High-intensity interval training has generated substantial research interest in the past decade, partly because its time efficiency makes it attractive, and partly because the physiological adaptations it produces are in some respects superior to those from moderate-intensity continuous training. For adults over 35 with limited training time, understanding what HIIT does, and what it does not do, is clinically useful.
The Cardiovascular Adaptations
A meta-analysis published in the British Journal of Sports Medicine covering 65 trials found that HIIT produced significantly greater improvements in VO2 max compared to moderate-intensity continuous training, despite lower training volume. The mechanisms include larger improvements in stroke volume, greater left ventricular remodelling, improved cardiac output at maximal effort, and greater capillarisation of working muscles. These adaptations are especially pronounced in previously sedentary adults, where the greatest gains are to be made.
The Norwegian 4x4 Protocol
The most researched HIIT protocol for cardiovascular benefit is the Norwegian 4x4: four intervals of 4 minutes at 90 to 95 percent of maximum heart rate, each followed by 3 minutes of active recovery, performed twice per week. Multiple studies from the Norwegian University of Science and Technology demonstrate that this protocol reverses age-related cardiovascular decline, improves VO2 max by 8 to 13 percent over 12 weeks, and significantly reduces cardiovascular risk markers in middle-aged adults.
The Role of HIIT Within a Balanced Training Week
- HIIT is most effective as a complement to zone 2 training, not a replacement; the aerobic base built through zone 2 work provides the platform for HIIT adaptations
- Two HIIT sessions per week is sufficient for most adults over 35; more than this generates systemic stress that impairs recovery without producing additional benefit
- HIIT should not be performed on days immediately before or after strength training if the goal is to maximise both adaptations
- Adults over 60 show the same proportional cardiovascular benefit from HIIT as younger adults, with appropriate intensity modification
- A thorough warm-up reduces injury risk and ensures the cardiovascular system is prepared for high-intensity effort
Sources
- Weston, K.S., et al. (2014). High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis. British Journal of Sports Medicine, 48(16), 1227-1234.
- Wisloff, U., et al. (2007). Superior cardiovascular effect of aerobic interval training versus moderate continuous training in heart failure patients. Circulation, 115(24), 3086-3094.
- Milanovic, Z., et al. (2015). Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements. Sports Medicine, 45(10), 1469-1481.