There is nothing new about kettlebells. In the age before steroids, 19th-century strongmen such as Arthur Saxon, Eugen Sandow and Ivan Poddubny used them to build lean, powerful, lightning-fast physiques that allowed them to perform incredible feats of strength and athleticism.
The kettlebell swing is the keystone in kettlebell training routines. The exercise appears simple, but requires coordinated, linked contractions of the thigh, butt, core and upper body muscles to do it properly. A study from the University of North Texas in Denton found that performing kettlebell swings for 12 rounds of 30 seconds resulted in near-maximal heart rates and substantial increases in testosterone, human growth hormones, cortisol and lactate.
Kettlebell workouts can potentially develop strength and aerobic fitness and create a metabolic environment conducive to improved athletic performance. (Journal Strength Conditioning Research, published online April 7, 2014)