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Components of Endurance Performance
What is Endurance? Endurance can be defined as the ability to withstand stress over prolonged periods of time. An endurance sport is therefore any sport in which there is a prolonged physical stress. An athletes level of ability, as an endurance athlete, will be determined by a number of key physiological components. The main requirement is the ability to sustain a fast pace over a prolonged period, without sustaining undue fatigue through the build up of lactic acid. Events of greater than 800 m are mainly aerobic, with an increasing aerobic demand as duration increases. The main aerobic source of energy is through the metabolism of carbohydrates (glycogen) and fats in the form of free fatty acids. Energy is also supplied from the anaerobic metabolism of glycogen to form lactate and an initial 4-6 seconds supply comes form the intra-muscular store of high-energy phosphates. In the 1500m, 5km, 10km and ˝ Marathon events the contribution of aerobic and anaerobic energy sources are approximately; 80% and 20% in the 1500m, 95% and 5% in the 5km, 97% and 3% in the 10km, and 99% and 1 % in the ˝ Marathon respectively (Maughan et al., 1997; Fallowfield and Wilkinson, 1999). Despite being small the anaerobic contribution may make a significant contribution to the relative exercise intensity sustained during the 1500m and 5km events (Fallowfield and Wilkinson, 1999) and even during longer races such as the ˝ Marathon it may come into play when working hard up a hill, during mid race surges or during a sprint finish. However the majority of a middle/long distance athlete’s training time would be devoted to developing and maximising their aerobic capacity and efficiency.
What is the Maximal Oxygen Uptake (VO2max)? The VO2max is a key determinant of athletic performance. The term VO2max simply represents the maximum amount of oxygen that can be consumed during physical activity. It is expressed in ml of oxygen per kg of body weight. Athletes must have the ability to consume large quantities of oxygen. A high VO2max will allow the athlete to work at higher work rates before their demand for oxygen exceeds supply, leading to a build up of lactic acid and eventually fatigue. Although there have been cases of elite athletes with relatively moderate VO2 max figures ( 60ml.kg.min-1), these are the exception, rather than the rule. Typically an elite athlete will have a VO2max of greater than 70ml.kg.min-1 (Noakes, 1991; Daniels and Daniels, 1992) but there is wide variation between individuals, even amongst elite athletes. An athletes VO2max appears to be limited by the pumping capacity of the heart (Poole and Richardson, 1997) and may be influenced by capillary density, aerobic enzymes concentration and the viscosity of the blood and concentration of red blood cells. Whilst a high VO2max reflects a high level of aerobic fitness it only gives an athlete the potential to perform at the highest level, as there are several other factors that contribute to athletic performance (Conley and Krahenbuhl, 1980; Morgan et al., 1989; Fallowfield and Wilkinson, 1999). A high VO2max may give an athlete the potential to become an elite athlete, but there performance will also be dependant on other factors such as the: economy of motion, sustainable %VO2max, Lactate threshold and the velocity at VO2max (vVO2max).
What is the Economy of Motion? The economy of motion relates to the quantity of oxygen (ml.kg.min-1) required to move at a given speed. Every muscular contraction requires a co-ordinated contraction of muscle fibres. The greater the level of co-ordination the lower the energy cost of that activity will be. A lower energy consumption implies a greater economy of motion or efficiency. Therefore an athlete that has a lower oxygen consumption, at a given speed, will have a greater potential to race at faster speeds. In many studies the economy of motion has been shown to be highly predictive of race performance (Morgan et al., 1989), with improvements leading to improved race performance (Conley et al., 1984; Jones, 1998). Conley et al., (1984) suggested that the greater running economy of US mile record holder Steve Scott, in comparison with former record holder Jim Ryun, allowed Scott to perform at a lower percentage of his maximal aerobic capacity at a given workload and accounted for the difference between the two athletes. As such many researchers have viewed this as the most important factor in endurance performance. However, it must be remembered that a good economy in motion cannot make up for a low VO2 max. The economy of motion has a far greater effect in sports that are highly technique orientated, such as swimming. There appears to be a strong genetic influence with regard to running economy with up to 20-30% variation observed amongst trained runners of similar ability (Morgan and Daniels, 1994). Factors that may influence running economy include; the percentage of Type-I muscle fibres (Horowitz et al., 1993), and the ability of a muscle and tendon to store elastic energy during a footstrike, this may be affected through neuromuscular co-ordination and possibly through inflexibility in the hip and calf regions of the musculoskeletal systems (Craib et al., 1996). A further variation in running economy can be seen in the slope of the line defining the relationship between the VO2 and running velocity. A shallow slope indicates greater efficiency at faster velocities whilst a steeper slope implies greater efficiency at slower speeds. For this reason it is important to assess running economy at race pace since a marathon runner may be economical at marathon pace but uneconomical, in comparison with a 1500m runner, at 1500m pace (Daniels and Daniels, 1992 ).
