Adaptation to higher stress level is that part of the training program that focuses upon the improvement of the body’s metabolic processes. Every coach should be familiar with the fact that the body has three metabolic systems for producing energy for muscular contractions.
These systems are:
- The anaerobic a-lactate system
- The anaerobic lactate system
- The aerobic system
1. Anaerobic a-lactate system
The muscles store within themselves small amounts of ATP. This ATP can be called upon for immediate use in efforts of maximal intensity but of very short duration (3-4sec). When existing stores of ATP are metabolized, ATP is reduced to ADP. The enzyme responsible for this reduction is ATPase. But ADP can be re-energized to ATP (phosphorylized). This can be done through two processes. The first utilizes stores of creatine phosphate (CP) in the muscles. The enzyme responsible for this process is creatine kinase. The second way how ADP can be re-energized is achieved through the action of the enzyme called myokinase, which combines two ADP’s to form one ATP. Maximal intensity of efforts will last up to 15sec prior ATP and CP reserves will be exhausted. During anaerobic alactate metabolism no lactic acid is produced.
Glycolysis is the energy pathway that the body is choosing, when the ATP and CP reserves in the muscles are insufficient to provide energy needed for a given activity (duration longer than 15sec). It is through the metabolism of glycogen (glycolysis) that the muscles produce ATP necessary for continued muscle contractions. When energy demands are very high (high intensity exercises 20-90sec in duration), then glycolysis proceeds rapidly and without oxygen which will cause a waste product called lactic acid (anaerobic lactate metabolism) and will interrupt the balance of muscle hydrogen ions. When energy demands are lower (low to moderate intensity exercises of duration 3min and longer), then glycolysis can proceed more slowly and utilize oxygen (aerobic metabolism).
So, an exercise is characterized as anaerobic, aerobic or mixed anaerobic/aerobic depending upon the rate which lactic acid is produced and the extent to which oxidative (aerobic) metabolic processes are utilized. The enzyme called Phosphofructokinase (PFK) is the most important one that regulates the rate of glycolysis. High concentrations of ADP in the muscle cells will stimulate its activity. When PFK activity is high, the rate of glycolysis is rapid.
Anaerobic lactate metabolism
When energy demands are very high and duration of exercise is longer than 15sec, muscles must use anaerobic lactate metabolism. High concentrations of ADP in the muscles will stimulate high levels of PFK activity. Glycolysis then proceeds very rapidly, using up glycogen reserves in the muscles and will produce lactic acid as waste. The concentration of lactic acid in the muscles tend to accumulate in this metabolism process faster that it can be combusted through the aerobic metabolic system. When lactate concentration will reach critical levels (16-18mM/l blood) endocrine and neuromuscular system will begin to shut down. This level will be reached within 35-50sec of maximal / near maximal efforts. Exercises continued beyond this point will do so at reduced intensity and so oxidative metabolic processes will begin to deliver energy. If intensity is not reduced, then lactate concentration will exceed critical levels, ATP will no longer be produced.
While aerobic metabolism proceeds more slowly than anaerobic lactate metabolism, its glycolysis process is more efficient. This is because lactic acid while it’s formed is easily combusted in cellular mitochondria. Through a very complicated energy pathway, the products of glycolysis are directed towards the mitochondria, where cellular (aerobic) respiration takes place. This requires the presence of oxygen. This energy pathway can be maintained indefinitely as long as the by-products of glycolysis (lactate and pyruvate) do not exceed the capacity of the mitochondria to metabolize them (this capacity van be measured and it is close to 2-4mm/l) and as long as the glycogen reserves in the blood can supply the muscles with fuel.
Aerobic metabolism is thus seen to be very important for blood lactate clearance, because of the oxidative metabolism of lactic acid.