No, not really. It doesn’t work like that.
The principle of reversibility can be summarized as “use it or lose it”. However, what happens after an individual stops training regularly is far more nuanced. How much one loses depends on how long they have been training, their overall fitness level, and what qualities we are observing.
Changes that are more temporary include changes that typically occur more quickly. Changes dependent on metabolic enzymes, energy sources, and blood volume reverse (and return) much more quickly than structural changes such as capillarization of muscle, tendon strength, muscle fiber composition, and strength.
There is also a big difference in reversibility when we look at a person’s training history. “Some detraining ‘effects’ are not the same when we compare the elite athlete with several years of training history to the previously sedentary, recently trained person who engages in activity for health-related purposes versus performance purposes.”
For the majority of casual exercisers, a few weeks off from exercise may mean that they need to start back at lower intensities, durations, or frequencies to reduce the risk of injury.
In my years of practice, I’ve never actually seen an individual permanently lose their strength or conditioning potential after months or even years off from regular exercise or training. A lot of that has to do with the fact that our abilities are often inherent, ie, training simply uncovered our strength or endurance potential. No, it doesn’t come back right away, but it does come back. From my observations, it can take 3-6 months to regain one’s abilities and recondition the body to handle the stresses of training again, but barring injury, that potential is usually just under the surface.
There is one caveat to all this: Many folks come back and find they can lift just as much weight or run just as fast as they did years before because their nervous system remembers. However, when the body isn’t used to regular mechanical stress, it can become structurally weaker. I’m talking about bones, muscles, and connective tissue. You have to be patient and give these structures time to regain their structural integrity or else you may find yourself with an injury.
Many folks begin a gym habit and feel great, but after being off for a few weeks due to vacation, holidays, or life in general what used to be easy can feel hard again. Don’t get discouraged, it will come back a lot faster than you think. However, be kind to your mind and body, take your time, and start from where you are standing.
“Detraining: The Loss of Training-Induced Adaptations in the Short Term,” SimpliFaster, January 16, 2017, https://simplifaster.com/articles/detraining-the-loss-of-training-induced-adaptations-in-the-short-term/; Iñigo Mujika and Sabino Padilla, “Detraining: Loss of Training-Induced Physiological and Performance Adaptations. Part I,” Sports Medicine 30, no. 2 (August 1, 2000): 79–87, https://doi.org/10.2165/00007256-200030020-00002.