What is the Lactate Threshold (LT)? The LT has also been shown to be highly predictive of endurance performance. The LT corresponds to the rate of aerobic energy expenditure at which lactic acid begins to accumulate in the bloodstream. During relatively low level activity, blood lactate levels remain low generally 1-2 mMol.l-1. The blood lactate will remain low until at a given intensity there will be a sudden rise in the blood lactate concentration. At the point of the LT, the production of lactic acid by muscles, is matched by the metabolic breakdown of lactate during oxidative-phosphorylation. At higher intensities than the LT, lactate will continue to rise over a time, even if the work intensity or speed is kept constant. The build up of lactic acid and the associated hydrogen ions leads to a reduction in the cellular pH, which at a certain level interferes with the normal functioning of cellular enzymes, impairing metabolism and eventually leading to fatigue. The LT and the Onset of Blood Lactate Accumulation (OBLA) have been shown to be good predictors of endurance performance accounting for a large proportion of variation between individuals of similar ability (Morgan et al., 1989; Coyle et al., 1991; Jones and Doust, 1998). There are several definisions of the LT including:
The LT is also called the Maximum Lactate Steady State (MLSS) and has wrongly been called the anaerobic threshold. The term anaerobic implies in the absence of oxygen. However, research has shown that this is clearly not the case and even at the highest exercise levels muscle cells are not starved of oxygen. The lactate threshold is an important factor in all endurance events, particularly those lasting over 30 minutes. An athlete that possesses a faster velocity at which the lactate threshold occurs will be able to race at faster velocities before they are forced to slow down due to the build up of lactic acid. The lactate threshold appears to be related to the percentage of type I (slow twitch) muscle fibres (Coyle et al., 1991) and possibly the ratio of type IIa to type IIb fibres (type IIa are a more fatigue resistant form of fast twitch fibre).
What is The Velocity at VO2max (vVO2max)? The vVO2max represents the velocity at which the VO2max occurs. This would typically be assessed during an incremental exercise test. The vVO2max represents an interplay between aerobic capacity (VO2max) and efficiency (Morgan 1989). The predicted velocity at which VO2max occurs (vVO2max) has been shown to account for a significant proportion of the variation across a homogenous group and was found to be at least as good a predictor of 10km race performance as the vOBLA (Morgan et al., 1989; Noakes et al., 1990). As such it can be used to explain the differences in race performance, between two athletes of equal VO2max. If two athletes have the same VO2max values or have a similar economy of motion then the athlete with the highest vVO2maxwill be running at a greater velocity for any given percentage of their VO2max. As long as there was no decrease in the sustainable %VO2max then any improvement in vVO2max would translate to improved performance.
What is The sustainable % VO2max? The Sustainable % VO2max is another important factor to consider as it explains the difference in race performance of athletes over varying race distances. Two athletes might be equal over a 10km race but have very different times over the marathon distance. Generally, if two athletes have equal VO2max and vVO2max then the athlete with the highest sustainable % VO2max will win in a race situation. It must be remembered that the sustainable % VO2max decreases as race distance increases and the rate of decrease varies from athlete to athlete. As the race duration increases it becomes harder to maintain a pace that is close to the maximal oxygen consumption. Athletes must be able to race at intensities that are increasingly close to their VO2max to maximise performance with the sustainable %VO2maxbecoming increasingly important as race duration increases (Coetzer et al., 1993; Fallowfield and Wilkinson, 1999). Marathon specialists tend to have a much greater sustainable % VO2max over the marathon distance than non-specialists but may have only equal or lower sustainable % VO2max over shorter distances. During 5km, 10km, ˝ Marathon and Marathon races the typical elite distance runner would be able to sustain approximately 98%, 92%, 85% and 78% of their VO2max respectively (Fallowfield and Wilkinson, 1999). Because the 5-10km runner will be racing at an intensity that is much closer to their VO2max they will have a more limited scope for improvement in their sustainable %VO2max in comparison with a marathon runner. The %VO2max sustained can be evaluated under laboratory conditions, by measuring oxygen uptake during a treadmill time trial, or through comparison of the average racing velocity against their laboratory measured VO2 for that speed. The %VO2max sustained seems to be dependant on the aerobic capacity of a muscle and the percentage of type I muscle fibres (Coyle et al., 1991) and appears to be strongly linked to the lactate threshold (Coetzer et al., 1993).
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