Avoid: Kettlebell training involves such powerful movements that Matthews says 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 weight into someone else's back). Strains are another problem. 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.
Do it right: 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. The movement should generate from your hip, while the arms provide stability instead of momentum, Matthews says.
Read more: Unexpected dangers to avoid at the gym