The reason VO2 max declines

We become unfit much quicker than it took to get in shape. To understand how the body becomes unfit, we need to understand how we become fit.

The key to becoming fitter is to do more than our body is used to. The stress from this makes us become more tolerant, leading to higher fitness levels. Some studies show that just six sessions of interval training can increase maximal oxygen uptake (VO2 max) and improve the body's ability to fuel itself.

Some gains in muscle force can appear in two weeks, but gains in muscle size won't be seen until around eight to 12 weeks.

12 Weeks without training causes a significant decrease in the amount of weight we can lift, but only a minimal reduction in the size of the muscle fibres. We will maintain some of the strength we gained. We lose muscle strength because the muscles are not put under stress, effectively becoming "lazy".

Cardiorespiratory fitness - shown by a person's VO2 max will decrease around 10% in the first four weeks after a person stops exercising. The rate of decline continues but eventually evens out.

Highly trained athletes (like a marathon runner) will maintain a VO2 higher than average. But the average person will fall back to pre-training levels in less than eight weeks.