Dr. Rhonda Patrick: Let us talk about how the heart and lungs work together as the cardiorespiratory system and how vigorous-intensity activity affects that system.

Brady Holmer: Higher exercise intensities increase metabolic and oxygen demand in working muscles. The body must deliver more oxygen, so the heart works harder and stroke volume increases. Stroke volume is the amount of blood the heart pumps with each beat. Heart rate also rises, and the lungs work harder to supply oxygen.

Vigorous exercise, and even zone 2 exercise up to a point, places more stress on the heart and lungs and forces the body to adapt. The heart becomes stronger. One of the key adaptations is an increase in stroke volume, which is a major contributor to a higher VO2 max. Greater stroke volume increases cardiac output, allowing the heart to deliver more blood and oxygen during exercise.

That helps explain why high-intensity interval training is effective. It creates greater oxygen demand, makes the heart work harder, and improves stroke volume. The lungs also adapt, although they may be less limiting in people without a condition such as asthma. Most people have considerable lung capacity, so the heart often provides the more important adaptation.

This is one reason vigorous exercise produced an efficient response in the study. Ben Levine has also discussed the need for higher intensities after age 50, 60, or 70. The exercise does not have to be HIIT, but it must be sufficiently vigorous to stimulate the heart and help prevent cardiac stiffening and fibrosis. Low-intensity activity alone may not provide that stimulus.

The study cohort ranged from 40 to 79 years old. That age range may help explain why vigorous activity had an especially large association with cardiovascular outcomes, including the approximately eight-to-one ratio discussed earlier.

Dr. Rhonda Patrick: Improving VO2 max and cardiorespiratory fitness matters because fitness is an important marker of longevity and lower all-cause mortality risk. We discussed evidence in our previous podcast showing that people with the highest cardiorespiratory fitness had approximately five years greater life expectancy than those with the lowest fitness. Another analysis found about an 80% lower all-cause mortality risk in the highest group. However you examine it, higher cardiorespiratory fitness is associated with longer life expectancy.

Brady Holmer: VO2 max begins to decline around the fourth or fifth decade and can fall by roughly 10% per decade. You can build a high level of fitness in your 30s and 40s, but without moderate-to-vigorous exercise at ages 40, 50, 60, and 70, VO2 max will continue to fall. More vigorous exercise is needed to maintain it or build it.

Another important benefit for older adults involves muscle-fiber recruitment. Muscle fibers are commonly divided into type I, or slow-twitch fibers, and type II, or fast-twitch fibers. Type II fibers include subtypes such as type IIa and IIx, but the broad distinction is sufficient here.

Low-intensity activities such as leisurely walking or washing dishes primarily recruit type I fibers. Endurance exercise also relies heavily on type I fibers. As exercise intensity or the force needed to move a load increases, the body progressively recruits more fibers and eventually engages type II fibers.

This may help explain the strong association between vigorous daily movement and health outcomes in the study. Vigorous movement gives type II fibers more stimulation. These fibers tend to atrophy and lose strength earlier with age than type I fibers. Picking up a heavy box, carrying a child, doing strenuous chores, or performing structured vigorous exercise can recruit them.

Type II fiber recruitment may affect several outcomes. Active muscle uses glucose, which can improve insulin sensitivity and lower fasting glucose. Type II fibers are also power-producing fibers. If you trip and need to catch yourself, they help generate the rapid force required to recover your balance.

Falls and hip fractures can lead to bed rest, disuse atrophy, and a catabolic crisis from which an older adult may not fully recover. Stronger type II fibers may reduce fall risk and thereby contribute to lower all-cause mortality. That is a plausible link in a cohort of more than 70,000 adults, although it is not proof of the mechanism. Power and strength decline with age, so preserving type II fibers is important.

Dr. Rhonda Patrick: Those are excellent points, Brady. I agree that they may help explain the reduction in all-cause mortality. They are also among the reasons I do CrossFit-style training. It combines functional movement, explosive power, strength, resistance training, aerobic exercise, and vigorous intensity. For me, it is a time-efficient way to train both cardiorespiratory fitness and muscle health.

That raises a question about special populations. Older adults often ask whether they can perform vigorous exercise. We are not talking only about HIIT. Cycling, running, or jogging can also qualify as vigorous activity. The study included adults up to age 79, and adults 65 and older also benefited from vigorous physical activity.

Someone who is older and has never done vigorous exercise does not have to begin at full intensity. The person can progress gradually, starting with interval walking, adding brief periods of light jogging, or accumulating one-minute sets of chair squats during the day. Cardiorespiratory fitness, muscle mass, and type II fiber function all decline with age, so there are many reasons for older adults to work toward some vigorous activity instead of relying only on the moderate-intensity portion of the guidelines.

Brady Holmer: Absolutely. Anyone who has never done HIIT or vigorous exercise, whether old or young, should ease into it and allow time to adapt. Many studies show that older adults can perform the Norwegian 4x4 protocol safely and effectively several times per week without burning out.

In graduate school, I participated in studies that used the Norwegian 4x4 to examine endothelial function. People around age 65 who had diabetes completed the protocol a couple of times per week at approximately 85% of maximum heart rate. It was their only structured exercise. Although 85% sounds high, the absolute workload can still be modest for someone with low fitness.

Older adults can do this exercise, and the cardiovascular stimulus becomes increasingly important with age. Without vigorous exercise, the heart may not receive enough stimulus to adapt and may continue to stiffen. Vigorous training is therefore feasible and may be necessary for some of these cardiac adaptations. The term HIIT can make people imagine Olympic-level training, but the workload is relative to the individual's capacity.

Dr. Rhonda Patrick: Thank you, Brady.

Brady Holmer: The clinical studies make that point clear.

Dr. Rhonda Patrick: Some people push back whenever I discuss the Norwegian 4x4. I am not recommending four sessions a day or five days a week. I am talking about one or two sessions per week. As you said, the target heart rate is relative to the individual. Reaching 85% of maximum heart rate can feel hard for an untrained person, but the absolute workload may still be low.

Brady Holmer: Right. On a bicycle, it might require only 50 to 100 watts, similar to a modest spin-class workload.

Dr. Rhonda Patrick: Yeah.