Watching people lifting weights, I've often wondered if folks who absentmindedly reel off repetitions -- while seemingly exerting little effort -- are getting much benefit.

They're not, according to a study in the current issue of the peer-reviewed Journal of Strength and Conditioning Research. And that finding might help explain why so many new gym-goers become former gym-goers so soon -- and may help you avoid becoming one of them.

Researchers asked 13 men and 17 women, all strength-training novices aged 17 to 21, to select a weight that would improve their muscular strength.

Both men and women chose a weight that was below what studies show to be the minimum level needed to gain muscular strength -- 60 percent of their one-repetition maximum. The weight they picked put them at 42 to 57 percent. Prior strength training research has shown that loads of at least 60 to 75 percent are required to gain strength.

The researchers at Grand Valley State University in Allendale, Mich., and Wayne State College in Wayne, Neb., tried to simulate gym conditions by showing participants how to use the machines first. (Yes, your gym orientation should include such instructions.)

The researchers then let participants try a few different weight settings before deciding which load they believed would yield muscular gains. All chose loads -- at this session and at a follow-up two to three days later -- that were too light, as measured by a subsequent assessment of each person's one-repetition maximum.

Lead investigator Stephen Glass, professor of exercise physiology at Grand Valley, told me, "At 60 percent of your maximum, you should be able to do about 15 to 20 reps before fatigue -- that is, before you cannot do another rep in proper form."

For example, if you can just barely squeeze out one military press with 100 pounds of weight, you should be able to do 15 to 20 reps with 60 pounds.

The American College of Sports Medicine (ACSM) recommends people lift 70 percent of their one-rep maximum, doing eight to 12 reps. (For curls where your one-time max is 40 pounds, you'd do eight to 12 reps with 28 pounds.) Glass says some novices may be more comfortable lifting at 60 percent, and still see benefits.

"The key is to monitor how many reps you are doing," provided you do them to fatigue, Glass said. "Do not focus on the weight itself." So if you are doing 17 reps to fatigue, the weight you've selected is in the right ballpark. When that task becomes easier -- to the point where you can do more than 20 reps with that weight -- increase the weight to a point where you can do only 15 to 20.

Glass advises doing two to three sets of each exercise, with two to three minutes of rest between sets. "If you are pressed for time, one set will yield benefits. But two or three [sets] will be better." (Doing the second set boosts the value of your workout more than the third set does.) You may not be able to do as many reps on your second and third sets; that is normal.

Asked if the data can be extrapolated to people older than 21, Glass said, "My guess would be that the older population would [choose] even lighter loads due to fear of injury. When people are not accustomed to lifting, it's hard to determine what is 'heavy' and what is 'fatigue.' "

Glass speculated that failure to see results could be one reason as many as half of new gym-goers drop out within weeks of their joining a gym. Working harder, he said, might change that outcome.

The researchers conclude that working with a trainer is the best way to ensure you are lifting enough. "But if you lack the money for that, this is good information for helping you self-select the right loads to work with," Glass said.

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-- John Briley