A cardio workout is any rhythmic, repeated physical activity that raises your heart rate and keeps it elevated for a sustained period. It uses your body’s large muscle groups, like those in your legs, back, and core, and relies on oxygen to fuel the effort. That’s why cardio is also called aerobic exercise, from the Greek word meaning “with oxygen.” Walking, running, cycling, swimming, and rowing all qualify, as long as they keep your heart pumping harder than it does at rest.
How Your Body Responds During Cardio
When you start a cardio workout, your breathing deepens and your heart beats faster. That increased breathing controls how much oxygen reaches your working muscles, where it’s used to convert stored energy into movement. The harder you work, the more oxygen your cells demand, and the faster your heart and lungs have to deliver it.
This is fundamentally different from short, explosive efforts like a heavy deadlift or a single sprint, where muscles burn fuel faster than oxygen can arrive. During cardio, you’re working at an intensity you can sustain because oxygen supply keeps pace with demand. That’s the dividing line between aerobic and anaerobic exercise.
What Cardio Does to Your Heart Over Time
Regular cardio reshapes your cardiovascular system in measurable ways. People who do long-term aerobic exercise develop resting heart rates between 40 and 60 beats per minute, well below the typical 60 to 100. This happens because the heart gets stronger and pumps more blood with each beat. In one study, swimmers had an average stroke volume (the amount of blood pushed out per heartbeat) of about 74 milliliters, compared to roughly 58 milliliters in a sedentary control group. A stronger pump means fewer beats needed to circulate the same amount of blood, so the heart works less at rest.
The benefits extend to blood pressure. A meta-analysis of randomized controlled trials found that endurance training reduced resting systolic blood pressure by about 6.9 mmHg and diastolic pressure by about 4.9 mmHg in people with hypertension. That’s a meaningful drop, roughly equivalent to what some blood pressure medications achieve.
Cardio also improves how your body handles blood sugar. Aerobic training increases your cells’ sensitivity to insulin, meaning they pull glucose out of the bloodstream more efficiently. Research in overweight, hypertensive adults showed significant reductions in insulin resistance after an aerobic training program, alongside lower levels of the stress hormone cortisol.
Brain Benefits of Aerobic Exercise
Cardio doesn’t just train your heart. It changes your brain. During prolonged exercise, your body produces a metabolic byproduct that accumulates in the hippocampus, the brain region central to learning and memory. This compound acts as a molecular switch, turning on the production of a growth protein that supports the creation of new neural connections, strengthens existing ones, and promotes the branching and turnover of brain cell structures.
Researchers have known for over two decades that physical activity boosts this growth protein in the brain, and that blocking its signaling eliminates the learning and memory improvements that exercise normally provides. The practical takeaway: regular cardio doesn’t just make you physically fitter, it makes your brain more adaptable and better at forming memories.
Types of Cardio Workouts
Steady-State Cardio (LISS)
Low-intensity steady-state cardio means maintaining a consistent, moderate pace for 30 to 60 minutes or longer. Think brisk walking, easy cycling, or a relaxed swim. Your target is roughly 50 to 65% of your maximum heart rate, sometimes described as a pace where you can hold a conversation without gasping. This approach is gentle on your joints, easy to recover from, and effective for building an aerobic base.
High-Intensity Interval Training (HIIT)
HIIT alternates between bursts of hard effort and brief recovery periods. During work intervals, you push to 85 to 90% of your max heart rate for roughly 30 seconds, then slow down before repeating. Sessions are shorter, typically 15 to 30 minutes, because the intensity is much higher. HIIT is time-efficient and improves cardiovascular fitness quickly, but it’s also more taxing on the body and requires more recovery between sessions.
Common Activities by Intensity
Not all cardio is equal in effort. Activities are often rated by metabolic equivalents (METs), a measure of energy cost relative to sitting still. Slow walking comes in around 3 METs, brisk walking at about 5.4, cycling at a moderate pace around 7, walking up stairs at roughly 4.7, and running at about 8.2. The higher the MET value, the more energy you burn per minute and the harder your cardiovascular system works.
How Much Cardio You Need
Current guidelines from the CDC recommend at least 150 minutes of moderate-intensity aerobic activity per week, which breaks down to about 30 minutes a day, five days a week. If you prefer vigorous exercise like running, 75 minutes per week provides equivalent benefits. You can also mix the two: a combination of moderate and vigorous activity across the week counts.
These are minimums. More activity generally produces more benefit, particularly for weight management and cardiovascular risk reduction.
Getting Started as a Beginner
If you’re new to exercise or returning after a long break, start with one to three low-intensity sessions per week. Walking, easy stationary biking, or elliptical training for 40 to 90 minutes at a comfortable pace builds your aerobic foundation without overwhelming your body. The pace should feel manageable, not punishing.
Once that feels routine, you can add short bursts of higher-intensity work. Start with just 5 to 15 minutes of harder effort (jump rope, faster cycling, bodyweight exercises like burpees or jumping jacks) two to four times per week. The key principle is gradual progression: increase duration or intensity in small steps, not all at once. Jumping ahead too fast leads to burnout or injury that sets you back further than patience would have.
Measuring Your Cardio Fitness
The gold standard for cardiovascular fitness is VO2 max, a measure of how much oxygen your body can use during maximum effort. It’s expressed in milliliters of oxygen per kilogram of body weight per minute. Many fitness watches now estimate this number.
For context, a man in his 20s with a VO2 max above 51 falls in the “excellent” range (80th percentile), while “superior” is above 55. For women in the same age group, excellent starts around 44 and superior around 50. These numbers decline naturally with age. A man in his 50s at 43 or a woman at 37 is still in the excellent category. Tracking your VO2 max over months of training gives you a concrete way to see whether your cardio routine is working, beyond just how you feel during a workout.