Kettlebells—rounded, thick-handled spheres of cast iron—have long been the training tools of choice for many Russian athletes and soldiers. Recently, they’ve gained popularity with American athletes, including skiers.

“After training with kettlebells, a ski stance feels perfectly natural,” says Jonathan Bischof, a 25-year ski coach  and owner of Core Awareness Training Center at Vermont’s Stratton Mountain Resort. “And they’re an efficient way to build stability.” That’s because the awkwardness of a kettlebell forces you to recruit all your stabilizing muscles.

Master these exercises, and you’ll understand how these time-tested fitness tools have become so popular—again.