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The Science of Performance |
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Traditional explanations for running performance are based on the belief that limitations in aerobic and anaerobic energy production limit performance. The energy requirements for maximal runs a few seconds in length are believed to be almost completely fueled by anaerobic energy production. Runs several minutes in length and longer are believed to be mostly powered by aerobic metabolism. Furthermore, it has been traditionally held that limitations in both anaerobic and aerobic energy production limit performance; limitations in anaerobic energy production limit maximal speed and performance in very short sprints, while limitations in aerobic energy production limit longer distance (endurance) performance. Exercise Physiologists McArdle, Katch, & Katch, in their exercise physiology textbook, explain anaerobic energy limitations this way. "Brief power activities lasting up to about 6 seconds rely almost exclusively on 'immediate' energy generated from the breakdown of the stored intramuscular high-energy phosphates, ATP and CP (i.e. anaerobic energy)." They explain that "during the 100-yard dash, for example, maximum speed cannot be maintained for longer than this 5- to 6-second period because of the slower rate of energy transfer via glycolysis.(i.e. short term energy system)"(1) Performance limitations due to aerobic energy production are expressed by exercise physiologist Pete Pfitzinger when he writes that "Your VO2max is important because it determines your aerobic capacity - the higher your VO2max, the greater your ability to produce energy aerobically." Mr. Pfitzinger then explains why this is important with his comment that "VO2max (i.e. maximal aerobic energy production) is the most important physiological variable determining performance in races of 1,500 meters to 5,000 meters."(2)
However, one of the things that has never been adequately explained by traditional views of aerobic and anaerobic metabolism is the lack of a relationship between aerobic power and performance during runs lasting longer than a few seconds and less than several minutes in length (i.e. intermediate and long sprints). All-out runs that fall between a few seconds and a few minutes cause a rapid increase in oxygen consumption at the beginning of the run, with oxygen consumption then remaining high throughout the duration of the event. Recent research (3,4) has revealed that aerobic energy production plays a much greater role in these events than previously believed. The data shows that aerobic energy production provides about 30% of the total energy used during sprints of 20 seconds in length, about 45% during sprints 50 seconds in length, and about 60% during 110 seconds of sprinting. Though aerobic energy production clearly plays a strong role in meeting the energy needs of intermediate and long sprints, exercise physiologists have not previously established a relationship between aerobic power and the speed at which these sprints are run. The lack of relationship between performance at these distances and aerobic power is curious considering the clear contribution made by aerobic energy production during these events.
In 1999 several exercise physiologists attempted to finally and accurately explain the above discrepancy.(5) They reasoned that since aerobic energy provided a major part of the energy used in intermediate and long sprints that reducing the amount of available oxygen (inducing hypoxia) during these events would result in decreased maximal speeds. In this way they could test "the importance of aerobic metabolism to human running speed directly by altering inspired oxygen concentrations and comparing the maximal speeds attained at different rates of oxygen uptake." They essentially set out to answer the simple question, "Is human sprinting performance impaired when the environmental oxygen and aerobic energy yields are limited?" They theorized that since the rate of oxygen use is high during sprints and that humans have little stored oxygen in the body that "moderate hypoxia would reduce maximal human running speeds during all-out runs lasting longer that 25 seconds."
They recruited athletic subjects (mostly trained runners) and had them participate in eight test sessions: four hypoxic (lowered oxygen) and four normoxic (normal amounts of oxygen). The normoxic sessions were used to establish baseline performances in all-out sprints of 15, 30, 45, 60, 75, 90, 120, 150, and 180 seconds in length. The hypoxic sessions were performed the same as the normoxic sessions, but a device was used to lower the amount of oxygen available to the subjects, resulting in a 30% decrease in maximal aerobic power, a level equivalent to that of sedentary subjects. Performances under the hypoxic conditions were compared to those from the normoxic conditions. The results of this study were both surprising and revealing.
Table 1 graphically illustrates sprint performance results under both hypoxic and normoxic conditions. As can be seen, there was no difference in performance at up to 60 seconds. Performance was significantly slower under hypoxic conditions for durations of 75 seconds or longer.
Table 1: Sprint performance under normoxic and hypoxic conditions
Recall that aerobic energy contributes significantly during all-out runs of 20 seconds and longer. If aerobic energy contributes such a large portion of energy during these events, why was sprint performance not affected at durations of up to 60 seconds and only affected to a small (but significant) degree at distances of up to 120 seconds? The researchers discovered that anaerobic energy increased to compensate for the loss of aerobic energy. "Rates of anaerobic metabolism increased sufficiently to fully compensate for the aerobic energy lost during hypoxic sprints of up to 60 sec and to partially compensate for that lost during hypoxic sprints of up to 150 sec."
What does this mean? Well, it means that during all-out sprints - sprints run as fast as you are capable of running - your performance is not limited by lack of energy. Though you are running at your fastest possible pace, your fastest pace is not limited due to lack of energy. The fact that anaerobic energy production increased during hypoxic running shows that rates of anaerobic energy production during normal conditions are not maximal. The researchers explain it like this,
Where, then, is the limitation found? In your muscle cells. Specifically, in the rate of contraction (known as cross-bridge cycling) at which your muscle fibers work. "...our results imply that the maximal metabolic rates under these circumstances are set by the rates of ATP hydrolysis at the cross-bridge level..." In other words, despite the availability of additional energy, your can't run any faster because your muscles are working as hard as they are capable of. You reach the limits of your muscular system prior to reaching the limits of energy production.
Where does this leave us? What limits high speed running performance? In opposition to traditional beliefs that performance is limited due to lack of energy, the evidence is that limitations in performance are found within the muscle cells themselves. Power Running has long proposed that muscles limit performance, specifically that the muscle factors of force of contraction, speed of contraction, and resistance to fatigue are the primary factors influencing running performance of any distance. This research adds to the body of evidence in support of this belief. Sprint run performance under hypoxic conditions demonstrate that muscles limit performance in sprint events and "that maximal metabolic power outputs during sprinting are not limited by rates of anaerobic metabolism and that human running speed is largely independent of aerobic power during all-out sprints lasting < 1 min."
References:
1. McArdle W, Katch F, Katch V., Exercise Physiology - Energy, Nutrition, and Human Performance, Williams & Wilkins, 1996, pg 121 and pg 393.
2. Pfitzinger P, Douglas S., Road Racing for Serious Runners, Human Kinetics, 1999, pg 16.
3. Spencer M, Gastin P., Energy system contribution during 200- to 1500-m running in highly trained athletes, Med Sci Sports Exerc, 2001, 33(1), 157-162.
4. Duffield R, Dawson B, Goodman C., Energy system contribution to 400-metre and 800-metre track running, J Sports Sci, 2005, 23(3), 299-307.
5. Weyand, P, Lee C, Martinez-Ruiz R, Bundle M, Bellizzi M, Wright S., High speed running performance is largely unaffected by hypoxic reductions in aerobic power, J. Appl. Physiol. 1999, 86(6), 2059-2064.
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