are demanding that the small muscles in your larynx create a degree of force nearly equal to the force coming from your abs!
(If this is in fact the case for the singer who wrote to me, he likely had built up some additional strength in the little muscles over time and was getting decent results until he had some extended down time. I wouldn't go so far as to say the laryngeal muscles atrophied, but they lost some of their tone. So when he returned to robust singing, they could no longer respond as they once had and the result was vocal fatigue. I suppose he could just build them up again, but this is a great opportunity to change his breath management strategies for the better.)
La lotta vocale
"La lotta vocale", or "vocal struggle" is the foundation of what the bel canto specialists call appoggio breathing technique. If you google these terms, you will find a variety of guidelines for achieving this. Here is Jean-Ronald Lafond's eloquent take on it, and here is another excellent description by Jack Li Vigni.
What all of these descriptions have in common is their emphasis on keeping your muscles of inspiration dynamically engaged while singing. The "struggle" implied by "lotta vocale" occurs when air that would normally just flow out on exhalation is restricted by the vacuum that the continued activity of your muscles of inspiration are creating.
I don't use bel canto terminology in my column on breath management, but the essential principles are the same. My emphasis is on maintaining a high, stable sternum and keeping the rib cage fully expanded while singing. (Please read my column for a more detailed description of the muscles involved and an exercise to begin developing skill in breath management.)
Hopefully you can see how releasing your breath while using your muscles of inspiration to hold back the flow concentrates and compresses the air, thus increasing your subglottal air pressure without involving the little muscles of your larynx. When you do this, you're not pushing any breath out. You're just providing your vocal folds a higher concentration of breath to set them vibrating, and you're doing it by working the relatively big muscles in your back and between your ribs.
Maintaining tension in your muscles of inspiration while exhaling is a bizarre thing to do. In fact, I can't think of any other human endeavor for which this coordination would be useful, with the exception of playing a wind instrument. For that reason, it's highly counterintuitive, and when you first begin to develop this skill you will likely find that the muscles involved are weak and possibly even difficult to engage and keep track of. But it's still the correct way to approach breath management, just as the correct way to lift involves using your legs rather than the more intuitive approach involving your back.
You will have to work slowly and consistently to build the requisite strength and stamina, just as you would have to stick to a program of resistance training to pump up your pecs and quads. The good news is that you will likely experience results pretty quickly, and that over time it is possible to build a great deal of strength in these muscles.
Develop the strength and coordination you need to manage your breath skillfully. Vocal fatigue will likely become a thing of the past, and you'll have more stamina than you ever imagined possible.
It's important to remember that anything that creates resistance in the voice can lead to fatigue. Make sure to rule out tongue, jaw or throat tension, inadequate resonance, and over-heavy registration before you focus on building up your breath management muscles.