When you have put in a lot of work into your fitness, there is a temptation to think the results are permanent and that you can now think of taking off some of that gym time.
Here is some bad news: It is actually very possible for your body to start regressing because of this more relaxed change in routine.
The biology of an exercise routine
The importance of routine in fitness is still often understated, but it can be boiled down to this: Your body gets stronger with more activity. Whether it is cardio or strength exercises, the parts that are being used in the activity will tap into more of its share of mass and energy resources.
More specifically, the key is to do it in bursts your body is not entirely used to. All those gains are the result of your body adapting to the new load it must undertake to perform the exercise. It repairs the stressed out areas of your muscles and blood vessels while replenishing spent energy.
Naturally, the reverse is true (although not quite in ways most people expect). Should there be a decrease in such activity, the body will also start losing its fitness edge because it no longer needs to use the same resources to perform.
For example, feats like running two kilometers in 20 minutes would require your body to produce a lot of blood to keep pumping in all the oxygen you need to keep going. But if you are no longer in the habit of needing to run that much, your body ceases producing that much blood for its new, more relaxed regular activity.
Also read: Dor Eckstein Discusses the Impact of Exercise and Nutrition in Fitness
Note - Body still needs rest from exercise
Now, while losing fitness gains can be problematic, it should not be a reason to have absolutely no breaks. Over-exercising can be just as problematic as taking too much time off.
It is important to remember that the growth and improvements that come with exercise are generally the result of your body repairing itself from the strain of activity. This regeneration, in turn, has its limits (usually defined by a number of factors such as your body type, genes and diet).
That is why it is more reasonable to understand how long a break should be. Taking an hour off a week would probably be fine for most people. But take off any form of exercise completely and all the fitness gains could be lost before the end of a month.
Transition is also important, whether you are adding or reducing the amount of exercise you do. If sudden health complications are making it harder to, say, do lifts, then start reducing the weights bit by bit. If you are used to biking only 30 minutes a day, try to extend that by just another 10 minutes instead of doubling it.
Lastly, there is nothing wrong with consulting doctors and professional trainers on how to adjust your routine. Being as educated as you can about your own fitness makes it more effective than any extreme change!
Also read: How Moderate Exercise Can Help You Manage Your Chronic Pain