NEW YORK (Reuters Health) - Adults with tight leg muscles can improve their flexibility, and may make their muscles stronger in the process, a study shows.

Most physically active people probably do a little stretching or know that they should. But little research has been done on the correlation between muscle flexibility and muscle performance, according to the authors of the new study.

To look at this question, the researchers recruited 30 young adults with tight hamstrings, the muscles at the back of the thighs. For six weeks, the volunteers performed a series of hamstring stretches five days per week; at the beginning and end of the study, they had their flexibility and thigh muscle strength tested using workout equipment.

In the end, the formerly tight volunteers loosened up and increased their range of motion. But their thigh muscles also became stronger, the researchers report in the Clinical Journal of Sports Medicine.

"Both muscle groups of the knee -- flexors and extensors -- were able to generate more work after the intervention, indicating that the tight muscles got stronger," explained study co-author Dr. Luci F. Teixeira-Salmela of the Federal University of Minas Gerais in Brazil.

The knee flexors refer to the hamstrings, which bend the knee, while the knee extensors refer to the quadriceps muscles at the front of the thigh, which straighten the knee joint.

The study results show that adults' muscle flexibility can be modified, Teixeira-Salmela and her colleagues write, and that this change might improve muscle performance.

One implication of this is that flexibility training may help prevent knee injuries, according to the researchers. However, it's important to regularly stretch not only the hamstrings, Teixeira-Salmela told Reuters Health, but other muscle groups as well -- particularly the ones that do the bulk of the work in any given activity.

SOURCE: Clinical Journal of Sports Medicine, July 2007.