Regular stretching and weight-machine use increases the risk of lower-back pain, according to a new study. Researchers also found that those who worked out with free weights and went to core-focused exercise classes reported less back pain than those who primarily stretched. "Using free weights likely helps develop stabilizing muscles more than weight machines," says Stacy Fritz, co-author of the study. While the researchers don't address the specific stretching that can cause back pain – whether it's yoga or calisthenics – proceed with caution. "It's easy to stretch too aggressively or incorrectly and injure muscles," says Fritz.