KU-developed app makes fitness testing possible for more patients

Sandra Billinger's decades of research now fit in clinicians' pockets

For nearly three decades, Sandra Billinger, doctoral professor in the Department of Physical Therapy, Rehabilitation Science & Athletic Training, pursued a simple but important question: How can clinicians accurately assess cardiorespiratory fitness in individuals who cannot easily perform traditional exercise tests?

That question led to the TBRS Submaximal Exercise Test, a validated alternative to testing on treadmills or upright bicycles for people who may not be able to use them safely or comfortably, and now a tool clinicians can carry in their pockets: the TBRS Submax app. Developed from KU research and supported by the KU Center for Technology Commercialization (KUCTC), the app transforms a validated exercise test into an accessible digital platform for healthcare professionals worldwide.

A vision years in the making

The story began in the late 1990s when Billinger was interested in exploring ways to accurately measure aerobic fitness in people who faced barriers to traditional exercise testing modalities such as a treadmill or upright bicycle. The total body recumbent stepper (TBRS) offered a promising alternative because it supported users while engaging both the arms and legs during exercise. Early research focused on maximal exercise testing, but Billinger recognized a challenge that extended beyond the lab.

“As a physical therapist, I kept coming back to a practical question: Would clinicians actually be able to do this?” Billinger said. “Maximal exercise testing provides valuable information, but it requires equipment, expertise, time and resources that aren't available in many clinical settings.”

That realization shifted the research focus for Billinger and her colleagues. The team developed the TBRS Submaximal Exercise Test, a scientifically validated method for estimating cardiorespiratory fitness without requiring patients to complete a maximal exercise test. The test was published in 2012 and attracted interest from clinicians and researchers seeking practical ways to evaluate fitness and prescribe exercise.

While the app seems like a recent innovation, the idea has been part of the technology’s journey for more than a decade.

After publishing the TBRS Submaximal Exercise Test, Billinger submitted an invention disclosure in 2014 to KUCTC and proposed that the assessment be translated into a mobile application. Over time, interest in the test continued to grow. Clinicians and researchers regularly discovered the team’s publications and instructional resources and reached out with questions about implementation.

Those conversations reinforced a recurring theme: Publishing the science was only part of the mission.

“Publishing the science was important, but making it easier to use could extend its impact,” Billinger said.

The tipping point came in 2024 when Billinger was teaching physical therapists in Singapore through a program with the Singapore Ministry of Health. During a discussion about exercise testing and prescription after stroke, a participant posed a straightforward question: “Can’t you make an app for that?”

That question reignited an idea that had been waiting for the right moment.

When Billinger returned to the United States, she and her team revisited the concept. With years of scientific validation behind the TBRS Submaximal Exercise Test and growing demand from clinicians, they saw an opportunity to transform the research into a tool that could reach a broader audience. Two years later, the TBRS Submax app became a reality.

Moving beyond publication

For Billinger, one of the most powerful motivations throughout the process was the continued interest from healthcare professionals who wanted to use the test in practice.

“Over the years, clinicians and researchers would contact me to teach workshops on how to use and implement the Submax test,” Billinger said. “Those conversations were reminders that the work mattered beyond the publication.”

Bringing the technology to the public required navigating challenges that extended beyond scientific research. Questions about software development, commercialization, distribution and revenue management required expertise from across the university.

Billinger credits the collaborative efforts of faculty, administrators and KUCTC for helping move the project forward for public impact.

“I had tremendous support from research leadership, teams on both the KU Lawrence and KU Medical Center campuses, and Taylor Escher in KUCTC, who helped facilitate the process,” Billinger said. “We didn’t have a roadmap, but we had people who were willing to help us create one.”

KUCTC worked alongside university stakeholders to help translate the research into a publicly accessible technology, demonstrating how university innovations can move beyond academic publication and into the world to benefit the lives of users.

Seeing decades of work come to life

One of the defining moments in the project’s development came when Billinger shared an early version of the app with colleagues around the world to test its capabilities.

The feedback was enthusiastic and constructive. Clinicians and researchers helped refine the user experience, allowing the team to improve functionality before launching the app. After multiple rounds of testing and revision, Billinger could finally see a direct path from years of research to a tool that could be widely adopted by clinicians.

Another memorable milestone arrived when the final app design was complete.

“The second most important moment was when I helped create and decide what the app icon would be,” Billinger said. “When the KU Communications team sent me the finished product, that was a moment I will never forget.”

The app launched in August 2026, and early reactions from clinicians and researchers familiar with the TBRS Submaximal Exercise Test have been encouraging to Billinger and her team. Many users immediately recognized the potential benefits of having the assessment available through an easy-to-use mobile platform.

What’s next

For Billinger, the TBRS Submax app represents decades of collaboration and persistence, as well as a new opportunity for others to build on that work.

“I would tell myself to keep believing in good ideas, even when the path forward isn’t clear,” Billinger said. “I’ve learned that when science has practical value, publication may actually be the beginning.”

Today, the app allows healthcare professionals and researchers to access a validated fitness assessment tool developed through KU research, extending the impact of the technology far beyond the laboratory.

“For me, this is truly the end of a research era,” Billinger said. “To see decades of research become something that people can now hold in their hands and use is difficult to put into words.”

The app illustrates how university research can evolve into practical solutions that serve clinicians, patients and researchers alike. It is also a reminder of KUCTC's mission: helping transform KU discoveries into innovations that create real-world impact.

“I am incredibly proud of what we created, and now I am excited to see where others take it,” Billinger said. “My hope is that someone, somewhere, uses this tool to help a person better understand their fitness, prescribe exercise more effectively, or ask the next research question that moves the field forward.”

Mobile device users can download the TBRS Submax app from the App Store or Google Play.

KUCTC is proud to support technologies that have real-world, accessible impacts on the public through health care, wellness and other industries. Explore KU's available technologies and see how researchers are developing innovative solutions for public benefit.