HIIT taxes and maximizes both aerobic and anaerobic fitness, while cardio addresses aerobic only. Aerobic respiration requires oxygen to generate energy in the form of ATP, while anaerobic respiration does not. HIIT affects muscle tissue at the cellular level, actually changing mitochondrial activity in the muscles themselves. Studies indicate as little as 27 minutes of HIIT three times per week produces the same anaerobic and aerobic improvement as 60 minutes of steady state cardio five times per week.
via Greatist
