Swimming vs running: what the studies measured for heart rate, oxygen and fitness

Swimming and running both train the heart and lungs; they do it in very different places. In the water you're horizontal and the water carries your weight; on land you're upright and land on one foot at a time. Several studies put the same people through a swim test and a treadmill test, and two trained groups on one or the other for weeks. This page covers what they found, what they didn't test, a calorie estimate for each, and how to choose or mix the two.

Swimming vs running at a glance

Heart rate peaks lower in the water

• About 11 beats lower at the top. In DiCarlo 1991, 34 college-age fitness swimmers, 19 men and 15 women, were tested to their maximum running on a treadmill and swimming against a tether. Peak heart rate swimming was 11 beats a minute lower than running and 13 below the age-predicted maximum (220 minus age). Blood lactate afterwards was about the same, 8.0 and 8.1 mmol/L. The authors suggested taking about 12 beats off a running or age-predicted maximum to estimate a swimming one.²
• Lower peak oxygen, too. In Hauber 1997, 11 fitness swimmers peaked at 174 beats a minute swimming against 183 on the treadmill, and at 3.58 against 3.97 L/min of oxygen.³ In 14 helicopter rescue paramedics, peak oxygen uptake was 45.5 mL/kg/min in a swim test and 48.5 in a run, a small difference that wasn't statistically significant, while maximal heart rate was 13% lower in the water, 162 against 186.⁸ In 13 trained triathletes, swimming VO2 max was 86.6% of their treadmill value.⁹
• But below the top, heart rate tracked oxygen the same way. In Hauber 1997, the line linking heart rate to oxygen use had the same slope in the pool and on the treadmill, and at the same submaximal oxygen use heart rate didn't differ significantly. The authors' reading: heart-rate ranges built on land work for fitness swimmers below their maximum.³

So if you set swim zones as a share of your maximum, DiCarlo's authors suggest a swimming maximum, not a running one. The heart-rate zone calculator takes whatever maximum you give it.

What weeks of each did for fitness

• From sedentary, a near tie. In Lieber 1989, 37 sedentary men aged 28 to 35 trained by running, by swimming, or not at all, three days a week for 11.5 weeks, at a heart rate matching 75% of their treadmill VO2 max. Treadmill VO2 max rose 28% in the runners, 25% in the swimmers and 5% in the controls; runners and swimmers didn't differ significantly.⁴
• For people who already swam, the gain stayed in the water. In Magel 1975, 15 male college-age recreational swimmers did interval swim training an hour a day, three days a week, for 10 weeks. Their swimming VO2 max rose 380 mL/min, but their VO2 max on a treadmill running test didn't improve significantly.¹⁰

The two started from different places: the sedentary men in Lieber 1989 improved on the treadmill with either, while the recreational swimmers in Magel 1975 improved mainly in the water. That's a reading of two small, old trials in men, not a tested rule.

Calories: an estimate, not a measurement of you

The Compendium of Physical Activities lists an energy cost, in METs, for more than a thousand activities. One MET is about 1.0 kcal per kilogram of body weight per hour.⁷ It lists slow, recreational freestyle laps at 5.8 METs, the crawl at about 50 yards a minute at 8.0 and fast, vigorous freestyle laps at 9.8;⁵ running at 5.0 to 5.2 mph at 8.5 and at 6 to 6.3 mph at 9.3.⁶ For a 70 kg (about 154 lb) person, 30 minutes works out at about 205 kcal of slow laps, about 280 of steady crawl, about 345 of fast laps and about 325 of running at 6 mph. How fast you swim moves the estimate more than which one you pick. The Compendium's own authors say its values are a starting point and don't reflect precise energy use for any one person.⁷

What these studies didn't test

Most were single test sessions in small groups of swimmers or trained people; the two training trials are from 1975 and 1989 and tested men only. None measured pain or injury, and this page makes no claim about either. Kaplan 2014 measured the load of running only.

How to choose, or mix them

As a practical guide, not a finding:

• You can swim comfortably and have a pool: swim, or mix the two. If you can't swim well yet, learn with an instructor before using it as cardio.
• No pool, or short on time: run, or walk-jog. Running vs walking covers the step down.
• Training for a run: most of it should be running. In recreational swimmers, swim training didn't carry over to the treadmill test.¹⁰
• Mixing: count both toward the World Health Organization's 150 to 300 moderate minutes a week, or 75 to 150 vigorous.¹¹ How to build endurance shows how easy and hard sessions fit together, and low-impact training has the other no-landing options.

If you have pain, an injury or a medical condition, stop and ask a doctor or a physical therapist.

Common questions

Is swimming or running better for cardio?

From a sedentary start, about the same in the one trial that compared them: treadmill VO2 max rose 25% with swimming and 28% with running, not a significant difference.⁴ In men who already swam, swim training raised swimming fitness but not running fitness.¹⁰

Does swimming burn more calories than running?

By the Compendium's estimate it depends on the pace: about 205 kcal for 30 minutes of slow laps and about 345 for fast laps, against about 325 for running at 6 mph, at 70 kg.^5,6,7 No study on this page measured calories for both in the same people.

Swimming or running for weight loss?

Lieber 1989 measured body composition in its runners and swimmers, but its abstract stops before saying how the two groups compared, so this page doesn't report it.⁴ No other study here measured weight or body fat, and this page makes no weight-loss claim for either. How to lose body fat covers what matters most. Cardio vs weights for fat loss has the trials that measured body fat over weeks.

Why is my heart rate lower when I swim?

It peaked about 9 to 11 beats a minute lower in the water in two studies of fitness swimmers, and 24 lower in rescue paramedics.^2,3,8 These abstracts measured it rather than explaining it, so this page doesn't give a reason.

Can swimming replace running?

For treadmill VO2 max from a sedentary start, it did about as well in Lieber 1989.⁴ For running fitness, the recreational swimmers in Magel 1975 didn't improve their treadmill VO2 max from swim training.¹⁰

If you'd rather have the week planned

Swimming or running is a small decision next to fitting either one around lifting, work and the rest of your life. Coaching is the version where your workouts are built around the days you actually have, and a weekly call looks at the week you just had and rebuilds the next one around what's coming. Coaching for a week that doesn't hold still explains how that works, and apply is the first step. Otherwise: swim 20 easy minutes once this week if you have a pool, run or walk-jog once, and keep the one you looked forward to.