Between work, family, activities and schedule changes, having a quick and effective workout ready to knock out can be very useful. High-intensity interval training (HIIT) workouts are meant to challenge the entire body and push intensity limits. While the intensity is meant to be high, adjusting for the energy level you have and/or injuries you may be experiencing is perfectly appropriate and recommended. HIIT training can be either high- or low-impact or a combination of both.