:-/
hum... i think it could be bcs of his psychic, like when a mother can hold a truck to save her child (bad exemple, but still an exemple ^^)
Human body never use his muscle strengh at full power, bcs it would be armfull, but in certain cases the psychic unlock that and full power is reached
so... Guts' is someone who don't care if he die, if his body is broken or not, so his psychic could unlock that state easily...
sorry if it's confuse my english isn't a it's top... :-[
or... peharps it's something about his mother or father...
why did SkullKnight really save him ? couldn't he be his father ? ;)
or perhaps Zodd ;D