Central extensions in semi-abelian categories

In the context of semi-abelian categories, we develop some new xaspects of the categorical theory of central extensions by Janelidze and Kelly. If image is a semi-abelian category and image is any admissible subcategory we give several characterizations of trivial and central extensions. The notion of central extension becomes intrinsic when image is the subcategory of the abelian objects in image. We apply these results to the category of internal groupoids in a semi-abelian category. As a very special case, we get the known description of central extensions for crossed modules.