EUGENE, Oregon, June 25 (Reuters) - Super-sensitive starting blocks frustrated athletes and spectators alike as temperatures soared at the U.S. Olympic trials in Eugene, Oregon, on Friday. "I"ve never had that many restarts ever, in my life," world silver medallist Sydney McLaughlin said, speaking after four recalls were needed to get the opening heat of the women"s 400 metres hurdles under way. "It was definitely a little bit of a mind game, having to refocus that many times," said McLaughlin, who still ran the fastest time of the rounds (54.07 seconds) to set the stage for a showdown on Sunday with world record holder Dalilah Muhammad, who clocked 55.51 in a later heat. McLaughlin also did it with a bloody knee. "My knee started bleeding a little just from the hot track," she said of the numerous restarts. In blazing sunshine, temperatures climbed to 95 F (35°C) and it was even hotter on the Hayward Field track. Even the slightest hint of movement by athletes in the blocks appeared to set off an electronic recall alarm, starting with the fourth round of the men"s 110 metre hurdles where five restarts were needed with only one official false start called. "Under the conditions, the rules of the game is to survive and advance," said Jarret Eaton, who moved on to Saturday"s semi-finals. "You use a lot of energy and mental focus at that moment." Four-time Olympic sprint medallist Ato Boldon, now an analyst for broadcaster NBC, blamed the sensitivity of the starting blocks for the misfires. As spectators booed and whistled, Boldon told viewers he didn"t agree with athletes having to get used to the blocks: the blocks need to be adjusted to the athletes, he said. "The blocks are set too sensitive." After the 400 hurdles, the trials went off without major miscues. But the weather will be an even bigger problem at the weekend, with weather forecasters issuing an excessive heat warning promising temperatures of 107 F (41.6°C).