Surprise risk: These tough, high-intensity group workouts, which combine weight training with cardio moves that send your heart rate through the roof, have been shown to be a highly effective and efficient way to burn calories. However, Matthews (a big fan of boot camps) says they usually involve unconventional props that can bruise more than your ego. The trickiest ones are probably the 20-pounds sandbags, which your instructor may require you to raise over your head or toss around the room. The sand inside shifts, which makes the bags harder to grip (good for the muscles you're using to hold them), and easier to drop (bad for the body parts that get caught underneath them).
Ouch! Factor: 2 to 4, depending on where the sandbag lands.
Sweat safely: Matthews suggests starting with 4-pound sandbags, and giving yourself time to adjust to how they affect your grip and your balance before progressing to bigger, heavier weights.