In continuing with the theme of heart rate, I thought I’d take some time to answer a question that I get a lot, which is something in the vein of:
“What will happen to my heart rate as I become more fit?”
To start, the aerobic system is VERY important (learn why below), and thus getting oxygen to the muscles is one of the most important things we consider when training for endurance sports. The body does this through what physiologists have dubbed the Oxygen Cascade. The Oxygen Cascade is the combination of the structures and functions that take oxygen from the atmosphere and deliver it to the muscles.
Through breathing, the body takes oxygen from the air, and puts it in our lungs. Next the oxygen enters the vascular system, also known as the the blood vessels. The vascular system is a loop, with oxygen-full blood going from the lungs, through the heart, down to the muscle, dropping that oxygen off at the muscle, and returning to the lungs to pick up more oxygen. The speed and amount of blood that makes it to the muscle (and thus the speed and amount of oxygen that gets there), is dependent on many things, but to answer today’s question, we’ll focus on the largest factor: the heart.
When you hold all other factors constant, the amount of blood reaching the muscles is determined by two things: heart rate, and what is called stroke volume, which is simply the amount of blood the heart can pump with one heartbeat. Heart rate goes up as you go harder, and down as you go easier, and the relationship of how hard you can hold a certain heart rate doesn’t really change with training. Your stroke volume, on the other hand, is the single largest physiological change with training. Endurance performance is largely determined by aerobic capacity (the amount of oxygen that is delivered to the muscle) and aerobic capacity is largely determined by stroke volume.
Lets see how this affects our heart rates in training and racing.
Let’s say I put a couch-potato on a treadmill at 5 mph for 5 minutes, and then (if they have anything left), I make the treadmill go 0.5 mph faster every minute until they can’t go any more. And let’s say they make it up to the 7 mph stage, and their heart rate tops out at 200 bpm as they step off the treadmill, exhausted.
If said couch potato becomes very fit, and I have them back on the treadmill to do the same test, I would see two main things:
- At 5 mph, their heart rate would be lower. Why? Because running 5 mph takes the same amount of oxygen, no matter how fit you are (assuming they didn’t lose significant weight). And if training increases the stroke volume of the heart, and it’s pumping more blood, then it needs fewer beats to deliver the same amount of oxygen, thus a LOWER heart rate at easy intensities.
- Former-couch-potato would make it a lot further in the stages, maybe up to 10 mph. And what is their HR at 10 mph? Still 200 bpm. Even though maximum heart rate doesn’t change with training, stroke volume DOES, and so at 200 bpm, since their heart is pumping more with each of those beats, they’re able to deliver MUCH more oxygen to their muscles and thus go significantly faster.
Now we can answer our question: “What will happen with my heart rate as I become more fit?”
- HR will decrease at all sub-maximal speeds.
- HR will stay the same at race speeds, but you’ll go faster!
Now that you’ve learned why heart rate is important, click below to learn which heart rate monitors are ideal for training.
About the Aerobic System:
All muscles require ATP (adenosine triphosphate) to contract. There’s a little ATP in the muscle, but not much. So when that ATP runs out, it has to be replenished. The body does the vast majority of that through two systems you’ve probably heard of; the aerobic system and the anaerobic system. The aerobic system uses oxygen to make ATP and the anaerobic system doesn’t. Because it doesn’t use the energy from oxygen, the anaerobic system creates byproducts (more on that later). Both of these systems are working together all the time. In really short/hard efforts, the anaerobic system supplies most of the ATP, and at longer/easier efforts, the aerobic system supplies most of the ATP. For example, in a 30-second sprint, the anaerobic system will supply more than 90% of the ATP needs. At about a two-minute-all-out effort, ATP comes from about 50/50 anaerobic/aerobic. A 10-minute all-out effort is about 90% aerobic. The aerobic system also has two other important functions. First, the aerobic system clears out the byproducts of the anaerobic system. This function is the reason that we can go fairly hard for a long time. At the same time that the anaerobic system is producing byproducts, the aerobic system is clearing. Second, and similarly, the aerobic system is responsible for recovery. This is why you breathe pretty hard AFTER you stop a hard effort. Your body is trying to deliver lots of oxygen to the aerobic system to speed up recovery!
