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
Learn about endurance performance including effects of water, carbohydrates, physiological determinants, and maximal power output during exercise. The video references several key factors associated with athletic performance related to VO2, muscle, nutrition, and physiological variants to consider for endurance training in cyclist athletes.
In this lecture, we’ll examine aspects of endurance performance and the potential mechanisms of fatigue during prolonged strenuous exercise. And here again, my favorite endurance athlete, Cadel Evans. If we look at the characteristics of endurance athletes, we say they’re characterized by a high VO2 max. But not always the highest VO2 max is the most successful endurance athlete.
Fraction of Utilization
And it’s the ability to maintain a high proportion of that VO2 max, or what we call the fraction of utilization, which might be just as important if not more important. A high-power output at lactate threshold, is important, and that’s related to the muscle oxidative capacity, and also the muscle fiber time. The ability to resist fatigue for some of the mechanisms that are associated with a reduction in performance during prolonged exercise. And given the duration of many of these endurance events, an efficient or economical technique can often be important. And the ability to oxidize fat at reasonably high power outputs is an important adaptation in endurance athletes.
- VO2 Max, Muscle QO2 & Endurance
- Lactate Threshold & Muscle Oxygen Consumption Rate (QO2)
- Determinants of Endurance in Well-Trained Cyclists
- Physiological Determinants of Endurance Performance
- Potassium Ion (K+) Balance & Fatigue
- Sarcolemma (SR) Calcium Ions (Ca2+) Release & Prolonged Exercise
- Muscle Glycogen & Fatigue
- Carbohydrate Ingestion & Fatigue
- Carbohydrate & Fluid Effects Independent & Additive
- Maximal Power During Prolonged Exercise – Effects of CHO & Fluid
- Hyperthermia & Fatigue
VO2 Max, Muscle QO2 & Endurance
As I said, both VO2 max and the muscle oxidative capacity, at least as we can estimate it from measurements such as lactate threshold, and I’ll talk more about that in a moment, are related to performance. As you’ll recall from the oxygen transport system, there’s a very close association between oxygen delivery and the VO2 max. If we look at muscle endurance or endurance performance, that seems to be more related to the muscle oxidative capacity. And here is a really nice study that was done in experimental rodents, and they were made iron deficient for over a number of weeks and that the consequence of iron deficiency is that they develop anemia because this not enough iron to maintain hemoglobin synthesis and red blood cell mass and also the electron transport system has iron-containing enzymes. With severe iron deficiency, you get problems with your muscle oxidative capacity.
These animals were then tested for their VO2 max and their endurance capacity. And then they were re-fed with iron and the iron levels were restored. And you can see in this study that as the hematocrit recovered, the VO2 max recovered. And that’s consistent with the notion that oxygen delivery is an important determinant of the maximal oxygen uptake. However, you can see here that the endurance performance is measured by time to fatigue on a treadmill, didn’t really recover until the muscle oxidative capacity had recovered. So clearly the adaptations in the muscle that increase mitochondria increase the ability to consume, substrates in the aerobic energy system are closely related to endurance performance.
Lactate Threshold & Muscle Oxygen Consumption Rate (QO2)
Now sports scientists for many years have attempted to look at the relationship between lactate threshold as a proxy of the muscle oxidative capacity. It’s difficult in a routine sense to obtain muscle biopsies from an elite athlete. And so lactate testing has in addition to VO2 max testing has become an important part of the sports science arsenal, if you like, in evaluating endurance athletes. And so you remember back to our ventilation lectures, the debate about the relationship between ventilation and lactate, but, at least you can measure the lactate threshold here, the point at which there’s a large increase in lactate, and this corresponds with a greater rate of glycogen breakdown in muscle. And you can see that there are close relationships between the lactate threshold, at the VO2 and the VO2 max, and the muscle oxidative capacity. In this case, measured in vitro. The correlation coefficient is higher for the lactate threshold. You can see the R-value here of .94, which means that about 80 odd percent of the variants in the lactate threshold can be accounted for by the variance in the muscle oxidative capacity. But why is that important?
Determinants of Endurance in Well-Trained Cyclists
Well, if we look at the relationship between an exhausted exercise test, in this case, in the left panel, time to fatigue, and the percent of the VO2 max, in which the lactate threshold occurred, and this was in a group of reasonably similar cyclists, at least in relation to the VO2 max, and you can see if you look at the performance, [14 compared cyclists], those cyclists with the highest lactate threshold were able to go for the longest period compared with those with the lowest. When they were studied during a standardized exercise bout at about 80% of their VO2 max they all had very similar VO2 max values, meaning that both the absolute and the relative exercise intensities were similar across the groups, you can say that those subjects who had the highest lactate threshold were the ones who used the least amount of glycogen during this standard exercise bout. Whatever reason the metabolic adaptations, changes in their technique, they are able to use relatively less glycogen in this standardized exercise bout. If glycogen is associated with fatigue, I’ll show you in a moment why it is, perhaps that’s part of the explanation for why lactate threshold and muscle oxidative capacity are related to endurance performance.
