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Muscle structure and contractions

Hadyn Luke posted this on Friday 12th of October 2012 Hadyn Luke 12/10/2012

Tags: Anatomy and physiology

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In our fitness blog this week, we’re looking at muscle structure and contractions.

The structure of a muscle

As every personal trainer will know, a bone is connected to a muscle via a tendon. Each muscle is made up of a series of sheaths, starting with the outer sheath, the epimysium, which covers the whole of the muscle (see diagram). Within the epimysium, there are bundles known as fascicles; each bundle is surrounded by another sheath, the perimysium, and within that there are further bundles of individual muscle fibres, surrounded by the endomysium.

The role of actin and myosin

The individual muscle fibres within the endomysium are called myofibrils; these are made up of two different types of cell – actin and myosin – which run parallel to each other.

Myosin has hook-like projections, which can bind to the actin to create a twisting rotation, which shortens the distance between the myosin and actin. This causes the muscle fibres to contract, shortening the muscle and allowing movement to occur. However, the myosin binding sites on the actin are covered by two chemicals: troponin and tropomyosin. These chemicals act as a kind of force field, preventing the myosin heads from binding on to the myosin binding sites on the actin. If they didn’t exist, the muscle would be constantly trying to contract.

What happens when a muscle contracts

If a personal trainer asks a client to carry out, for example, a bicep curl, the following process is set in motion:

  1. The body sends an impulse from the nervous system to the bicep muscle.
  2. When the electrical impulse reaches the synapse of the nerve – the neuromuscular junction, where the nerve meets the muscle – it creates an action potential.
  3. This causes the release of a neurotransmitter called acetylcholine, which causes sodium to move through the membrane of the muscle.
  4. The result is depolarisation, a reversal of electrical activity, which in turn causes a cascade of calcium from the sarcoplasmic reticulum into the sliding filament mechanism, where the actin and myosin are found.
  5. The calcium causes the troponin and tropomyosin to uncover the myosin binding sites on the actin, allowing the two filaments to bind and shorten, which in turn allows the muscle contraction to take place.
  6. Almost as quickly as that occurs, the calcium disappears, the myosin heads can no longer bind and the muscle relaxes.

 The role of motor units in strength training

When a personal trainer is working with a client, the amount of neural output the client’s nervous system puts out will dictate how many motor units they will recruit. For example, one motor unit may be connected to 10 muscle fibres, which will be enough to lift a small weight. To lift a heavier weight, they will need to recruit another motor unit.

A motor unit consists of one nerve and the muscle fibres that are connected to it. Once you activate that neuromuscular junction, all the muscle fibres connected to that nerve will have to fire – this is known as the all-or-nothing law.

So if a personal trainer has a client who wants to increase their strength, work should be done to improve the client’s nervous system’s ability to fire a nervous impulse to the muscle (code of firing). Equally, the muscle will need to improve its ability to recruit more and more motor units. This is why the effectiveness of the central nervous system is so important in strength training.

 

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