Instead of "after each number" did you mean "after each number, EXCEPT the first number, where the number is also before the first number?" That's what Star and Adam's solutions do. Or do you really mean what you said, instead of the "result" you gave, and so there would be no 1 at the beginning?
Just to be complete and cover all ambiguities, I'll give the other solution. If you really want what you said in words, rather than the numerical result you gave, then use this code:
result = [a; ones(size(a))]
result = result(:)'
The result is:
10 1 2 1 40 1 58 1 100 1 90 1 40 1 30 1 4 1
Note: 1's are only after the numbers and not before the first one.