What Cardio Actually Does Inside Your Body
Cardiovascular exercise — commonly called cardio — refers to any sustained, rhythmic activity that raises your heart rate and keeps it elevated for an extended period. Running, cycling, swimming, brisk walking, and rowing all qualify. The defining feature is that your body relies primarily on its aerobic energy system, using oxygen to convert stored fuel (glucose and fat) into usable energy. For a deeper look at how that system works, see our article on aerobic vs. anaerobic exercise.
The most direct adaptation from regular cardio is a stronger, more efficient heart. Over time, the heart's left ventricle expands slightly, allowing it to pump more blood per beat — a measure called stroke volume. Your resting heart rate drops, your lungs become more efficient at extracting oxygen, and your body improves its ability to clear metabolic waste during effort. These changes collectively improve what exercise scientists call cardiorespiratory fitness, or VO₂ max.
Beyond the heart and lungs, cardio training improves insulin sensitivity, supports healthy blood pressure and cholesterol profiles, and is associated with reduced risk of type 2 diabetes and cardiovascular disease. It also has well-documented effects on mood, largely through changes in neurotransmitter activity and stress-hormone regulation.
| Criterion | Cardio Training | Strength Training |
|---|---|---|
| Primary system trained | Heart, lungs, aerobic system | Muscles, bones, nervous system |
| Energy source during exercise | Aerobic (oxygen + glucose/fat) | Anaerobic (phosphocreatine, glycogen) |
| Effect on muscle mass | Minimal direct muscle growth | Significant hypertrophy over time |
| Effect on bone density | Moderate (weight-bearing types) | Strong, direct stimulus |
| Resting metabolic rate | Modest, indirect effect | Raised by increased muscle mass |
| Cardiovascular health | Strong, direct benefit | Meaningful but less direct |
| Calorie burn during session | Generally higher per session | Lower during session; elevated post-workout |
| Equipment needed | Often minimal (walking, running) | Varies (bands to free weights) |
What Strength Training Actually Does Inside Your Body
Strength training — also called resistance training — involves working your muscles against an opposing force: free weights, machines, resistance bands, or even your own bodyweight. Unlike steady-state cardio, most resistance work is anaerobic in nature, meaning individual sets are short and intense enough that your body can't rely on oxygen delivery alone. To learn how different tools compare in building that strength, see resistance bands, free weights, and machines.
At the cellular level, lifting causes microscopic damage to muscle fibers. During recovery, the body repairs and rebuilds those fibers slightly thicker and stronger — a process called muscle hypertrophy. Over repeated training cycles, this produces meaningful increases in muscle mass. Alongside that, mechanical loading signals bone-forming cells (osteoblasts) to lay down new bone tissue, increasing bone mineral density — an important buffer against osteoporosis.
One often-overlooked effect: muscle is metabolically active tissue. A pound of muscle burns more calories at rest than a pound of fat, so gaining muscle raises your resting metabolic rate. For a detailed side-by-side of how these two training modes affect energy expenditure, see our article on aerobic exercise vs. resistance training and energy expenditure.
Strength training also improves muscular endurance, joint stability, posture, and functional movement — the kind of capacity that makes daily tasks like carrying groceries or climbing stairs easier and safer as you age.
How They Compare: A Side-by-Side Look
Cardio and strength training are not competing programs — they improve different but overlapping aspects of fitness. The table above maps the key differences. For readers thinking about how these fit into a broader picture, our guide on what each major workout type does for your body covers cardio, strength, flexibility, and balance together.
150–300 min
Recommended weekly aerobic activity for adults
The U.S. Physical Activity Guidelines for Americans recommend this range of moderate-intensity cardio per week for substantial health benefits.
2x per week
Minimum strength sessions recommended
The same federal guidelines specify that adults should perform muscle-strengthening activities on two or more days per week, targeting all major muscle groups.
~1–3%
Annual bone density loss without resistance exercise
Research on aging populations suggests bone mineral density can decline at this rate after peak bone mass is reached, with resistance training being a key protective factor.
Research consistently shows that relying exclusively on one modality creates gaps. What happens when you only do one type of workout explores those imbalances in detail. The U.S. Department of Health and Human Services' Physical Activity Guidelines recommend that most adults get both aerobic activity and muscle-strengthening activity each week — not one or the other.
For most people, the practical question isn't which is better — it's how to structure both within a realistic schedule. Beginners can start with just two to three sessions per week of either type and build from there. Consult a qualified fitness professional or healthcare provider before starting a new exercise program, especially if you have any existing health conditions.
