If you think it’s hard to keep patients exercising, it may comfort you to know that even exercise researchers have problems getting people to stick with it. Even in carefully designed exercise studies, attrition rates can range from 25% to 50%, and the average adherence rate for those who do complete the study is about 66%.

The motivational message of physical activity always focuses on the positives: Here’s what will happen if you start exercising — stronger heart and lungs, better balance, flexibility and mobility, lower blood pressure and lipid scores, improved brain health, quality of life — and more!

How long before gains reverse? How long to become deconditioned? How long will it take to bounce back to reach previous levels? Your cells tell the tale. Not much would happen in the first couple of days, possibly within the first week, said Andrew M. Jones, PhD, DSc, professor of applied physiology at the University of Exeter, Exeter, England. That’s especially true if you’ve been training intensely before you stop, which could help you get more fit as your body recovers and adapts.

After that point, however, things do start to go downhill, Jones said, starting with plasma volume. “When you start to exercise, one of the first things that improves is that you get a larger blood volume, and that’s helpful to your cardiovascular system,” he said. And while there’s not as much evidence around detraining — and genetics and other factors may determine exactly how quickly the process occurs — he suspects that plasma volume would also be the first thing to go when someone stops training. This, in turn, may start to lower your maximal oxygen uptake. Similarly, Jones suspects that after 5-7 days, the activity and expression of various oxidative enzymes in the skeletal muscle that control those processes of mitochondrial respiration would decline, which would affect your ability to use oxygen when it arrives in the muscles.

Within a couple of weeks, you would likely see a decline in maximum strength, said Jamie Pasquin, MS, CSCS, RSCC, CPSS, exercise physiologist at Hospital for Special Surgery in New York City, although research in Sports Medicine shows this could happen as soon as 5 days after complete bed rest. Structural changes, such as muscle size and muscle volume, tend to become more apparent with longer periods of detraining, although some muscle loss can occur earlier if, again, you’re completely immobilized vs simply not getting in your regularly scheduled lifts.