Creatine is one of the most studied nutritional supplement and ergogenic aid available to athletes. Many studies have noted that increasing muscle creatine through creatine supplements can elevate muscle creatine levels, and boost exercise and training performance. Accordingly, creatine is one of the most beneficial and popular nutritional supplement and ergogenic aid available for athletes and is time and time again proved itself to be one of the most effective, safe and useful nutritional supplement to increase muscle mass, strength and performance.
During short explosive exercises lasting in the range of 0-15 seconds, the energy supplied for the rephosphorylation of adenosine diphosphate (ADP) to adenosine triphosphate (ATP) depends to large extent on the amount of phosphocreatine stored within muscle. While exercising, phosphocreatine stores are being depleted and the available energy is reduced due to the inability of ATP to be resynthesized at a sufficient rate. As a result, the capacity to maintain maximal effort while training diminishes. Since the availability of phosphocreatine in muscle significantly influences the amount of available energy during short bursts of high intensity exercise, it can be deduced that by increasing muscle creatine by consuming creatine supplements we can increase available phosphocreatine levels and allow for quicker resynthesis of ATP both during and after intense short-duration exercises. Creatine supplementation while exercising may lead to pronounced adaptations to training due to an enhanced quality and larger volume of work that is being performed.
Creatine is a naturally occurring compound that is found in muscle. Approximately 2/3 of creatine that is found in muscle is stored as phosphocreatine while the rest is stored as free creatine. The total creatine content (both phosphocreatine and free creatine) within muscle is on average about 120 grams for a 70 kilogram individual. The body is capable of storing up to 160 grams of creatine depending on the circumstances. The body on average breaks down about 1-2% of total creatine into creatinine per day (in muscle tissue) which is then excreted in urine. Depleted creatine is replenished in two ways, first approximately half of the creatine is obtained from the diet by consuming foods which contain creatine and second, the other half is synthesized from 3 amino acids: arginine, glycine and methionine. Foods which contain creatine include salmon and beef which has about 1-2 grams of creatine per pound of uncooked meet. Normal creatine synthesis and dietary intake of creatine from food consumption keeps creatine levels at around 120 grams for an ordinary sized individual.
Most of the research which has been done on creatine has examined the effects of creatine supplements on the levels of high-energy phosphates in muscle and the roles it plays on exercise volume as well as recovery during sprinting which is dependent on the phosphagen energy system. Nonetheless, recent studies have focused on the role of creatine supplements on creatine kinase (CK) in muscle tissue and the transferring of high-energy phosphate groups within cells or the creatine phosphate shuttle.
CK is an crucial cellular enzyme which facilitates energy transduction within muscle cells. This is done reversibly by catalyzing the transfer of a high energy phosphate group between ATP and phosphocreatine. There are numerous variants of CK which work simultaneously in the rapid interconversion of phosphocreatine and ATP. The CK enzyme is composed of two types of subunits: M (Muscle subunit) and B (brain subunit).The three isozymes of CK include MB-CK, BB-CK and MM-CK. Moreover, another isozyme of CK, Mi-CK, is located on the outer portion of the inner mitochondrial membrane. CK within skeletal muscle is almost completely MM-CK (also known as myofibrillar CK) where the activity of CK is the greatest. In skeletal muscle MM-CK is bound to myofibrils and is localized on the A bands and also scattered across the whole filament.
Mi-CK is coupled to oxidative phosphorylation and the on the outer surface of the inner mitochondrial membrane catalyses the phosphorylation of creatine to phosphocreatine. Fast-twitch muscle fibres have more CK activity which also contain greater amounts of MM-CK compared to slow-twitch muscle fibres which have a greater amount of Mi-CK. Studies indicate that dietary and availability of creatine by muscle can influence CK activity.
From an athletic performance and ergogenic perspective, the resynthesis of phosphocreatine can be considered the factor of utmost importance during intense and sustained exercise. The functional mechanism behind this phenomenon is the creatine phosphate shuttle which functions primarily as an energy buffer. The CK-phosphocreatine system has been proposed to function in energy transport based on the association of CK isozymes and the sites of ATP production and hydrolysis. In respects to the creatine phosphate shuttle postulation, phosphocreatine and creatine serve as shuttle molecules between the inter mitochondrial membrane space and the cytosol. Mi-CK aids in the formation of phosphocreatine from creatine and the ATP formed from oxidative phosphorylation. Phosphocreatine is thought to diffuse from the mitochondria to the myofibrillar M band where it replenishes ATP via catalysis by MM-CK. Creatine then diffuses back across the membrane to the sites of ATP synthesis to be rephosphorylated again. Since creatine plays a role in shuttling high energy phosphate groups from the mitochondria to the cytosol or sarcoplasm, it may also be involved in increasing performance in intense aerobic exercise in addition to aerobic exercise.