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The following article will guide you about how muscles move bones at a joint in human body.
Muscles work in the following way:
(i) They can pull but never push.
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(ii) They pull only when they contract.
(iii) When they contract, they decrease in length.
(iv) When returning to their original length, they relax (not ‘expand’).
(v) They are attached to bones by tendons which do not stretch. They are arranged in antagonistic pairs, and apply their force either side of the bone they are required to move.
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(vi) One muscle of the pair (the flexor) contracts to bend the limb at the joint. As it does so, the other muscle in the pair (the extensor) relaxes.
(vii) The extensor contracts to straighten the limb at the joint, and as it does so, the flexor relaxes.
The arrangements of the antagonistic muscles at the hinge joint of the elbow are shown below:
