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Action of muscles - moving a smaller force will keep it...

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Action of muscles Muscles do not suddenly snap from a state of relaxation to one of contraction. At a given time some functional units (motor units, groups of fibres of various size) will be contracting, some relaxing and some in stasis, the resultant providing muscle tone. If the proportions doing each stays constant, so will muscle tone, although individual units will cycle. When an individual fibre contacts it tends to approximate its ends, but whether or not this results in contraction depends on the force generated and the forces opposing contraction. The net result for the whole muscle may be contraction, relaxation or stasis. A muscle trying to initiate contraction is opposed by 1. passive internal resistance of muscle 2. ditto articular tissues 3. opposing muscles 4. opposing soft tissues 5. inertia of whatever it is trying to move 6. load 7. gravity If the force generated exceeds the sum of all these then the limb is accelerated from rest: once
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Unformatted text preview: moving a smaller force will keep it moving. A muscle doing this is sometimes called a prime mover or agonist . It is often opposed by antagonists which can stop the movement. When both groups act together nothing moves, or the movement is moderated or controlled. If the movement is abolished the real result is that the joint across which the muscles act will be stabilised - often cannot be done wholly by close packing or gravity - which are preferable as they use little energy. Movement is always opposed or aided by gravity, and this is used wherever possible. In placing a weight on a table the extensor of the arm is not triceps but gravity, controlled by slow relaxation of the flexors. The action of a prime mover often exerts a little unwanted movement. For example the flexion of the fingers by long flexors also flexes the wrists: this is opposed by wrist extensors....
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