Exercise Physiology Flashcards
What does the energy source for muscle contraction depend on?
Source of the ATP depends on the type of muscle fibres involved and the duration of contraction.
Describe ATP immediately available as an energy source.
The amount of ATP immediately available in a muscle fibre is only sufficient to sustain contraction for a couple of seconds at most. This ATP needs to be replenished from other energy stores in the muscle fibres.
Describe phosphocreatine as an energy source.
The next most readily available source is phosphocreatine, which can replenish ATP very short term but this will be depleted in less than 10 seconds.
Describe glycolytic metabolism as an energy source.
For short bursts of muscle contraction, such as in sprints, the main source of ATP is provided by glycolytic metabolism that produces lactic acid as a by-product and is therefore known as the lactic acid system. This can sustain muscle contraction for up to a minute.
Describe oxidative metabolism as an energy source.
For longer periods of muscle contraction, over a minute, the muscles become increasingly dependent on aerobic mechanisms – oxidative metabolism for ATP production.
Distinguish the energy available for shorter and longer duration exercise.
There is much more energy available for short burst of muscular activity, which can sustain high amounts of cross bridge cycling and therefore higher levels of force production than longer duration activity.
What is the role of creatine kinase?
Creatine effectively stores energy from ATP in the form of high energy phosphate bon in phosphocreatine, a reaction that is catalysed by the enzyme creatine kinase.
Many factors can influence the activity of creatine kinase but majorly is the ratio of ATP: ADP.
When is there increased and decreased ATP production from phosphocreatine?
When high ATP levels inhibit the production of further ATP from phosphocreatine, whereas high levels of ADP will push the equilibrium to the right by receiving the high energy phosphate from phosphocreatine.
What is the ATP-phosphocreatine system?
- Muscles contain 3-4 times more phosphocreatine than ATP.
- Functions as an ATP buffer, providing an immediate energy reserve that smooths out change in ATP in muscle fibres, especially at the onset of exercise.
- Depleted phosphocreatine stores can be replenishes to 70% of maximum levels in only 30 seconds.
- Important for providing energy for high intensity but short bouts of exercise of the order of seconds, such as jumping, rapid escapes or birds taking off.
Name the 3 main fuel sources for ATP generation.
- Carbohydrates mobilised from glycogen stores, either locally or in the liver.
- Proteins can be catabolised to amino acids and following deamination by the liver and can either be converted to glucose in gluconeogenesis or can feed into glycolysis ad citric acid cycles.
- Oxidative metabolism of fats
How are carbohydrates mobilised from glycogen stores?
Can be catabolised to monosaccharides and feed into glycolysis and then via acetyl coenzyme A into ethe citric acid cycle and oxidative phosphorylation.
How can fats be metabolised?
Fats are catabolised into the glycerol backbone which can enter the glycolysis and fatty acids. This can be metabolised to acetyl coenzyme A to enter the citric acid cycle.
Compare efficiencies of glycolysis and oxidative metabolism.
For every molecule of glucose, only 2 ATP molecules are produced by glycolysis/anaerobic metabolism. 36 ATP can be produced by oxidative metabolism. So oxygen delivery to muscles is a limiting factor to oxidative ATP production, which generates more energy for muscles to sustain high work rates.
What are the properties of fast twitch fibres?
- Can contract rapidly, achieving maximum tension in 30-50ms.
- Fast speed required high myosin ATPase activity, with an efficient sarcoplasmic reticulum to handle the rapid calcium ion release and reuptake.
- Have low myoglobin levels, so do not have much of an oxygen store but highly active glycolytic metabolism.
- They are active during high and rapid force demands such as printing and jumping.
Distinguish type IIa and type IIb fast twitch fibres.
Type IIa are rapid shortening fibres with well developed glycolytic and oxidative metabolism and are fatigue resistant.
Type IIb are the most powerful and fastest contracting and have the highest glycolytic capacity but fatigue rapidly.
