About 10 years ago, a friend of mine -- who was a strength coach for a Major League baseball team -- told me about an all-star outfielder who was absolutely crazy about fitness. So much so that the player did barbell squats in the locker room. While standing on a Swiss ball. Buck naked. Which isn't just crazy -- it's scary.

Thank goodness naked Swiss-ball squats never went viral. But I've found three current fitness trends that are just as scary. (If you don't count the nudity.)

Scary Fitness Trend #1: Hopping For Heavyweights
If you’ve ever watched a reality weight-loss show, you might have seen 300-pound contestants jumping onto boxes to blast calories. It’s often an awkward jump that looks dangerous, even to the casual observer. Of course, they wouldn’t show it on TV if it was a bad idea -- right? Wrong. (Exhibit A: The Real Housewives of Beverly Hills.)

Is jumping an effective way to burn calories? Yes. But is it a good idea for people who are significantly overweight? No. And that goes double if the person is just starting a fitness regimen.

“While jumps of any kind are a great tool for increasing power, building strength, and blasting your cardio, they will increase your injury risk,” says Robert dos Remedios, C.S.C.S., strength coach at College of the Canyons and author Men’s Health Power Training. “For an obese person, this injury risk will sky-rocket simply due to the large deficit in their strength to body-weight ratio.” So combine a lot of extra weight with muscles that aren’t well trained, and you have a recipe for disaster.

“I rarely even use box jumps with my athletes for fear of mishap and injury,” says dos Remedios, who has been training collegiate athletes for 18 years. “I can never imagine putting an obese client in this situation.”

Dos Remedios acknowledges that low-level jumps, such as jumping jacks, partial squat jumps, and even some jump rope drills can have a place in an obese client’s fitness routine. But he emphasizes that he would always weigh the risk of such activities versus the reward. “There are so many other ways to burn calories and boost your metabolism,” says dos Remedios. “My goal is to minimize risk and maximize results.”

One of his favorite ways to do that: the kettlebell swing. Not only has this exercise been shown to be a great calorie-torcher, a new study from the University of Waterloo shows that it may help prevent lower-back injuries. But you have to do it right. Watch the video to make sure you perform the kettlebell swing with perfect form. (And to use the kettlebell swing to melt flab fast, check out The World’s Simplest Fat-Loss Routine.)

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Scary Fitness Trend #2: Marathon-Training For Fat Loss
Whenever someone tells me they’re going to run a marathon, I ask a single question: “Why?” I don’t ask sarcastically; I’m truly curious.

The most common reason? “I want to lose weight.”

My first thought: “Wrong answer.”

While completing a marathon is a truly admiral accomplishment, it’s not necessarily an ideal way to lose your excess baggage. In fact, I would argue that if you want to do a marathon, you should first lose the weight before you start your marathon training.

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Fitness expert Rachel Cosgrove agrees. Cosgrove is the co-owner of Results Fitness in Santa Clarita, California, and is also an Ironman triathlete. Read: She’s not only completed a marathon, but also a 2.4 mile swim and 112-mile bike ride right before that marathon. The upshot: As a both a trainer and an athlete, she knows a lot about marathon-training -- and plenty about fat-loss training, too.

“Running is actually an advanced activity,” says Cosgrove. “Before you ever start to train for a marathon, you should first work on getting strong and fit.”

Cosgrove explains: “Every step you take running is a hop onto one leg. And hopping on one leg is considered a plyometric activity. Plyometrics are an advanced exercise that you should do only after you’ve built up a base strength to handle them.”

Think of it this way: Running just one mile can amount to doing 1300 to 1600 hops. “If you hired me to train you and on day one, I said, ‘We’re going to do 1500 hops on one leg,’ you'd think I was nuts,” says Cosgrove. “But that’s exactly what you’re doing when you head out for a run.” And just like Scary Trend #1, this can significantly increase your risk for injury.

Yes, people have successfully used marathon-training to drop weight. But probably more people have suffered ankle, knee, and hip injuries because they weren’t prepared for the stress that training for the event put on their bodies.

The take-home message: Make peak fitness your top priority. And once you achieve that goal, you can consider making the marathon your next challenge. One way to know you’re ready: Try The Men’s Health Spartacus Workout, created by Cosgrove herself. If you can handle this intense routine, you can handle just about anything.

Scary Fitness Trend #3: Extreme Workouts That Are Too . . . Extreme
It’s cool to push yourself in the gym. In fact, you should push yourself. What’s not so cool: To push yourself beyond your limits. It’s a quick way to get injured. But in fitness circles, this practice is trending. The idea is this: You work yourself to complete exhaustion, no matter how sloppy your form gets. At the end of your workout, you feel an air of satisfaction, knowing you gave it your all.

But here's the problem: As fatigue starts to set in during an exercise, form starts to falter. That's a fact. "When this happens, your body starts to compensate, altering the muscle fibers that are recruited as well as delaying reflexes," says Men's Health fitness adviser Bill Hartman, P.T., C.S.C.S, co-owner of Indianapolis Fitness and Sports Training. "This causes overload to joints and soft-tissues, and makes fatigue a key component of injury potential."

How do you know when you've taken an exercise too far? One sign is that you achieve "technical" failure. This is the point at which your performance starts to decline, and it can be identified in two ways, says Hartman.

1. You can't maintain perfect form. The easiest gauge is when your posture changes -- for instance, you have to excessively arch your back to complete a bench press, or you need to lean your torso backward to complete an arm curl. Another indicator: You stop performing an exercise in a full range of motion. So instead of performing a squat as deep as you can, you "stop short" and do partial squats instead. (To make sure you know perfect form, check out The Men's Health Big Book of Exercises and The Women's Health Big Book of Exercises, where you'll find full-color photos of more than 500 exercises.)

2. You aren't in total control of the weight. In this case, the speed at which you lift a weight slows down as you pass your "sticking point." So if the rate at which you do a pushup starts to slow as you press yourself up, you've achieved technical failure. The other yardstick: You aren't able to lower a weight back to the starting position at the same rate from top to bottom. That is, it feels as though the weight overtakes you. When either of these conditions occurs, you've reached technical failure.

Unlike in absolute failure, in which you can't perform even one more repetition, you'll probably feel as if you could pump out a couple more. But the truth is, once you've hit technical failure, fewer target muscle fibers are firing during each repetition thereafter -- so you've achieved already maximal benefit from that exercise. "You'll get better overall results if you rest and add another set, than if you push past what you're capable of doing with good form," says Hartman.

Keep in mind, that beyond the immediate injury risk, you also need to consider the ramifications down the line. “Even though you may be able to gut through an exercise in the short-term, this action could be bringing long-standing issues closer and closer to threshold,” says Eric Cressey, C.S.C.S., owner of Cressey Performance in Hudson, Massachusetts, where he trains dozens of professional athletes.

“We know that the overwhelming majority of people have something structurally wrong on MRIs and X-rays -- whether it's a disc herniation, rotator cuff tear, or degenerative changes in the knee," Cressey says. "Good training should prevent these issues from ever getting to the point that they cause symptoms, but just plowing through exercises with terrible form can instead bring these issues to the forefront."

Be smart: When it comes to your fitness plan, don't strive be an absolute failure. (And make sure to use this no-fail plan when looking for the best presents to give -- or receive: The Men's Health Holiday Gift Guide.)

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