If I were to tell you that there was an entire industry that overcharged the vast majority of its customers, but those customers were fully aware they were being robbed, and that was the only way to make the business viable, what would you guess?

If you’re a member of a gym, you will be aware that for the first month of the year the place is horribly packed out with sweaty and unfit people, all the classes are booked up and you can’t get on any of the machines you want. If your interaction with the keep-fit industry is more along the lines of walking past the gym on the way to the cake shop, you might be more aware of the equally curious fact that commercial gyms always seem to have a heavily advertised ‘special’ membership deal going on. Paying the full whack listed rate at a gym is actually a pretty difficult thing to do — much more so than paying full freight rack-rate for a hotel room — unless you do the single most expensive thing you can do in physical culture, and join the gym shortly after the Christmas holidays.

SWEATY BETTY Having seen the books of a gym chain or two, we can tell you that the ‘Sweaty January’ phenomenon is not an urban myth or a joke — it’s absolutely fundamental to the economics of the industry and it’s basically impossible to run an economically viable gym without taking it into account. Usually about 75 per cent of all gym memberships are taken out in the month of January. Not only this, but the economics of the industry absolutely depend on the fact that a very great proportion of January joiners will not visit more than three or four times in total before their membership comes to a floundering flop of weight not lost at the end of the year. The founder of Colman’s Mustard used to claim that his fortune was based on the bit of mustard that everyone left behind on their plate, but gym memberships have really pushed things to the limit when it comes to this model of making people pay for a lot more of the product than they have any likelihood of using.

On the bizarre economics of gyms. The spatial inefficiency of gyms is something I hadn't ever spent much time thinking about.

Human nature being as immutable as it is, most gyms are great investments (other than Bally Total Fitness, which reached too far, too fast). In fact, human nature is so predictable that a company like Planet Fitness can come along and offer memberships for just $10 a month and still not be overrun with people. It's found money.

If you're feeling particularly fitness motivated this month, maybe wait a month and see if the impulse passes along with the January prices.