Weightlifting can exert extremely large forces on the body\'s joints and muscles

Question

Weightlifting can exert extremely large forces on the body's joints and muscles. In the strict curl event, a standing athlete lifts a barbell with two hands by moving only their forearms, which pivot at the elbow. During the lift there are forces acting in both the elbow joint and the bicep muscle. The record weight lifted in the strict curl is over 2.00E2 lbs (9.00E2 N). If the athlete's right hand that holds the weight is 35 cm from the elbow and the lifting bicep muscle attaches 4.0 cm from the elbow in the same direction, then what is the tension in the right bicep muscle while the barbell is held stationary in this position? What is the force exerted at the elbow?

Explanation / Answer

Before starting, let me note that I'm assuming the arm is in a horizontal position. The lower arm being diagonally will change my result. Let FB and FE be the forces in the bicepts and elbow, respecitvlely. Frist note that the elbow is the piviot point, so we have 35cm * 9.00E2 N for the torque of the weight, 0cm *FE=0 for the torque of the elbow, and 4.0cm *FB for the torque of the bicept. Thus, for the torque equation, we can ignore the elbow and we have 4.0cmFB - 35cm * 9.00E2 N = 0. The force the bicept exerts is 35*900/4.0 N = 7,875N Since the distances were to 2 significant digits, our final answer is 7,900 (or 7.9E3) N. For FE note that everthing is stationary, so the sums of Fx, Fy, etc. =0. With the assumption of a horizontal lower arm, the bicept (and weight) forces are vertical, so all we have to worry about is Fy = 0. This gives FE + 7875 N - 900 N = 0, and FE = -6975 N, which rounds to -7.0E3 N.

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