You shell out money for a gym membership, but pushing weight by yourself may not be the best way to pack on muscle and shed that winter belly. New research from the University of California, Los Angeles, found that exercising with a personal trainer can increase your strength, cardiovascular fitness, and lean body mass more effectively than flying solo at a health club.

For the study, 34 healthy men at the same Equinox gym exercised at least three days a week for 12 weeks. Half of the men worked out alone, following their own program, and the other half worked out with a personal trainer. At the end of the 12 weeks, men working with a trainer chest pressed 42 percent more weight and leg pressed 35 percent more than they had at the beginning. The self-trained guys increased their chest press 19 percent and their leg press 23 percent.

Also, when asked to run on a treadmill until exhaustion, men in the personal trainer group saw their cardiovascular performance improve by 7 percent from the study's start. The solo group saw no gain. The personal trainers also helped their patrons increase lean body mass—which helps you look toned and strong—by 2.8 pounds, while the solo group stayed stagnant.

It wasn’t that the self-trained guys spent their time at the snack bar. They actually worked out 22 minutes more each week than the trainer-assisted guys. So why didn’t their workouts pay off as much? When you’re observed by a professional, it makes you work harder and smarter, says study author Thomas Storer, Ph.D. Plus, a trainer will pick up on small—but important—cues as you exercise and tailor workout routines to fit your needs. They can help you break through plateaus, stay injury-free, and challenge your muscles in new ways, says Storer.

While hiring a personal trainer can increase your gains, not just any trainer will do. You need to pick an effective trainer—otherwise you’ll waste your time and money for an injury.