Cardiorespiratory Fitness, Longevity, and the Evolving Role of High Intensity Interval Training

Cardiovascular and cardiorespiratory health are increasingly recognized as central pillars of not only athletic performance but also long-term performance and disease prevention. While muscle mass, bone density, and mobility often take centre stage in conversations around healthy aging, it is one's cardiorespiratory fitness (CRF)—most often measured cardiopulmonary exercise testing (CPET)—that may offer the most powerful insight into overall physiological reserve. In fact, growing literature supports the idea that CRF is one of the strongest predictors of all-cause mortality, surpassing even traditional risk factors such as smoking or diabetes. This is particularly relevant when one's VO₂ max—the maximum amount of oxygen consumption your body can utilize during intense exercise—falls into the lowest percentiles.

VO₂ max represents the body’s capacity to uptake, transport, and utilize oxygen during maximal exercise, reflecting the efficiency of the heart, lungs, blood vessels, and skeletal muscles working in synergy. While elite athletes have long trained to improve this metric, what’s often underappreciated is its relevance to all of us—especially those entering midlife or managing chronic conditions. Literature suggests that even modest improvements in VO₂ max have been associated with meaningful reductions in mortality risk, which is a compelling reason to prioritize aerobic conditioning beyond performance goals. Cardiologists and exercise physiologists are increasingly emphasizing that CRF should be treated as a clinical vital sign, particularly because it is modifiable through structured interventions that are readily available for most people.

The Spectrum of Intensity: Zone 2 Aerobic Training and High Intensity Interval Training

From a training perspective, two approaches stand out as particularly effective for improving CRF and VO₂ max: low-intensity training (commonly referred to as Zone 2 heart rate training) and high-intensity interval training (HIIT). These two strategies sit on opposite ends of the intensity spectrum but are complementary when used together. Zone 2 training, performed at an intensity where the body primarily uses fat as fuel and blood lactate measurements remain below threshold, promotes mitochondrial biogenesis, improves aerobic efficiency, and supports recovery. It’s the foundation upon which sustainable aerobic capacity is built, and it plays a key role in metabolic flexibility, cardiovascular health, and endurance.

HIIT, on the other hand, is described as short bursts of near-maximal effort followed by recovery intervals. HIIT pushes the cardiovascular system to operate at higher intensities, which is shown to lead to improvements in VO₂ max, insulin sensitivity, and vascular function. Research from McMaster University Professor Dr. Martin Gibala—a leader in the study of HIIT on CRF—suggests that HIIT can produce comparable or even superior cardiovascular benefits to longer-duration moderate-intensity continuous training in a fraction of the time. He and his team have also shown that implementing “exercise snacks” in the form of sporadic, short <1min bursts or near maximal activity interspersed throughout the day can also have a meaningful improvement in CRF markers for those who are pressed on time. In one of their studies, they used intense stair climbing as their intervention.

Historically, there have been concerns around safety when applying high-intensity protocols to aging populations and those with known cardiovascular disease. However, evidence suggests that HIIT is well tolerated and extremely safe when appropriately screened and supervised in most populations. HIIT actually shows a greater positive benefit compared to moderate intensity continuous training in multiple systematic reviews and meta-analyses in those with known disease. Additionally, in speaking with Dr. Paul Angaran—a cardiologist and electrophysiologist at St. Michael's hospital—he confirmed that intense exercise can be safely prescribed to those with known cardiovascular disease pending there is no immediate red flag indicator. The thought here is that those with known disease are at more risk of having another event if they continue to do nothing, than they are when doing something active and therefore reducing future risk. This was a topic discussed in detail last November when I invited Dr. Gibala and Dr. Angaran to speak at our annual Sports Conference. It was refreshing to hear that they both had some reinforcing insights and recommendations from a research and clinical perspective.

Cardiovascular Exercise and Musculoskeletal Health

Beyond its metabolic and cardiovascular effects, aerobic training—especially when programmed strategically—has meaningful implications for musculoskeletal health. Aerobic activity supports synovial fluid circulation, joint nutrition, and tissue perfusion. HIIT specifically recruits fast-twitch, Type 2 muscle fibres and challenges neuromuscular coordination, while Zone 2 work helps improve fatigue resistance and muscular endurance via Type 1 muscle fibres (for a refresher on muscle fibre types, click here). These adaptations are critical for maintaining balance, strength, and mobility as we age. In active individuals and athletes, the combination of aerobic conditioning and strength training leads to musculoskeletal resilience—reducing injury risk and supporting recovery.

Cardiorespiratory Health and Brain Function

Cardiovascular exercise also plays a role in preserving cognitive function, emotional regulation, and sleep quality. Aerobic training has been associated with increased hippocampal volume, improved executive function, and reduced symptoms of anxiety and depression. These benefits are amplified when CRF is elevated, which further reinforces the value of targeting VO₂ max improvements—not just for physical health, but for cognitive and emotional well-being.

Cardiorespiratory Fitness and Physiological Flexibility

For individuals looking to improve their overall health and physiological resilience the integration of lower heart rate zone aerobic training and HIIT into a structured weekly program allows for physiological variability. Not only does changing the exercise type and intensity allow for recovery of different muscle groups and fibre types throughout the week, but it also provides novel stimuli at different times of your training week and cycle. This, in a way, “keeps our body and brain guessing” which keeps our nervous system malleable and helps us retain the ability to cope with different stressors and stimuli.

If we were to solely train HIIT or solely Zone 2 aerobic function, we would essentially be a “one trick pony”. This may be good if you’re training for a highly specific event or sport, but when it comes to metabolic flexibility, recovery and a general reduction of all-cause mortality, we know that a combination of training types from different domains and intensities seems to be the recipe for long term success.

Clinically, this approach offers a scalable, evidence-based intervention for reducing long-term disease risk, improving quality of life, and optimizing physical performance. Whether you’re recovering post-operatively, recovering from a previous cardiac episode, a healthy person looking to remain active or even an athlete looking to optimize performance, the principles remain consistent: improve oxygen and nutrient delivery, preserve muscle, and reduce metabolic and cardiovascular decline through intentional exercise.

As a practitioner, I have a unique opportunity to use emerging exercise science to inform personalized, precision-based strategies that are not only effective but sustainable for folks like you who are reading this. For you, you’re likely looking for strategies to improve your personal, business, and physical performance, so I applaud you for taking the steps to improve your own health literacy. In that, I hope this educational content helps you live, train and recovery more meaningfully.

Conclusion

In summary, cardiorespiratory health should be viewed not simply as a marker of fitness but as a fundamental determinant of neurological and physical longevity. VO₂ max is a modifiable predictor of mortality and disease, and improving it through a combination of endurance and high-intensity interval work is both safe and effective across a broad spectrum of diseased and healthy populations.

With proper professional oversight, HIIT can be integrated into a variety of care models—offering patients and athletes alike a pathway toward greater performance, resilience, and healthspan. If you are unsure of where to start, and how to apply these strategies, please connect with me directly. I'd be thrilled to help you towards your goals.

Yours in good health,

Andrew

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