What is Cardio?

What is Cardio?

Cardio is any continuous, rhythmic activity that raises your heart rate and breathing for a sustained period — running, cycling, swimming, rowing, and brisk walking are all common forms. The name comes from 'aerobic', meaning your muscles use oxygen to produce energy during the activity, which is what allows it to be sustained for minutes at a time rather than just seconds.

How it works

Cardio works by placing a sustained demand on your heart, lungs, and muscles that they meet using aerobic metabolism — burning fuel with oxygen rather than the short, oxygen-independent bursts used for a maximal lift. Over weeks of consistent training, your heart's stroke volume increases, capillary density around your muscles improves, and your cells become more efficient at using oxygen, which together lower your resting heart rate and raise how much work you can sustain before fatiguing.

Types

Worked example

Here's one way to hit the CDC's 150-minute weekly moderate-cardio guideline without one long session.

That totals 150 minutes for the week — split however fits your schedule, since the guideline counts cumulative minutes, not one continuous block.

Cardio vs HIIT

Most well-rounded programs use both — steady-state cardio for volume and recovery days, HIIT for time-efficient intensity.

Common misconceptions

• "Cardio makes you lose muscle." Moderate cardio alongside adequate protein and resistance training does not meaningfully reduce muscle mass; only very high volumes of cardio combined with a large calorie deficit and low protein pose a real risk.
• "You need to stay in a specific 'fat-burning zone' to lose fat." Total energy balance across the week matters far more than the exact heart-rate zone; higher intensities burn more total calories per minute even though a slightly smaller percentage comes directly from fat.
• "More cardio is always better for health." Benefits rise sharply up to about 150-300 minutes of moderate cardio per week, then continue to add smaller returns beyond that — extreme volumes without recovery can raise injury and overtraining risk.

Related terms

Cardio FAQ

How much cardio should I do per week?

The CDC and AHA recommend at least 150 minutes of moderate-intensity cardio, or 75 minutes of vigorous-intensity cardio, or an equivalent mix, spread across the week for general cardiovascular health benefits.

Does cardio burn muscle?

Moderate cardio combined with adequate protein intake and resistance training does not meaningfully reduce muscle mass. Only very high volumes of cardio paired with a large calorie deficit and low protein intake pose a real risk to muscle.

What counts as moderate-intensity cardio?

Moderate-intensity cardio raises your heart rate enough that you can still talk in full sentences but couldn't sing — brisk walking, easy cycling, and swimming at a steady pace are common examples.

Is it better to do cardio before or after lifting?

Doing cardio after lifting, or on a separate day, generally preserves more strength and power for your lifts, since fatigue from cardio beforehand can reduce the weight you can move on your main lifts.

What's the difference between cardio and HIIT?

Cardio is the broad category of aerobic exercise that raises your heart rate. HIIT is one specific style of cardio that alternates short near-maximal efforts with recovery periods, rather than holding one continuous, moderate pace.

References

1. What Counts as Physical Activity for Adults. CDC
2. Aerobic Exercise: What It Is, Benefits & Examples. Cleveland Clinic
3. Updated exercise guidelines showcase the benefits to your heart and beyond. Harvard Health
4. Cardiovascular Effects and Benefits of Exercise. PMC. PMC6172294