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VOLUME 2 ISSUE 2 Journal of International Society of Swimming Coaching grams daily for 4 to 6 days. The 20 grams are ingested in four doses of 5 grams daily, separated by 4 to 5 hours. This has been shown to increase the CP content of muscles by an average of 11% (19). Athletes with the lowest initial levels of CP tend to store more while trained athletes and those with high initial levels will store less. Creatine supplementation appears to be most effective for improving performance in events lasting up to 3 minutes”(19). It also seems to improve performance when subsequent efforts are repeated with short recovery periods. Numerous reports from athletes indicate that creatine supplementation also enables them to train harder because they recover faster. In addition, there have been many reports of increased muscle tissue and strength with creatine loading. This effect has not been universally supported, however. Louis and associates (24) found no such effect in their research, while, at the same time, Safdar and colleagues (32) showed that creatine supplementation increased the expression of genes involved in muscle growth. On a negative note, it has been suggested that sustained creatine supplementation will reduce its uptake by muscle cells over time so that the performance enhancing effect of creatine loading will become minimal at best (39). Increases of Pi and ADP as causes of muscular fatigue. Other suggested causes of muscular fatigue are increases of inorganic phosphate (Pi) and adenosine diphosphate (ADP) during muscular contraction. ADP is produced, and increases in muscles, when ATP is split for energy during exercise. Both ATP and creatine phosphate (CP) metabolism result in increases of inorganic phosphate (Pi) during exercise. As shown in figure 6, the splitting of ATP and release of its energy results in the formation of ADP (adenosine diphosphate) as well as inorganic phosphate, which probably accounts for the fact that both are associated with muscular fatigue (34). Also diagrammed in figure 6 is the inorganic phosphate that is freed during creatine phosphate metabolism. ATP ADP + Pi + free energy CP C + Pi + free energy Figure 6. The splitting of ATP, the release of free energy and the subsequent formation of ADP and Pi. Also diagrammed is the splitting of Creatine Phosphate (CP), and the release of inorganic phosphate and free energy that can be used for regenerating ATP from ADP. The amount of free inorganic phosphate in muscles will also increase when glycogen is used to regenerate ATP. ADP also increases because the rate of ATP regeneration slows as the amount of creatine phosphate in muscles is reduced. Journal of ISOSC http://www.isosc.org 18
VOLUME 2 ISSUE 2 Journal of International Society of Swimming Coaching Both inorganic phosphate and ADP may contribute to fatigue through their effect on calcium release in muscle fibers. Calcium is stored in the sarcoplasmic reticulum of muscle fibers and is essential to muscular contraction because it allows the myosin cross-bridges in myofibrils to attach to the actin filaments and pull them inward so that they shorten, or, to use the proper term, contract. This was a partial description of the very popular sliding filament theory of muscular contraction. This process is pictured and described further in figure 7. The effects of increased inorganic phosphate (Pi) on muscular fatigue. Contraction force may be reduced when an increase of inorganic phosphate reduces the release of calcium in muscles. This is because a reduction of calcium interferes with the coupling of actin and myosin so that fewer and weaker bonds are formed. When this happens contraction force will be characterized as “weak” rather than “strong”. Obviously, when “weak” contractions occur, more fibers will have to contract to maintain a particular level of performance and fatigue will soon follow. Increases of inorganic phosphate, like those of hydrogen ions, are temperature and fiber type dependent. In one study, an increase of inorganic phosphate reduced muscle contractile force by 54% and 50% in slow and fast twitch muscle fibers respectively at a temperature of 15 0 C. This effect was reduced, but still significant in slow twitch muscle fibers (-19%) at a temperature of 30 0 C. The contractile force of fast twitch muscle fibers was much less affected at a high muscle temperature, declining only 5% at 30 0 C. Muscle contraction velocity was not reduced by a high level of inorganic phosphate at either low or high temperatures. On the other hand, peak power was reduced 49% for slow twitch and 40% for fast twitch fibers at 15 0 probably because of the considerable loss of contractile force at this muscle temperature. Muscle contractile power was reduced only 16% for slow twitch and 18% for fast twitch fibers at 30 0, , however (11). These results were obviously due to smaller reductions of force and no reduction of contractile velocity at the higher muscle temperature. Despite the reduced loss of power at 30 0 , it is to be expected that reductions in fast and slow twitch muscle power of 16% and 18% would have a detrimental effect on performance. Journal of ISOSC http://www.isosc.org 19
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