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Forearm and the biceps muscles

This animation shows the behaviousr of the biceps muscle when the arm is bent. The biceps muscle shortens and thickens to raise the forearm. If you consider the distance of the point of attachment of the biceps muscle and the centre of mass of the forearm from the elbow (the pivot) it is clear that the force in the biceps muscle is much greater than the weight of the forearm.

 
 
 
© Keith Gibbs/John Bourne 2009