This occurs when an exercise is performed totally or partially anaerobically. Then this happens PCr stores deplete and lactic acid builds in the muscles. Oxygen is then needed to break down the lactic acid to convert it back into pyruvic acid. This is why heart rate and respiratory rate must remain elevated after the main exercise has been completed in order to allow the pay- back of oxygen.
After a bout of exercise ATP must be replenished, PCr stores must be replenished, lactic acid need to be removed, myoglobin must be replenished with oxygen and glycogen stores need to be replenished. ATP and PCr can take place with 20 minutes of stopping exercise however oxygen replenishment and glycogen refilling can take 24- 48 hours. The oxygen debt consists of the Alactacid and the Lactacid system
This oxygen debt does not involve lactic acid as the aerobic system is used to produce the ATP needed to replenish PCr and ATP stores.
ADP + P + oxygen = ATP
ATP + Cr + P = PCr + ADP
Around 50% of ATP replenishment occurs within 30 seconds after exercise however full recovery can take up to 3 minutes.
The higher the fitness level of the person the higher the oxygen debt this is because training increases the PCr content within the muscle cells. However the recovery time is reduced as they have superior methods of oxygen delivery, such as increased capillarisation which will increase the rate of ATP production from the aerobic energy system.
The Lactacid oxygen debt takes much longer to complete depending on the intensity of the exercise. The process involves oxygen, which is required to break down lactic acid into pyruvate. This can then enter the aerobic system and eventually be broken down into carbon dioxide and water.
Lactic Acid + Oxygen= Pyruvate
Neuromuscular Fatigue
Neuromuscular fatigue means that muscles are either not able to receive signals from the central nervous system that stimulate muscle contraction or that the muscle tissue is unable to function properly. This is because exercise can eventually interfere with calcium release, which is required for muscle contraction. Exercise cannot continue indefinitely because of neuromuscular fatigue. This is because of
- Reduced PCr
- Reduced glucose and glycogen
- Increased Lactic Acid
- Increased Carbon Dioxide
- Reduced oxygen, haemoglobin and myoglobin
This means that in order for the body to return to its pre- exercise state the person must rest.
Muscle fatigue may be due to the depletion of high energy phosphates PCr and glycogen stores and to the accumulation of metabolites, such as lactate, and carbon dioxide. There is considerable evidence that supports the importance of accumulation of lactic acid and carbon dioxide as a limiting factor in performance. Fast- twitch fibres are capable of generating large forces but fatigue occurs mire rapidly as opposed to slow twitch fibres which are low- force fibres, and is confined to the contractile mechanism. This may be due to their low aerobic capacity and energy creation via glycolysis, which leads to lactic acid accumulation. Fatigue resulting from endurance based exercise is probably due to depletion of glycogen stores in both the fast twitch and slow twitch muscle fibres. Total fatigue includes factors such as low blood glucose level, liver glycogen depletion and electrolyte loss in sweat. Fluid loss decreases plasma volume and therefore blood pressure, which in turn reduces blood flow to the skin and muscles. To overcome a reduction in plasma volume the heart has to work harder and, because blood flow to the skin is reduced the body retains more heat energy and muscle temperature rises.
If exercise continuous active cells will eventually run out of PC, so replacement of ATP by ATP- PC mechanism stops and the alactic- lactic threshold is reached. Not enough ATP us produced aerobically so glycolysis takes over as the predominate method of ATP supply with its rapid depletion of muscle glycogen and production of lactic acid as a by- product. Oxymyoglobin stores supply a limited amount of oxygen to muscle to generate ATP anaerobically. Eventually, ATP production via the anaerobic system will by used up and exercise must. During the exercise period an oxygen deficit is created. This is because the muscle requirement for oxygen is larger than the oxygen supply. The oxygen deficit for a specific exercise regime is calculated as the difference between the oxygen required and oxygen actually consumed. \
The breakdown of glucose or glycogen produces lactate and hydrogen ions - for each lactate molecule, one hydrogen ion is formed. The presence of hydrogen ions makes the muscle acidic that will eventually halt muscle function. As hydrogen ion concentrations increase the blood and muscle become acidic. This acidic environment will slow down enzyme activity and ultimately the breakdown of glucose itself. Acidic muscles will aggravate associated nerve endings causing pain and increase irritation of the central nervous system.
