Serena Williams made headlines this week with her return to professional tennis at age 44, entering the doubles draw at the HSBC Championships in London after nearly four years away from the sport. But beyond the fanfare, what does her comeback actually demand of her body?

Matthew Armstrong, assistant professor in the Department of Exercise Science in the David B. Falk College of Sport, says the cardiovascular realities of competing at 44 are significant—and measurable.

The most concrete factor, Armstrong explains, is maximum heart rate (HR), which declines approximately 0.7 to 1 beat per minute per year—a rate that holds regardless of sex or training background. Over two-plus decades of a career, that adds up.

“A 20-year-old would be able to achieve a max HR about 24 BPM higher than a 44-year-old Serena,” Armstrong says. “That is a large delta that would make a difference.”

As for the reconditioning process itself, Armstrong says the fundamentals don’t change much at the elite level. “Realistically, the training is the same. It’s just hard work that needs to be done in order to get to a level that you can compete at.”

One key variable, he notes, is how active Williams remained during her hiatus. “If she has done very little training during her break then the comeback will be difficult—but if she has been training consistently, even though it would not be to a pro athlete standard, then this would make it easier.”

Elite athletes do carry certain cardiovascular advantages into later career stages—structural adaptations like enlarged chamber size and increased stroke volume that make the heart more efficient—but those advantages only go so far against the natural decline in aerobic capacity that comes with age.

With the HSBC Championships beginning June 8, Armstrong says the most fascinating question may be one that only time—and performance data—can answer: whether Williams’ speed, power and endurance on court today match what she brought to the game in the past.

Professor Armstrong is available for interview. If you’d like to connect with him, send written questions or learn more, email Daryl Lovell: dalovell@syr.edu.