Kettlebells—those cast-iron weights that look like cannonballs with handles—are meant to be swung with momentum, not slowly lifted against gravity. Because kettlebell training involves such powerful movements, Matthews says that people often get carried away and lose control of their form—or of their weight (she's heard of a gym-goer accidentally swinging a kettlebell into another person's back). Strains are another problem. She says that one of the trickiest kettlebell exercises to do correctly is the single-arm swing, which is a combination of a squat and an arm swing. Matthews sees people putting too much of their upper body into the lift, which can lead to shoulder and back strains, as well as torn rotator cuffs.
Be safe: Hold an 8- to 15-pound kettlebell in one hand. Brace your core, squat down with your weight on your heels, and push upward from the legs as you stand and swing your arm up. This isn't a shoulder exercise; it targets the glutes and quads, so the movement should generate from your hips. "The arms should be providing stability, not momentum," says Matthews.