Science
At Endurance Science we think that our experiences as runners and endurance athletes have made us better physiologists. We know that our understanding of physiology has helped us and our clients to become better athletes with the minimum of wasted training and the lowest possible risk of injury and overtraining.In speaking with other endurance athletes, we often find that the use of jargon replaces real understanding of the training process. This is a pity, because understanding a few simple concepts can greatly enrich your training, and help you appreciate the nuances of a great training plan as well as the dangers of a poor one.
Let’s start off by discussing some of the basics of physiology that apply to all of us, whether we are training to become better athletes or not.
Oxygen Delivery and Utilization as a Model of Performance
By far the most prevalent model used today to think about endurance physiology is the “Cardiovascular-Aerobic Model”. This model conceives of the heart as a pump that delivers oxygen (via blood) to working muscles, and defines improvements in performance as improvements in either the delivery of oxygen or improvements in the utilization of oxygen by working muscles. While this model isn’t perfect, and can’t explain everything that happens during training or racing, it is by far the most powerful tool that we have for analyzing performance and maximizing the performance of athletes. This is the model that we use at Endurance Science to manage our athletes and fine tune their progress.
The Heart is a Pump
The role of the cardiovascular system in endurance sports is to act as a pump that supplies oxygen to exercising muscles. The heart’s ability to do this depends on how powerful it is, and how much oxygen there is in the athlete’s blood.
The output of blood from the heart is called Cardiac Output (symbolized in scientific notation as “Q”), and is the product of the athlete’s heart rate and the stroke volume (the amount of blood ejected by the heart with each beat).
Q = SV X HR
The average resting adult has a heart rate of around 70 beats/minute and each beat has a stroke volume of roughly 75 ml of blood. So the cardiac output in this situation is 75 x 70 = 5250ml/min, or 5 and a quarter litres. As you train, your stroke volume gradually increases (let’s say to 85 ml/stroke). Now, to generate the same cardiac output (5.25L/min), the heart only has to beat 62 times per minute (5250/85). This is one of the reasons that trained athletes tend to have a lower resting heart rate than the untrained.
Hemoglobin is the body’s oxygen delivery vehicle. It is the chemical that makes red blood cells red. Normally, our red blood cells are fully saturated with oxygen when they leave the heart and lungs. This oxygen is stripped off of the red cells as they pass through the microcirculation, and is used to help generate energy in cells to support their functions. The concentration of red blood cells in blood varies between individuals depending on their gender (women tend to have slightly less), nutritional status, environment and genetics.
The amount of oxygen delivered by the heart to the rest of the body depends on the cardiac output, as well as the concentration of hemoglobin in the blood, and the amount of oxygen bound to the hemoglobin.
So the amount of oxygen delivered by the heart can be described as:
Oxygen Delivery = HR x SV x Hgb x Saturation
This is a central equation in understanding training physiology. It shows us the various ways that we can increase oxygen delivery – by increasing heart rate, by increasing stroke volume, by increasing the amount of hemoglobin we have and by increasing the saturation of that hemoglobin. All of these variables can be manipulated to some degree and form the cornerstones of how we train to perform better.
A Few Words on Lactate
Unless you have lived as a hermit for the last twenty years, it is impossible not to have heard about lactate, and lactate’s role in athletic performance. Conventional training wisdom is that lactate is a product of anaerobic metabolism that causes the painful sensations when you run too fast and that it is responsible for the post run stiffness we feel after a long run. Neither of these “facts” is true.
Lactate is produced in all exercising cells and is normally rapidly either converted back into glucose, or shuttled out of the blood into other, less active cells. It is the product of anaerobic (“without oxygen”) metabolism. When you are at rest, virtually all of your energy is derived from aerobic (“with oxygen”) metabolism. When slowly jogging, more than 95% of your energy is from aerobic metabolism and only a very little is from anaerobic metabolism – thus very little lactate is produced during slow jogging. The body quickly metabolizes this lactate so that if we were to measure lactate levels in our blood, they would be very low. As we increase the speed of our run, more energy is produced from anaerobic metabolism and more lactate is produced. Our body still attempts to convert metabolize this lactate but eventually as our speed increases and our lactate production increases, the increasing amounts of lactate produced exceed our body’s ability to get rid of it, and the lactate levels begin to climb. We call this point the “lactate threshold”. When this point occurs can be greatly affected by training and is one of the key measures of an effective training program. There is a great deal more on this in the Training section.
