They sat a subject in a chair and fastened a cable to his wrist with a wide strap. When the second hand of a clock reach a given point the subject pulled against the cable in what amounted to a static contraction (isometric) biceps curl. Readings were taken from a tension meter connected to the cable.
The subjects given alcohol and adrenaline had improvements considered statistically insignificant. The most profound effect was from hypnosis where the subjects were given the suggestion of either feeling great strength or of great weakness. These subjects demonstrated either 26.5% more or 31.7% less strength while hypnotized. (!) Subjects given amphetamines saw a 13.5% improvement. Subjects who had a starter’s pistol (firing blanks) shot unexpectedly while lifting had a 7.4% increase in strength. And the subjects who shouted during their exercise garnered a 12.2% increase in strength.
For most of us, taking amphetamines, having a personal hypnotist at the gym with us or firing off a handgun during our leg press is just not going to be practical, to say the least. But the cheap and simple act of yelling your own Barbaric Yawp during your 5 second static exertion might well garner a 12.2% increase in your performance. Wow!
The researchers suggested that this phenomenon might be explained by acting to remove internal inhibitions. Somehow the Vikings must have known that when they ran ashore, bare-chested wielding battle axes and screaming their own, truly barbaric, yawp. But you can get the same benefit without the messy bloodshed.