Warming up before a workout with stretching exercises actually weakens runners' endurance and makes them less efficient.
While previous studies have illustrated the effects of stretching on anaerobic activities, this was the first one to show the effects on runners.
The study took 10 fit middle and long-distance runners (all male) and had each of them do the same run on two separate days with a 72-hour recovery period in between.
The researchers divided the run into two 30-minute parts, with the first testing for caloric expenditure and the second assessing endurance. One day a participant did a stretching routine before running; on the other, he simply sat quietly prior to his workout.
During the first interval, participants ran at 65 percent of their VO2 max, keeping a constant pace.
The researchers found that when the runners stretched before the workout, they burned, on average, 5 percent more calories during the run than when they didn't stretch.
For the second half, participants were told to run as far as they could on the treadmill for 30 minutes. When the runners didn't stretch, they went 3.4 percent farther than when they did.
"Just asking runners, they seem to think stretching would enhance performance," ABC News quoted study co-author Jacob Wilson of the University of Tampa as saying.
"The thought is that if you can loosen up and you feel looser, you can perform better," he said.
The study published was in the Journal of Strength and Conditioning Research.