Physiological Determinants of Endurance Performance
And if we look at all of the factors that we’ve examined during this course that might determine or influence endurance exercise performance we can see them all summarized here. And so ultimately we’re interested in how quickly we can run or how much power we can develop. And this is going to be influenced by the aerobic energy production, influenced by the lactate threshold and the VO2 max. And as we saw in the earlier lectures, the VO2 Max is going to be influenced by the whole range of morphological factors, e.g., maximal cardiac output, hemoglobin mass, and the mass muscle oxidative capacity. And then there will be some factors around the anaerobic key points of transition, and factors related to efficiency, how well you can transduce the metabolic energy into the mechanical work, that’s going to be influenced by technique, perhaps a little bit by fiber type, and various other factors, and you can see it’s quite a complex indirection and here many factors which go in to making a successful endurance athlete.
Potassium Ion (K+) Balance & FatigueAgain, let’s look at some selected fatigue mechanisms during prolonged exercise. Here although the changes are not as dramatic during prolonged strenuous exercise you can see that there is a progressive release of potassium from contracting skeletal muscle. And this manifests itself as a slow rise in the arterial plasma potassium. And indeed when measurements have been made before and after prolonged exercise bouts, there’s a slight reduction in the intra-muscular potassium levels. And just as we saw with the very intense exercise which clearly occurs with the much shorter time frame, with prolonged exercise there may be enough loss of potassium to impact the excitability of the sarcolemma during prolonged exercise.
Sarcolemma (SR) Calcium Ions (Ca2+) Release & Prolonged Exercise
Investigators have seen changes in calcium release following prolong exercise, now these measurements are made under optimal conditions in the test tube, and suggest that there are some long-lasting perhaps conformational changes in some of the release mechanisms within skeletal muscle, and this might be further potentiated by changes within the muscle, changes in the local metabolite concentrations, ATP and inorganic phosphate hydrogen iron. So at least using this essay system there were reductions in calcium release during that type of exercise.
Muscle Glycogen & Fatigue
A lot of attention is focused on glycogen. I mentioned earlier that the relationship between lactate threshold and fatigue may be related at least in part to a lower rate of glycogen use. You can see here the association between the decreasing glycogen during prolonged strenuous exercise and the point of fatigue.
And here, you can see the increase in a breakdown product inosine monophosphate (IMP). Now you may recall from the fuels lecture, that the enzyme adenosine kinase (ADK) can covert two adenosine diphosphate’s (ADPs) into adenosine triphosphate (ATP) and adenosine monophosphate (AMP). And if the rate of adenosine triphosphate (ATP) re-synthesis is falling slightly behind the rate of adenosine triphosphate (ATP) breakdown you can get the accumulation of adenosine diphosphate (ADP) and adenosine monophosphate (AMP). AMP activates an enzyme called AMP deaminase, and the product of that reaction is IMP. And IMP, in this context, is often used as a marker of this imbalance between ATP re-synthesis and ATP breakdown. So the higher inosine monophosphate (IMP) here that corresponds with this lower muscle glycogen has been interpreted as a challenge to the ability to maintain high energy adenine dinucleotide levels within a fatigued muscle.
And as I showed you in the first slide of this series, it’s not just the absolute amount of glycogen that is critical, it could be where that glycogen is. And so, glycogen, both within the muscle and at key spots around the T-tubule and the Sarcolemma (SR), where it might be an important substrate for excitation-contraction coupling processes. It might be the location of that glycogen, which is just as important as the total amount.
Carbohydrate Ingestion & Fatigue
The other important carbohydrate fuel source for contracting skeletal muscle blood glucose and glucose is also important for the brain. You recall from the first side in fatigue, or the first lecture in fatigue, the evidence that glucose supplied to the brain might influence central motor drive. Well here is what happens during prolonged exercise. If you don’t ingest carbohydrate you get a progressive decline in blood glucose. If you ingest a carbohydrate-containing beverage you can see that the carbohydrate, or the carbohydrate ingestion, maintains the blood glucose level. And that’s associated with an increase in exercise time. But importantly, it doesn’t prevent fatigue. So carbohydrate ingestion delays, but doesn’t prevent fatigue, so clearly there are many factors contributing to fatigue during prolonged strenuous exercise. But its generally believed that carbohydrate ingestion can improve exercise performance and its thought to do this by maintaining blood glucose levels. Which in turn, will maintain muscle carbohydrate oxidation and glucose supply to the brain. And interestingly, some recent experiments have suggested that even just the presence of carbohydrates in the mouth might activate some sensory nerves which then impact the central nervous system. And there are many athletes who comment that just having something sweet in the mouth gives them a little bit of a boost.
