Overview of Principles of Exercise Physiology
Exercise Physiology | Muscle Contraction | Muscle Fibers | Muscle Adaptations | Exercise Fuels | CHO Metabolism | Fat Metabolism | Oxygen Uptake | Cardiovascular Exercise | Respiratory Responses | VO2 Max | Temperature Regulation | Heat | Fluid Balance | Fatigue | Sprinting | Endurance | Genes | Practical Case Example
Practical Case Example – Cadel Evans
In the last week of the course, I thought we would bring together many of the concepts that we’ve looked at during this course. And what better way than to examine some of the physiological and metabolic responses in an elite athlete. In this case, I’ve chosen the time trial, a practical case example, which is on the day of the 2011 Tour de France when Cadel Evans really rode himself to victory.
And so let’ look at some of the changes that take place during such an event. Let’s think about what happens when Cadel Evans undertakes a time trial that lasts just under an hour. He’s probably going to be exercising close to 85, 90% of his maximum oxygen uptake. And as he comes close to finishing, he’ll increase his exercise intensity. He could well be close to or above his VO2 max. So, let’s think about all of the things that we’ve covered during this course and try and put them together in this example.
So, oxygen uptake — recall that oxygen uptake will increase to a steady-state level depending on the exercise intensity. In this time trial, as I said, it could be as high as 85, close to 90% of the VO2 max. During the longer stages, when Cadel Evans is in the group, his energy expenditure is much less. And one of the advantages of having a team that rides in front of you is that they can block the wind resistance and markedly reduce the energy expenditure associated with riding. So, we often think of the Tour de France as you think of the individual winner. But it’s very much a team event, where that team works together to protect the lead rider to conserve energy over the three weeks of the tour. So, they’ve got plenty of energy for this individual time trial.
Think about the mechanisms that increase oxygen delivery to the contracting muscles, increase oxygen consumption by those contracting muscles. Over time, as I said, his VO2 might go up a little bit, during the time trial, perhaps not as much but in a more extensive effort that VO2 will drift up a little bit. So, think back to what happens in the transition. We remember this period of oxygen deficit and the other energy systems that are contributing, and, of course, in the end, sprint here, the so-called aerobic sources again will be recruited to supplement the aerobic energy production to really get that final sprint to the line.
Now, Cadel Evans’ VO2 max is pretty high. He’s been studied at the Australian Institute of Sport over the years and it’s routinely measured in the 80s. And that’s quite common for these well-trained cyclists. And you’ll recall that we went through some other factors which influence the VO2 max. The other important factor that we spoke about, in the endurance performance lecture, was the lactate threshold. And where that occurs on the ability to maintain a high proportion of that VO2 max with a relatively modest production of lactate. During a time trial, at this intensity, there’ll be a fairly high rate of lactate production because the rate of glycolysis is so high.
Now, let’s think about the components of that oxygen uptake. The factors that contribute to the delivery of oxygen to those contracting muscles. So, ventilation; the amount of air in another of lungs each minute. That’s going to increase with exercise and then slowly increase over time. This slight upward drift in ventilation. If we look at the heart rate, it will follow a similar pattern. And again, you’ll see this increase in heart rate, the so-called cardiovascular drift. The reason for that is that there are increases in body temperature, increases in circulating adrenaline, which stimulates the heart to beat that little bit faster.
Over time, if we look at stroke volume, the stroke volume will increase and will be relatively well-maintained and it may even decrease slightly. In this event only lasting less than an hour probably doesn’t decrease that much if at all. In longer, you’ll recall that as you begin to lose body fluids as the heart rate goes up, the stroke volume will go down a little bit as well the cardiac output.
And we spoke about some of the potential advantages of fluid ingestion during the more prolonged exercise is to maintain the blood volume, maintain the stroke volume, maintain the cardiac output. And so, in preparing for the final quiz, think about these various factors that work together to deliver oxygen to the contracting muscles.
Muscle Fiber Type
Now, those muscles are contracting, they’re generating force, they’re consuming energy. In the first week, you’ll recall we spent a bit of time looking at the muscle fiber types. I’m not aware of whether Cadel Evans has ever had a muscle sample taken or whether he’s had fiber type measured. And there’s a fair chance there’s a higher proportion of slow-twitch fibers. And the 2a fibers that are there are likely to be very well-adapted for endurance type exercise with an increase in oxidative capacity.
If you look at fiber type recruitment during a strenuous, prolonged exercise bout like this, if we look at the slow-twitch fibers and if we measured say, glycogen as one marker of the recruitment of these fibers, you’ll recall in one of the early lectures on fiber recruitment, glycogen depletion patterns used as measure of the recruitment of particular fibers. So, the slope to which fibers will lose their glycogen and then over time you’ll probably see the recruitment of the 2a fibers. And then maybe towards the end, the few type 2b fibers is that Cadel Evans has perhaps in the final sprint might be recruited as well. So, think about the fiber types, the characteristics of those fiber types and how they might be recruited during exercise.
