Type IIx muscle fibers do not benefit from capillarization, and capillarization is by definition a low frequency stimulation.
What I’m saying above is explained in the book Application of Muscle-Nerve Stimulation in Health and Disease, Springer 2008, Vrbova, Hudlicka, Schaefer Centofanti. Muscle fibers IIx, by definition, do not utilize aerobic mechanisms to convert energy into muscle contraction. Capillarization instead increase the amount of capillary in a particular muscle fiber, thereby increasing the amount of oxygen that can be supplied to that muscle fiber. But if you want to utilize that fiber for anaerobic activities, you wouldn’t be able to take advantage of the increased blood supply. Therefore you would not benefit from increased capillarization.
Low-frequency stimulation is instead prevalently used for endurance training; research on animals and humans has shown that low frequency stimulation has the capability of increasing capillarization in muscle fibers, no matter what the fiber type was at the beginning (of course it takes several weeks to achieve this).
I hope this answer your question.