(CNN) At exactly 8:46 a.m. every September 11 -- the moment when the first hijacked plane struck the Twin Towers in 2001 -- firefighters around the nation begin their ritual.

In stadiums, public buildings and, in some cases, on the StairMasters at their gyms, the first responders walk up 110 flights of stairs -- dressed in full firefighting gear.

It's a tradition meant to honor the 343 firefighters who lost their lives in the line of duty that horrific day. The 110 flight-climb is meant to simulate the heroic distance those fallen men and women had to travel inside the World Trade Center that day.

They do this every year to mark the somber occasion, and today was no different.

The firefighters at the gym