Been asked this a great deal lately, and have discussed it with singers, teachers and colleagues alike!
This could be a long and intense treatise, but if we can simplify, let me try!
Singing is a physical and athletic activity. It does not mean you have to be good a sports! I certainly wasn't! But it demands a level of natural co-ordination and accessibility to the muscular balance of the body.
I am amazed at the LACK of body strength, and body co-ordination that I see - especially in this younger generation. Just simple balanced posture seems to be lacking these days.
So - if you want to be a singer you better learn how to get your body is shape to do it well.
Singing strength doesn't come from body building! Strength comes from cardio balance, from intrinsic balance, from co-ordination and breathing while doing so (!), and from building muscle stamina and power with lengthening and engaging the process!
Get thee to the gym is only part of it!
Singing (as acting and dancing!) requires strength, elasticity, engagement, lengthening, and GROUNDING. A body needs to build on these things. The body in its physicality and athleticism needs to be engaged to support FULLY at all times and is never rigid or shortened, but rather, in suspension and stretch!
Are you strong enough?
Are you co-ordinated enough?
Are you aware of your breath WHILE you stretching?
Are you aware of the strengths and conditioning of the many parts of your instrument and physicality, and do you know how to co-ordinate all those parts with your breath in order to support and function as an athletic singer? (and is there any other kind?!)
I am amazed at how many wannabe singers don't have a clue about their legs and how they function. Standing on one leg doesn't support you - especially when you are trying to balance your body to find the elasticity of the voice!
The slumping of the ribcage - or "runway body" as I call it (!) is also completely negating the lengthening and active energy of the support system as a singer. Other muscles that do not and should not take on severe responsibilities, will begin to. Then the downward spiral of begins.
Why wouldn't we, as singers, want to find the most effective and most efficient way to build the athletic and physical instrument? Why not recognize the issues, and develop stronger physical behavior to support the voice?
It is YOUR responsibility to find that athleticism and PRACTICE IT. Make it behavior. Find the co-ordination and DEVELOP it. Create new and positive and STRONG physical behaviors so the physical body has access to the strength necessary to sing WELL and in a healthy manner.
Build power, build elasticity, build co-ordination...the body reflects what the voice COULD do. Why would you ignore that?!