Any aerobic activity will take away from gains if you are trying to bulk. However, it is, in my opinion, a good idea to strive for overall health by exercising the heart and lungs as well as the glamour muscles. Therefore, doing 20 minutes or so of low impact aerobics three times per week, wherther or not you are doing HST or SD, is certainly a good idea. Jogging on a hard surface, however, would not be my choice as it will eventually get to your lower back and knees.