Endurance is the ability to perform for a length of time without muscle fatigue. People who excel at endurance sports mostly do not achieve such good results in strength sports and vice versa. The endurance properties of muscles are down to their structure, which is largely genetically determined. This is mainly caused by the ratio of fast-twitch and slow-twitch muscle fibres. People with a better predisposition to endurance sports have mostly weaker, yet more enduring, slow twitch fibres. These fibres have a high aerobic capacity and can therefore effectively utilise various energy sources, such as carbohydrates and fats.
For the contraction, the muscle needs energy and the energy is derived from the adenosine triphosphate (ATP) present in muscle The amount of ATP in muscle is limited and when is depleted, it has to be resynthesized from other sources - creatine phosphate or muscle glycogen (see below). The human body is also able to resynthesize ATP from lipids (free fatty acids). The energy coverage depends on the intensity and duration of the exercise. During low-intensity exercise, slow-twitch muscle fibres are primarily recruited and the oxidative system predominate. It is the primary energy source for activity ranging from 3 minutes to over several hours. The system starts working slower and it takes one minute to start producing a sufficient amount of energy for ATP. In this system, carbohydrates and fats are the primary energy sources converted into ATP and this process takes place in the mitochondria of the cell. This system is also known as the Krebs cycle. Athletes on the Tour de France will have a much higher mitochondrial density than the average person. The density is also genetically determined.
This is one of the dietary supplements most commonly used by endurance athletes. L-carnitine is involved in the metabolism of fatty acids (fats). Although our bodies can make it themselves, increased supplementary intake helps muscles gain more energy from fats and therefore saves the relatively limited glycogen stores in the muscles that should not be completely exhausted. Some scientific studies have also shown that L-carnitine supplementation accelerates muscle regeneration after exercise. It is also interesting to note that L-carnitine is present in seminal fluid, where it promotes sperm mobility. Study results have also shown that carnitine intake can have a positive effect on sperm quality.
Glycogen is a substance which is made up of many glucose molecules and is the animal equivalent of starch. It serves as the storage form for glucose found in a variety of tissues. It is found in the liver and muscles, where it serves as a supply molecule. During intense exercise and throughout prolonged physical activity, muscle glycogen is broken down, freeing glucose molecules that muscle cells then oxidize through aerobic (anaerobic) processes to produce the ATP molecules needed for muscle contraction. Endurance athletes who train for hours at a time will also experience a marked decline in muscle glycogen at a slower rate of degradation than the sprinter.
The amount of glycogen in the muscles is limited, and, if used up, muscle performance degrades significantly. For this reason, maintain the recommended daily intake of carbohydrate is necessary with additional carbohydrates during physical activity (a value reflect the daily training load). Less-than-optimal daily intake is likely a result of demanding training schedules, busy lives or inadequate understanding of basic sports nutrition.
Carbohydrate supercompensation is a method that can be temporarily used to increase glycogen reserves in the muscles and improve their performance. The principle of this method is first to exhaust the supply of glycogen through a low-carbohydrate diet (carbohydrates make up less than 55% of the overall energy intake) and intense training, after which it is supplemented by increasing carbohydrate intake (carbohydrates make up over 70% of the overall energy intake) and reducing training load. Our bodies try to prepare for the next lack of glycogen by increasing reserves in the muscles.
Carbohydrate supercompensation can prepare the muscle for a race or other intense load, where it is applied largely for endurance sports such as marathons, triathlons, endurance swimming, mountain climbing, etc. There is a variety of plans (e.g. 7-day or 3-day) through which the glycogen content in your muscles can be increased. However, it is important to note that since glycogen binds to water in the muscles, carbohydrate supercompensation can lead to a temporary weight increase.
Location of glycogen storage
Whole-body glycogen is approximately 600g and reflects the body mass, diet, fitness or recent exercise. Skeletal muscle cells represent the largest storage of glycogen. The content of glycogen in liver cells varies throughout each day and depends on the carbohydrate content of the diet, the time between meals and the intensity and duration of physical activity.