We’ve all had to take some time off from exercising. It is usually related to an injury or a change in schedules at home or work. It can be frustrating to know that all the gains in fitness you made are slowing going away as you miss workout after workout. But how long does it really take to lose all that you had gained? Once you get back to running, cycling or lifting again, do you get back to your previous level of fitness much faster than it originally took to get there? I wanted to know the answers to these questions, so I started doing some research.

It’s important to note that “fitness” can take on two forms. Aerobic fitness is the measurement of how efficiently your body can absorb oxygen into the blood and transport it to your muscles. In running this means your ability to maintain a certain level of performance, essentially your endurance. Structural fitness ​relates to the strength of your muscles, and for runners, this translates into how well your body can absorb and tolerate the impact of running. Structural strength is essential for avoiding overuse running injuries.

Both research and personal experience shows that an experienced runner who has built up running endurance over many years will retain most of their aerobic fitness for several months, while a beginner runner’s overall aerobic fitness will drop sooner (within a week) following a period of inactivity.

Structural fitness will start to decline after seven days or more of inactivity, but the big difference being it will take longer than aerobic fitness to build back up again. You're demonstrating to your body that you don't need those muscles anymore. Over time your body will revert to a stable state that's adapted to the workload that you're giving it. Your body will also start shifting more attention to type 1 muscle fibers away from the high burning Type 2 muscles. At this point, it really depends on who you are and how well you're trained. Some athletes see a loss of about 6% muscle density after three weeks. Some power lifters see losses of as much as 35% after seven months. Young women who trained for seven weeks and gained two pounds of muscle mass, lost nearly all of it after detraining for seven weeks.

In 2016, there was a study (https://www.ncbi.nlm.nih.gov/pubmed/29212672) done with 21 people that had trained and raced the Boston Marathon. Immediately after completing the race, they were only allowed to exercise a total of two hours or less a week with no more than 3-4 miles total running per week . During their training leading up to the race, they averaged nearly 32 miles a week. So these were some well-conditioned athletes.

After the first four weeks, the runners had significant drops in blood volume and plasma volume. These changes can be thought of as the body’s blood-pumping system deteriorating, both in terms of the size and power of the pump. The result of this would make a given running pace feel harder, because less oxygen would reach working muscles per heart beat than had been the case at the time of their marathon. The changes that were experienced in the first four weeks stabilized and didn’t get much worse over the second four weeks.

At the end of the 8 weeks, a treadmill test found that the average time to exhaustion decreased by 5 to 6 seconds per week. This test matches most runners’ observations that their ability to sustain a hard pace had declined more than their ability to maintain their easy pace. It’s worthy of noting that there was a lack of decline in VO2 max and haemoglobin mass. According to the researchers, this suggests that with the resumption of training, you would see a fairly rapid return to form.

When researching cycling detraining, I found a review of over 60 studies from The Journals of Medicine and Science in Sports & Exercise and Sports Medicine.

They summarized what can occur during a 2 to 4 week break from training. They found the same drops in blood and plasma volume as the Boston Marathon runners study. Other effects of detraining include a loss of flexibility, a decrease in your lactate threshold pace, and large reductions in your muscle glycogen concentration and aerobic enzyme activity. Interestingly, the fitter you are, the greater these losses tend to be.

They found that endurance performance tends to remain the same or actually improve after just a few days without training. This is not surprising because when you are in hard training you are perpetually fatigued, so a short break allows your body to recover and adapt to your previous training (i.e. a taper before a race). Between 1 and 2 weeks off from training, however, the benefits of recovery start to become outweighed by a loss in fitness. Performance is likely to decrease by about 3-5% after 3 to 4 weeks of detraining.

From what I found, it appears that the body will lose its ability to tolerate the impact of running sooner so than its loses its ability to run. This can be scary for runners that are returning from injury. Your aerobic fitness might be telling you a six-mile run is fine, whereas your body’s ability to actually tolerate six miles of running has decreased due to the decline of structural fitness.

The better shape you were in, the less time it will take to get back into shape. Your muscle memory remains for a long time after your muscles have faded. Your body remembers how it was able to run and lift, you just have to remind it and get those muscles, blood vessels, and lungs back in shape to make it happen again​. Of course, it will take more time the longer you go without exercise. So slowly build the mileage back up as you are more at risk of injury because of the decrease in structural fitness. This applies to both beginner and experienced athletes.