Describe the physiological changes that occur in muscle when it is trained for endurance - What are the most effective ways of causing these changes?

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Mark Halford 330256

Describe the physiological changes that occur in muscle when it is trained for endurance. What are the most effective ways of causing these changes?

Exercise Physiology (02 08027)

Endurance is “the ability of muscle groups to perform submaximal contractions for extended periods of time” (Zachazewski, 1996). Endurance is compromised of two parts; muscular and cardiorespiritory endurance. Cardiorespiritory endurance is compromised of the cardio and respiratory systems and refers to the body as a whole, usually referred to as the aerobic system. Aerobic reactions become the main source of energy after several minutes of exercise. Muscular endurance refers to individual muscles.

The physiological changes in muscle can be categorised by aerobic changes and anaerobic changes. Anaerobic endurance training induces significant increase of ATP, Phosphocreatine and glycogen stores within muscles themselves. The muscle also has an increased amount and activity of key enzymes i.e. ATPase, that control the anaerobic phase of glucose metabolism. An increase in levels of glycogen and glycoltic enzymes help generate high levels of blood lactate.

Aerobically trained muscles have a greater blood supply due to an increase in the number and density of capillaries around skeletal muscle, which provides a greater surface area for increased oxygen exchange. The stimulus for capillary development with training may be due to vascular stretch and shear stress on the vessel walls caused by the increased blood flow in aerobic exercise.  To make full use of the better blood supply, aerobically trained muscle contain larger and more numerous mitochondria. Kiessling et. Al. (1971 in Fox and Matthews 1981) found a 120% increase in the number of mitochondria in vastus lateralis following a 28 week, 5 days per week training program of distance running. The increase in mitochondria within muscles complements an increase in myoglobin within the muscle tissue. Myoglobin is responsible for the exchange of oxygen between the haemoglobin in blood and into the muscles. Therefore an increase in myoglobin increases the rate of oxygen movement into muscle fibres. Additionally Myoglobin is has a key roll in the short-term storage of oxygen within the muscles. Muscles that have been trained aerobically also have an increased capability to mobilise, deliver and oxidise fatty acids for energy during submaximal exercise due to an increase in fat-mobilising and fat metabolising enzymes. In humans the increase in Succinate Dehydrogenase (SDH) and cytochrome in just 8 weeks of training is nearly 40% (Henriksson and Reitman 1977 in Fox and Matthews 1981). Costill et. al. (1976 in Reilley, 1990) showed the activity of Malate Dehydrogenase was found to be twice as high in elite distance runners compared to untrained men in the gastrocnemius, and SDH was 3.4 times more active. The increase of utilisation of Free Fatty Acids (FFAs) as a direct result of endurance training means there is an increase in contribution of fat as an energy source. The increase in the ability of muscles to oxidise fat following endurance training is a result of three factors; firstly an increase in FFA from adipose tissue into the circulation; secondly increased intramuscular stores of triglycerides (Hopper et. al. found there to be 150% more intramuscular fat stores in endurance trained males compared to untrained males); lastly increased activity of enzymes related to activation, transport and breakdown of FFA. Endurance training promotes an increase in glycogen storage, Gollnick et. al  found an increase of 250% due to an increase in the activity of glycogen synthesis and the glycogen branching enzyme. The issue of whether fibre type changes is still controversial. Studies have shown that muscle fibres can change into slow twitch fibres in animal tests but the question of can endurance training induce such a change in humans is uncertain.

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Fibre hypotrophy is another field in uncertainty. For a muscle to undergo hypertrophy, it must be put under a level of tension not often found in endurance training. However a study produced by Costill et. al. (1976 in Reilley 1990) found that not only where the male elite runners fibres that he was testing more numerous, which we would expect, but that the fibres themselves were 30% larger than those of an untrained male. Again conflicting results from this study and others, make the issue of whether muscle hypertrophy occurs as a result of endurance training uncertain.

Aerobic training doesn’t ...

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