You often hear of the importance of incorporating cardio, such as running, biking or swimming, into your exercise routine. In fact, to maintain a healthy cardiovascular system, the American Heart Association recommends you exercise at least 150 minutes per week of moderate-intensity aerobic activity or 75 minutes per week of vigorous aerobic activity.

But injuries, illnesses or just plain old lack of time happens, and we stop exercising. If you’ve been following an exercise program and suddenly stop, how quickly do you return to square one?

Below, experts reveal just how fast you lose your cardio fitness — and how long it takes to get it back.

First, a quick refresher on your cardiovascular system. Your cardiovascular endurance or aerobic fitness is defined as the ability of your lungs to efficiently take in oxygen from the air and move it to your bloodstream, where your pumping heart and blood vessels deliver it to your muscles.

Aerobic fitness is often measured in V̇O2 max, which looks at oxygen consumed per minute. The more your exercise, the more efficient your cardiovascular system becomes and the higher your VO2 max. To put it simply, how efficient is your body in allowing you to move (such as run or bike) before you have to stop due to fatigue?

“How long it takes for you to ‘lose’ your cardiovascular conditioning is dependent on a few factors,” Grayson Wickham PT, DPT, CSCS, founder of Movement Vault, tells LIVESTRONG.com. “The first factor is what your fitness level and/or cardiovascular conditioning was before you stopped your cardio workouts.”

Wickham says other important factors include how long you have been exercising consistently (several years versus just starting out) as well as your current overall health status.

“If you have healthy blood glucose and insulin levels, healthy cardiovascular biomarkers and low levels of inflammation, you will likely maintain your cardiovascular fitness longer than if you are in poor overall health,” he says. “Sleep and stress levels also play a big role here.”

Another factor is the amount of movement you are performing during the time period in which you are stopping your cardio workouts, according to Wickham. If you are moving and walking a lot during your normal day, he says, you will maintain your cardio much better than if you were lying in bed all day.

Keeping all those factors in mind, several studies have looked for ways to put a definite time frame on how detraining — or stopping and/or decreasing exercising — affects your body.

An April 2018 study in the ​Journal of Applied Physiology​ looked at recreational runners who stopped running for eight weeks. The researchers saw that after four weeks, there were negative changes in the runners’ hearts, including reduced plasma volume (the liquid component of blood that delivers nutrients, hormones and proteins to parts of the body that need it) and ventricle thickness (chambers of the heart that pump blood out).

By eight weeks, those reductions in plasma volume and ventricle size were more pronounced. (Remember, your heart is a muscle and the more you exercise it, the stronger and larger it gets!)

It’s not just those who are athletes that are affected by stopping cardio exercise. A July 2021 study in ​Topics in Stroke Rehabilitation​ looked at the cardiovascular effects of stopping exercise in those who have had a stroke. The researchers saw significant reductions in just one month of stopping exercise.

Losing cardio fitness actually starts before the four-week mark, however — changes in VO2 max and plasma volume actually begin showing up as early as two weeks of stopping exercising, according to the Centers for Disease Control and Prevention.

All of the factors discussed above come into play with this question, Wickham says, referring to your previous level of cardiovascular health, underlying medical conditions and how much movement you get on average each day.

“If you are checking all of the boxes above, you can regain your cardio fitness relatively fast — in as little as three to four weeks. The amount of time you take off of your cardio workouts is a big determining factor here. If you stop your workouts for six months, it will take a much longer period of time to regain your cardio fitness, versus only taking four weeks off,” he says.

Keep in mind your age as well. After the age of 30, you lose 2 percent of VO2 max each year, according to UC Davis Health. You can offset this decrease by maintaining a consistent exercise program or by not going longer than two weeks without exercising.

“To make the most progress in the shortest amount of time, you need to be smart and consistent with your workouts,” Wickham says. “The smart aspect means you need to be following a well-designed training program based on your goals. This program will progressively get more challenging over time, which will cause your body to make adaptations in the form of greater cardiovascular fitness. Once you have this program, you need to stay consistent with it.”

There are two main approaches to improving cardiovascular fitness: long-duration, lower-intensity workouts and short-duration, higher-intensity workouts, according to Wickham. Both will improve your cardiovascular fitness when performed consistently, he says.

“Generally speaking, for lower-intensity, longer-duration cardio workouts, you should aim for 30 to 60 minute workouts. And higher-intensity, short-duration workouts, aim for 10 to 30 minutes workouts.”

Incorporate aerobic activities, such as running, sprinting, biking, rowing, fast walking, hiking, swimming and various HIIT workouts, Wickham recommends.

It’s totally OK to take breaks away from exercise, but if you’re concerned about losing your cardio fitness, keep these breaks as short as possible, according to Wickham.

“If following a well-designed program, you can have periods of time, such as a week or two, in which you are decreasing your workout intensity as much as 50 percent,” Wickham says. “You may even take a full week off of training. You cannot train at 90 to 100 percent intensity consistently all of the time, or you will burn out, leading to overtraining.”

Try to keep your exercise break (or detraining) period no longer than two weeks and don’t stop exercising altogether, according to a landmark August 2007 review on the effects of overtraining in ​Medicine & Science in Sports & Exercise.​ You can decrease your training volume by 4o to 60 percent, and, at minimum, try to get in some cardio at least twice a week.

It takes three to four weeks before your muscle strength decreases, so you can go a bit longer resting from weight lifting, according to a May 2013 review in ​Sports Medicine​, but keep your cardio a priority.