A hearty second to the rowing machine! It's quite refreshing if you're bored with endlessly punishing your legs for cardio
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Ordinarily running is my drug of choice, whether it's in the open outdoors or on the track (but I don't like treadmills much). My knees have been complaining a bit recently, though. Anyone know a good solution to that? (or if it's an involved topic I can move the thought elsewhere lest I threadjack).