Vasoconstriction
Vessels can also shut down blood flow to tissues, which can temporarily lessen blood supply. This process is known as vasoconstriction and involves a decrease in the diameter of blood vessels by contraction of the involuntary muscle fibres in the vessels walls, resulting in the reduction of blood flow. For example, kidney function shows the capacity to control renal blood flow. At rest renal blood flow accounts for about 20% of cardiac output during maximal exercise, renal blood flow decreases to about 1% of cardiac output.
Thermoregulation
Steady state exercise increases sweating. This beneficial initial response is because of the significant increase in plasma volume that happens during steady state exercise. Increased plasma volume supports sweat gland function during heat stress, and maintains the correct plasma volume for the cardiovascular demands of the exercise. Therefore, a trained person will store less heat early during steady state exercise, reaching a thermal steady state sooner and at a lower core temperature than an untrained person. The training advantage for thermoregulation happens only if the individual fully hydrates during exercise.
Increased vanous return
Veins solve the potential problem related to the low blood pressure of venous blood. Values spaced at short intervals within the vein permit one way blood flow back to the heart. Veins compress because of low venous blood pressure or in the case of steady state exercise, muscular contractions. Other alternate venous compression and relaxation, combined with the one way action of valves, provides a ‘milking’ effect similar to the action of the heart. Venous compression gives a lot of energy for blood flow, whereas a relaxation of the blood vessels allows blood to move towards the heart. Without valves, blood would stagnate or pool in the veins and athletes would faint or pas s out every time they did exercise.
Starling’s law
Starling’s law states that the stroke volume of the heart increases in response to an increase in the volume of the blood filling the heart. The increased volume of blood stretches the ventricle wall causing cardiac muscle tissue to contract more forcefully. The stroke volume may also increase as a result of more contractions in the cardiac muscles during exercise. Therefore, the reduced heart rate of a trained athlete allows a greater filling during the longer diastole, so the amount of stretch of the cardiac muscle is greater. This in turn increases the stroke volume.
Respiratory responses
Respiratory rate and tidal volume
Increases in respiration rate maintain alveolar ventilation during steady state exercise. During steady state exercise, trained athletes achieve the required alveolar ventilation by increasing their tidal volume and only minimally increasing breathing rate. With deeper breathing, alveolar ventilation usually increases from 70% of minute ventilation at rest to over 85% of total ventilation during exercise. This increase happens because deeper breathing causes a greater percentage of the incoming tidal volume to enter the alveoli.
Effects of pH and temperature on the oxygen dissociation curve
The oxygen dissociation curve is a graph that shows the relationship between the percentage of oxygen saturation of blood and the partial pressure of oxygen. During steady state exercise, increased temperature and lower blood pH concentration affect the oxygen disassociation curve in such a way that more oxygen can be unloaded to supply the active muscle. In prolonged high intensity exercise, large amounts of lactic acid enter the blood from active muscles. At exhaustion, blood pH can approach 6.8. Only after exercise stops does blood pH stabilise and return to its normal pH level of 7.4.
(fig1)
P4
Neuromuscular system
Muscle spindles and golgi tendon organs provide sensory information in relation to the intensity of the exercise undertaken, providing smooth, co-ordinated movement patterns.
Increased pliability of muscles
Muscle spindles are located within the muscle fibres. When the spindle is stretched, nerve impulses are generated and information relative to the extent of the stretch is sent to the central nervous system. The central nervous system sends back the information concerning how many motor units should be contracted in order to apply a smooth movement. The more the body is used to a particular steady state exercise, the more efficient the muscle spindles become at transmitting the same information, and the more pliable the muscles become.
(fig2)
Increased transmutation rate of nerve impulses
Golgi tendon organs are located within the tendons and are also sensitive to stretch. The information that the Golgi tendon organs send to the central nervous system is concerned with the strength of the muscle contraction. The Golgi tendon organs complement the muscle spindles and together ease the process of efficiency and movement.
Energy system
On arrival at the skeletal muscle, energy is consumed with or without oxygen as the muscles convert chemical energy to mechanical energy.
At any point in time, one of the three energy systems is dominated in contributing the energy needed for the resynthesis of adenosine triphosphate (ATP). The contribution of each energy system is dependent on the intensity and duration of the exercise. The continuous interaction of the three systems is known as the energy continuum.
Adenosine triphosphate (ATP)
In general, in order to enhance your body ATP production and ATP energy transfer capacity requires repetitive, intense, short duration exercise.
The training activities chosen should engage the muscles in the movement of which the athlete desires improved anaerobic power. An athlete wishing to improve on their speed and power would undertake sprint training and weight training. This achieves two goals:
∙ It enhances the metabolic capacity of working muscle tissue fibres
∙ It improves the neuromuscular adaptations to the specific movement required for a sport
When ATP is split, some of the energy is used to power muscle contractions, but as no energy is 100% efficient, some is always lost through heat. This is why vigorous exercise produces large amounts of heat that must escape from the body.
