Reasons for rapid loss of strength (muscle cell death) in some people?

[diane hein]

1.  Does anyone know why a person might lose more than 1/2 pound
of muscle tissue per year after the age of 30 (this is
from Joyce Vedral, Ph.D. fitness trainer)?  She was on
the television program, "The Today Show" this week with
Marie Shriver.

   For example, would a deficiency of a certain amino acid
   or mineral or hormone cause more rapid loss of muscle
   cells leading to a more frail state that we see in
   some elderly persons?

2.  Does anyone know if there any specific known disease states
that are associated with this rapid loss of strength?

3.  And lastly, does anyone know of any researchers who might be working in 
this area?   (snail mail or email addresses or university researcher center 
addresses would be GREATLY APPRECIATED too!)

Thank you for any help you can give me!

D. Hein