Exercise Physiology
Task 4: Mechanisms of Fatigue
In this assignment I will be listing the mechanisms of fatigue, including neuromuscular fatigue. I will be listing the role of metabolites, and identifying the role of oxygen and other factors in the recovery process.
(Wilmore and Costill, 1988) described Fatigue as a general sensation of tiredness as well as a decrease in muscular performance. Muscular fatigue occurs in athletes after a tough training session. This description is very general and does not explain what causes fatigue.
(Answers.com, 2005) believe fatigue is the decreased capacity or complete inability of an organism to function normally because off excessive stimulation. This description is better then the first as it states that excessive stimulation, even lack of sleep, will cause the body to fatigue. It is the body's way of saying it needs rest.
(Gandevia et al., 1995; Hagberg, 1981; Hawley et al., 1997) describes fatigue as the inability to continue functioning at the level of one's normal abilities. This is also very general but it is clear in what it describes.
Neuromuscular fatigue
Neural fatigue
Neural fatigue means a disruption of neuromuscular events. Fatigue occurs because of a decrease in calcium production. A failure of acetylcholine generation reduces the chance of an action potential. The CNS (central nervous system) may identify fatigue prior to physiological fatigue. This stops you from doing anymore exercise to avoid you from injury, which could be muscle soreness.
Muscular fatigue
Muscle fatigue means a decrease of muscular performance. It is an inability to maintain the standard power output. Muscle fatigues when there is a depletion of PC stores e.g. sprinting. Therefore there is an accumulation of lactic acid and a decrease in pH.
During endurance there is a depletion of energy stores, which are fat and carbohydrates. Carbon dioxide accumulation results in an increase in pH in the blood. During exercise fluid is lost; this could be through sweating, which results in lower blood pressure, and therefore the heart will have to work harder. Little as 2-3% of water loss can reduce a performance.
Effects of metabolic bi-products
Lactic acid is the main bi-product of anaerobic glycolysis. Blood always contains small amount of lactic acid, however, during high intensity exercise this increases greatly. The increased production of lactic acid results in the pH of the blood decreasing. A blood pH of 6.4 or lower affects muscle and neural functions.
Onset blood lactate accumulation (OBLA)
OBLA is also known as the anaerobic threshold. OBLA is the point at which lactic acid begins to accumulate in the muscles. OBLA is considered to occur somewhere between 85% and 90% of your maximum heart rate.
The recovery process
Oxygen debt
Oxygen debt occurs when the exercise performed is completely or to some extent anaerobic. When this happens, PC stores are depleted and lactic acid builds up inside the muscle. Oxygen is then required to break down the lactic acid and convert it back to pyruvic acid. Therefore, the heart rate and respiratory rate should remain elevated for a period of time after the main exercise has been complete in order to allow for the pay back of the oxygen debt.
After a session of dynamic exercise, five events must happen ...
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The recovery process
Oxygen debt
Oxygen debt occurs when the exercise performed is completely or to some extent anaerobic. When this happens, PC stores are depleted and lactic acid builds up inside the muscle. Oxygen is then required to break down the lactic acid and convert it back to pyruvic acid. Therefore, the heart rate and respiratory rate should remain elevated for a period of time after the main exercise has been complete in order to allow for the pay back of the oxygen debt.
After a session of dynamic exercise, five events must happen before the muscle can operate again:
* ATP must be replaced
* PC stores must be replenished
* Lactic acid must be removed
* Myoglobin must be replenished with oxygen
* Glycogen stores must be replenished
The replacement of ATP and PC, and the removal of lactic acid take place within 20 minutes of stopping exercise, but the oxygen replenishment of myoglobin and refilling the glycogen stores take between 24 and 48 hours. If the exercise session was of a very high intensity then it will take longer to recover, however, the fitter you are the faster you will recover. The faster the debt can be repaid, the more quickly the performer can exercise again. The oxygen debt consists of two separate components: the alactacid debt and the lactacid debt.
Alactacid oxygen debt is the process of recovery that does not involve lactic acid.
