CABIN ATTENDANT INJURED DURING TAXI OUT WHEN AUTO BRAKES ENGAGED AT MAX WHEN ARMED FOR TKOF. ENGAGEMENT DUE TO SPD BRAKES INADVERTENTLY LEFT EXTENDED AFTER PREVIOUS LNDG; A CONDITION WHICH SATISFIES THE REQUIREMENTS FOR AUTOBRAKE ACTIVATION AT THE LEVEL SELECTED.

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TAXIING OUT TO RWY 21R AT DETROIT. WE WERE ON VECTOR ABOUT TO MAKE THE TURN ONTO TXWY M WITH A GND SPD OF NO MORE THAN 10 KTS. THE FO WAS DOING HIS TAXI OUT FLOWS WHEN ALL OF A SUDDEN WE STOPPED. I LOOKED DOWN AND SAW THAT THE FLT SPOILERS WERE DEPLOYED AND KNEW RIGHT AWAY WHAT HAPPENED. SOMEHOW IT WAS MISSED DURING THE FLOW THAT THE SPOILERS WERE DEPLOYED PRIOR TO SELECTING THE AUTOBRAKES TO MAX. THIS WOULD AUTOMATICALLY ACTIVATE THE BRAKES. ALTHOUGH IT WAS A SLOW SPD; THE SUDDEN MOVEMENT KNOCKED DOWN A FLT ATTENDANT WHO WAS IN THE AISLE. WE CALLED THEM BACK TO SEE HOW EVERYONE WAS AND IT WAS DETERMINED TO CONTINUE TO DENVER BUT THAT THE FLT ATTENDANT WANTED MEDICAL ATTN UPON ARR AND WOULD PROBABLY NOT BE ABLE TO FLY THE NEXT TRIP. WE LANDED IN DENVER; DEPLANED AND THE PARAMEDICS CHKED ON THE FLT ATTENDANT'S NECK AND BACK INJURY. WE WERE NOT IN A HURRY AS A CREW ON TAXI OUT. I THINK IT WAS JUST SOMETHING THAT WAS MISSED DURING THE FO'S FLOW. FROM NOW ON; I WILL ADD A SCAN TO THE FLT CTLS AS I AM TAXIING TO VERIFY THE SPOILERS ARE STOWED PRIOR TO THE ACTIVATION OF THE AUTOBRAKES.