Exercise is as old as the ancient Greeks...and popular as ever. Probably one of the best examples is the marathon (which is derived from Pheidippides' supposed 40-kilometer run from Marathon to Athens): In 1977, more than 4,000 people signed up to run the first Chicago Marathon; in 1995, 9,000 did. Then, in 2007, it sold out with 45,000 entries-six months before the race.

Because fitness has become so interwoven into the fabric of society, it has gone through various trends. Coaches and trainers implement new science into their repertoires, companies invent the new workout apparatus, and the consumer decides what works and what doesn't. With the help of Rafael Torres, owner of Torres Fitness in New York City, we identified the top six fitness trends for 2009. (Hint: The first half is economic-themed.)

1. Group sessions. In the time of a weak economy, people are more likely to choose a group class over a one-on-one session. "This began around Thanksgiving," says Torres. "Suddenly people couldn't afford their personal trainers and were more likely to work out with a group either at the gym or in the park."

2. Boot camps. Typically, these consist of small (4-10 people) group sessions held in a public area with an intense, motivating instructor. "These can offer more bang for your buck," Torres says. You feel like you're getting personal attention, but at a fraction of the price.

3. Traditional training. This stems from both the economy and a mindset that pushes against functional, "like the pros" training of the past few years. "People are going old-school," Torres says. Think a half-hour jog in the park followed by a circuit of bench presses, military presses, and squats.

4. Inertia-free machines. By preventing the snap-back you get from a traditional cable machine and by being both easier to use and more effective in isolating muscles, gyms are purchasing them for health and safety reasons. Examples include Keiser machines and the Nautilus Freedom Trainer.

5. Brazilian Ji-Jitsu. The emergence of mixed martial arts as a mainstream sport has helped push people toward kickboxing and karate, but Brazilian Ji-Jitsu has everything: its moves are easy to learn, it requires less strength than technique, it is more "down-and-dirty" than meditative, and it has an enormous effect on the cardiovascular system. Torres says, "I've never burned as many calories as I have doing Brazilian Ji-Jitsu."

6. Back-to-the-basics. Interestingly, Torres can trace this to kettlebells. As the unique Russian training tool became popular in America, people became influenced to try it without understanding how best to use it. But those who weren't properly conditioned to did not experience gains. The result has been a primary focus on strengthening the core muscles with basic exercises (leg extensions, bicep curls, pull-ups) that can lead to an ability to execute more difficult exercises like kettlebells.