Many writings talk about how Elvis exercised constantly. He practiced Racketball. He rode a stationary bike at Graceland. He practiced Karate.

Audio file
..//downloads/elvis-vocalizing.mp3
"I exercise everyday. I vocalize everyday. And practice ... whether I'm working on... or not. I just try to stay in shape all the time. Vocally and Mentally." He is asked which is harder. "Well both is tough. You gotta work at them. I don't mind it, you know. It's worth it."
Exercise
The first thing he said was "exercise". You can't sing at your peak if you don't exercise. Elvis did cardio. Elvis also strengthened his stomach muscles. Doing situps (or something comparable) will up your singing breath control. Breath is half of singing.
Stomach Exercised
Elvis installed a stomach exercising rack so he could concentrate on exercising his stomach. He also used it for streaching.
In the 70’s he started playing racquetball with the “Memphis Mob.” He started playing at the local Y and at Memphis State. His two favorite things to do was to ... go to the movies and play racquetball. In the summer of 1975, Elvis decided to bring his racquetball hobby closer to home. He began planning to build a racquetball court at Graceland, and construction of the building began in September 1975. He played racquetball almost daily.
Frank Sinatra
Frank Sinatra played golf and tennis constantly for exercise.
Opera
A famous opera singer practiced yoga 2 hours every day. Other opera singers would say that he was the best singer they had ever heard. Caruso was a deep diver and long distance swimmer. All great singers exercised constantly.

