BRIDGING THE GAP
RESEARCH and PRACTICALITY
For intervals, it is not imperative you get your HR up the entire time. When you increase your pace (speed - or candence on a spin bike) you stress your cardiovascular system more than your muscular system and get a larger increase in HR.
When you increase the grade (similar with an increase in tension on a spin bike) more stress is placed on your muscles - and the muscular system - than the heart and while fatigue sets in there is less of a rise in HR.
One other possibility is your body was slightly fatigued going into your workout. When you are tired it is much harder to elevate your heart rate. In addition, your resting heart rate will be higher (an indication of stress and possible fatigue).
Written by Dr. Sternlicht for www.jeffshealthclub.com on 2.06.06