You don't have to panic if you have to skip one workout because you have to study, or have to work overtime. Skipping a training session can also be good for you because it helps your body to recover from previous training sessions. Recovery is critical to become stronger and fitter from the training you performed. Without sufficient recovery your body can't adapt to the training stimulus.
When you don't train for a longer period your fitness decreases
Detraining should not be confused with the recovery of one training or training period and takes place after the recovery process. When detraining starts is dependent on the intensity and volume of your previous training sessions. If you are fully recovered and you don't give your body a new training stimulus, detraining starts and your fitness decreases.
How quickly will your fitness decrease?
After three to five days of inactivity, the first signs of detraining will slowly become visible. Around the fifth day of inactivity, the blood volume decreases, making it more difficult to transport oxygen from your lungs to your muscles. This is also the reason for the higher heart rate at a certain intensity. The distribution of oxygen through your body becomes less efficient, so your heart has to beat faster to deliver enough oxygen-rich blood to your muscles.
After several days of inactivity, the lactate metabolism is also negatively affected. In trained athletes lactate levels will be higher at the same absolute intensity, which means that the lactate threshold is reduced. If you have not trained for five days your glycogen stores will also decrease to normal levels.
What happens in your body when you can't train for a longer period?
After about ten days without training, your VO2max decreases. This decrease continues after you stop exercising for a period of 6-8 weeks. Although VO2max decreases, trained athletes will remain a higher VO2max compared to inactive people.
Your muscles are also affected when you're not able to train for a longer period of time. After 2-3 weeks without training, the number of mitochondria (energy factories) in your muscles decreases. Although, in the first weeks, there are no changes in the proportion of fast and slow-twitch muscle fibres, the amount of slow-twitch muscle fibres will decrease after eight weeks.
All this implies that your muscles will have a lower ability to produce energy for high intensity exercise. After a period of detraining you can rebuild your fitness again.
Adjust your diet if you can't train for a few days
If you can't train you also use less energy. To prevent weight gains you should make sure that your energy intake isn't higher than your energy expenditure, therefore you should eat less on days you're not training. After a few days without training your body will use more carbohydrates than fat during exercise. If you have not trained for a long time, about two weeks, changes take place that favour fat tissue storage.
Despite the decrease in your fitness, a rest period after the season remains important
A period of rest after the season is very important for an athlete. At least two to three weeks without exercise helps your body to completely recover, small injuries will heal, and you will be mentally fit again. You will start the new season fit, injury free, and with a lot of motivation. To prevent that your fitness completely drops, you can participate in other sports during this rest period.