No doubt about it: BodyPUMP is the hottest thing to hit health clubs since Spinning. Imported from New Zealand just three years ago, these weight-training classes are now offered at more than 800 fitness clubs nationwide. But some experts question whether the program, which involves doing dozens of repetitions with light weights, lives up to its claims.

The program's Web site makes a bold statement: "BodyPUMP will improve your fat-burning ability and help build lean muscle and strength. Quite simply, it's the fastest way in the universe to get in shape." Is it? To find out, Shape commissioned researchers at California State University, Northridge, to track men and women in a BodyPUMP class. Although the study had its shortcomings, such as a small sample size, results weren't impressive. After eight weeks, subjects didn't show significant strength gain or body fat loss. The only measurable benefit was a gain in muscle endurance.

BodyPUMP promoters and scientists believe the study was too short to adequately assess the program. "If [the study] had followed the subjects longer they'd have seen more-dramatic changes," says Terry Browning, vice president of The STEP Company, U.S. distributor of BodyPUMP. The researchers maintain that eight weeks was enough to test the claim that it's "the fastest way in the universe to get in shape."

Outside experts who've reviewed the study say that eight weeks is considered a minimum acceptable length for studies of this type. "It would've been ideal if the study had gone on longer," says exercise physiologist Daniel Kosich, Ph.D., fitness consultant to the Aurora Cardiology Practice in Denver. "But there are eight-week studies that have shown much greater changes in strength." (See "Weighty Findings.")

Maximum effort, modest returns

CSUN research subjects took an hour-long BodyPUMP class twice a week and avoided other weight training. "We asked participants to continue with their usual aerobic exercise and dietary routines," says Eve Fleck, M.S., the study's lead author, who did the study for her master's thesis. Before the program began and after the eighth week, the researchers measured subjects' strength on the bench press using a one-rep max test (the most weight the subjects could lift once) and muscular endurance (how many times they could bench press the amount of weight prescribed by the YMCA endurance test: 35 pounds for women, 80 pounds for men).

While 27 subjects began the program, only 16, a mix of novice and experienced lifters, finished it. (Several dropped out due to time conflicts, one because the program aggravated her arthritis.) After eight weeks, the only measurable change was an increase in the number of bench-press repetitions subjects could do. "The average increase was significant, about 48 percent," says Fleck. Also, three of the four novices gained strength, an average of 13 percent.

Fleck attributes the endurance and strength increases partly to improved neural coordination typically experienced by novice lifters. She says she wasn't surprised that the group on average didn't gain strength, since it's harder for experienced lifters to do so. To gain strength, the American College of Sports Medicine recommends lifting 70-80 percent of your one-repetition maximum. But in a typical BodyPUMP class, subjects lifted an average of just 19 percent of their max.

BodyPUMP promoters defend the use of light weights. "The reason for the light weight is that the program is designed to improve muscular endurance," Browning says. (Muscular endurance, experts agree, is important for activities lasting several hours, such as biking, hiking and skiing.) Browning says the Web site's increased-strength claim applies only to beginning exercisers, but this disclaimer doesn't appear on the site. Fleck says she'd need more novice subjects to determine if beginning lifters really gain strength with BodyPUMP. A significant limitation of the study, experts agree, is that the subjects' weight-training experience was too diverse. "With such a small sample size split into different fitness levels, it's hard to get statistical power," Kosich says.

A risk of injury?

BodyPUMP promoters maintain that muscular endurance is best achieved by doing dozens of repetitions of each exercise. However, research shows that doing the traditional eight to 12 repetitions develops plenty of muscular endurance, while also building strength, bone and enough muscle mass to boost metabolism. "When you gain [muscular] strength you automatically gain [muscular] endurance, but apparently the opposite isn't true," says Wayne Westcott, Ph.D., fitness research director at Boston's South Shore YMCA.

Doing dozens of repetitions is not only unnecessary, Westcott says, but may increase risk of overuse injury. None of the CSUN study subjects reported new injuries. "But [such] injuries may take longer than eight weeks to develop," says William C. Whiting, Ph.D., director of the biomechanics laboratory at CSUN and one of Fleck's advisers.

The researchers were also concerned that so many repetitions (up to 100 for some exercises) might foster sloppy technique. Fleck said she routinely saw poor form, especially among newcomers. They tended to load the bar with too much weight, and by the 40th repetition could barely lift it. She noted that the instructors involved in her study rarely corrected participants who were lifting incorrectly. "Even after eight weeks, all of our subjects used poor wrist, back, elbow, shoulder and knee alignment," says Fleck. Browning points out that BodyPUMP instructors offer 15-minute technique workshops before class and that newcomers are urged to attend at least one before taking a class.

Clearly, BodyPUMP classes are a lot of fun. Participants report that they love lifting weights to music and find the program motivating. But are the classes worth taking? "For a novice, it's a way to be initiated into weight training," Fleck says, noting that several subjects had been too intimidated to lift weights until they tried BodyPUMP. But she suggests that if you do BodyPUMP, have instructors demonstrate technique for each exercise outside of class and reduce the number of repetitions you do in order to reduce injury risk.

If you're looking to build muscle, increase your metabolism and strengthen your bones, Fleck says, stick with a traditional weight-training program. Meanwhile, BodyPUMP may help you maintain muscle strength, and, she adds, "It's something fun to throw into your routine once in awhile."