Thin filaments are made up from two chains of actin spiral wound around each other. Thin filaments also contain other proteins including tropomyosin and troponin that directs contraction.
The release of many myofilaments produces the myofibril, which shows the striations. The sarcomere is the basic contractile unit of a myofibril. Each myofibril contains many sarcomeres.
- The Z-disk delineates the borders of sarcomeres.
- Thin filaments attach to a Z-disc at their plus ends
- Thick filaments are centred between the Z-discs.
Levers
A lever is a mechanical system that creates movement. It either has a pivot or a fulcrum, a force that generates the movement, a resistance load or weight that is moved, a lever arm which applies.
The application of a force to the lever arm causes the arm to rotate around the pivot or fulcrum, causing the load or resistance to move.
Class 1 lever:
The fulcrum is in between the load and the effort.
E
F
L
Note that if the system is in balance the principle of movement applies. An example from a human joint complex is the action of the triceps muscle on the elbow joint
The effort lies in the muscle, the fulcrum at the elbow joint and the load at the hand exerting a force.
Class 2 lever:
Where the load lies between the effort and the fulcrum
In this type of lever the fulcrum is at one end of the lever arm, the effort at the other end, the load is between the fulcrum and the effort.
Note: because of the rotational nature of a lever system load and effort must move in opposite direction – clockwise or anti-clockwise.
An example of this type of lever in the human body is the ankle joint.
Class 3 lever:
The fulcrum and load are at opposite ends of the lever arm, with the effort somewhere in the middle. In this case the effort is always larger than the load, since the effort is nearer the fulcrum.
This is the most common class of lever to be found in the human joint complexes. For example the biceps curl.
Task 2
Describe the process of muscle contraction from the neural stimulation to the final movement.
“A muscle can only contract when only a nerve ending is stimulated by outgoing impulses from the central nervous system (CNS), which consists of the brain and spinal cord.”
The contractile system for muscles is organised into distinct parts, each are controlled by a single motor neurone, and each of the motor neurone controls a large number of muscle fibres. A group of fibres and its neurone is called a motor unit.
All the skeletal muscle reacts to an electrical stimulus, which is conducted from the brain to the muscle via nerves. The central nerves system are the brain and spinal cord and the nerves that carry information from the central nerves system to the skeletal muscle are called motor or efferent nerves. Motor nerves form part of the somatic nervous system, which then forms part of the peripheral nervous system.
Central nervous System
(Brain and spinal cord)
Peripheral nervous system
(Cranial and spinal nerves)
Somatic nervous Autonomic nervous system
System (under (under involuntary control)
Voluntary control) Sensory neurones/
- Sensory neurones/ visceral receptors
Afferent nerves Motor neurones/
- Motor neurones/ sympathetic and
Efferent nerves parasympathetic nerves
Davis. B, Bull. R, Roscoe. J, Roscoe. D; Physical Education and the study of sport; 1997 third edition; Mosby; London’ p 40
A motor neurone is started of as a cell body within the grey matter and its axon passes out of the ventral root of the spinal cord to innervate muscle cells. The muscle cell consists of three major parts that include a cell body containing the nucleus, mitochondria and other organelles, cellular extensions called dendrites and an axon. Dendrites look like fine branches with twigs extending out from a neurone. Following sensory stimulation a relay neurone transmits neural impulses to the dendrites of the motor neurone; they are specialised to receive electrical impulses and conduct these towards the cell body.
Axons are segmented tube-like extensions of a neurone. They arise from the thickened area of the cell body and emerge out of the ventral root of the spinal cord. The function of the axon is to transmit neural impulses away from the cell body towards muscle tissue or a gland. The surrounding myelin sheath acts as an insulator this will speed up the transmission of the impulse. Motor neurone typically divides up into several branches as it reaches the muscle bed. The branches then connect motor neurone to the muscle fibres by distinct structures known as motor end plates. Control by the brain is possible because of the ascending and descending fibres to the motor cortex.
Information is relayed from the brain to the muscle via a nerve impulse. A nerve impulse is an electrical current running the length of the nerve, starting at the brain and passing down the spinal column tot he relevant cell body. The cell bodies of individual motor neurones are located in various regions of the anterior horn of the spinal column. These collections of cell bodies are called motor neurone pools. The cell bodies are positioned in relation to the muscle they stimulate, for example the circumflex nerve, which stimulates the deltoid, is found in the fifth cervical vertebra.
