Something

package {
	import flash.display.*;
	import flash.events.*;
	
	public class DocumentClass extends MovieClip {
		
		public function DocumentClass() {
			playerChar.legs.stop();
			
			addEventListener(KeyboardEvent.KEY_DOWN,pressKey);
			addEventListener(KeyboardEvent.KEY_DOWN,releaseKey);
		}
		
		private function pressKey(event:Event) {
			playerChar.legs.start();
		}
		
		private function releaseKey(event:Event) {
			playerChar.legs.stop();
		}
	}
}