Good luck getting into that spinning class this month...or next month, or even in March. Gyms fill up in January, as optimistic out of shape folks resolve to get in shape, once and for all, for real this time. But while January does bring in more members than any other month for America's gyms, some clubs don't clear out until spring starts.

More than 12 percent of gym members join in January, compared to an average of 8.3 percent per month for the full year, according to the International Health, Racquet, and Sportsclub Association (IHRSA).

Still, it's not necessarily just a January bump; the entire first quarter of the year is a busy time for health clubs. February comes in second.

That can mean that the New Year's rush to the gym can last much longer than one month, with traffic getting even heavier in later months.

 New Year's resolutions aside, longer-term trends point to more Americans packing into athletic clubs.