"You can only go so far, and gradually the strength gain begins to drop," said researcher Matthew Rhea, a graduate student at Arizona State University.
The review sums up the findings of 140 separate studies of varying weight training programs involving everyone from beginners to regular exercisers who had been working out at least a year. The analysis was an attempt to find what the studies had in common and calculate the size of strength increases.
Results were published in the March issue of the American College of Sports Medicine journal, Medicine and Science in Sports and Exercise.
People at the very start of an exercise program do best at "pretty low intensity," about 40 to 50 percent of the maximum they can lift at one time, Rhea said. On the effort scale of 1 to 10 -- 10 being the best effort -- it would feel about 5, he said.
These untrained people can start with one set -- typically eight to 12 repetitions of an exercise -- for each muscle group, done three times a week, Rhea said.
One reason is that muscles and nerves need to get used to lifting. Other researchers have found that much of the early increases in strength is due to practice, much as piano players can play a piece faster if they practice it more.
"Your body has to learn to work differently than it used to," said Debbie Leoni, manager at Main Street Fitness Center in Jackson.
The body will adjust as it gets accustomed to the workouts, she said.
But the study indicates that after the break-in period, the workload needs to become harder to get the best results. Exercisers should increase gradually to lifts of 60 percent of their one-time maximum, doing four sets of 12 repetitions, it said.
This argues against some starting exercisers' tendencies to pile on more weight immediately, figuring that lifting more will make muscle grow faster, Rhea said.
Fitness coordinators and trainers at Universal Health and Fitness Center in Cape Girardeau help people determine when to add more weights to their workout routine. It's always best to add more weight instead of just adding more repetition with a lesser weight, said Shawn Nesler.
People who have never lifted weights before do better with an initial fitness assessment and routine designed just for them, he said. "If they don't meet with a trainer, there a better chance they will lapse and fall out of the program."
Make a plan
The key to getting good, consistent benefits is to follow a standard plan, Leoni said. She recommends using FITT, an acronym for frequency, intensity, time and type.
When people want to make a change in their workout routine, the best way is to consider exercising more often, increasing the intensity and duration of the workout, and adding new types of exercises.
Weight training helps people feel strong and helps their posture but it also needs to be paired with some cardiovascular exercise, Leoni said.
The new exercisers also need to avoid overtraining that can lead to injuries, the study said. However, the study did not track injuries, and there is little research that does, Rhea said. Nor did the study of strength growth also examine which regimens would give the most health benefits.
The analysis also argues against the idea that people who have been exercising for less than a year will get their best results by continuing to do just one set. One set over 12 to 16 weeks might result in a 10 to 15 percent gain in strength, but four sets could result in a 20 to 25 percent gain, Rhea said.
Multiple sets are a good idea in principle, but the practice should be approached cautiously with new exercisers, said Mike Merk, executive director of program development at the YMCA of Greater Cleveland.
"What typically happens is, we let them know they will start slow," Merk said. "We know our goal is to get them to three or four sets, but you don't want to tell them that because it would get their anxiety levels up."
After a year of exercising, weight trainers would do best with four sets of eight lifts each at around 80 percent of their maximums, and only twice a week, the study found.
Doing only one set might gain these veterans 5 to 10 percent in strength, while four sets could result in 15 to 20 percent gains, Rhea said.
That's a lot less dramatic than the gains they got as beginners. But these exercisers are beyond the point at which they can lift more weight simply because they practiced doing it. Their benefits now come from the arduous work of building muscle.
This is expected, said researcher William Kraemer of the University of Connecticut, who was not a part of the study. "When you are a beginner, you can do anything and get better," he said.
Features editor Laura Johnston contributed to this report.
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