Endurance sport is appealing to many because the effect of training is noticeable quickly. In other words, hard work and determination is rewarded and the benefits are seen after a short time. This is motivating and encourages athletes to set new goals and attempt to improve performance further, creating a cycle of improvement and motivation to set new goals. However, to get the most from the training, it must be informed by a knowledge of the factors limiting the performance in the first place, and the training should be targeted at addressing those limitations. These limitations are the determinants of performance, since by addressing them, performance improves. As the athlete’s performance gets better, further gains are dependent on more precisely determining the limiting factors and targeting them through training.
Performance is determined by the ability to produce high power over a long period and the ability to convert that power into forward velocity. The sustainable power is determined by the ability to convert oxygen and fuel (fat or carbohydrate) into energy. The efficiency of turning the energy into muscular power creates the forward velocity, whether that is swimming, cycling or running. Clearly technique is then important in how effectively the power is turned into forward velocity, but the focus here will be on development of high sustainable muscular power and mechanical efficiency or economy, and therefore the physiological determinants of endurance sport performance.
Sustainable, high rates of energy production in the active muscles are a product of a high ability to transfer oxygen from the atmosphere to the working muscles, and this requires a high maximal oxygen uptake. This is sometimes referred to as maximal aerobic power, and is the ability of the body to take oxygen from the atmosphere into the lungs (ventilation), then carry that oxygen in the blood to the working muscles (circulation), and then take up and use the oxygen in the working muscles (respiration). Taken together, all these steps create whole body oxygen uptake, and the rate is measurable at the mouth. When exercising, the working muscles account for the vast majority of the whole body oxygen uptake. The maximal rate is a critical determinant of endurance performance, but it is not the maximal rate alone which is important. Equally important is the fraction or percentage of the maximal rate that can be sustained.
So, what determines the percentage of the maximum that can be sustained over a long duration typical of endurance sport events? It is the ability of the body to produce energy for the working muscles without involving the anaerobic metabolic pathways so much so that the muscles become too acidic. Some acidity can be tolerated and dealt with, but too much induces fatigue quickly, effecting the contractile apparatus and also the neural signals that innervate them. The acidity is caused by a build up of hydrogen ions (charged particles). When produced, lactate is also produced, hence why we often measure lactate as an indicator of the extent of the acidity and how well it is being dealt with by the body as a whole. The acidity also stimulates ventilation, so it is also possible to look at minute ventilation as an indirect indicator of whole body acid status, hence terms such as respiratory threshold, as well as terms such as lactate threshold or break points.
So, when an athlete has a high maximal oxygen uptake, along with a high sustainable percentage of that maximum (as indicated through a high ability to avoid the acid conditions), the only other major physiological determinant is the efficiency of turning the energy into useful work or power (the rate of doing work). This is known as mechanical efficiency and is determined by measuring the oxygen uptake required at set power outputs, and is easy to do during cycling. If running, economy is determined instead, and is determined by measuring the oxygen uptake required at set velocities. Although harder to measure oxygen uptake during swimming, it is possible, and economy is the oxygen uptake in relation to set swim velocities.
Any one of these three physiological determinants of endurance performance might be the athlete’s weakness or limiting factor. For example, in studies comparing athletes of similar performance levels, there was a reasonable degree of variability in the three determinants of performance. There was a general tendency for athletes excelling in the longer endurance events (for example, in running, marathon versus 10 km) to have greater mechanical efficiency or economy. This is also the case for cyclists. Also, the better athletes show less of a deterioration in mechanical efficiency or economy as fatigue ensues. In terms of training to target each of the determinants, this is more controversial. Generally, the intensity needs to be high to stimulate the adaptations to improve maximal oxygen uptake, and is normally achieved through interval training approaches. On the other hand, to improve the sustainable percentage of maximal oxygen uptake, lower intensity and long duration exercise at less than 70% maximum is advocated. Finally, there is no consensus on the best methods to improve mechanical efficiency or economy, and this remains one of the great challenges for exercise physiologists.
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