Even when you have the best of intentions, life sometimes gets in the way of a fitness routine. And whatever the reason behind it, the absence of workouts will cause your body to lose some of the progress it had made. Here’s how an exercise hiatus impacts your body—and what to do to get back on the plus side—for five common scenarios.

THE SITUATION: You had a crazy month at work and stopped your usual four-day-a-week gym habit cold turkey.
THE EFFECT ON YOUR BOD: Doing a mix of strength training and cardio is optimal for weight loss or control, muscle building, and aerobic health. Stop for a month, and you may notice that some areas get softer, that you're not able to lug as many heavy groceries, and that you get winded a little faster from taking the stairs. "In a study of beginners who exercised for two months, their strength increased by 46 percent, and when they stopped training for two months, they lost 23 percent—half the gains they'd made," says exercise scientist Wayne Westcott, Ph.D., who points out that they were still ahead of where they'd be had they never trained at all. Further, the more fit you were to start, the slower the loss; a triathlete on a break may only drop five to 10 percent of her fitness level in a month or two. Still, when getting back into it, go easy. For strength training, start with about 75 percent of the resistance you'd been using—and increase as you feel you can. You’ll be back to where you were in probably half the length of time that you took off.

MORE: The "Get Back in Shape" Workout Plan

THE SITUATION: You used to weight train like crazy, but for the past several months, all you’ve fit in is a few sessions a week on the treadmill.
THE EFFECT ON YOUR BOD: In this case, your aerobic health should be in good shape, though you may notice that your strength and muscle tone have diminished some. Without weight training, you've likely lost muscle mass and gained some fat, even if the number on the scale stays the same. "Surprisingly, research shows that longtime endurance runners lose muscle mass at the same rate—five pounds per decade—as everyone else, including the sedentary," says Westcott. "Running and other cardio activities don't build or maintain muscle mass." Add some strength back to your bod, and into your routine, to remedy that in short order by following that 75 percent guideline mentioned above.

THE SITUATION: You ran a half-marathon, which you trained for like a fiend, then gave yourself a few weeks to recover.
THE EFFECT ON YOUR BOD: A break like this isn't a major problem aerobically for someone who was in really good cardio shape. "You'll be down from your competitive edge, but it won't take long to come back," says Westcott. "Just don't expect to come back at full-speed right away." He recommends easing back in using your heart rate (the zones may have changed from when you were at your peak) and perceived exertion—a seven on a scale of one to 10. He also recommends strength training as a muscle-building complement to your cardio workouts.

MORE: How to Make an Exercise Comeback

THE SITUATION: You've been really into yoga but now miss the CrossFit you stopped a few months ago.
THE EFFECT ON YOUR BOD: Swapping one workout for another isn’t a bad thing, necessarily. Just know that if you go back to program "A" after doing program "B," you may not be able to bring your A-game to "A" as you once could. "Unfortunately, training is very, very specific," says Westcott. He points out that at the peak of his cycling career, Lance Armstrong was (very arguably) the best athlete in the world, yet when he took up marathon running, his first race was a respectable-but-not-remarkable three hours. In the case of bodyweight training (yoga) versus weight training (CrossFit), expect your strength to be down when you first return to the gym. Which isn’t to say you should stop your Oming—no reason you shouldn’t have both in your repertoire.

THE SITUATION: You got injured and haven't been able (or wanted) to work out at all for six months.
THE EFFECT ON YOUR BOD: In this case, you've definitely lost muscle and gained fat (as if getting hurt wasn’t enough of a bummer!), especially if your everyday activity level was affected in addition to the lack of workouts. "Once you're cleared to exercise, you need to return very slowly, very light," says Westcott. "Half or less of what you once lifted may be too much; go way down and find a resistance you can do with good form and without pain for 10 to 15 reps." If you know you’re going to be sidelined (or currently are), he recommends upping your protein intake in your diet to help reduce loss of muscle mass during your time off.

MORE: Q&A: How Do I Stay in Shape While Injured?

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Amy Roberts is a certified personal trainer.