When it comes to cardiovascular fitness and muscle strength, the adage is true: Use it or lose it. While regular exercise can improve heart health and increase strength and mobility, taking weeks or months off can reverse many of those benefits.

That’s not to say that rest days are not important. In general, short breaks can help you physically and mentally recharge, but whenever possible, you should avoid extending your time off for too long so that hopping back on the wagon doesn’t feel too daunting or miserable.

“Your body adapts to the stimulus you provide,” said Dr. Kevin Stone, an orthopedic surgeon and the author of the book “Play Forever: How to Recover From Injury and Thrive.” “Your muscles become used to the stress and the testosterone, the adrenaline and endorphins — all the wonderful things that circulate from exercise. When you take that away, the body initiates a muscle loss program.”

What does it mean to lose fitness?

To understand the phenomenon of fitness loss, it is helpful to think about how activity and, therefore, inactivity, affects your cardiovascular system and muscle strength. Because regular exercise helps your body to deliver oxygen and nutrients to tissues in a more efficient way, one of the first things that declines when you become inactive is your cardiovascular endurance, said Edward Coyle, a professor of kinesiology and health education at the University of Texas at Austin.

After just a few days of inactivity, the volume of blood plasma circulating in your body decreases, Dr. Coyle said, leading to a series of other cardiovascular changes. After 12 days, studies show that the total amount of blood the heart pumps every minute decreases, along with the amount of oxygenated blood available to muscles and other cells — measured as the VO2 max.