Of course we all have periods we don’t run. For example because of holidays or – even worse – facing an injury. After how many days or weeks do you really need to start worrying about your running fitness? According to this study – dating back to 2008 – that’s pretty soon.
If you don’t run for two weeks, there would already be a huge decline in endurance; your enzyme levels in your blood that are associated with endurance performance decline by fifty percent and your VO2max drops by 7 percent at the same time.
Don’t be afraid; the biggest decline occurs in the first few weeks and after that it fortunately slows down. Even after two/three weeks of no running, there is still a decline in enzyme levels, but the declines are already flattening out.
After three months of running inactivity not so much happens anymore; experienced runners are back to their general fitness level and will not quickly drop below that level. That only happens after about a year of complete non-running. Incidentally, this is mainly about endurance; you retain your speed for a longer time. In addition, this research is based on complete inactivity, so there is no swimming or biking.
