Defining a math macro

[Angus Leeming]

Is it possible to define a math macro so that the body is in bold but 
any sub/superscripts are not?

Currently, I have:
        \newcommand{\Matrix}[1]{\boldsymbol{#1}}
        \Matrix{m}_{B}=\left[\ldots\right]
but would prefer
        \Matrix{m_{B}}=\left[\ldots\right]

Can this be done?

-- 
Angus