Anaerobic glycolysis
Anaerobic glycolysis involves the breakdown of glycogen in the absence of oxygen, with the formation of ATP plus lactate (lactic acid).
The accumulation of lactic acid stops the use of this energy system after 40-60seconds of maximum effort. This means it is the system called upon by athletes whose sports demand high-energy expenditure for up to 60seconds, such as 400metre runners and those in compound sprint sports such as rugby and squash.
Aerobic energy system
This aerobic energy system involves the oxygen transporting system and the use of mitochondria in the working muscle for oxidation of glycogen and fatty acids. Due to this systems reliance on oxygen, it is called the aerobic energy system. This system is involved in prolonged work at low intensity and is of increasing importance the longer the sport goes on. Only the lack of fuel, overheating or dehydration will end exercise using this system.
The system for burning fuel in aerobic exercise varies according to the duration and intensity of the exercise. In prolonged aerobic exercise, the preferred fuel for energy is fatty acids because glycogen stores are limited in comparison to the large amounts of fat stores in the body. Unlike glycogen, fatty acids can only be used in the aerobic energy system, whereas higher intensity exercise involving aerobic and anaerobic energy systems prefer glycogen as fuel.
Mitochondria
Mitochondria are the site of aerobic respiration. Pyruvate oxidation and the Krebs cycle take place in the matrix (fluid) of the mitochondria, while the electron transport chain takes place in the inner membrane.
(fig3)
Krebs cycle
The Krebs cycle is a series of aerobic chemical reactions occurring in the matrix in mitochondria. The main purpose of the Krebs cycle is to provide a continuous supply of electrons to feed the electron transport chain. This cycle begins when the 2 carbon acetyl CoA joins with a 4 carbon compound to form a 6 carbon called citric acid. Citric acid (6C) is gradually converted back to a 4 carbon compound ready to start the cycle once more. The carbons removed are released as CO2. The hydrogen’s, which are removed, join with NAD to form NADH2.
(Fig4)
Electron transport chain
The electron transport chain is a series of biomechanical reactions during which free energy contained within hydrogen (coming from the Krebs system) is released, so that it can be used to synthesis ATP during aerobic metabolism. The electron transport system chain occurs in the cristae in mitochondria. Each reaction involves a specific electron carrier molecule that has a particular draw to hydrogen. The final link in the electron transport chain is oxygen, which combines with the hydrogen and electrons to form water.
(fig5)
P5
Fatigue
Fatigue involves the exhaustion of muscles coming from prolonged exercise. The body cannot exert itself forever because of neuromuscular fatigue , which occurs as a result of different methods and systems. The symptoms of fatigue are:
- Depletion of energy stores such as creatine phosphate and glycogen
- Increase in lactic acid
- Dehydration
- The loss of electrolytes
Exercise puts demands on the body, and as a result changes happen in the body such as:
- Oxygen levels fall
- Carbon dioxide and lactic acid levels increase
- Body temperature increases
- Blood glucose and glycogen levels fall
- Fluid and electrolytes are lost as you sweat
During short term maximal exercise, not enough oxygen and/or an increased lactic acid build up can bring about fatigue.
Depletion of energy sources
The body need energy in order to function properly. As an athlete begins to exercise, the body has to ensure a supply of energy so that the heart rate increases, forcing more blood to the skeletal muscles so they can contract more frequently in response to the exercise undertaken.
The energy needed comes from the food we eat.
- Carbohydrates (pasta, bread, rice and potatoes) is broken down into glucose in the body to provide energy.
- Fats (cheese, butter, oils) are broken down into fatty acids to provide energy.
- Proteins (meat, fish, eggs) are broken down into amino acids that provide energy but only when all other energy supplies have gone.
The breakdown of all three of these fuels in the body produces ATP, the only substance the body is able use to provide energy. Everything the body does requires energy in the form of ATP. So therefore if an athlete doesn’t take on board enough carbohydrates, fats or protein, it is likely they will deplete their energy sources quickly when exercise is undertaken.
Creatine phosphate
Creatine phosphate is made in the liver and transported to skeletal muscles for storage. It is used to form ATP from adenosine diphosphate (ADP), and is particularly important for intense physical exercise with the time limit of approximately 10 seconds e.g. weightlifting.
Muscle and liver glycogen
A reduction in muscle and liver glycogen and blood glucose during sub maximal exercise can happen despite the availability of enough oxygen and ATP. Once glycogen stores are depleted, muscles stop contracting, even during steady state exercise, as the body is unable to use fat as a sole source of fuel. Marathon runners particularly must be careful not to deplete their glycogen early in a race by running to fast. The early depletion of these stores brings the athlete to the point of exhaustion. To stop this happening marathon runners run at a pace that metabolises fats so that the rate at which the glycogen depletes is lessened.
