Muscle Groups (Part 2)

Microscopic Muscle Structure

Muscles are made up of the following components (from largest to smallest):

  • Muscle belly
  • Fasciculi – a group of muscle fibres that are surrounded by connective tissue
  • Perimysium – the name given to the connective tissue that surrounds the fasciculi
  • Epimysium – encases the muscle to hold everything together. As it nears the end of the muscle it thickens to eventually become the tendon
  • Myofibrils – muscle fibres are made up of myofibrils and are similar to the many wires that make up a cable
  • Sarcomere – myofibrils have many units known as sarcomeres. These are arranged end to end for the entire length of the fibre
  • Z lines – These lines mark where one sarcomere starts and another finishes.
  • MyofilamentsMyofibrils can be further divided into myofilaments. There are two types of myofilaments – actin (thin filaments) and myosin (thick filaments)
  • Sarcolemma – the cell membrane that surrounds the muscle fibre
  • Sarcoplasm contains the following:
  1. Mitochondria
  1. Myoglobin
  1. Fat, glycogen, ATP, PC
  1. Enzymes

Sliding Filament Theory

For contraction to occur, a nerve impulse must be sent from the brain. Down at the motor end plate, calcium ions are released which effectively act as a power switch. This power switch activates the crossbridges (which connect thick and thin filaments).

ATP energises the crossbridges, which allows a pulling action to occur, resulting in the thin filaments sliding over the thick filaments. This occurs all down the length of the fibre (at each sarcomere), and the result is acontraction of the muscle.

What happens to each of the following when a muscle contracts?

  • Actin – thin filaments: slide over the thick filaments; whether they slide toward or away from one another depends on whether it is a concentric or eccentric contraction
  • Crossbridges – oar-like structures that are attached to the thick (myosin) filaments. These structures reach out and are attracted to the thin (actin) filaments. When contraction occurs, these oar-like structures drag the thin filaments in toward the centre of the sarcomere and the muscle contracts (concentric contraction)
  • I band – distance between the z-line and the edge of the A band
  • H zone – the gap between the thin filaments before contraction. The H zone disappears upon full contraction
  • A band – the length of the thick filament

Make sure you have a good look at a diagram of the sliding filaments (actin): one is on the previous page. It is quite possible that a question with a diagram will be used in the exam.

Muscle Fibre Recruitment: Muscular Contraction

The nerve that goes to the muscle is called a MOTOR NEURON. Motor neurons are responsible for stimulating a number of muscle fibres – together they are called a MOTOR UNIT.

When precise movement is required, 1 motor unit may stimulate only 1-2 fibres, e.g. movement of the eye.

When gross movement is required, 1 motor neuron may stimulate 80-100 fibres, e.g. quadriceps muscle.

The all or nothing principle of muscle contraction

When an electrical impulse reaches a threshold, all muscle fibres of that unit contract maximally. It is not possible for a muscle fibre to contract partially – it’s all or nothing.

The intensity of muscular contraction is governed by the number of motor units recruited. The stronger the impulse, the more motor units (nerve cell and muscle fibres) are activated. The greater the frequency of arrival of impulses, the greater the force developed by that muscle. This is how we are able to lift a glass one minute and a dumbbell the next.


Types of Muscle Contractions

1. Isotonic – the most common type of contraction. For example, bicep curl.

By definition, the muscle length changes as tension develops.

There are two types of isotonic:

  • Concentric – muscle shortens while contracting. For example, a pull-up exercise.
  • Eccentric – muscle lengthens while contracting. For example, lowering back down from pull-up.

2. Isometric – contraction, but no change in length of muscle fibres; e.g. a contraction against an immovable object such as pushing against a wall.

3. Isokinetic – Tension created from contraction is maximal throughout the range of movement. In an isotonic concentric contraction, the tension is not even throughout the range of movement. It is easier in the last phase of a pull-up than in the first part. With an isokinetic contraction, the tension is the same throughout. Many gym equipment pieces such as ‘Hydra’ gym operate on this principle. The force applied by the machine is equal to the force exerted by the muscle. In other words, the harder you push, the harder it pushes back. This type of contraction supposedly gives you the greatest muscular improvement.

Agonists & Antagonists

Muscles work in pairs. The AGONIST is the working muscle; for example, the muscle contracting (biceps muscle). The ANTAGONISTis the relaxing muscle. Here, the bicep is theagonist (or prime mover) and the tricep is theantagonist. This process of one muscle relaxing and one contracting is called reciprocal inhibition. This relaxation/contraction is automatic and occurs without the conscious control of the athlete.