A muscle is composed of 50% fast-twitch motor units and 50% slow-twitch motor un

Question

A muscle is composed of 50% fast-twitch motor units and 50% slow-twitch motor units. The muscle is tetanized by stimulating the motor nerve at a very high frequency and relatively high voltage. The contractile strength declines over time (fatigue) as shown below. With two additional lines (marked as shown in the legend), show the changes in force generated by the fast-twitch (FT) motor units and the slow-twitch (ST) motor units over the period of time indicated. Explain your answer. 100 % max force whole muscle- FT motor units ST motor units 1 2 3 4 5 6 7 8 9 10 20 30 minutes

Explanation / Answer

The changes in force generated by the Fast twitch motor units and Slow Twitch motor units

Skeletal muscle has two main fiber types: slow twitch and fast twitch. Fast twitch and slow twitch fibers vary in different muscles and in different individuals – these proportions tend to be genetically determined. As the fibers have distinct characteristics, this will affect performance in certain sporting activities.

For example, a marathon runner may have almost 80% slow twitch fibers which are designed for long periods of low intensity work; whereas, a sprinter may have 80% fast twitch fibers, which can generate extremely large forces, but fatigue easily.

Each muscle contains both types of fibers, but not in equal proportions. The fibers are grouped in motor units, with only one type of fiber in any given unit.

1) Slow twitch fibers are referred to as slow oxidative. They are red in color, contract slowly, are aerobic (supplied with oxygen), are used in endurance based activities because they can contract repeatedly, but they exert less force.

These are slow twitch fibers because they contract more slowly than type II fibers. The myelin sheath of the motor neuron stimulating the muscle fiber is not as thick as that of the fast twitch unit, and this reduces the amount of insulation, slowing down the nerve impulse. Slow twitch fibers do not produce as much force as fast twitch fibers, but can easily cope with prolonged bouts of exercise. They are more suited to aerobic work as they contain more mitochondria and myoglobin and have more blood capillaries than fast twitch fibers. Slow twitch fibers have the enzymes necessary for aerobic respiration and are able to break down fat and carbohydrates to carbon dioxide and water. This is a slower process than releasing energy anaerobically, but it does not produce any fatiguing by products.

2) Fast twitch fibers are white in color, contract rapidly, are anaerobic in nature (no oxygen supply), and are used for strength/speed based activities because they can exert great forces, but they are easily exhausted.

Fast twitch fibers are further divided into Type 2a and Type 2b

3) Type 2a fibers are referred to as fast oxidative glycolytic. These fibers take on certain type 1 characteristic through endurance training. They therefore tend to have a greater resistance to fatigue, and are used in activities which are fairly high in intensity and of relatively short duration. The motor neuron stimulating the type Ilia fiber has a thicker myelin sheath than the slow twitch fibers, so it can, therefore, contract more quickly and exert more force. The amount of force produced by these fibers is greater than slow twitch ones because there are more muscle fibers in each motor unit. They can produce energy both aerobically and anaerobically by breaking down carbohydrates to pyruvic acid, but it is more suited to anaerobic respiration, allowing it to release energy very quickly. The rapid buildup of lactic acid (a by-product of anaerobic respiration) lowers the Ph (due to increased acidity) and has a negative affect on enzyme action, causing the muscle fiber to fatigue quickly.

4) Type 2b are pure fast twitch fibers and are referred to as fast twitch glycolytic. These are used for activities of very high intensity and have a much stronger force of contraction. This is because the motor neuron that carries the impulse is much larger. There are generally more fibers within a fast twitch motor unit, and the muscle fibers themselves are larger and thicker. The motor neurons supplying this fiber type are large, and this increases the speed of contraction. The neuron also activates a greater number of muscle fibers allowing each motor unit to produce a far greater force than slow twitch fibers. These fibers are very quick to contract, and can exert a large amount of force. They rely heavily on anaerobic respiration for releasing energy as they have very few mitochondria. Energy is, therefore, released rapidly, but the muscle fiber is also quick to fatigue.

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