Lisa was running late．Lisa,25, had a lot to do at work, plus visitors on the way : her parents were coming in for Thanksgiving from her hometown．But as she hurried down the subway stairs, she started to feel uncomfortably warm．By the time she got to the platform,Lisa felt weak and tired--maybe it hadn"t been a good idea to give blood the night before,she thought．She rested herself against a post close to the tracks． Several yards away,Frank,43, and his girlfriend,Jennifer, found a spot close to where the front of the train would stop．They were deep in discussion about a house they were thinking of buying． But when he heard the scream, followed by someone yelling, " Oh, my God, she fell in!" Frank didn"t hesitate．He jumped down to the tracks and ran some 40 feet toward the body lying on the rails．"No! Not you!" his girlfriend screamed after him． She was right to be alarmed．By the time Frank reached Lisa, he could feel the tracks shaking and see the light coming．The train was about 20 seconds from the station． It was hard to lift her．She was just out．But he managed to raise her the four feet to the platform so that bystanders could hold her by the arms and drag her away from the edge．That was where Lisa briefly regained consciousness ,felt herself being pulled along the ground,and saw someone else holding her purse． Lisa thought she"d been robbed．A woman held her hand and a man gave his shirt to help stop the blood pouring from her head．And she tried to talk but she couldn"t, and that was when she realized how much pain she was in． Police and fire officials soon arrived, and Frank told the story to an officer．Jennifer said her boyfriend was calm on their 40-minute train ride downtown--just as he had been seconds after the rescue,which made her think about her reaction at the time． "I saw the train coming and I was thinking he was going to die," she explained． What was the most probable cause for Lisa"s weakness?