Cardiorespiratory Fitness Adds a Missing Measure to Heart Health

Strength training builds capacities that matter throughout life, from preserving muscle and bone to making everyday movement easier. It can help people stay capable as they age, but it doesn’t describe every part of physical health. A large treadmill-testing study points to another measure that deserves attention: how well the heart and lungs support exertion. For lifters who treat cardio as an obstacle to muscle, that’s a useful reminder that strength alone doesn’t capture the full picture of health. The two forms of fitness answer different questions.

Published in Medicine & Science in Sports & Exercise, the study examined 562,234 adults who had completed clinical treadmill tests. Researchers assessed whether cardiorespiratory fitness could improve estimates of who would die during the next decade, beyond factors such as age, sex, high blood pressure, diabetes, cholesterol problems, and smoking. It did: fitness added information that those familiar risk markers didn’t provide. That matters because common health checks tend to focus on measurements taken at rest, while a treadmill test captures how the body responds when physical demand rises. It’s a different view of risk, though one that depends on a person’s ability to complete the test.

What treadmill fitness revealed

The team expressed exercise capacity in metabolic equivalents, or METs, which compare the energy required for an activity with the energy used at rest. Participants in the least-fit fifth scored below five METs. After adjustment for conventional risk factors, their estimated 10-year mortality risk was 32.9 percent, compared with 13.4 percent among fitter participants. Put plainly, the study found a substantial difference between the groups even after accounting for several familiar markers. But the figures describe estimated risk within this study’s population, not a personalized prediction for any one reader.

That gap is striking, but the study population matters. These adults had been referred for clinical exercise testing, so they don’t represent a random sample of healthy people. People sent for testing may differ from the general population in ways that affect both fitness and health, and the results can’t erase that selection. The findings show how fitness relates to risk in the group studied; they don’t supply a direct forecast for every gym-goer. This is more significant than it looks as a measure of added information, but the setting limits how broadly the numbers can be applied.

Each additional MET of fitness was associated with a 16 percent lower hazard of death during follow-up. That association doesn’t establish that increasing fitness by one MET will itself reduce someone’s risk by that amount. The analysis was observational, and people with better health may be able to exercise harder in the first place. Adjusting for several major risk factors helps, but it can’t account for every possible difference between participants, including factors the researchers didn’t measure. The result is a strong association to investigate, not a guaranteed payoff from a specific training change.

Fitness also made mortality estimates somewhat better, rather than turning them into precise predictions. Adding cardiorespiratory fitness raised the model’s discrimination score from 0.662 to 0.682. The improvement was statistically meaningful, though modest. In practical terms, the added measure helped the model distinguish between people at different levels of risk, but it didn’t make the estimate exact. The useful point is that exercise capacity appears to add context alongside measures such as blood pressure and cholesterol, especially when clinicians are assessing a person whose risk isn’t fully captured by those markers.

Cardio can fit alongside strength training

The findings also challenge an old split in fitness culture: lifting for size and strength, cardio for everyone else. Muscle and strength support health, mobility, and independence. Aerobic fitness reflects a different capacity, and this study suggests it carries information relevant to longevity. Treating the two forms of training as competitors leaves part of the picture out. If you’re working in this space, the more useful question is how to make both capacities part of a routine someone can sustain.

That doesn’t mean every lifter needs to run long distances. Cycling, rowing, swimming, and incline walking can raise breathing and heart rate without requiring a pounding stride. The aim is to build enough aerobic capacity that ordinary exertion feels manageable, then increase the work gradually. Starting with a form of activity that fits a person’s body and schedule may make consistency more realistic than treating cardio as a separate athletic project. People with symptoms or known heart disease should seek medical guidance before pushing hard.

A strong deadlift is worth having. So is the ability to carry groceries upstairs without turning the trip into a test of endurance. For competitive bodybuilders, cardio may already be part of the training plan; keeping it in the routine after competition makes sense too. For everyone else, the study adds to the case for pairing strength work with aerobic exercise as part of a broader approach to staying capable over time. A little more breathing room in daily life.

What the findings mean for training

The study doesn’t prescribe a training program, and it can’t tell an individual how much cardio to add. Its contribution is narrower: treadmill-measured fitness offered information about mortality risk beyond several standard factors. That distinction matters. A marker can help describe risk without proving that changing the marker will produce a particular outcome, and the study’s design leaves that causal question open.

And yet, the result gives people who prioritize strength a reason to take aerobic capacity seriously. Cardio doesn’t have to displace lifting to count; it can complement it by training a different system and making sustained effort less demanding. The practical test isn’t whether every workout becomes a race. It’s whether a routine builds strength while also leaving room to improve how comfortably the body handles exertion. For many people, that’s a more useful measure of fitness than a single lift or a treadmill score alone.