The company last week released an app that will enable it to test the Apple Watch’s ability to track irregular heart rhythms as part of a study done in collaboration with Stanford University researchers. The study is the latest aiming to tap the ubiquity of mobile devices to monitor or improve health, and it is Apple’s most aggressive attempt to elbow into the $3.2 trillion U.S. health-care market.

While Apple has sold 30 million watches since the product made its debut in 2015, it hopes to boost sales of the device by bolstering its health-care capabilities in addition to its timekeeping and fitness-monitoring functions. The company also could push further into medical-records management, glucose monitoring and more.

“Everyone is looking at the fact that about a fifth of the economy is health care,” said Brandon Ballinger, co-founder of Cardiogram Inc., which uses wearable technology to conduct health diagnostics.

Since 2009, at least 940 clinical trials using smartphones, watches or other mobile devices have been registered with ClinicalTrials.gov, a database run by the National Institutes of Health. About 290 of these were registered so far this year. Researchers are deploying the technologies to detect epileptic seizures, prevent cardiovascular disease and help U.S. combat veterans adjust to civilian life.

Researchers say the digital tools can make participating in medical studies more convenient for healthy volunteers and patients, helping to enroll them more quickly than in many conventional clinical trials. And built-in features such as heart-rate monitors and accelerometers can record data more accurately than study participants relying on recollection or making entries into log books, they say.

But researchers also see limitations, including high dropout rates by participants, as well as patient demographics that don’t always represent the broader population—partly because of the high cost of some devices. The Apple Watch costs between $249 and $399.

Concerns also have emerged about the privacy of participants’ health data because many of the devices and apps involve third-party vendors, said Charlene Wong, an assistant professor of pediatrics at the Duke University School of Medicine who has studied the use of technology in pediatric health care.

Apple will face those challenges firsthand with the study of the app’s ability to detect irregular heartbeats—a condition known as atrial fibrillation that often goes unnoticed and can lead to strokes. Anyone 22 and older with an Apple Watch will be able to participate and choose to wirelessly share their heart data with Apple and researchers at the Stanford University School of Medicine.

The study will combine the watch’s LED lights, which monitor blood flow through the wrist and already were able to measure heart rate, with the app’s software algorithm that can detect irregular rhythms. If there is an issue, participants will be notified and given a free consultation with a study doctor as well as an electrocardiogram patch that adheres to the skin for more monitoring.

The project is being led by Apple’s growing health team. Since at least 2012, LinkedIn shows the company has hired or reassigned hundreds of engineers, clinical researchers and doctors to an internal team focused on health. The company also has acquired health startups such as Gliimpse Inc., which worked on shareable medical records.

People at Apple with knowledge of the research study said the company plans to submit the app, known as the Apple Heart Study, to the U.S. Food and Drug Administration for approval as a regulated software that could be used to identify irregular heart rhythms.

The company doesn’t need to submit the watch itself to the FDA, however, because the agency plans to regulate medical software and the sensors in wearable devices—but not the actual wearables, an FDA spokeswoman said.

Sanford C. Bernstein & Co. analyst Toni Sacconaghi says efforts to accelerate sales of the Apple Watch rest on proving it is capable of more health-monitoring functions than tracking heart rate and steps. Capabilities such as blood-pressure monitoring or tracking glucose would help encourage health insurers or doctors to encourage watch use, he wrote earlier this year.

To date, Apple has primarily looked to others to prove the watch is capable of more.

Launching a study of its own is a departure for Apple. In 2015, just before it launched its smartwatch, Apple introduced ResearchKit, a software system medical researchers could use for creating apps to use for studies.

A host of researchers have taken Apple up on that, placing the smartwatches and iPhones at the center of research into everything from epilepsy and chronic pain to diabetes and exercise. Many are encouraged by the devices’ potential to improve health.

Nathan E. Crone, professor of neurology at the Johns Hopkins University School of Medicine, is optimistic his “EpiWatch” study will show the Apple Watch can detect an epileptic seizure using heart-rate and motion-sensor data. It could then alert patients, though if they have lost awareness, it could alert family members and care givers.

Researchers have been encouraged by their ability to enroll patients quickly using Apple’s software tools. Stanford researchers have been helping to run a separate heart study called MyHeart Counts using participants’ iPhones or watches and an app to detect patterns that might be linked to cardiovascular health. Anyone 18 and older is eligible to enroll by downloading the free app from the Apple app store.

“You can sign up for the study while waiting for coffee in the Starbucks line,” said Euan Ashley, professor of cardiovascular medicine at Stanford and a leader of the study.

But the ease of enrolling has a flip side: It is just as easy to drop out. In the first seven months of the study during 2015, nearly 49,000 people downloaded the app but only about 4,990 of the participants stuck with it. That dropout rate is higher than the normal dropout rate across all clinical trials, which Forte Research Systems Inc., a research-services firm, estimates at 30%.

Other researchers have been reluctant to use the Apple Watch because it hasn’t received FDA approval as a medical device. Instead, they opt to use FDA-approved activity and sleep monitors from companies such as ActiGraph LLC, believing that medical experts will trust their research claims more if they use a an FDA-validated system, said Christine Lemke, co-founder of Evidation Health Inc., a startup that does health research using technology.

Mr. Ballinger of Cardiogram said the success of Apple’s push into health hinges in part on its ability to shed some of its secrecy and become more transparent. The company has said it is more secretive than the Central Intelligence Agency, but doctors and researchers want transparency around studies and devices—such as the publication of data in peer-reviewed medical journals. Mr. Ballinger said that would require a significant cultural change at a company like Apple.