So now that we have looked at the basics of oxygen delivery and exercise metabolism, let’s see how things look when the rubber hits the road...
Let’s go for a run
I think that the easiest way think about the major issues in endurance physiology is to imagine going for a run (or ride, or paddle, or swim, or climb…). Now imagine that we were able to instantaneously measure all of the internal physiologic and chemical changes that are going on inside of you during this run.
As you start off at an easy jog (let’s say a couple of minutes per mile slower than your marathon pace), your heart rate and oxygen delivery are only slightly elevated, your breathing is easy. Your ability to metabolize lactate exceeds your production of lactate at this point, so your lactate levels stay low and constant. If we drew a graph that demonstrated these variables, it might look something like Figure 1. Along the bottom/horizontal axis is increasing pace, and above that are lines representing heart rate, lactate levels, breathing rate and cellular oxygen consumption.
The Aerobic Threshold
As you slowly pick up the pace (imagine running at your marathon pace), all of these measured parameters begin to increase. You breathing becomes a bit quicker, your heart rate and O2 delivery are both higher, and your lactate rises slightly (although not as quickly as the other parameters). This point is often called the Aerobic Threshold – the pace at which your work is being fuelled primarily by aerobic metabolism, but beyond which we start to accumulate lactate at a faster rate. Subjectively, you feel like you are running well, at a pace that could be maintained for a long time. You can still speak easily, and you might rate your exertion as “moderate”. Your physiologic parameters now look like Figure 2.
When you increase your pace from your marathon to your half marathon pace (“comfortably hard”) your heart rate and oxygen delivery increase at a consistent rate. What happens around this point though is that your breathing rate suddenly increases due to your need to exhale more metabolic waste products. This point is sometimes called the Ventilatory Threshold. Similarly, your lactate levels begin to rise more quickly than they had before. At around this pace, your lactate levels will start to increase rapidly. This often happens right around half marathon pace, or a bit higher. This has often been called the Lactate Threshold. You can see these changes depicted in Figure 3.
As you start to push the pace, and approach the maximum speed at which you could run a 10 km race, your heart rate and O2 delivery continue their steady climb. You are well over the Ventilatory Threshold and you breathing is rapid (you certainly couldn’t get more that a few words out at a time!). You lactate production now significantly exceeds your body’s ability to clear it, and lactate levels begin to rise rapidly. Things are starting to get uncomfortable. In Figure 3 we can see these changes occurring.
As you continue to pick up speed and approach the speed at which you can run a mile dash, the suffering really starts. You are now breathing at close to your maximal rate, your O2 delivery and heart rate have continued their climb and now can’t go any higher. Your heart, lungs and circulatory system simply can’t deliver any more oxygen to your working muscles. Because your heart can’t force out any more blood, you have reached your maximum ability to deliver oxygen to the cells. We call this point your “VO2max”. The running speed at which you first achieve this maximum cardiac output is called your Velocity at VO2max” or “vVO2max”. Your lactate levels are sky rocketing and your legs are burning. You have a strong desire to slow down. Because your cells aren’t getting all of the oxygen they need to produce energy, they generate a significant amount of energy from the much less efficient anaerobic pathway. As you try to push the pace a little higher (towards your 400m race pace), you struggle to hold on. Your production of lactate swamps your body’s ability to clear it, and levels climb rapidly. At this point you have to slow down, as your desire to run is limited by the pain, and your need to reestablish stable physiology.
For any athlete, these events all occur in roughly the same order and with great reliability. However, the actual running speeds at which they occur is obviously very different between runners. By measuring when key events in this predictable series of changes occurs, we can accurately prescribe a training program that maximizes your ability to reach your genetic limits while not wasting any training time through inefficient methods.
The application of this science to an individual athlete is a detailed process that requires close cooperation between the athlete and scientist, but it can bear remarkable dividends in a short period of time. How we do this is outlined in the Training section.