Carbohydrate & Fluid Effects Independent & Additive
Now carbohydrate is often ingested together with fluid in the form of a drink. Of course, there are gels and other sports supplements, solid foods that contain carbohydrates, but most athletes will ingest their carbohydrates in the form of a sports drink and so they are ingesting fluid. And you recall from our lectures on fluid, that fluid ingestion is also associated with increased exercise performance.
Here’s an interesting study looking at the relative contributions of carbohydrate and fluid to endurance performance. And in this study, well-trained cyclists exercised at 80% of their VO2 max for 50 minutes, and this was set by the investigators in the laboratory. In the last 10 minutes, the subjects themselves were able to vary the exercise intensity, either up or down, depending on how they were feeling, but they were required to complete the same amount of work as if they continued for 10 minutes at 80% of VO2 max. So clearly, if the exercise time is longer than 10 minutes, they’ve turned the intensity down and they’re fatigued somewhat. If its less than 10 minutes, then they were able to turn up the intensity and go at a higher power output. And they did that on four trials.
One occasion where they had a placebo where they gave them capsules and a small volume of water about 200 mills. On another occasion where they gave them fluid about 1.1 liters of fluid which matched the sweat losses during the 50 minutes of exercise or close to the hour of exercise, but it was sweetened and flavored to make the subjects think that they were ingesting carbohydrate. On another occasion, they ingested a very low volume, about 200 mills of a highly concentrated carbohydrate solution. And in the fourth trial, they ingested 1.1 liters of fluid that contained carbohydrates. And so this enabled the investigators to compare the effects of no carbohydrate with carbohydrates. Or a small volume of fluid with a large volume of fluid, whether it contained carbohydrate or not.
Now interestingly if you looked at those two comparisons carbohydrate improved performance by about 6%. You can see here as reflected by the lower exercise time. And the fluid ingestion increased performance by about 6%, and they acted independently of each other. So perhaps the carbohydrate was, was acting on metabolic supply, on the perception of fatigue in some way, and maybe the fluid ingestion was acting maybe, perhaps on the cardiovascular system.
But the really interesting comparison then is to look at the two extremes. So here is the placebo, which is the low volume, no carbohydrate, and here is the high volume with carbohydrate, and that improvement in performance there was 12%. And so we can see from this study that it suggests that the effects of carbohydrates are independent but they’re also addictive. And that forms the rationale for the inclusion of carbohydrates in sports drinks that are designed to deliver both carbohydrate and fluid, and in many instances some electrolytes as well.
Maximal Power During Prolonged Exercise – Effects of CHO & Fluid
And these state investigators went on and looked at this in the context with a similar series of supplements but with a slightly different model where they had subjects exercise for 2 hours and they measured the maximal power output at half-hour intervals. And you can see here that over time, there was a decline in the power output. But that decline was least when there was water and carbohydrate, somewhat lower when there were water and the low volume, and the carbohydrate with the absence of water and the low volume of carbohydrate really didn’t do much as well. So the combination of carbohydrate and fluid, at least seemed, to maintain the ability to generate power during prolonged exercise.
Hyperthermia & Fatigue
And finally, just to reiterate, as we did, as you saw in the heat and fluids module, an increase in body temperature is associated with fatigue. So during prolonged strenuous exercise increases in body temperature can impact exercise endurance performance. And this is probably related in part to the direct effect of hypothermia perhaps on the central nervous system, perhaps on the muscle function. But also the consequences of hypothermia and any associated dehydration on cardiovascular function, the ability to maintain an optimal cardiac output and oxygen delivery to contracting muscles. And so endurance performance is really dependent on many factors. The maximal oxygen uptake sets the upper limit, if you like, for aerobic metabolism. The lactate threshold is a marker of the ability to maintain a higher fraction of that, and endurance athletes have adaptation, as we’ve seen during this semester, to spare carbohydrate to increase their ability to oxidize fat, to increase their maximum cardiac output, and all of these adaptations work together to assist the endurance athlete really work at a high intensity during this prolonged strenuous efforts..
Are you a sports performance coach?
If you are a qualified strength coach or a sports performance coach, we want to hear from you! If you would like to help folks visiting our website, please contact us today. We welcome you to TribeLocus — where people find or share health, fitness, and exercise solutions for quality of life and experiences of a lifetime.℠