Cardio vs. Strength Training: What Each One Actually Does for Your Body

What Cardio Actually Does Inside Your Body

Cardio — short for cardiovascular exercise — is any sustained movement that raises your heart rate and keeps it elevated. Running, cycling, swimming, and brisk walking all qualify. The defining feature is that your muscles are primarily powered by oxygen, which is why this type of exercise is also called aerobic (meaning "with oxygen").

The adaptations cardio produces are concentrated in the cardiovascular and respiratory systems. Over time, regular aerobic exercise causes the heart to pump more blood per beat (increased stroke volume), lowers resting heart rate, and improves the body's ability to deliver and use oxygen during activity — a measure known as VO2 max. These changes reduce the workload on the heart during everyday tasks and are strongly associated with lower risk of cardiovascular disease.

Cardio also has measurable effects on mental health. Research consistently links regular aerobic exercise to reduced symptoms of anxiety and depression, largely through its influence on neurotransmitters and stress hormones. For a deeper dive into how exercise intensity affects these processes, see our article on aerobic vs. anaerobic exercise.

What cardio does not do particularly well is build significant muscle mass or meaningfully increase resting metabolic rate. It burns calories during the session, but that effect is largely temporary.

What Strength Training Actually Does Inside Your Body

Strength training — also called resistance training — involves working muscles against an opposing force: free weights, machines, resistance bands, or even your own bodyweight. The goal is to create enough muscular tension to stimulate adaptation. For more on bodyweight-only approaches, see our guide to bodyweight training.

When you lift, you create microscopic damage to muscle fibers. The body repairs and rebuilds those fibers slightly thicker and stronger — a process called hypertrophy. Accumulated over weeks and months, this increases lean muscle mass. More muscle mass raises your basal metabolic rate, meaning you burn more calories at rest, though the magnitude of this effect is often overstated in popular fitness media.

Strength training also places mechanical stress on bones, stimulating them to increase density. This is one of the most important long-term benefits, particularly for women and older adults at risk of osteoporosis. Hormonal responses — including the release of growth hormone and testosterone — also occur during resistance exercise, supporting tissue repair and adaptation. Our article on strength training after 40 covers how these responses shift as you age.

Where the Two Approaches Overlap — and Where They Don't

The most common misconception about cardio and strength training is that they compete. In practice, they produce largely complementary adaptations. A well-functioning body benefits from both a strong cardiovascular system and adequate muscle mass, and neither form of exercise reliably delivers what the other specializes in.

There is some middle ground. Higher-rep strength circuits performed with minimal rest can elevate heart rate enough to provide modest cardiovascular stimulus. Similarly, very prolonged cardio at low intensity can cause muscle loss over time if calorie intake is insufficient — but this is an edge case, not typical recreational exercise.

The bigger practical question is sequencing and volume. Current guidelines from bodies such as the U.S. Department of Health and Human Services suggest adults aim for at least 150 minutes of moderate-intensity aerobic activity per week, plus muscle-strengthening activity on two or more days. That framework treats both types as essential, not interchangeable. If you're curious about the low-intensity end of cardio, our overview of Zone 2 training explains a method increasingly popular among everyday exercisers.