Now, glycogen is used as a proxy because it’s utilized to a great extent during exercise like this. And so you’ll remember the pattern of glycogen depletion a very rapid reduction in muscle glycogen concentration at the early part of the exercise and it slows down a little bit as exercise continues. It’s unlikely that there’ll be complete glycogen depletion in the muscle during an exercise event like this. But there could be parts of the muscle or within particular fibers where the glycogen does reach a critically low level. And so think about the factors that increase glycogen use during exercise. We spoke about the local factors within the contracting muscle. The increase in calcium, the increase in organic phosphate, and at this high intensity with a very large increase in sympathetic activity, there’ll be a heavy reliance on muscle glycogen. The increase in adrenaline will stimulate that glycogen breakdown.
Glucose will also be utilized during this type of exercise. So, let’s look at glucose uptake you will see that it will increase during this exercise and increase over time. It won’t contribute as much to the overall carbohydrate oxidation as the glycogen will. But it will increase progressively. In a one-hour time trial, it’s unlikely that Cadel Evans would be drinking many carbohydrates. There are some studies suggesting a beneficial effect in time trials of about an hour duration. But he’s probably ensured that his glycogen stores are maximized prior to this exercise. And probably doesn’t take too much effort to drink carbohydrate-containing beverages during an exercise bout like this. In the longer exercise durations in the several hours that ingesting carbohydrate, eating carbohydrate-containing foods is a strategy to maintain blood glucose levels. That maintains glucose supply to the brain and it maintains glucose supply to the contracting muscles so that you can maintain carbohydrate oxidation.
Now, fat oxidation is probably relatively minor contributed to overall energy expenditure. But it is important nonetheless. And you’ll probably find an increase, a slight increase in the plasma free fatty acid levels. They may even go down slightly because of the increased uptake, and if there’s at high intensities, the blood flow to the subcutaneous adipose tissue might just mean that the mobilization from the adipose tissues reduced somewhat. Muscle triglycerides become more important. And certainly, there’s good evidence that during prolonged or during strenuous exercise there will be some reduction in the muscle triglyceride levels. And the fat will contribute a little bit. But as I said earlier, the major contributed to energy metabolism in this sort of event would be glycogen with a little bit of a contribution from blood glucose.
Exercise in the Heat
Now, because the energy expenditure is so high in these sorts of events the body temperature is likely to rise quite quickly. Remember the main factor, if we look at the body core temperature, the main factor driving the increase in core temperature is the rate of energy expenditure, which will raise very rapid at the onset of exercise and then it might drift up over time, and you’ll remember in this early period there’s a mismatch between the rate of heat production and the rate of heat loss. But as the heat loss mechanisms kick in, there will be an increase in the loss of that body heat.
The body core temperature will tend to level off and then only rise very slowly. In this sort of event lasting only about an hour, it’s unlikely that you’ll get to a critical core temperature or really high levels. But, it can still be quite a significant increase. The evaporation of sweat you’ll recall is the major factor that facilitates the loss of body heat. In this event where there’s a very large convective loss because of the airflow over the body, that will also contribute. And as you’ll recall, depending on the environmental temperature if it’s a little bit hotter, then sweating clearly becomes a major mechanism. And that sweat is associated with fluid loss, which has implications for the physiological function and performance.
As I said, it’s probably unlikely that in an event like this there would be a large amount of fluid ingested if, if at all in some cases. But in a practical case, certainly during the longer event, the longer duration exercise then fluid ingestion is important to try and prevent some of the physiological changes that are associated with dehydration. So, in reviewing all of the material that we’ve covered during the six weeks of this course, think about all of those factors working together and all of them contribute to the integrated physiological and metabolic response to exercise.
Environment & Genetics
We spoke briefly last week about some of the factors that limit endurance performance, and in the practical case of Cadel Evans, having a high VO2 max, and that’s going to be a function of choosing his parents wisely so that has the right genes to set him up for that sort of activity. But in order to optimize that genetic potential, he has to work hard and train hard. And then in the other factors, the environment, the nutrition, which will impact on performance.
We saw that maximal oxygen uptake is a key factor that influences or determines the maximum rate of aerobic energy production. But the lactate threshold is an important biomarker that is used in exercise and sports science to look at the capacity for endurance-type exercise. And that’s related in part to the utilization of glycogen. And as we saw in one endurance performance lecture the higher the lactate threshold, the lower the glycogen utilization at a given power output. Or forgiven glycogen utilization, you can work at a higher power output. And all of these factors, the inherent biological genetic characteristics together with the years and years of hard work, are going to combine to contribute to successful endurance performance.
Like Cadel Evans, you’ve made it to the finish line of this exercise physiology course. I hope you found it interesting and I hope it stimulated you to learn a bit more about exercise physiology. And also, maybe you had a greater understanding of the athlete within . While there is no final exam, we hope this series inspires you to continue your exercise physiology education, and apply science-based critical thinking and practical coaching.
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