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Joints and muscle contraction
Muscles bend (flex) and straighten (extend) to allow you to work. Muscles bring about movement at
a joint. They can only pull they cannot push, so at least two muscles are needed to move a bone to
and fro. A pair of muscles that work in this way are described as antagonistic. A muscle that
contracts to cause extension of a joint is called an extensor; the corresponding flexor muscle
contracts to reverse the movement.
The hip, knee and ankle joints are examples of synovial joints; the bones that move are separated
by a cavity filled with synovial fluid, which acts as a lubricant. Synovial joints have the same basic
Muscles are made up of bundles of muscle fibres. Each fibre is a single muscle cell and can be very
long. They are multinucleate because a single nucleus would not be able to control the metabolism
of such a long cell. They are also striped which is important for contraction.
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Inside the muscle fibre there are also numerous myofibrils; each is composed if repeated contractile
units called sarcomeres.
A sarcomere is made up of 2 protein molecules; thin filaments made of actin (light band) and thicker
ones made of myosin (dark band). Contractions are brought about by the co-ordinated movement
of these 2 filaments in conjunction to each other.
How the sarcomere shortens
Actin molecules are associated with 2 other protein molecules called troponin and tropomyosin.…read more
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This hydrolysis causes a change in the shape of the myosin head. It returns to its upright
position. This enables the cycle to start again.…read more