It's January. The bars are empty and the gyms are full of people with good intentions. By February or March, of course, that situation will reverse, as new year resolutions begin to flag and it becomes increasingly difficult to justify that extortionate gym membership.

Like dieting, the biggest problem with getting fit is sticking with it over time.

That's the problem Tokyo-based trainers Chris Colucci and Allan Wooding aim to solve with TokyoFit. Their classes, which I have been attending since 2013, are a far cry from those at the city's typically pristine and pricey gyms: Held in a small park under a highway bridge in Hiroo, Shibuya Ward, students thrust barbells, swing kettlebells, and thrash battle ropes. Passersby are often surprised by the sight of grown-ups chasing each other to yank red velcro strips off their partners in a warm-up game called "Steal the Bacon."