And medical researchers are always looking for ways to increase the population samples reflected in research surveys, where getting hundreds of participants can be a trial.

That’s where 6th Vital Sign, a new study by the Duke Clinical Research Institute, fits in. The project involves the free ResearchKit app, which can be downloaded from the iTunes store and used to measure a person’s walking speed, with the data transmitted to researchers if the person agrees.

“This is an opportunity for us to monitor people objectively,” said Janet Bettger, associate professor in orthopedic surgery at Duke.

After downloading the app to a phone, the person is asked to answer a few questions about his or her general health, take a two-minute walk as the app records walking speed, then answer follow-up questions.

“I wanted to be part of it,” said Elaine Miller, 64, a Durham resident who’s intent on remaining active and healthy, despite some symptoms of genetically related diabetes.

“With diabetes, the more you move, it can help control that,” she said.

The goal is to repeat the measurement over time, assembling a large database of people’s walking speeds for use as a key health indicator, along with blood pressure, temperature, heart rate, breathing rate and pain. One key to the effort is the ease of use; participants don’t have to return to a large medical center for followup.

“You can be around the block, you can be at the beach,” Bettger said. “The largest study done in a clinic was with 1,200 people. We wanted to have more than that, and both male and female, and of all age spans.”

Duke researchers are already in touch with partner organizations that should add to the total of people monitored. In addition, users can track their own progress in walking speed.

“Like other vital signs, it’s a way that we can self-regulate that we are doing better,” Bettger said. “For people that have something going on or getting worse, there’s a tremendous benefit.”

For older people especially, a person’s mobility can reflect overall health, and immobility can reflect problems with muscles, heart, digestive system, joints, and even mood. Data from the app can also be delivered to the user’s doctor, or other interested party, as a means of monitoring ongoing health.

“It will show a doctor whether you are slowing down,” Miller said. “They can ask, ‘Were you short of breath? Is there a reason you are slowing down?’”

Miller heard about the study through her son, Dr. Ryan Shaw, an assistant professor in the Duke nursing school.

“We’re conducting this study because we don’t know enough about the walking speed across the human lifespan,” Miriam Morey a Duke professor of medicine, said. “As we get older, our systems start to decline and we have age-related declines that are normal. However, we can influence the pace of this decline by how we take care of ourselves.”

Walking speed can be used to track a person’s recovery from illness or injury, declines in health, or risks for falls, depression and even survival, researchers said.

A 2011 study in the Journal of the American Medical Association – by University of Pittsburgh Dr. Stephanie Studenski and colleagues – concluded that predictions of survival using age, sex and walking speed as indicators matched the accuracy of forecasts based on “age, sex, chronic conditions, smoking history, blood pressure, body mass index, and hospitalization.”