You can use the fieldnames function and then use parentheses around the fieldname when called with a variable, see the code below:
s = struct('f1', [1 2; 3 4], 'f2', [5 6; 7 8]);
fn = fieldnames(s)
for idx = 1:length(fn)
mv(idx) = mean(s.(fn{idx})(:));
end
