Those were probably -100 series.
Those are ground spoilers. The control in the cockpit is a toggle switch called the "Flight/Taxi switch" It is normally selected to "Taxi" on the ground which stows them. Immediately prior to flight, they are selected to the "Flight" position. They will deploy whenever 1) there is weight on the wheels and 2) the power levers are near the flight idle position. Therefore, what you are seeing, when they come up prior to take-off is the FO selecting the switch to "Flight" on his line-up checks. They then retract as the power levers are advanced on the takeoff roll. They will not deploy again until the aircraft lands.
Having the Flight/Taxi switch in the "Flight" position is a requirement on the -100 series. The -200, -300 and -400's do not have ground spoilers (experience with the -100's showed it was more trouble than it was worth). If the crew fails to remember to select the Flight/Taxi switch to the "Flight" position, when the power levers are advanced it will automatically snap into the flight position with an audible click in the cockpit.
Another thing you may not have wanted to know, is that many airlines paint their ground spoilers black or red-- the most outboard ones can be seen from the cockpit and are an approved representive surface for a "last chance" detection of ice or snow build up prior to takeoff.
Does that answer your question?