// Metabolic demand and heart rate training
by Gary McCoy
As health and fitness professionals we’re all guilty of providing our clients with broad estimates instead of facts when it comes to burning fat and training for performance.
Our intentions are honourable; we want to get the most out of every training session and use whatever resources we have, including information learnt in college or training courses and articles we’ve read. In fact, we don’t have to go too far to have the information reinforced, it’s there in a multi-coloured chart on the wall of the gym, and embedded into most cardiovascular equipment. What are we talking about?
Heart rate zone trainingBut how accurate is the ‘fat burning zone’ on these charts – and does it work for everyone? If you’ve prescribed exercise based on the information on these charts, the chances are you’re already skeptical about their effectiveness. In fact, recent research shows that these estimates can be off by over 30 beats per minute(1).
Where did maximum heart rate and zone training begin? And why are we using these methods if they are flawed?
In 1968, physiologist William Haskell and cardiologist Sam Fox were on their way to a World Health Organisation conference and were charged with the task of coming up with a guideline for patient exercise intensities during cardiac rehabilitation. Armed with a few pages of research based on 80 subjects, they determined that maximum measured heart rates appeared similar among the subjects. It appeared that most 30-year-old subjects had a maximum measured heart rate of 190 beats on average, most 20-year-olds up to 200 beats and so on.
‘220 minus age’ appeared to be a good ceiling for max HR prescription.
In 1971 the works were published, and concluded that ‘220 minus age’ seemed to be a good overall fit for rehabilitation prescription. It made sense to the fitness industry that a percentage of this max HR could affect the type of training outcome we were looking to affect. Sixty-five per cent of max HR was declared the ‘fat burning zone’, and that’s where we have stayed for over 30 years. Certainly, we can individualise the theory by using the Karvonen formula to take individual resting heart rates into account, but the fact remains that Karvonen is still founded on the age-based max heart rate.
Back to basics
We measure heart rate because it gives us valuable feedback on how hard we are training. But what makes our heart rate increase and decrease? By answering this, we can see that we may be putting the cart before the horse when designing a heart rate training program.
Demand for oxygen is what makes the heart pump. Oxygen demand is increased by the need to develop energy in the working muscle cells.
Oxygen couples with a caloric substrate and through a complex series of chemical events, we develop ATP (adenosine triphosphate) for cellular energy. These are the basics of metabolism, and it is metabolism (or the metabolic rate of activity) that makes the heart pump faster or slower. When we design heart rate training programs (other than those for cardiac rehabilitation), we are trying to affect metabolism. Consider the fat burning zone, the cardiac improvement zone, the fitness zone; these are all effectively looking at metabolic factors that are 'artificially observed' by age-based heart rate.
Metabolism: the driving source for human movement
Metabolism can be defined as both resting metabolic rate (RMR) and exercise metabolic rate (EMR), both of which affect weight loss and performance.
RMR can simply be defined as the energy required to sustain life. A good analogy is that of a car which has its engine left idling all night in the garage, burning a fair amount of fuel. That’s resting metabolism, and its effect on weight management is critical, but not the focus here.
Your resting heart rate supplies oxygen to your subconscious working organs throughout the day and night, and the rate at which it works is dependent upon factors such as genetics, demand, disease and fitness levels.
So, what about exercise and EMR?
Exercise requires energy above the demands of basic life functions, and this is where heart rate training comes in. At differing exercise intensities there is a differing need to generate ATP for cellular energy. The available fuels vary as well, driven by their ability to couple with oxygen and the rate at which they are needed.
Think about the zones for a moment; ATP-PC (phosphogen system) – lactic acid system – aerobic system. We ‘systematically’ shift substrate utilisation based upon the time-work demand of the activity.
Now think again about oxygen demand and use. Are all 35-year-olds the same? Should Lance Armstrong – poster boy for physiological excellence at 35 – train the same as Mary in your club who is the same age, but has just got off the sofa after 15 sedentary years of watching Days of Our Lives and packing on 42 per cent body fat? Of course not. So why do we look at heart rate charts which suggest this is what we should be doing?
Heart rate is demand driven by metabolic activity. The chart below shows what actually happens at various heart rates, outlining zones based upon a gas exchange, or metabolic assessment, of a 21-year-old female athlete. The black line intersecting the zones shows fat utilisation. Notice the end range measured heart rate – a whopping 237 beats! (compared to the 220 ceiling indicated by Haskell and Fox). The maximum ranges of fat burning are somewhere around 142 beats, with a threshold for efficiency at 197 beats.
This information is critical for assessing what is happening at the cellular level. If we have this information we can accurately create heart rate training programs based upon the desired outcomes; increased fat burning across multiple heart rates, or improved efficiency (anaerobic) threshold. Without this information, we are guessing rather than accurately planning.
Heart rate : pump by demand
Heart rate is affected by many factors, all of which encompass the delivery of oxygen to the working muscle cells. Heart rate can be increased or decreased by;
• the way in which oxygen is ‘pumped by demand’ through the blood stream.
The availability of oxygen increases or decreases respiration (think of altitude training) and heart rate
• environmental temperatures and the need for regulation
• the ability of the oxygen to permeate the working muscle cell
• blood viscosity, caloric availability, ability to buffer lactic acid and all the products of metabolism.
One of the keys to the rate at which the heart pumps at the local level is the ability of the left ventricle to eject blood efficiently through the venous system.
This cardiac output is a key proponent to the rate of cardiac contraction. A volumetrically larger and stronger left ventricle (stronger by definition of the overall volume of blood that it can eject in a single beat) will beat less frequently than a smaller volume left ventricle. The oxygen demand therefore will be met by the volume of oxygenated blood expressed. This is a reason why a low heart rate should never be assumed to be healthier than a high rate when comparing different individuals. Each has a different left ventricle capacity, and even if the ages are the same, it is possible for one individual with poor fitness to have a low heart rate, while another person could be in great shape and have a higher heart rate. Let’s return to the engine analysis; this is like comparing the tuning of a Harley Davidson to that of a Kawasaki. Is the high rate of the Kawasaki or the low beating hum of the Harley better? The fact is, they are two different engines, and are beyond comparison without further analysis.
Re-evaluating heart rate training
Without becoming another evangelist regarding heart rate training, taking a look at this level it is clear to see that not everyone of the same age should train according to the same heart rate chart. While the charts provide us with a safe guideline, they rarely, if ever, provide a solution to the weight loss and performance training program. In response to this problem, more fitness facilities are introducing ‘metabolic assessments’, a tool which can more accurately help ensure success for personal training clients, and for you as a health and fitness professional.
1. Tanaka, H., Monahan, K.D., & Seals, D.R. (2001). Age-predicted maximal heart rate revisited. Journal of the American College of Cardiology, 37, 153-156
Gary is a strength, conditioning and performance coach for athletes globally. The former director of education for CYBEX International and vice president of IDEA, residing in the US, Gary is the performance coach for the Australian national baseball team and Florida Marlins baseball club.
NETWORK MAGAZINE • SPRING 2009 • PP52-53