It wouldn't have been easy: Somewhere between the years of Houston mesmerizing fans with the resonating "you" in "I Will Always Love You" and the demise of the reality TV show "Being Bobby Brown," Houston's voice had deteriorated.

What is the normal life span of a voice? Can training or techniques prevent aging of the vocal cords, and can surgery -- or a special gel -- correct it?

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Think of a singer as an athlete, experts suggest.

"Just like any other muscle, it's a physical thing," said Andrea Leap, a professional singer and voice instructor at the MacPhail Center for Music in Minneapolis. "It depends on the use. If you stopped walking up the stairs every day, it would get harder. It's exactly the same thing for the voice. Muscles do lose strength and agility as they age, so more effort is required in continuing that."

Opera star Placido Domingo is still belting out arias at age 71, because he's in terrific shape vocally, Leap said.

"The voice is not a finite thing; it's not something you use up," Leap said. "When you're singing, you're training your voice at a more intense level than talking. It's like going to the gym and lifting weights as opposed to putting groceries away."

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In fact, overexertion is such an issue that the opera singers' union maintains strict rules about the frequency an opera singer can perform.

Preserving their general health, getting good rest and hydration, also helps keeps singers' voices in shape. Houston's overall health was clearly poor.

"If you're a smoker, it's going to be harder; if you're drinking every night, it's going to be harder," Leap said.

"Anything that you put into your body is going to go right past your vocal cords. I know some people who won't drink soda for that reason. I'm sure with Whitney Houston the larger issue was her overall health. That voice was an unbelievable instrument; it was going to take a lot to really undo it."

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Even with good health habits, however, vocal cords stiffen with age.

"As the vocal membranes are used more,they become fibrous and stiff with a diminished amplitude of vibration," said Dr. Steven Zeitels, Professor of Laryngeal Surgery at Harvard Medical School.

"Consequently you have to use more air pressure from the lungs to drive the vocal cords into vibration. This occurs from decades of voice use so that the vocal cords become worn out as an individual ages."

Many singers develop growths or nodules on their vocal cords that can bleed and eventually scar. Scarring makes the voice hoarse.

Advances in technology have made surgeries to remove those growths much more common.

Zeitels estimates he's performed about 75,000 voice operations, 500-700 of those on singers -- including Adele, The Who's lead singer Roger Daltrey, and Aerosmith's Steven Tyler. Most of the singers he has performed on have been opera singers, because of the substantial demands on their voice.

"You can't do marathons to train for marathons," Zeitels said. "There is a simple amount of mileage that vocal tissues can handle."

Zeitels has been developing a special gel that he hopes will allow singers to restore and preserve their voices.

When Zeitels explained the idea of the biogel to Julie Andrews during dinner one night, he mentioned that one of his reservations was that he didn't know how long it would work.

"Well, for people like me, even if it would last for a while we could utilize that and get things done," he remembers Andrews saying. "So, we came to think that the way to go at it was not to go for a home run, the perfect fix, but to get it to first base."

Now, he says the gel is ready for human trial.

"The holy grail is to inject a biogel into the vocal cord and restore it. So from a performing perspective, these folks who so-called can't sing anymore are totally fit to sing. Julie Andrews could sing beautifully tomorrow if she had the biogel. It's not that she's too old to sing."

Both Andrews, who went to Zeitels after losing pliability in her vocal cords due to a poor surgery, and Tyler encouraged Zeitels and MIT scientist Bob Langer to go forward with the biogel project, even contributing money to the development.

"It's not some magic potion," Zeitels said. "We're just making the vocal cords pure."

The gel is currently in a primitive form, but Zeitels is confident it will work in humans; trials will tell the degree to which the current form will work.