Is there a relationship between heart-rate over time and calories burned, or is it just duration? What about a non-cardio workout?
Both, and here's why.
The answer here is pretty obvious. The longer you workout the more calories you burn.
Of course, if you're trying to maximize the amount of calories burned you need to look at more than just the calories burned during exercise.
Resting metabolic rate (RMR) is defined as the energy expenditure necessary to maintain the physiological processes in the post-absorptive state and, depending on the level of physical activity, may represent approximately 60 to 70% of total energy expenditure.
Diet Induced Thermogenesis (DIT) refers to the increase in metabolic rate above resting levels due to food intake (i.e. high protein foods) and corresponds to approximately 10% of total energy expenditure.
Physical activity is the most variable component and is related to the energy expenditure necessary for skeletal muscle activity. In sedentary individuals it represents approximately 15% of total energy expenditure, whereas in physically active individuals this can reach 30%
If you consider that exercise alone only accounts for up to 30% of overall calorie consumption then how are you supposed to achieve greater results.
The answer is simple, increase your RMR (Resting Metabolic Rate) and burn a lot of calories through exercise. Which leads me to talk about intensity.
Researchers compared duration and magnitude of metabolism after exercise in a typical resistance exercise session with that of aerobic exercises with same duration (27 min) and intensity. Results showed that oxygen consumption (a measure of metabolism) remained significantly elevated up to 90 min after terminating the resistance exercises and only 30 min after the aerobic activity. Calories burned after exercise was higher during the first 30 min after resistance exercises than after the aerobic exercise, representing an additional energy expenditure of 95kcals for circuit training and 64 kcal for aerobics
And... don't forget about EPOC (Excess Post-Exercise Oxygen Consumption).
Let’s take a closer look at this for caloric burn and also from what is called EPOC, or excess post oxygen consumption. EPOC generally accounts for the energy expenditure during recovery from the exercise bout or the “post exercise burning" of calories (9). Typically this EPOC is fueled by fat and the intensity of work performed. The higher the intensity, the higher the EPOC. When compared to post exercise fat oxidation, moderate to low intensity exercise barely compares. Take for example the work done by Tremblay et al. This study compared an aerobic group and an anaerobic group of subjects for caloric burn and fat loss. The aerobic group trained for 20 weeks while the anaerobic group (interval) trained for only 15 weeks. The results showed that although the aerobic group burned nearly 50% more calories, the anaerobic (interval) group burned nine times more subcutaneous fat than their counterparts (11). For those not paying attention, in summary, that is five weeks less work and nine times the fat lost. Imagine becoming the trainer who is known for giving clients better results in a shorter period of time!
Last but not least, don't forget that lean muscle mass requires energy. The more you have, the more you burn.
The caloric utilization of tissues in the body differs too. The old adage that “muscle burns fat” is not entirely true. But when compared, muscle tissue burns 7-10 kcal/kg/day whereas adipose tissue only consumes 2-3 kcal/kg/day. Look at the figure below. Which one of these athletes looks the leanest? Now, which one of these athletes looks like they have more muscle mass? Lastly, which athlete is a sprinter (high intensity interval) and which do you think is the aerobic (jogging, LSD) runner? If you guessed the runner on the left, you were correct on all three accounts.
As you can see by the images, the athlete in Photo 1 appears to have a larger percentage of lean mass than does the athlete in Photo 2. With this statement and the aforementioned data, it seems feasible that the sprinter possesses a higher resting metabolic rate (RMR) as well due to the higher level of lean mass. In fact, there is an underlying correlation with RMR and fat free mass (FFM). Although FFM accounts for muscle tissue, bones, and also organ tissues, muscle is the only of these three that can be altered to any varying degree and thus alone contributes to nearly 22% of RMR (2). Some of the additional benefits that the high intensity interval trained may experience in conjunction with increased fat loss include, greater improvements in VO2max, increased growth hormone response (due to lactate accumulation), and positive blood pressure response (4, 5).
Intensity means everything in when it comes to burning fat mass. Duration definitely also plays a big part, but only when it comes to how you fine tune your workouts to your personal ability (ex. out of shape = shorter workout duration).
I know it goes against the conventional wisdom taught by most personal trainers today (being that aerobic workouts are supposedly the best types of workouts to burn fat) but the results have been proven and backed by scientific fact.
Plus, take a look around at the typical gym. How many people do you see who are overweight or out of shape putting in their 40 minutes on the treadmill 2-3 times a week vs the few who are doing weight or resistance training?
I always thought it was insanity that there were so many people at my local gym who would be there before I arrived and leave after me who had so little to show for how much time/effort they put into exercising. Whereas, I would do my little 30-45 minutes of a high intensity sprint (2-4 miles) with some focused bodyweight and resistance/weight exercises twice a week at most and have great results to show.
Of course, higher intensity means you need to be strict about taking rest days but the best part is, recovery takes an excess amount of energy too (say hello to elevated RMR).
The better way to understand it is the load you are putting your body under. If you exercise long enough (duration) you can get to the same place as you would working harder (intensity) for a shorter period of time.
Load = Intensity x Duration
You can find that for your current ability you can find the sweet spot once you know how long you can maintain any given intensity. But it also shows that if you only have so much time available, you can increase the load by increasing the intensity.
Lower intensity (i.e. lower heart rate) burns more fat percentage wise, and higher intensity (i.e. higher heart rate) burns more sugar percentage wise. The volume of calories burned can outweigh the benefits of a low heart rate workout assuming you only have so much time.
The above is a simplification of everything, because our bodies don't linearly progress in the number of calories burned as you increase your intensity. You get to a certain threshold and the number of calories burned jumps way higher.
The higher your heart rate, the more calories you will burn. However, a person is not able to maintain a high effort/ high burn rate for as long as a lower effort /lower burn rate.
However, depending on the effort, the calories will be burned from different fuel sources. For more information on this specific topic check out the "Aerobic versus anaerobic exercise" section at http://en.wikipedia.org/wiki/Aerobic_exercise