Physical exercise is vital to vocal health and stamina. To paraphrase an old adage, "move it or lose it!" ... "it" being your voice!
If you have a habit of singing with correct technique, you'll notice you don't get vocally tired. But you'll also notice that you do get physically tired! That's because singing correctly will use the big muscles of your core (abs, back, thighs, buttocks) and minimize incorrect over-use of the little muscles of your throat. The state of your body at any given time will affect your vocal ability - for better or for worse.
It should go without saying that part of the state of your body has to do with your rest, stress, hydration and nutrition levels. However, for this article I want to concentrate on the levels of flexibility, coordination and strength of your muscles.
I have noticed that people who don't do much physical exercise don't contract their lower abdominal muscles when they sing. This means they won't be encouraging the abdominal contents to shift upwards, which is vital for supporting the upper movement of the diaphragm. This results in all kinds of vocal limitations.
These sedentary people also don't get good breath in. Their shoulders tend to be rounded forward, ribcage slumped, trunk sort of compact and arms hanging limply at the sides like rib anchors.
They also tend to be rather numb in performance. It takes physical energy to communicate.
Great vocal exercises are powered by your physical core if you do them properly. If you're not used to it, your abs and back muscles can become sore. (Just be careful not to tense abs or any other muscles near your diaphragm, which should stay widely stretched by the ribcage.) Doing physical exercises to work out your core will enable you to do your vocal exercises more correctly. You'll find yourself singing longer without strain. Remember- once physical fatigue sets in, vocal fatigue can quickly follow.
* Find an exercise routine you will actually do regularly - at least 3 or 4 times a week.
* If you go to a gym, consider hiring a personal trainer for at least one round of lessons. Let them know you are a singer or public speaker. When holding your breath to push against a weight, be careful not to put too much pressure on your vocal cords. (Don't grunt hard).
* Find a routine you can take with you wherever you go. I used to use Cindy Crawford tapes in my hotel room before my concerts. I didn't know why then, but I knew from experience that I sang better after exercising at least 30 minutes before a show.