Try this 4-minute rowing circuit: Begin with 20 seconds of rowing followed by 10 seconds of rest. Look at how many meters you traveled in that time. (Don't get off the rowing machine or even let go of the handle when you rest, says Penfold.) Repeat this eight times, trying to beat your distance each time. When you're finished with this four-minute circuit, row a fast 500 meters and note how long it takes you. "That's the number you'll want to match or beat during your next rowing session," says Penfold.
For our purposes here, high-intensity cardio falls between about 80 to 90 percent of your maximum heart rate (MHR) or, if you're not using heart rate zones, about a 6 to 8 on a 10-point perceived exertion scale. What this translates to is exercise at a level that feels challenging and leaves you too breathless to talk in complete sentences. But you're not going all out, as in sprinting as fast as you can. There's no doubt that some high-intensity training work can be helpful for weight loss as well as improving endurance and aerobic capacity.