Fitness trackers routinely measure physical activity such as running and cycling and encourage people to stand up and walk around throughout the day. But what if you are in a wheelchair? Apple recently announced that an update expected later this year to the Apple Watch will include a fitness tracker for wheelchair users.

Instead of standing breaks, people in wheelchairs will be prompted to wheel or spin their chairs around regularly. Apple will also start tracking distance, speed and calories burned during wheelchair use, just as it does for walking or running.

Jeff Williams, Apple's chief operating officer, said the feature isn't about market opportunities.

"We want to make products that serve every walk of life," Williams said in an interview. "We realize that while it was great for messages on the wrist, we wanted to offer this group of people the same opportunity to get healthier using Apple Watch."

Apple largely had to start from scratch because past scientific studies on burning calories weren't designed with wheelchair subjects. The company couldn't simply translate formulas meant for walking and running.

For one thing, people push their wheelchairs differently when approaching a ramp or circumventing an obstacle. Apple also had to factor in different seat and wheel heights and different surfaces, like carpeting or asphalt. And some formulas change depending on whether the disability is from a spinal-cord injury or muscular dystrophy.

"The more you look into it, the harder and more challenging you realize it was," said Ron Huang, Apple's director of software engineering for location and motion technologies.

The feature is part of a free software update, watchOS 3.0, expected later this year. No new hardware is required.