What are the properties of slow twitch fibres?
- Take longer to contract reaching peak tension in 50-110ms.
- This is due to low myosin ATPase activity, which is associated with slower calcium handling and a less well developed glycolytic system.
- Instead, they have a metabolism that is adapted to oxidative phosphorylation, with high myoglobin levels, providing a high muscle store for oxygen and high capillary density for delivery of oxygen and a high density of large mitochondria.
When are slow twitch fibres utilised?
- Used in activities that require sustained low levels of force production, such as standing and walking.
- Their resistance to fatigue makes them particularly important for prolonged endurance exercise.
What is the muscle fibre composition of ocular muscles?
Need to contract rapidly and briefly to generate eye movements. They rapidly achieve maximum tension and have a high proportion of fast twitch type II fibres.
Describe the muscle fibre composition of soleus muscle.
Major antigravity muscle of the leg that requires sustained, but slow contraction to maintain standing posture and during walking. High proportion of slow twitch, type I muscle fibres, which reaches 100% in some species such as cats.
Describe the muscle composition of gastrocnemius muscle.
Intermediate in contraction speed between the 2 extremes an more typical of most muscles. It has a more balanced mix of different fibre types and is important for running and jumping.
What is the link between species and fibre type?
Different species have different adaptations of their muscle fibre compositions to match their lifestyle requirements. For instance, there is a general linear relationship between the sprinting speed of different species and the percentage of fast twitch, glycolytic, type IIb fibres in tehri muscles.
What is the importance and limitations of glycolytic/lactate system?
Important for rapid, forceful muscle contractions that require lots of ATP quickly. ATP production by glycolysis rises over the first 10 seconds of activity, peaks at around 2 minutes into exercise.
But energy production by glycolysis will ultimately be limited to lactate production, which will eventually build up and result in metabolic acidosis.
What is the process of lactate shuttle?
- Muscles with a high oxidative capacity can take up the lactate that is being produced by the fibres with low oxidative capacity via monocarboxylate transfer proteins.
- Lactate is transported into the mitochondria again via MCT where it is oxidised to pyruvate via lactate dehydrogenase.
- Pyruvate can enter the citric acid cycle and oxidative phosphorylation, which spares the consumption of other fuels.
- This redistributes lactate from cells that can’t use it to those that can. so, during intense exercise, the sole fuel source for cardiac muscle comes from circulating lactate.
What is the Cori cycle?
- Can regenerate glucose and glycogen a pyruvate.
- In exercise, circulating adrenaline is going to be high and so glucose will be released back into the bloodstream, where it is available to be taken up by active muscles as a fuel.
- It may seem wasteful that this cycle effectively has a net consumption of 4 ATP but it effectively shifts the metabolic burden from the muscles to the liver.
How is lactate correlated with exercise intensity?
At low exercise, the blood level of lactate is low and remains constant.
As intensity of exercise increases, the capacity of the body to metabolise the circulating lactate us exceeded and it starts to accumulate in the blood.
How can training increase lactate threshold?
- Training results in a variety of changes, which increase the body’s ability to take up and metabolise lactate.
- Blood lactate concentration can be measured directly form blood samples.
- Alternatively and more commonly, respiratory variable can be monitored to provide an indication of lactate threshold.
- This is because the lactate accumulation in the blood leads to metabolic acidosis, which drives an increase in ventilation rate and increase in expired CO2.
Explain oxygen deficit.
This difference between demand and supply of ATP is made up from short term ATP stores, such as the phosphocreatine system and by glycolytic metabolism. Leads to an oxygen deficit and the lactate produced by glycolysis will need oxygen to be metabolised. So rate of oxygen uptake will remain elevated after exercise stops and only recovers gradually over time as the oxygen deficit is repaid.
What is the VO2 max?
