Cardio vs. Strength Training: What Actually Happens to Your Body

Understand the physiological differences between cardio and strength training so you can make informed choices about your exercise routine.

What Each Type of Exercise Actually Does

When you lace up for a run or pick up a set of dumbbells, your body launches into very different biological processes depending on what you're doing. Understanding those differences — in plain terms — can help you make exercise choices that genuinely fit your goals. For a quick primer on fitness terminology, see our plain-language fitness glossary.

Cardio: Training Your Heart and Lungs

Cardiovascular exercise — walking, jogging, cycling, swimming, dancing — is any sustained activity that raises your heart rate for an extended period. During cardio, your heart pumps harder to deliver oxygen-rich blood to working muscles. Over time, this makes the heart stronger and more efficient, lowers resting heart rate, and improves your body's ability to use oxygen (a measure researchers call VO2 max). Your lungs also adapt, moving air more effectively with each breath.

Strength Training: Building and Preserving Muscle

Strength training — using body weight, free weights, resistance bands, or machines — creates small amounts of stress on muscle fibers. Your body repairs those fibers during rest, making them slightly thicker and stronger. This process, called muscle protein synthesis, gradually increases lean muscle mass. More muscle tissue means your body burns more calories even at rest, supporting metabolic health over the long term.

Criterion	Cardiovascular Exercise	Strength Training
Primary system trained	Heart, lungs, circulatory system	Muscles, bones, connective tissue
Calories burned during session	Generally higher (sustained effort)	Moderate (varies by intensity)
Effect on resting metabolism	Modest, short-term boost	Longer-term increase via muscle gain
Bone density benefit	Moderate (weight-bearing types)	High — direct bone-loading stimulus
Equipment needed	Often none (walking, running)	Some (weights, bands, machines)
Mental health benefits	Well-documented, especially for mood	Significant, particularly for anxiety
Recovery time	Lower for moderate intensity	48–72 hours per muscle group

The Health Benefits — Beyond the Surface

Both exercise types deliver meaningful health benefits, though they emphasize different systems in your body.

What Cardio Supports

Heart and circulatory health: Regular aerobic activity is linked to lower blood pressure, reduced LDL cholesterol, and a lower risk of cardiovascular disease, according to guidance from the American Heart Association.
Blood sugar regulation: Cardio helps muscles absorb glucose from the bloodstream more effectively.
Mental wellbeing: Aerobic exercise is associated with reduced symptoms of anxiety and depression, partly through the release of endorphins and other mood-regulating neurochemicals.

What Strength Training Supports

Bone density: Resistance exercise places mechanical load on bones, stimulating them to stay dense — an important factor as we age.
Metabolic rate: Muscle is metabolically active tissue; building more of it raises the number of calories your body burns at rest.
Functional strength: Everyday tasks — carrying groceries, climbing stairs — become easier when your muscles are conditioned.

150 min

Recommended weekly moderate aerobic activity

The U.S. Department of Health and Human Services Physical Activity Guidelines for Americans recommend at least 150 minutes of moderate-intensity aerobic activity per week for most adults.

2x/week

Recommended muscle-strengthening sessions

The same federal guidelines suggest muscle-strengthening activities on two or more days per week, targeting all major muscle groups.

3–8%

Metabolic rate increase with strength training

Research published in peer-reviewed exercise science literature suggests consistent resistance training can raise resting metabolic rate by approximately 3–8% through increases in lean muscle mass.

Both forms of exercise also reduce the risk of chronic conditions including type 2 diabetes and certain cancers, according to research reviewed by the CDC. You can read more about how these needs shift across life stages in our guide on staying active as you age.

This article provides general health information for educational purposes and is not a substitute for personalized medical advice. Please consult a qualified healthcare professional before starting any new exercise program, particularly if you have an existing health condition.

Do You Have to Choose? The Case for Combining Both

Here's the encouraging truth: you don't have to pick one. Major public health guidelines — including those from the U.S. Department of Health and Human Services — recommend that adults get both aerobic activity and muscle-strengthening exercise each week. These two types complement rather than compete with each other.

If you're just starting out, the most important thing is building a consistent habit rather than optimizing the perfect split. Even modest amounts of movement deliver real benefits. As you grow more comfortable, you can layer in variety. Our article on common exercise myths can help you let go of all-or-nothing thinking that often derails new routines.

When you're ready to think about where to work out, the home workouts vs. gym membership comparison walks through how to weigh your options practically. And don't overlook the value of structured classes — our overview of group fitness classes shows how formats like yoga, spin, and water aerobics can make movement more enjoyable and social.

A Note for Special Populations

If you're pregnant, managing a chronic condition such as heart disease or osteoporosis, or are returning to exercise after an injury, please consult your healthcare provider before beginning or changing your exercise routine. General guidelines may not apply to your specific situation, and a qualified professional can help you exercise safely and effectively.