User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-16281343-20150414050518/@comment-26209278-20150414101055

The only reason Issei has any strength is due to 1)Boosted Gear, and 2) that, like Sairaorg, he increases his physical strength due to a lack of any demonic power residing in him. Remember he couldn't even use the magic circle in the beginning? It's only possible now because his physical strength gives him the magical power to jump (kind of, could be explained better)