Continuing the Gym Instructor Level 3 articles, this relates to muscle contraction and fibre types.
When stimulated by a nerve message (see The Nervous System for more on this) the myosin (golf-club) heads attach to the actin, then pivot and bend. This pulls the actin filaments over the myosin filaments towards the centre of the sarcomere. The myosin heads then detach from the actin, relax back to their original position and prepare to attach to the actin again, only now further down the molecule.
In a regular (concentric) contraction the myosin continues to move along the actin, pulling each filament towards the centre of the sarcomere in what is a very complex, strenuous and energy demanding process. But the muscle can also be static (isometric) or even lengthening (eccentric) during a contraction, which puts even more stress on muscle tissues and the delicate myosin heads.
For any type of contraction to take place a continuous supply of energy is needed in the form of ATP (adenosidetriphosphate). An ATP molecule is used every time a myosin head pulls and pivots on an actin filament, which can be billions of times during exercise.
Muscle Fibre Types
Although muscle cells are very similar they are different in 2 important ways:
• The speed at which they contract
• The way they prefer to generate ATP (for energy)
Muscle cells are therefore either:
• Slow Twitch (type 1) cells
• Fast Twitch (type 2a and type 2b) cells
Slow Twitch Fibres
Slow twitch fibres get their ATP mostly in the presence of oxygen which is why they are also called Slow Oxidative (SO) fibres, which have the following characteristics:
• When they contract they do so smoothly and gradually, taking about 40-50 milliseconds to contract maximally and about the same time to then relax.
• They have an abundant supply of capillaries bringing them oxygenated blood because of the way they generate ATP.
• They are very effective at extracting the oxygen from the blood supplied to them due to an abundance of myoglobin, which is an oxygen-binding protein, so they are similar in colour to red blood cells.
• Mitochondria are needed as aerobic generation of ATP can only happen inside them, so Type 1 slow-twitch fibres have a lot of them.
• Producing ATP aerobically leaves only water (H2O) and carbon dioxide (CO2) as end products which is easily dealt with by the muscle cells, which combined with their slow rate of contraction makes Type 1 fibres resistant to fatigue.
Fast Twitch Fibres
There are actually 2 types of fast twitch fibres.
Type 2b fibres mostly generate their ATP anaerobically (without oxygen) via a process called glycolysis, which is why they are known as Fast Glycolytic Type 2b (FG) fibres.
Type 2a fibres are basically halfway between type 1 and type 2b as they can generate their ATP via a combination of aerobic and anaerobic methods. They are known as Fast Oxidative Glycolytic (FOG) fibres.
Type 2 fibres have the following characteristics:
• They reach their maximum contraction rapidly in about 5 milliseconds which results in greater power (power is a combination of strength and speed).
• Their method of ATP generation is a lot quicker than type 1 fibres and a good supply of oxygen is not required, which means it can occur even at near-maximal heart rates.
• They do not have a lot of mitochondria or myoglobin, or a good blood supply, which is why they are white.
• The glycolytic way in which type 2 fibres make ATP can only use glucose as its energy source, which is a finite resource and produces lactic acid as a by-product. This lactic acid prevents further production of ATP which is why they fatigue at high intensities. If this is done regularly however, the muscles become more efficient at resisting its effects and dealing with it.
Although lactic acid inhibits the production of ATP in type 2 cells, type 1 cells can actually use it for ATP production. Also, when there is sufficient blood flow it can be transported to the liver to be converted back to glycogen, which is the storage form of glucose.
Everyone is different in the amount of slow twitch and fast twitch cells they have. While this can be changed through exercise (or lack of), the amount is pretty much determined by genes. Those with a greater amount of fast twitch cells will always have a greater athletic potential than those with a greater amount of slow twitch cells. However, it has no bearing on fitness which is determined by the efficiency of the heart and lungs. For example, a 100m sprinter is fast twitch while a marathon runner is slow twitch, but both are extremely fit.
To test how well you’re doing, click here for Gym Instructor Level 3 Questions – Muscle Types, Fibres And Contraction
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