The rowing machine doesn't burn as many calories as running.

The reason using an indoor rowing machine doesn't burn as many calories as running is that rowing is mainly an upper-body exercise, involving a smaller amount of muscle mass than running. (The reason it feels so difficult is that the fatigue is also concentrated in a smaller area.) As a rule, lower-body exercises, which utilize the large muscles in the leg and buttocks, burn more calories. The cross-country ski machine is the greatest calorie burner because it employs both upper and lower body activity. If you enjoy rowing and don't burn out too quickly from the effort, keep doing it; otherwise, if you're looking to burn as many calories as possible, you'd be better off running or using a ski machine or stairclimber.