By now, YouTube's reputation for the wild and just plain weird (cats flushing toilets and a question-answering ninja) is well established. What you may not know is that its stash also includes fitness videos. Which raises the question: If you've grown bored with your regular fitness routine, could this be a source for a new workout?

 Well, yes ... and no.

 On days when time's too short for the gym or you're dying for a new routine but don't want to spring for a studio lesson, YouTube might pose an offbeat solution. Assuming you have access to high-speed Internet, JavaScript and Macromedia's Flash Player, you're ready to roll. But you have to be willing to put up with a few drawbacks.

 Take video length. Many videos are short - under three minutes. Like what you see? Hit replay. You say once was more than enough? Move on. Only problem is you have to stop moving, at least briefly, to do so.

 Not only does production quality vary, but also the instruction quality. Some presenting themselves as fitness coaches know what they are doing and offer detailed instructions and safety advice. Others make a conscious parody of conventional fitness videos. But sometimes, it is tough to tell the pros from the put-ons.

 Although working with a video is usually a one-way experience, via YouTube's internal messaging system you can get feedback, though delayed, from your instructor.

 Here's a look at a handful of YouTube fitness videos, with the help of three experts: Sarah

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 Camhi, a doctoral candidate in exercise physiology at the University of Maryland; Todd Miller, assistant professor of exercise science at George Washington University and a strength and conditioning specialist certified by the National Strength and Conditioning Association; and clinical exercise physiologist Michael Jones, with the Cooper Institute in Dallas.

 YOGA AND PILATES ON THE FLY

 Web site:  www.youtube. com/watch?v=2-RMa5Vj0vI

 Overall:  When Les Romack crawls to and from his Web cam, getting within inches of the lens, it is jarring. The San Diego resident provides quick snippets of yoga and Pilates moves. Romack's focus on technique and his commentary, like where shoulder blades should be when in plank position, testify to his certification from the American Council on Exercise.

 Expert take:  Romack generally receives good marks on technique. But, Camhi notes, "his movements are very dynamic and fast, and yoga is typically slow and controlled." Plus, such details as "elbows in or out, palms wide or narrow - which makes a big difference in the level of difficulty" - are not fully explained when Romack demonstrates push-ups.

 WORKOUTS FOR WEAK WOMEN

 Web site:  www.youtube.com/ watch?v=y8UxJXIG2SU

 Overall:  The funky beat, polished production by the folks atDiet.com  (Related)   and all-female cast may entice some viewers to tune in to this strength-training instructional. Others may appreciate that the women look merely average, don't bare too much skin and train in a typical gym.

 Expert take:  This is Camhi's favorite video. Miller and Jones disagree. Jones saysDiet.com  (Related)  's exercises are most effective when partnered with each other (the total-body exercise routine is chopped into segments), but it may be too easy for some. Miller says dumbbells are light and the instructors "don't appear to exert themselves in the least." He continues, "If you want a workout that will almost guarantee you zero results, these are the types of exercises you would choose."

 HARD-CORE LIFTING

 Web site:  www.youtube.com/ watch?v=yG66ZOj1CK8

 Overall:  Dave Bellomo, owner of a Pennsylvania gym, and his production partner, Rob Rupp, have posted intense workouts for their lifting techniques series. A former power lifter, Bellomo is filmed in his grassy back yard lifting sewer grates, chunks of concrete and anything else he can scrounge up.

 Expert take:  Caution. These workouts are geared toward experienced athletes, warn our experts, since the thick-muscled Bellomo does not explain lifting technique or address warm-ups. Camhi adds that vigorous exercisers should rest 24 to 48 hours between workouts. Miller says this is his favorite of the workouts and Bellomo's spiel about how he trains is enlightening. If you "lift the same amount of weight or run at the same speed ... with no increase in intensity, this results in no gain in performance," Miller says.

 HYPERACTIVE AEROBICS

 Web site:  www.youtube.com/ watch?v=7DW1W5Aeoj0mode

 Overall:  Pittsburgh trainer and public access station show host Paul Eugene comes at you like a ball of fire. A smile plastered on his face, he yelps like a 1980s Michael Jackson and echoes the kinetic energy of his bright stage set. The instructor, certified by the Aerobics and Fitness Association of America, exudes infectious enthusiasm, incorporating fluid old-school dance moves in aerobics routines.

 Expert take:  There's no denying Eugene is fun, our experts agree. It's "not the most challenging aerobic routine, but it could be a lot of fun for some people," Jones says. "And having fun, as opposed to struggling to survive a workout, could be what it takes to motivate a person."

 SPORTS TRAINING WITHOUT WARNING

 Web site:www.youtube.com/  (Related)   watch?v=Kotr9rDceRw

 Overall:  Filmed on a Tampa, Fla., high school's grassy field, Kurt Bierek, former professional baseball player (and brother ofShapefit.com  (Related)  's president and founder Kris Bierek), executes plyometric exercises (meant to prepare the body for short, explosive bursts of movement) and other sports training drills. The no-nonsense Bierek runs through an exercise and finishes the segment out of breath.

 Expert take:  Jones says the Bierek video may provide a good change of pace for exercisers "bored with the standard treadmill, bike, elliptical trainer workout." But Miller says plyometrics are best for athletes: The National Strength and Conditioning Association recommends advanced routines, such as side box jumps, only for people strong enough to barbell squat 1½ times their body weight. Camhi, too, says these exercises "can cause injury if not done carefully with good form, and this should be made very clear!"

 "GET MORE FITNESS!"

 Web site:  www.youtube.com/ watch?v=EEz0B2B_BHw

 Overall:  With his sweatbands, dangling silver medal and barely-there black tank top over his hairy chest, Mr. Fitness is a sweet satire of cheesy workout videos. The episode depicts Mr. Fitness using, um, everyday objects to exercise. For example, he lifts a 30-pound cat to work his biceps and pectorals. (Blaise Hemingway, who plays Mr. Fitness, is quick to say that no animals were harmed in the production of this video. But please don't try this at home.) Hemingway, a member of Los Angeles-based Cinema Mavericks, says Mr. Fitness is inspired by "the characters that take fitness a little too seriously and wear terrifyingly revealing outfits that leave little to the imagination."

 Expert take:  While they appreciate that the video was primarily intended for a chuckle, Jones and Camhi can't help noting that exercising with everyday objects - not generally including household pets - may be a good and practical tip.