The equation is simple: the more exercise you do, the fitter you become. The message has been pumped into us by government campaigns, shouty PE teachers and celebrities on the make. NHS guidelines state we should be doing 150 minutes of moderate exercise or 75 minutes of vigorous exercise a week - a one-workout-fits-all regime.
We've watched some of our greatest screen legends flex and pant - Olivia Newton-John, Jane Fonda - to tempt us into training. All the while we've become fatter, slower and lazier as morbid obesity spills out of our waistbands and into our hospitals.
And for those who have failed to get fitter? Guilt. Shame. Blame. Because the fitness dream is unlike all other human strata: it's meritocratic. You get out what you put in. Anyone could have Madonna's body if they just rolled off the sofa and on to a treadmill. All of this, it transpires - thanks to a new genetic test - is guff: not just simplistic and unfair on those who don't have the resources to exercise, but scientific mumbo-jumbo.
The test, called XRPredict, costs NZ$390 and has been developed by Dr Jamie Timmons, professor of systems biology at Loughborough University, and is processed by his company, XRGenomics. He has isolated the genes responsible for your body's adaptations to aerobic exercise. In other words, how much fitter you become, which can be measured by how efficiently you transport oxygen from outside the body to the muscles within.
"About 10 years ago, we started trying to come up with diagnostics that explained the differences we saw when we trained people," explains Dr Timmons. "We were seeing huge variations in response to the same exercise programme."
By 2010 their findings had been patented. They make grim reading for the terminally unfit, the Department of Health and for personal trainers everywhere.
Approximately one in six of the general population are "high responders" - following the NHS guidelines, for example, will spark a marked increase in fitness levels, of about 25-50 per cent. One in three are "medium responders", who'll enjoy a 15 to 25 per cent boost in aerobic fitness. Another one in three are "medium-low responders", who'll only see a 5 to 15 per cent increase. But, shockingly, another one in six are "low responders", who, at the very most, will only see a 5 per cent rise in their fitness levels.
Most of these will see no improvement whatsoever - no matter how much time they spend sweating on a cross-trainer, they will never be fitter. Worse still, about a sixth of the low responders - 3 per cent of the general population - will actually become less fit. In other words, 1.8 million Britons are effectively allergic to exercise. How can this be?
"So far, the only explanation we have for this is that somewhere in their system, between the mouth inhaling air and the oxygen, probably in the periphery of the muscles, they're maladapting to exercise so that it becomes harder for them to get oxygen to the muscles," says Dr Timmons.
"The analogy with allergy is interesting as allergies are partly driven by inflammation and it's possible that that is one of the problems we have with these people."
Unfortunately, a solution has yet to be found. Dr Timmons suggests this maladaption could be due to the wrong form of exercise, or the wrong type of regime, and that another approach could prove beneficial. "We're not sure what to do yet," he says. "The solution could even be combining exercise with an anti-inflammatory drug.
Resistance training - using weights - could also be an option, "though we don't know yet whether this group would also be non-responders to building muscle," Dr Timmons says. "But there are many common features between aerobic and resistance training in terms of what's happening at a molecular level, which suggests to me a correlation. But we can't be sure yet."
Earlier this year, Dr Timmons and his team published an article about the biology of those who don't gain muscle tissue after a 20-week resistance-training programme. It's about 25 per cent of the population.
I decide to take the test - there must surely be an explanation for why each time I stick to a gym habit for several months I enjoy relatively minor gains and so eventually give up. There's a dreary form to fill out about my medical particulars, height and weight, current and former exercise habits, and a saliva sample (obtained by scraping the inside of my cheek with a swab) that I sent off to their lab.
Several weeks later, the results are in: I'm a "medium-low responder". My DNA is NBG. Thus, the chances of me making Rio 2016 are skeletally slim. According to the report, if I performed 20 weeks of training I would only see up to a 15 per cent improvement in fitness levels.
Should I bother?
"It depends on your goals. If vanity rather than aerobic fitness is your motivation [it absolutely is] then we suggest you try out resistance training, which will also reduce your chance of developing type 2 diabetes. Something like 10 to 15 minutes per day with a kettle bell."
What? That little? Do I not have to make horribly unflattering grimaces for at least an hour five times a week to make a difference?
"There's been a public health myth touted around that we've been evolved to be continually running around, which is based on very little science. If we were perpetually active, we would also be perpetually needing food, so the evolutionary strategy would be too inefficient - running around consuming as much as possible so that we can carry on running around. It makes no sense."
For those, however, who do want to make the most of their genes - as long as they're not low-responders - and reach the zenith of their capacity for aerobic fitness, the guidance is simple. "Train like a middle-distance athlete: high-intensity interval training."
But there's something else bothering me: I don't get the post-workout "buzz" that is often discussed around exercise. As well as differences in physical adaptations are there mental differences, too?
"It seems there are. Some show a robust improvement in mental health and some don't. The reason that message has got out there is partly because many of the trials use a self-selecting sample: in other words, the people who make themselves available for research into the effects of exercise are often people who enjoy exercising."
The key, he says, to all this, is not only to discover your genetic capacity and tailor your workout as best you can but, crucially, to follow the simplest and perhaps the most effective exercise advice of all: do something you enjoy.