44 Forces on the Forearm while Curling

In this unit we use the example of holding a 50 lb weight in the hand to learn more about various forces acting within the body,  how they are generated, and how to determine their and direction.

Figure is a schematic drawing of a forearm rotated around the elbow. A 50 pound ball is held in the palm. The distance between the elbow and the ball is 13 inches. The distance between the elbow and the biceps muscle, which causes a torque around the elbow, is 1.5 inches. Forearm forms a 60 degree angle with the upper arm.
The elbow joint flexed to form a 60° angle between the upper arm and forearm while the hand holds a 50 lb ball . Image Credit: Openstax University Physics

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The force in the biceps is trying to lift and rotate the forearm around the elbow joint. The is pulling down on the ball and the forearm, causing rotation in the opposite direction to that caused by the biceps. Our intuition tells us that the muscle tension needs to somehow counteract the of the ball with the forearm acting as a sort of . That’s where we’ll start to solve this example.

We know the ball weighs 50 lbs. If we wanted to include the effect of the of the forearm in our example problem we could look up a typical forearm weight and also look up where the of the forearm is located.  Instead let’s take this opportunity to practice making justified . We know that forearms typically weigh only a few pounds, but the ball weight is 50 lbs, so the forearm weight is about an (10x) smaller than the ball weight[2].  Therefore, let’s assume the forearm weight doesn’t matter much and continue solving our biceps force problem only including the weight of the ball, which acts at the of the ball.

 


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