While muscle mass and strength levels stay more or less the same for quite a while, one of the first things you will notice when you stop training is a decrease in your endurance. Running up the stairs or making a dash for the train may still be something that you can do, but you will soon feel a difference as you don’t seem to have the ‘wind’ that you used to, and you might have to stop and catch your breath a little more than you used to. Unlike strength losses, which are mostly psychological, endurance declines relatively quickly after training is stopped. One of the adaptations from intense exercise is a significant increase in aerobic capacity or peak oxygen uptake— commonly referred to as VO2 max. After a period of intense regular exercise this value goes up as your heart’s capacity to pump blood increases and the volume of blood sent out with each individual beat of the heart (cardiac output and heart stroke volume respectively).[19,26] This increased efficiency translates into a tangible boost in endurance— however in keeping with the reversibility concept, these increases deteriorate over time. A rapid average decline of 7% of peak oxygen uptake was observed in a study of experienced athletes who stopped training within 12 to 21 days, with a further drop in VO2 max values of 9% during the following 21- 84 days of inactivity. The initial 7% decline during the first 12 days of inactivity was linked to a reduction in maximal heart stroke volume whereas the decrease in VO2 max during the 21-84 day period was associated with a decline in maximal arteriovenous oxygen, (which is the technical way of referring to the maximum amount of oxygen is present in your blood).