If you’re searching for a form of cardio that’ll have you sucking wind without punishing your joints, then look no further than the rowing machine, aka the erg. Unlike running on a treadmill or jumping rope, with the erg, your knees are spared. Also, since you’re really working only during the concentric phase—or top portion of the movement—you won’t be as sore the next day, which means your future workouts won’t be hindered, either. Row incorrectly, though, and you’ll risk an achy back and an all-around inefficient cardio workout. So avoid the following five mistakes to get the most out of the erg.