Indoor rowing machines, or "ergs," are designed with a seat on a rail that travels back and forth, to simulate rowing on a boat, says Annie Mulgrew , program director at CityRow in New York City. On the machine itself, there's a monitor with two metrics that you'll need to keep track of while you row: speed, measured in strokes per minute (or SPM), and split time, which is a ratio of how many meters you can row in a period of time, and represents your stroke intensity, Mulgrew says.