Smartphones can go where no doctor has gone before — they are by your side 24/7. That alone should help give health and health research a tremendous boost. The Instant Blood Pressure app spent over five months as one of the best selling iPhone apps.

But health apps don't always work as well as they're supposed to. A study published earlier this year estimated that the Instant Blood Pressure app would falsely reassure nearly 80% of those who have high blood pressure that their blood pressure was normal. In another study, three of four smartphone apps tested weren't very accurate at screening for melanoma.

People routinely overestimate how much physical activity they get. At least that's what their motion sensor readings said.

Yet the promise of using your cell phone to improve your health remains, and a group of Stanford researchers have begun testing how to best integrate smartphones into heart health research. “People check these devices 46 times a day,” said senior author, Euan Ashley, “From a cardiovascular health standpoint, we can use that personal attachment to measure physical activity, heart rate and more.”

Even small amounts of physical activity are good for health, and more is better. Yet there is still little certainty about the best pattern of physical activity. Current recommendations are for 150 minutes of moderate intensity physical activity a week but say nothing about how those 150 minutes are best spread out.

Using software developed by Apple, Stanford researchers set out to show the feasibility of measuring physical activity entirely by smartphone — an iPhone in this case — and to begin answering questions about how patterns of physical activity affect heart health.

The app is called MyHeart Counts, as is the study. From March to October 2015, nearly 50,000 people consented to participate. Over 40,000 actually uploaded data.

Participants were asked to keep their phone with them as much as possible and were also asked to provide some basic health information — age, weight, blood pressure, cholesterol levels and heart risk factors. They also completed surveys about their diet, physical activity and heart health status. This allowed the app to provide them with feedback on their chance of developing heart disease. The phone's motion sensors were used to estimate physical activity.

Preliminary findings showed that, as suggested by other studies, people routinely overestimate their amount of physical activity. At least that's what their motion sensor readings said. And patterns of activity do seem to matter. Among people with similar activity level, those who were active throughout the day rather than in a single relatively short burst reported better heart health, with less chest pain and fewer heart attacks.

The authors are hoping for a lot more from future studies. They estimate that it would take just a few weeks to conduct a new study collecting the results of more people's 6-minute walk tests (a measure of their exercise capacity) than have previously been collected in any single study.

The few bugs that need addressing are not technological, but human: people sign up, but the dropout rate approaches 90%. And participants in the feasibility study were overwhelmingly young (average age 36), healthy and from California. A more diverse group is needed.

The researchers hope to broaden the reach of their phone-based approach by developing an Android version of the app. Ultimately, they'd like to explore the psychological components of activity. It's one thing to know that you should be more active than you are and another to actually become more active.

The study appears in JAMA Cardiology.