Physical activity

Depletion and replacement of substrates What are energy substrates?

Energy substrates are molecules that provide starting materials for various bioenergy reactions such as phosphagens (ATP and phosphocreatine), glycogen, lactate, glucose, amino acids and free fatty acids.

Using Energy Substrates

Energy substrates are commonly used and are of great importance in exercising. The substrates used will depend on the type of activity and the intensity of the exercise we do.

During actions of varying intensity and duration, energy substrates can be used selectively. Therefore, the volume of energy production through bioenergy systems is decreasing.

When we do something and notice fatigue, it is regularly associated with depletion of phosphagens and glycogen.

In contrast, depletion of substrates such as free fatty acids, amino acids or lactate usually does not occur to the extent that it limits performance. The nature of replacement, depletion of glycogen and phosphagen after activity, is important in the bioenergy of exercise.


Exercise fatigue is believed to be related to a decrease in phosphagen levels, at least in part. As a result of intense anaerobic exercise, the concentration of phosphagen in the muscles decreases earlier than with aerobic exercise.

During the first part of high-intensity exercise, phosphocreatine can be significantly reduced (50% to 70%). As a result of high intensity exercise to exhaustion can be almost completely eliminated. Although the concentration of ATP in the muscle does not fall by more than 60% in relation to the initial values.

With repetition of resistance exercise, dynamic contractions use more metabolic energy. This usually reduces phosphagens more than isometric muscle contractions.

After training, the replacement of phosphagens is short, from 3 to 5 minutes, leading to a complete re-synthesis of ATP. Full resistance to phosphocreatine after exercise can occur in no more than 8 minutes. Phosphagen replenishment is largely a result of aerobic metabolism. Rapid glycolysis can also promote ATP resynthesis after intense exercise.


The amount of stored glycogen available for use during exercise is limited. 300-400 g are stored in the muscles of the body. glycogen and in the liver from 70 to 100 g.

Diet and exercise can affect liver and muscle glycogen concentrations. Research shows they can increase resting muscle glycogen concentration in both aerobic resistance training and anaerobic training, as well as strength training and sprinting.

Exercise intensity is related to the level of glycogen depletion. In high to medium intensity exercise, muscle glycogen is a more important source of energy than liver glycogen. Instead, liver glycogen appears to be more important than muscle glycogen during low-intensity exercise. Even with prolonged exercise, the metabolic contribution of glycogen in the liver increases.

Glycogenolysis Rate

The rate of muscle Glocogenolysis increases with increasing relative exercise intensity when there is maximum oxygen uptake, increasing glycogen available for the glycolysis pathway.

Muscle glycogen is becoming an increasingly important energy substrate in relative exercise intensity with oxygen uptake of over 60%. Therefore, it may happen that during exercise, the glycogen content of some muscle cells is depleted.

This can lead to significant muscle glycogen depletion (20% to 60% reduction), high-intensity intermittent exercise such as mid-court basketball or strength training, with relatively little exercise.

While phosphagens can be the main limiting factor during strength training with low reps or multiple sets, muscle glycogen can become the limiting factor during strength training with many total sets or higher total loads. This type of exercise can limit performance and can cause selective depletion of muscle fiber glycogen. Greater depletion in type II fibers.

The rate of muscle glycogenolysis depends on the intensity of strength training, as with other dynamic exercises. Although, apparently, the same amount of total work results in an equal amount of glycogen depletion regardless of the relative exercise intensity.

Glycogen Replacement

Replacing muscle glycogen during recovery is associated with post-workout carbohydrate intake. When taken internally 0.7 to 3.0 g of carbohydrates per kilogram of body weight every two hours after training, recovery appears to be optimal.

However, if you eat enough carbohydrates, muscle glycogen for 24 hours. fully replenished. If the exercise contains a lot of eccentric content, such as muscle damage from exercise, it takes longer to completely replace it.

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