How long does it take to get 'fit'?

The question

How long does it take to get "fit?"

The answer

The pair of "before and after" pictures is a staple of fitness hucksterism. Follow our patented program for a few weeks or months, the ads say, and your body will be transformed.

Intrigued by such ads, Megan Anderson and her colleagues at the University of Wisconsin-La Crosse put 25 subjects through an intense six-week exercise program, modelled on claims made by companies such as Bowflex and Body-for-LIFE, in a 2004 study published in the Journal of Strength & Conditioning Research. The subjects' before-and-after attractiveness was rated by a panel of six judges, who were unable to detect any change whatsoever.

Does that mean six weeks isn't long enough to reshape your body? Not necessarily.

"The 'time course' of fitness changes depends on the training stimulus: intensity, duration and frequency," says Friederike Scharhag-Rosenberger, a researcher at the University of Potsdam in Germany who published a study on the topic in Medicine & Science in Sports & Exercise earlier this year.

Under the right circumstances, six weeks may be enough. But even if it takes longer, the benefits of exercise start long before you begin to bulk up or run faster.

Here's what you can expect from different kinds of workouts:

Resistance training

There are two basic ways that strength training makes you stronger. The first and most immediate is neural adaptation: After you've tried bench-pressing a few times, you learn to recruit more muscle fibres and make them contract in sync. The result is that you're able to lift heavier weight before you've even gained any muscle. The second is hypertrophy: The muscle fibres you're stressing will get bigger.

The traditional view has been that neural adaptations dominate for the first six or seven weeks, after which hypertrophy kicks in. In the past decade, a number of studies have shown evidence for hypertrophy after as little as two weeks, and research by McMaster University's Stuart Phillips has suggested that even a single strength-training session can spur the addition of force-producing muscle proteins without changing the muscle size.

Still, Dr. Phillips concludes, "there is almost no doubt that neural gains in strength are the predominant reason for early changes in strength during these types of resistance-training programs."

There's an important caveat, though. The Wisconsin-La Crosse study that found no body changes had subjects working out three times a week for an average of 34 minutes per session. The studies that seek the earliest possible hypertrophy, in contrast, use higher intensity programs (six two-hour workouts a week, for example) designed and supervised by professionals.

Cardiovascular training

The key change in your body that allows you to run a little farther or pedal a little faster is an increase in the amount of mitochondria present in your muscles. According to a recent review in Sports Medicine by Vernon Coffey and John Hawley of RMIT University in Melbourne, Australia, it's possible to increase mitochondrial protein content by 50 to 100 per cent with about six weeks of endurance training.

These muscle proteins are constantly being replaced, with a half-life of about one week, so you have to keep training to keep your elevated mitochondria levels.

Just as with resistance training, though, the less visible benefits of cardiovascular exercise start much earlier. A single bout of exercise improves insulin sensitivity for as long as 48 hours, and regular endurance training produces longer lasting changes that lower the risk of diabetes and related conditions.

For both resistance and cardiovascular training, then, it takes about six weeks to see tangible changes under optimal conditions. Realistically, the vast majority of recreational exercisers will need longer than six weeks to pump up - how much longer depends on how hard the program is.

But it's crucial to remember that real gains for both health and performance show up within a few days of starting an exercise program, even if those gains aren't obvious at the beach.

Alex Hutchinson blogs about research on exercise and athletic performance at http://www.SweatScience.com.