The nerve impulse is passed along the axon of the motor neurone. The axon is covered in a myelin sheath. This sheath is mostly made up of fat to insulate the nerve, the sheath is not continuous. The node of Ranvier is place where there are gaps in the myelin sheath. The impulse is passed from one node of Ranvier to the next so that the impulse can travel faster, it slow down if it would need to travel the whole length of the axon. Saltatory conduction is the name of this nerve impulse propagation method. Also the thicker the myelin sheaths the faster the nerve impulse that can be conducted.
Once the impulse the end of the axon the nerve transmits the information to the muscle by releasing a chemical transmitter called acetylcholine, at the neuromuscular junction.
The nerve impulse is when information is relayed from the brain to the muscle via a nerve impulse. A nerve impulse is an electrical current running the length of the nerve, starting at the brain and passing down the spinal column to the relevant cell body. The cell bodies of individual motor neurones are located in various regions of the anterior horn of the spinal column. These cell bodies are referred to as motor neurone pools. The cell bodies are where the muscles are they stimulate for example the circumflex nerve.
Task 4
Describe in detail the contractions in one specific area of activity and the muscle and movement types involved.
The biceps curl:
In this exercise there are three joints involved the shoulder, elbow and wrist. Then to do the action there is three movements upward, top and downward movement.
Task 3
Examine the different types of muscular contraction and the movements that they produce
The 'All or none law' is that all the forces within a motor unit will contract maximally or not at all.
Generally there are three ways or categories of muscle contraction. They are isotonic, isokinetic and isometric. There are two more that are called concentric and eccentric.
Isotonic contraction is the most common form of muscular contraction. It occurs when a muscle is acting as a prime mover and shortening under tension, creating movement around a joint.
The definition of isotonic contraction is the force applied to a movable object or it is a dynamic movement. An example of this is picking up an object. When you pick up a bag you applied a force to a movable object. When you are lifting weights you are also applying force to a movable object. It is possible that isotonic contraction can turn into isometric contraction. An example of this if you try to pick up a heavy bag. You move the bag slightly upwards (isotonic contraction), but then you stop and you can’t stand up completely erect while holding the sack. At the point when the movement stopped, contraction became isometric.
In isotonic contractions the muscle contracts and shortens, giving movement. Nearly all the training you do is isotonic.
Advantages
- Strengthens a muscle throughout the range of movement.
- You can choose isotonic exercises to match the actions in your sport.
Disadvantages
- Can make muscles sore, because of stress while they lengthen.
- The muscle gains most strength at the weakest point of the action, rather than evenly throughout.
Isometric contraction is forced applied to an immovable object. To show this, you stand in a doorway. Place your hands on the door jams. Now push outwards on them. You will notice that your arm muscles are contracting, but there is no movement so you will not be able to push the doors apart.
In isometric contractions the muscle contracts but does not shorten, and therefore there is know movement being produced.
Advantages
- Isometric exercises develop static strength - the strength you need to push or pull a heavy object or hold it up.
- They are quick to do and don't hurt.
- They don't need expensive equipment.
- You can do them anywhere.
Disadvantages
- The muscle gains strength only at the angle you use in the exercise.
- During an exercise the blood flow to the muscle stops, blood pressure rises, and less blood flows back to the heart. It could be dangerous if you have heart problems.
Isometric training is not sufficient on its own. You need to combine it with isotonic training.
Isokinetic contractions are when the muscle contracts and shortens at constant speed. An isotonic contraction is different to an isokinetic contraction because it is usually slowest at the start.
For isokinetic training you need special equipment that detects when a muscle is speeding up, and increases the load to slow it down again.
Advantages
- The muscle gains strength evenly all through the range of movement.
- It is the fastest way to increase muscle strength.
Disadvantage
- The equipment is very expensive so most gyms cannot afford it.
Concentric contraction occurs when a muscle shortens in length and develops tension. Eccentric contraction involves the development of tension whilst the muscle is being lengthened.
Bibliography
Davis, Bull, Roscoe, Roscoe; physical Education and the study of sport; 1997 3rd edition; Mosby; London
Bibliography
Davis, Bull, Roscoe, Roscoe. Physical education and the study of sport (third edition)
Publisher: Geoff Greenwood
Published: 1997
Place of published: Lynton House
John Honeyborne, Michael Hill and Helen Moors. Physical education and sport (second edition)
Publisher: S. Thornes
Published: 2000
Place of published: Cheltenham
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