The aerobic energy system is used to produce the ATP required to replenish the PC stores and ATP stores in the body:
ADP + P + Oxygen --> ATP
ATP + C + P --> PC + ADP
Around 50% of the replenishment occurs during the first 30 seconds, while full recovery occurs at about three minutes.
The alactacid oxygen debt ranges between 2 to 3.5 litres of oxygen. The fitter you are, the greater the debt because training increases the PC content within the muscle cells. However, the recovery time of a fitter person is reduced because they have enhanced methods of oxygen delivery such as increased capilliarisation and an improved cardio respiratory system, which will increase the rate of ATP production from the aerobic energy system.
The lactacid oxygen debt takes much longer to complete and can last for minutes or hours, depending on the severity of the exercise. The process involves oxygen, which is required to break down the lactic acid produced during anaerobic glycolysis into pyruvate. Pyruvate can then enter the aerobic energy system and eventually be broken down into carbon dioxide and water.
Lactic acid + oxygen = pyruvate
Lactic acid can also be converted in the liver to glycogen and stored either in the liver or in muscle tissue. Research has shown that an active recovery increases the rate of removal of lactic acid, so walking or slow jogging after exercising will help to decrease the time it takes to rid the body of lactic acid. An active recovery keeps the heart rate and breathing rate up, which has the effect of increasing the rate of delivery of oxygen to the working muscles, which will then help to rid the body of the lactic acid. Therefore, a cool down is very important after any form of activity in order to maximise recovery. Failure to cool down adequately means that the levels of lactic acid will remain elevated. It is thought that this acidity level affects the pain receptors and contributes to muscle soreness which people may feel some times after having exercise. This muscle soreness, termed 'delayed onset of muscle soreness' (DOMS), is at its most uncomfortable 36 to 48 hours after exercise has ceased.
Muscle glycogen stores must also be restored. This is attained through a high carbohydrates diet and rest, and can take several days to recover, depending on the intensity of the exercise.
Excess post-exercise oxygen consumption (EPOC)
Oxygen debt is occasionally called EPOC. EPOC refers to the total oxygen consumed after exercise in excess of a pre-exercise baseline level. EPOC occurs when a person exercises at high intensity, when oxygen cannot be supplied by the anaerobic energy systems, which results in lactic acid production. When the person stops exercising, extra oxygen is breathed in order to break down lactic acid to carbon dioxide and water, to replenish ATP, phosphocreatine and glycogen, and to pay back any oxygen that has been borrowed from haemoglobin and myoglobin.
Training and its effect on fatigue
Training has the effect of increasing the body's ability to exercise for longer without tiring. However, it is necessary to carry out specific training in order to ensure the body adapts to the type of exercise an athlete is competing in, i.e. a marathon runner would have to run persistently for long distances in order to adapt their body to increase the aerobic production of ATP. A sprinter, on the other hand, would have to train anaerobically in order to increase their tolerance to lactic acid and increase their PC stores.
Training and its effects on fatigue.
Energy store fatigue
* High intensity exercise-ATP and Pc become depleted.
* Muscle fibers most frequently recruited become individually depleted of glycogen-Reduces number of fibres avaliable
* Association between fatigue and lactic acid has been recognised since the 30,s
* Lactic acid is partly responsible for fatigue in short term high intensity exercise.
* Sprinting cycling swimming all result in a large accumulation of lactate- anaerobic glycolysis
* 65% of lactic acuid is converted to carbon dioxide and water, 20% into glycogen,10% into protein and 5% into glucose
The elevation of lactic acid within the blood and muscle negatively affects both meduim and long term exercise (Karlson 1971). This only occurs when there is a build up of lactic acid and with training tis can occur much later.
Stafford brown et al (2003) states training has the effect of increasing the bodys ability to exercise for longer without tiring. The training must be specific to the sport the athlete is competing in. A sprinter must train anaerobically in order to increase their tolerance to lactic acid and increase PC stores. If a marathon runner did the same training it would not be beneficial to their sport.
