"Temporary loss of consciousness"? not likely nor safe - never happened. However... the catapult stroke on HMAS Melbourne was 100 feet (lengthened c.1971) at 6 G and it all happened in less than two seconds (quoted and I was not counting) to around 100 knots wheel speed for an A4G Skyhawk. The effect I liken to a punch in the chest that was painful but no loss of consciousness. The G loading at launch is fore and aft; the Skyhawk was rated to 9 G maximum in that direction specifically for max. load catapults in difficult environments (hot day - nil wind etc.).
And to comment on the link info below: there is a thread about F-35C catapult already mentioned recently whilst both hands of the F-35C pilot are on the towel racks during catapulting. Specifically in the case of the Skyhawk a drop down handle was provided that was clutched together with the throttle at full power for catapulting so that the throttle could not be reduced during the launch. The right hand was cupped in the stomach to catch the stick moving back during launch. Scroll down the page at 'Canute' URL below for a more detailed description.