the Boston Marathon the most famous road race in America for some the ultimate test of endurance but where does that ability to endure come from what's the physiological difference which we know that can run over 26 miles and those that can only watch in all can an average person standing on the sidelines be transformed into an athlete crossing the finish line nova intends to find out chosen from hundreds of applicants these are the participants in a real-life experiment they come from all walks of life but share a common dream to run the Boston Marathon to make sure the runners get off their seats safely nova collects some baseline health information as tsommo runs on a treadmill her exhaled breath is captured by a mouthpiece and then analyzed for oxygen content revealing how much oxygen she's using about 30 seconds I'm going to increase that incline in the speed okay the goal is to find the maximum rate that Samas body can consume oxygen her vo2 max vo2 max is really the measure of the person's body's ability to extract and utilize oxygen during exercise and it's the best measure we have of a person's cardiovascular or aerobic fitness are you doing alright here's why to run your leg muscles must continually contract and relax the source of the movement lies deep inside each muscle cell where tiny proteins grab and release each other but each grab and release requires energy this energy comes mainly from fat and carbohydrate mixed with oxygen they keep going you're doing great good job so to keep running Samas body must move large amounts of oxygenated blood from her lungs all the way to those tiny filaments inside each muscle cell when we measure somebody's vo2 max it's a very interesting number because it is really complicated and there are a lot of different factors so it's how well the heart is beating it's how well the vessels are expanding how elastic they are how many capillaries there are to bring the oxygenated blood to the muscles so with one number that shows us an overall good health of the entire cardiovascular system okay stop right here okay you're a little bit below what we think of for the average but one of the things that determines this is really how much physical activity we get how much exercise you do okay so as you become more fit and do more training we would expect this vo2 max value to go up the team's first run was two miles by the fall they were at five December 10 when we look at the gains from July to December when they ran the ten-mile run I believe that probably 90% of the fitness happened in that time period for most of the Nova team what's happened what's changed the runners hearts are more efficient filling up faster between beats and pumping more blood with less energy they might even be slightly bigger certainly the heart is working a bit better but by far the majority of the changes are happening with the vessels the plumbing of the body arteries and veins have become more elastic easing blood flow and down at the level of the muscle cell itself there are more tiny capillaries meaning faster delivery of oxygen even inside the cell energy production has been ramped up by mitochondria the structures that transform fat carbohydrate and oxygen into energy as you become more and more trained the muscle actually starts making more mitochondria and also making them larger so that they can actually process and break down more fuels for energy from their hearts to the tiniest enzymes in their cells these bodies were transformed the human body is an amazing organism and what we see is that when you don't use things you lose that body tissue the first day that you go out and exercise you are healthier than the day before because the body all of a sudden realizes oh I've got to use those muscles I have to make them stronger I have to make them bitter I have to make them so that I can cover more ground so it's a survival mechanism that our body is responsive to the demands that we put upon it you