Spark image

The forearm as a lever

We can consider the forearm as a simple level. The pivot (or fulcrum) is at the elbow joint, the load is held in the palm of the hand and the effort is applied in the biceps muscle. In the arrangement shown in the diagram the forearm is horizontal and the force in the biceps muscle acts vertically.

(N.B the blue force lines represent the direction of the forces and NOT their relative magnitude)

Using the value shown in the diagram and taking moments about the fulcrum (F):
Anticlockwise moments = effort x 50
Clockwise moments = 20x150 + 10x400 = 7000
Effort x 50 = 700 and so
Effort in biceps muscle to support the forearm and a 10N load in the hand = 7000/50 = 140 N

(See: Forearm animation)
© Keith Gibbs 2011