These are not stems. The stem of "跪<ひざまず>く" is "hizamazuk" and the stem of "敷<し>く" is "shik". "跪き" and "敷き" are the traditional form of "て形te-form" . "て形te-form" was formed by adding "てte" to the traditional form.　Nowadays we usually use "て形te-form" , especially in colloquial sentences. But we use the traditional form even now, especially in written sentences or colloquial sentences to express formality or gravity.