Tom Brady says he can't commit physically to football anymore. Here's how many times he's been hit.

• Tom Brady announced his retirement on Instagram on Tuesday.
• Brady said he can't physically commit to the game of football anymore.

Tom Brady is calling it a career.

The 44-year-old NFL legend announced his retirement via Instagram post on Tuesday, and said a big reason for his departure is the full-body commitment it would take to keep playing.

"There is a physical, mental, and emotional challenge every single day that has allowed me to maximize my highest potential," Brady wrote. "I am not going to make that competitive commitment anymore."

Brady has been one of the most committed athletes of the 21st century, playing in the NFL for 22 seasons and maintaining a notoriously strict diet and lifestyle to extend his career, which he detailed in his 2017 book "The TB12 method."

But most notably, Brady's commitment to football has required him to take a physical punishment over the last two decades, which he says can no longer do at this point in his life.

Brady has taken more sacks than any player in NFL history

Over the course of his career, Brady has taken 543 sacks — more than any player in NFL history.

Still, despite the hits Brady has managed to avoid injuries for the most part. He hasn't missed a game to injury since 2008 when he missed the final 15 games of the season with a torn ACL, which ultimately means he's been on the field to take more hits.

Research on NFL players has drawn a link between that repetitive mid- to lower-body hits and osteoarthritis which could cause complications for Brady years down the line.

Other retired quarterbacks have shared their own experience with pain in retirement, including San Francisco 49ers Hall-of-Famer Joe Montana, and Green Bay Packers Hall-of-Famer Brett Favre.

Montana told USA Today he has arthritis after playing 15 years and winning four Super Bowls, and it's so severe he can't even run because of it. Before Brady, it was Favre who held the record for most sacks taken in the NFL. He told Sports Illustrated that the pain drove him into an addiction to painkillers.

Having played across three different decades, Brady has seen more than one generation of quarterback hits as well.

It's likely Brady's hits have been harder than Montana's and Favre's, too. Players are subjected to more force these days, NPR reported, citing the book "Football Physics: The Science of the Game." That's partly due to the increased size and speed of players, which has been documented by NFL scouting combine trends.

While the hits earlier in Brady's career may not have been as intense, they may have been harmful in another way, since the rules were different back then.

Before the NFL instituted rules to protect quarterbacks, defenders used to be able to hit quarterbacks in the knees and vulnerable leg areas, roll over them after the play was over before those moves were banned in 2006 and 2009. Brady had already won his first three Super Bowls by the time those rules were changed, having started his career in 2000.