To first understand what happens to our body when we stop working out, we must understand what occurs when we start working out. When you begin your fitness journey, you must encounter a greater force that you aren’t typically used to. For instance, if you are used to doing squats, adding a load, such as weights, or some sort of resistance will force your body to adapt to the greater force, thus allowing you to reach higher levels of fitness. However, what happens to our bodies when we discontinue our fitness journey?
Depending on what category of fitness you are into (e.g., strength training or cardio), your body will respond differently when you stop working out. According to the BBC, a marathon runner who discontinues their training will experience a decrease in their VO2 max (the maximum amount of oxygen one can use during exercise) by 10% within the first four weeks. Those who are highly trained athletes will still experience this, but it tends to even out after the four weeks have passed.
As for strength training, individuals begin to notice a decrease in the amount of weight they can lift at around 12 weeks. However, according to a study published by the National Library of Medicine, individuals can actually maintain some of the strength they gained during training. Not only that, but the study also shows that there was a minimal decrease in the size of the muscle fibers. Despite this, the number of muscle fibers actually begins to decrease by 12% around two to three weeks of discontinued training.
Overall, your body will begin to feel a difference from the discontinued training at around two to three weeks if it is cardio training, and six to ten weeks if it is strength training.
SOURCES
[Strength and skeletal muscle adaptations in heavy-resistance-trained women after detraining and retraining - PubMed (nih.gov)](https://pubmed.ncbi.nlm.nih.gov/1827108/)
[How soon do we become unfit when we stop exercising? - BBC Future](https://www.bbc.com/future/article/20210603-how-soon-do-we-become-unfit-when-we-stop-exercising)