What is the recommended heart rate level for an efficient cardio exercise?

Target heart rate: A guide for a target heart rate during a good workout is the formula:: 220 minus your age and then take 85% of that value. I recommend working gradually to that goal if you haven't been exercising regularly. If you want to engage in very vigorous workouts, have a treadmill stress test first. A physical trainer could guide you so you won't hurt yourself with improper technique.