Effects of waste products
The main waste products of exercise are urea, carbon dioxide, water and lactic acid. Urea and water are filtered through the kidneys and expelled from the body. Carbon dioxide is carried in the blood to the lungs, where it passes into the alveoli and is then expelled from the body.
Blood lactate accumulation, carbon dioxide and increased acidity
Raised levels of carbon dioxide increase (from exercise) the level of blood acidity. Carbon dioxide is carried in chemical combination in the blood. In red blood cells, enzymes speed up the reaction of carbon dioxide and water to form carbonic acid.
H2O + CO2 → H2CO2
Carbonic acid breaks down into hydrogen ions (H+) and bicarbonate ions (HCO3-).
H2CO3→ H+ + HCO3-
The increase in hydrogen ions is responsible for the increase in blood acidity. Metabolites other than lactic acid are disposed of by oxidation. Lactic acid is disposed of as follows:
- Muscle lactate is disposed of first by oxidation to private and then dissimilation to carbon dioxide and water
- Some blood lactate is taken in by the liver, which comes back as glycogen
- The remaining blood lactate diffuses back into the muscles or other organs, to be oxidised the dismantled. This oxidation forms carbon dioxide, which is later excreted by the lungs.
Neuromuscular fatigue
Acetylcholine is a neurotransmitter that is released to stimulate skeletal muscle and the parasympathetic nervous system. Its effect is short term because it is destroyed by acetyl cholinesterase, an enzyme released into the sarcolemma of muscle fibres, preventing continued muscle contraction in the absence of more nervous stimulation.
Recovery
Four processes have to be satisfied before the exhausted muscle can perform to its optimum level again. These are:
- Rebuilding of muscle phosphogen stores
- Removal of lactic acid
- Replenishment of myoglobin stores with oxygen
- Replacement of glycogen
Excess post-exercise oxygen consumption (EPOC)
The need for additional oxygen to replace ATP and remove lactic acid is known as oxygen debt or excess post-exercise consumption (EPOC). The two major components of EPOC are:
- Fast components (alactacid oxygen debt) this is the amount of oxygen required to synthesis and rebuild muscle phosphagen stores (ATP and creatine phosphate)
- Slow components (lactacid oxygen debt) this is the amount of oxygen required to remove lactic acid from the muscle cells and blood.
Bodily processes do not immediately return to normal or resting levels after exercise. After light exercise such as walking, recovery to a resting state takes place quickly and generally without realising it. With steady state exercise however, it takes time for the body to return to normal.
EPOC defines the excess oxygen uptake above the resting level in recovery. It means the total oxygen consumed after exercise is in excess of pre-exercise baseline levels. Oxygen consumption after exercise restores the energy demands used during exercise.
Fast components
Restoration of muscle phosphogen stores
Alactacid oxygen debt (without lactic acid) represents that portion of oxygen used to synthesise and restore muscle phosphogen stores, which have been almost been completely exhausted during high intensity exercise. During the first three minutes of recovery, EPOC restores almost 99% of the ATP and creatine phosphate used during exercise.
Slow components
Removal of lactic acid
The slow component of EPOC is concerned with the removal of lactic acid from the muscles and the blood. This can take several hours, depending on the intensity of the exercise and whether the athlete was active or passive during the recovery period (continuous activity can significantly speed up recovery). Around half of lactic acid is removed after 15minutes, and most is removed after an hour.
Lactacid recovery converts most of the lactic acid to pyretic acid, which is oxidised via the Krebs cycle to create ATP. Once exercise is over, the live synthesises lactic acid into glycogen while the rest of the body can remove small amounts of lactic acid through respiration, perspiration and excretion.
Replenishment of myoglobin
Myoglobin is an oxygen storage protein found in muscle. Like haemogloin, it forms a loose combination with oxygen while the oxygen supply is plentiful, and stores it until the demand for oxygen increases. Muscle has its own build in oxygen supply. However, during exercise, the oxygen from myoglobin is quickly used up. After exercise, additional oxygen is required to pay back any oxygen that has been borrowed from myoglobin stores.
Replacement of glycogen
The replenishment of muscle and liver glycogen stores depends on the type of exercise. Short distance, high intensity exercise may take two or three hours, whereas long endurance activities such as marathon running, may take days. Replenishment of glycogen stores is most rapid during the first few hours after training. Complete restoration of glycogen stores is speeded up with a high carbohydrate diet.
Bibliography
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(fig2)- http://content.answers.com/main/content/img/oxford/Oxford_Sports/0199210896.Golgi-tendon-organ.1.jpg 23/11/09
(fig3)- 23/11/09
(fig4)- http://uwstudentweb.uwyo.edu/a/ateeter/krebs_cycle.gif 24/11/09
(Fig5)- 24/11/09