During intense training, the levels of lactic acid in most racers range from 12 µL/kg to as much as 20 µL/kg; lance Armstrong doesn't go above 6 µL/kg. The result is that less lactic acid accumulates in Armstrong's system.
Training and recovery of ATP PC system
To work the ATP-PC system to its full potential the movement you use to train must be specific to you sport. Training this system is very intense and involves working at maximal levels. Work per rep must not exceed 10 seconds other wise this starts to train the lactic acid system. Work rest ratio should be 1:3 this means for every 1 minute of work you should get 3 minutes rest.
A set of exercises should not exceed more then 1 minute so 6 x 10 second reps in set. After each set there should be about 3-10 minute rest to allow the energy system to recover properly. The Energy systems can be trained every other day as they recover quickly. A training program for the energy systems should last 8-12 weeks.
To develop this energy system, sessions of four to seven seconds of high-intensity work at near peak speed are required. For example, a training session might consist of:
* 60 m runs performed 15 times with a 60 second recovery period
* 20 m shuttle runs repeated 20 times with 45 seconds recovery.
By following this type of training, the body will produce more enzymes that make ATP via the PCr energy system and may even be able to store more PCr in the muscles.
Those link to specificity, the body:
* Stores of ATP increase
* Stores of pc increased
* Increase in amount of anaerobic enzyme-creatine kinase
* Overall leads to the maximum peak power output of a muscle
Lactic acid system/anaerobic glycolysis
This energy system requires no oxygen but as a side effect produces lactic acid.
This system uses the break down of muscle glycogen as energy. This system lasts between 10 seconds and 2 minutes peak power occurs around 30 seconds.
This system is trained using interval training the intensity of the exercises should be maximal or near maximal. The work to rest ratio is 1:2 so if you run for one minute you should rest for 2 minutes. The total volume of work should not exceed 10-12 minutes per set. The rest between sets should be between 10-15 minutes long.
Light aerobic work, speeds the recovery and removes lactic acid. Training can be done every other day the training program should last 8-12 weeks.
Example sessions that will help to train this energy system are:
* 150 m intervals with 20 seconds recovery until pace significantly slows.
* 300 m repeated eight times with three minutes recovery.
By performing this training, the body will be able to withstand higher levels of lactic acid and produce more enzymes that will help in the production of ATP via the lactic acid system.
Exercises of over 60 seconds, can overload the lactic acid system. This system adapts by:
* Glycogen stores within muscles increased, due to increase in size or number of mitochodria.
* Cells learn to use and store more rapidly
* Increase in glycotic enzymes
* Increase in lactate dehydrogenase aids convertion of pyruvic acid
* Buffering capacity of muscle against lactic acid increased
* Can work longer before hydrogen inhibits enzyme action
Aerobic system
This system requires oxygen and most adaptations from exercise occur in this system. There is no lactic acid and can even burn lactic acid as fuel. The main sources of fuels are complex carbohydrates and fats as fuels.
When training this system you should Increase duration of the exercise then increase the intensity. This allows both the capacity to use and deliver oxygen to improve.
Total work volume should be between 15-60 minutes for both continuous and interval training. Continuous training works best between 30-60 minutes. Interval training a rep may be between 10secs up to 7 minutes Work rest ratio is 2:1 so for 10 minutes work get 5 minutes rest.
Training should occur at least every other 6 days and at most every second day. The training program should last between 3-6 weeks.
Bibliography
http://en.wikipedia.org/wiki/Acetylcholine
http://www.resistance-training.com/article1.html
http://www.unm.edu/~lkravitz/Exercise%20Phys/deficitepoc.html
http://www.digital-humans.org/Report2004/Documents/12-ModelingPhysiology_files/image075.jpg
http://jp.physoc.org/cgi/content/full/536/1/1
http://sportsmedicine.about.com/cs/exercisephysiology/a/aa053101a.htm
http://www.brianmac.demon.co.uk/lactic.htm
BTEC National Diploma Sport and Exercise Science
Jake Heath-Grey
