newcommandUnderset[2]{underset{textstyle #1}{#2}}

documentclass{article}
usepackage{amsmath}
newcommandUnderset[2]{underset{textstyle #1}{#2}}
parindent0pt
begin{document}

Using verb|underset|:
[ underset{0le xle 1}{max} f(x) ]
Using verb|Underset|:
[ Underset{0le xle 1}{max} f(x) ]

end{document}