Cardiorespiratory fitness, your heart and lungs’ ability to sustain prolonged physical activity, is the single most important fitness component for reducing your risk of early death. People with high cardiorespiratory fitness have a 41% to 53% lower risk of dying prematurely compared to those with low fitness. No other fitness component comes close to that level of protection, which is why the American Heart Association formally recommended in 2016 that cardiorespiratory fitness be treated as a clinical vital sign, measured alongside blood pressure and heart rate.
But “most important” doesn’t mean “only important.” Muscular strength runs a strong second, and the combination of both delivers results neither can match alone. Here’s what the evidence actually shows for each component and how to use that information.
Why Cardiorespiratory Fitness Ranks First
A 2024 overview in the British Journal of Sports Medicine pulled together data from over 20.9 million observations across 199 separate long-term studies. The finding was consistent: people with high cardiorespiratory fitness had roughly half the mortality risk of people with low fitness. For people already living with heart disease, the protection was even more dramatic. Those with high fitness had a 73% lower risk of dying from cardiovascular causes compared to those with low fitness.
These numbers dwarf the risk reductions seen with most medications and lifestyle changes. High cardiorespiratory fitness is linked to lower rates of heart disease, type 2 diabetes, hypertension, and several cancers. It’s the closest thing to a universal health protector that exercise science has identified.
What makes cardiorespiratory fitness so powerful is the sheer number of systems it touches. Your heart pumps more efficiently, your blood vessels stay flexible, your body regulates blood sugar better, and inflammation stays in check. These aren’t isolated benefits. They cascade through nearly every organ system.
Muscular Strength Is a Close Second
Strength isn’t just about lifting heavy things. It’s an independent predictor of how long you’ll live. In a large prospective study of men, those in the top third of muscular strength had a 36% lower risk of dying from any cause compared to those in the bottom third. Among men over 60, that gap widened to 56%.
What’s especially notable is that strength still matters after you account for cardiorespiratory fitness. Even after adjusting for aerobic capacity, strong individuals had a 34% lower risk of death. This means strength protects you through mechanisms that go beyond just having a healthy heart and lungs.
One key reason: skeletal muscle is the primary tissue responsible for pulling sugar out of your bloodstream after a meal. When your muscles are larger and more metabolically active, they absorb glucose more efficiently, which directly protects against insulin resistance and type 2 diabetes. Losing muscle as you age (a process that accelerates after 50) undermines this system and raises metabolic risk. Strength training is the most direct way to slow that loss.
The best outcomes in the research belong to people who score high on both fitness and strength. Men in the top third for both had a 51% lower risk of death compared to those who were weak and unfit. That combination outperformed either quality alone.
The Other Three Components
The five standard health-related fitness components are cardiorespiratory endurance, muscular strength, muscular endurance, flexibility, and body composition. The first two carry the strongest evidence for longevity. The remaining three matter, but in different ways.
Muscular endurance (how long your muscles can sustain repeated effort) overlaps heavily with both strength and cardiorespiratory fitness. Improving either of those tends to improve muscular endurance as a byproduct. It’s rarely studied as an independent predictor of health outcomes because it’s so intertwined with the other two.
Body composition (the ratio of fat to lean tissue in your body) clearly affects health. Excess body fat, particularly around the organs, raises blood pressure, disrupts blood sugar regulation, and increases cardiovascular risk. But body composition is largely an outcome of the other components. Building cardiorespiratory fitness and muscular strength reliably shifts body composition in a healthier direction. It’s more useful to think of it as a result you’re tracking rather than a component you train directly.
Flexibility (range of motion around your joints) has the weakest link to major health outcomes. Research reviews consistently describe the evidence connecting flexibility to mortality or chronic disease as “unclear” and “limited.” That doesn’t mean stretching is useless. Adequate flexibility helps you move well, avoid injury, and maintain independence as you age. It just doesn’t carry the same life-or-death weight as aerobic and strength fitness.
How to Train for What Matters Most
The American College of Sports Medicine recommends at least 150 minutes per week of moderate aerobic activity (or 75 minutes of vigorous activity) plus at least two days per week of resistance training. But the guidelines for strength training remain vague compared to the aerobic recommendations, with no specific duration in minutes.
A year-long supervised trial found that splitting exercise time equally between aerobic and resistance training (30 minutes of each per session, three times a week) was optimal for reducing chronic disease risk and improving longevity markers. That’s a practical framework: if you have an hour to exercise, spend half on cardio and half on strength work.
If you’re starting from zero and can only pick one thing, prioritize cardiorespiratory fitness. Walk briskly, bike, swim, or do anything that elevates your heart rate for sustained periods. The mortality data is unambiguous: moving from “low” to even “moderate” aerobic fitness delivers the biggest jump in protection. Once that habit is established, add resistance training to capture the additional, independent benefits of strength.
Tracking Your Progress Without a Lab
The gold standard for measuring cardiorespiratory fitness is a lab-based test that measures your peak oxygen consumption during exercise. That requires specialized equipment most people don’t have access to. But simpler markers track reasonably well with actual fitness changes. Your resting heart rate (lower is generally better), how long you can sustain a treadmill or running effort before fatigue, and your performance on a timed distance (like running or walking a set route) all reflect real changes in aerobic capacity.
A study comparing these surrogate measures to lab testing found a strong correlation (r = 0.60) between improvements in training performance and actual gains in peak oxygen uptake. That’s not perfect agreement, but it means if your pace on a regular route is getting faster or your heart rate during the same effort is dropping, your cardiorespiratory fitness is almost certainly improving.
For strength, the simplest measure is whether you’re progressively handling more resistance over time. Grip strength is widely used in research as a proxy for whole-body strength and is one of the strongest predictors of health outcomes in older adults. If you can squeeze harder, carry heavier loads, and get up from a chair more easily, you’re on the right track.