Serious Strength was a powerup that was supposed to let the player toss objects much larger than they were for a short amount of time. It would look like a gold statue of Sam flexing his muscles. The amount of time the player could use it is unknown.
Interestingly enough, Serious Strength managed to get into the game's manual, suggesting that it managed to get pretty far in development before it got cut.
It was likely scrapped because there was no real need for it. There is nothing larger than the player that has usable physics.