The rate of oxygen uptake during exercise, which involves the whole body, increases as intensity of exercise increases until it reaches a maximum level. This is the maximum rate of consumption of oxygen by the body and is an estimate of an individual’s capacity of aerobic ATP synthesis.
What are the VO2 max values for humans, thoroughbred horses and greyhounds in mL/kg/min?
Human = 40-50
Human athlete = 94
Thoroughbred = 220
Greyhound = 240
Do differences in fibre type contribute to thoroughbred high oxidative capacity?
No – thoroughbreds still need the mix of muscle types that give the optimum combination of speed and endurance. Instead, the explanation is that thoroughbred muscles have a greater muscle mass. This is due to more and larger mitochondria in type I and IIa fibres. The greater surface area for oxidative phosphorylation provides increased ATP generating capacity and therefore increased oxygen requirements to sustain higher intensities of exercise.
Describe the contribution of different fibre types during locomotion.
- Low levels of activity: force generation is low and primarily provided by type I slow twitch fibres using fat fuel.
- Speed increases: more type IIa fibres recruited to provide the greater force required using fats and lactate as fuel.
- Speed increases further: type IIb glycolytic fibres recruited to provide maximum force. Unable to utilise lactate or fats due to low oxidative metabolism so primarily uses carbohydrates from glycogen stores and blood glucose form the Cori cycle.
- Lactate produced: used by other muscle fibres to maintain high rates of ATP production in type IIa and type I fibres.
- Fast speed cannot be maintained for long before muscle glycogen stores begin to be depleted and fatigue sets in.
What is the mechanism of fatigue?
- Increased transmural pressure on blood vessel sin the muscle, which can decrease blood flow.
- Resulting ischaemia can lead to the accumulation of metabolites.
- Muscle fibre swelling and mitochondrial swelling, impairing the ability of muscle fibres to contract.
- Prolonged anaerobic metabolism of type IIb fibres can also lead to a decrease in ATP substrates, again limiting contractile function.
Describe fatigue on a muscular level.
- Can result in depletion of muscle glycogen stores, which limits the work rate.
- Extracellular potassium ion accumulation can also impair action potential firing and muscle contractility, as can an increase in muscle temperature in hyperthermia.
- Prolonged exercise can lead to dehydration and electrolyte loss due to sweating, which is needed to avoid hyperthermia.
Horses are more prone to this than other species.
What is central fatigue?
- Results from a decrease in drive to exercise from eth brain.
- Due to inhibitory feedback from group III (A-delta) and group IV (C) afferents within the muscle.
- These can signal the work being done by the muscle, as well as the accumulation of metabolites, which decrease performance by decreasing the central motivation to keep going.
Describe the phases of increased ventilation at steady state exercise.
- At the onset of exercise, the pulmonary ventilation increases steeply within a single respiratory cycle.
- This is far too rapid to be a results of any changes resulting from the exercise itself.
- If the animal is expecting exercise, there is often an anticipatory phase, in which pulmonary ventilation increases before the start of exercise, driven by activity in motor cortex and respiratory centres in the medulla to directly increase ventilation rate.
- Initial phase of respiration may be followed by a dip before the second pages of the ventilatory response.
- This gradually increases over a period of several minutes to match pulmonary ventilation with the oxygen carrying capacity and oxygen demand.
- Mammalian respiratory system is very good at this matching and pulmonary ventilation is never a limiting factor to exercise performance in normal animals.
- This increase is not driven by a change in blood glasses, which remain pretty stable in most species.
- Instead this matching phase of the increase in pulmonary ventilation is driven by the mechanoreflex and metaboreflex.
What is mechanoreflex?
- Driven by increased activity in type III (A-delta) sensory afferents from the body of the muscle.
- These afferents have free nerve endings which are squeezed and distortion by muscle contractions.
- Provide information that the muscle are doing work to drive the medullary ventilatory response.
- Afferent information from joint receptors also contribute, as passive movement of limbs have to be sound to increase pulmonary ventilation in the absence of muscle contractions.
